AGE AND TECTONIC EVOLUTION OF THE AMDO BASEMENT: IMPLICATIONS
FOR DEVELOPMENT OF THE TIBETAN PLATEAU AND GONDWANA
PALEOGEOGRAPHY
by
Jerome Hamilton Guynn
_________________________
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
2006
2
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
As members of the Dissertation Committee, we certify that we have read the dissertation
prepared by Jerome Guynn
entitled “Age and Tectonic Evolution of the Amdo Basement: Implications for
Development of the Tibetan Plateau and Gondwana Paleogeography”
and recommend that it be accepted as fulfilling the dissertation requirement for the
Degree of Doctor of Philosophy
_______________________________________________________________________
Date: 11/20/06
Dr. Paul Kapp
_______________________________________________________________________
Date: 11/20/06
Dr. Pete DeCelles
_______________________________________________________________________
Date: 11/20/06
Dr. George Zandt
_______________________________________________________________________
Date: 11/20/06
Dr. Mihai Ducea
_______________________________________________________________________
Date: 11/20/06
Dr. Clem Chase
Final approval and acceptance of this dissertation is contingent upon the candidate’s
submission of the final copies of the dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared under my direction and
recommend that it be accepted as fulfilling the dissertation requirement.
________________________________________________ Date: 11/20/06
Dissertation Director: Dr. Paul Kapp
3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an
advanced degree at the University of Arizona and is deposited in the University Library
to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided
that accurate acknowledgment of source is made. Requests for permission for extended
quotation from or reproduction of this manuscript in whole or in part may be grated by
the head of the major department or the Dean of the Graduate College when in his or her
judgment the proposed use of the material is in the interests of scholarship. In all other
instances, however, permission must be obtained from the author.
Signed: Jerome H. Guynn
4
ACKNOWLEDGMENTS
Many, many people have supported me throughout the last seven years as I switched
careers from engineering to geology. First and foremost is my mom, Trish Casper, who
wholly supported my decision to go back to school and has always been there to talk to
during bumps along the way. My sister, Sierra, and I have been in graduate school about
the same time as me and so she has been a great resource and a wonderful support during
this time. She also was the first to suggest I go back to school in geology after the great
trip we had to New Zealand. My friend, Mimi Ashcraft, has always wanted me to pursue
geology, ever since I earned the geology merit badge with her in high school. She is
always interested in hearing about my research and has been highly supportive.
I want to give a big “thank you” to my advisor, Paul Kapp, who has provided me with
opportunities, advice, constructive criticism and encouragement throughout the time I
have earned my Ph.D. He has always been available for discussing my research and
helping me through problems and I have learned a great deal about doing science from
him. I have also learned a lot from Pete DeCelles, in classes, in the field and in
discussions about Himalayan and Tibetan geology. George Gehrels has been a great help
and I am indebted to him for his time and assistance with U-Pb geochronology, which has
formed a large part of my research. I would also like to thank my other committee
members: George Zandt, Mihai Ducea and Clem Chase. I also want to thank Matt
Heizler of New Mexico Tech for his 40Ar/39Ar analyses. Finally, I thank Ding Lin of the
Chinese Academy of Sciences in Beijing for his support with our field work, including
permits, colleagues and travel arrangements.
A special “thanks” goes to Alex Pullen and Ross Waldrip who assisted me in the field
in Tibet. John Volkmer, Shundong He and Ross Waldrip have been terrific graduate
student colleagues and friends. Other fellow graduate students and undergraduates who
have provided assistance and discussion include Joel Saylor, Dave Pearson, Jen Fox, Jen
McGraw, Kelley Stair and Jen Pullen. Ken Dominik of the Lunar and Planetary
Laboratory provided invaluable assistance with the electron microprobe.
My roommates for the past year and a half, Kevin Anchukaitis and Tm Shanahan,
have been a great support as we have all struggled through finishing our Ph.D. I will
miss the commiserating and comradery that we have shared over many beers and burgers
at Gentle Ben’s the past year. Thanks to Katie Davis for our many conversations and the
good advice she always has. I also have a long list of Arizona friends who have been
very supportive and a great social network for escaping the craziness of grad school.
Many thanks to Aly, Andy, Anna, Britt, Dave, Jessica R., Jessica C., Kevin J., Lynette,
Scott, Toby, Tom, Megan, Lara, Tashana, Jana and Morgen.
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DEDICATION
This dissertation is dedicated to my mom, Trish Hamilton Casper; my sister, Dr. Sierra
Guynn; and my friend, Mimi Ashcraft.
6
TABLE OF CONTENTS
ABSTRACT....................................................................................................................12
INTRODUCTION..........................................................................................................13
PRESENT STUDY.........................................................................................................17
Timing of Bangong suture tectonics as revealed by the Amdo basement...........17
Metamorphism of the Amdo basement..................................................................18
Jurassic and Cretaceous evolution of the Bangong suture...................................19
Geochronology of the Amdo gneisses and paleogeography of the Lhasa and
Qiangtang terranes ..................................................................................................20
REFERENCES CITED .................................................................................................21
APPENDIX A: Permission for Reproduction from the Geological Society of
America...........................................................................................................................25
APPENDIX A: Tibetan basement rocks near Amdo reveal “missing” Mesozoic
tectonism along the Bangong suture, central Tibet ....................................................27
ABSTRACT .............................................................................................................28
INTRODUCTION ...................................................................................................29
GEOLOGY ..............................................................................................................31
GEOCHRONOLOGY ............................................................................................32
HISTORY OF METAMORPHISM AND COOLING ........................................33
DISCUSSION AND CONCLUSIONS ..................................................................35
ACKNOWLEDGEMENTS ...................................................................................37
REFERENCES CITED ..........................................................................................37
7
TABLE OF CONTENTS - Continued
FIGURE CAPTIONS ..............................................................................................42
FIGURES..................................................................................................................45
Data Repository Item DR2006094 Supplementary Geochronologic and
Thermochronologic Data .......................................................................................49
Summary of analytical methods for zircon and titanite U-Pb analysis
(University of Arizona) ......................................................................................49
Summary of analytical methods for mica 40Ar/39Ar analyses (University of
California, Los Angeles). ...................................................................................50
Summary of analytical methods for hornblende and feldspar 40Ar/39Ar
analysis (New Mexico Geochronological Research Laboratory)...................50
K-Feldspar multi-domain diffusion modeling methods .................................51
References Cited.................................................................................................52
Figures and Tables.............................................................................................54
APPENDIX B: Metamorphism and exhumation of the Amdo basement, Tibet:
Implications for the Jurassic tectonics of the Bangong Suture zone ........................74
ABSTRACT .............................................................................................................75
INTRODUCTION ...................................................................................................76
REGIONAL GEOLOGY AND PREVIOUS WORK ..........................................80
Basement Rocks ................................................................................................82
THERMOBAROMETRY ......................................................................................84
Garnet-kyanite schist thermobarometry ........................................................86
8
TABLE OF CONTENTS - Continued
Mafic amphibolite thermobarometry .............................................................88
Thermometry of non-garnet bearing amphibolites .......................................94
Samples without thermobarometry .................................................................95
U-Pb METAMORPHIC GEOCHRONOLOGY .................................................97
DISCUSSION ................................................................................................................100
Thermobarometric results ...............................................................................100
Tectonic implications ........................................................................................104
ACKNOWLEDGEMENTS ...................................................................................108
REFERENCES ........................................................................................................108
FIGURE CAPTIONS .............................................................................................116
FIGURES .................................................................................................................122
TABLES ...................................................................................................................137
APPENDIX C: Jurassic and Cretaceous Tectonic Evolution of the Bangong Suture
Zone near Amdo, central Tibet ....................................................................................145
ABSTRACT .............................................................................................................147
INTRODUCTION ...................................................................................................149
REGIONAL GEOLOGY .......................................................................................151
GEOCHRONOLOGY ............................................................................................155
Igneous Zircon Analysis ...................................................................................156
Detrital Zircon Analysis ...................................................................................157
Jurassic flysch ...............................................................................................158
9
TABLE OF CONTENTS - Continued
Red arenites and shale ..................................................................................158
GEOLOGY OF THE AMDO AREA ....................................................................160
Rock Types and Ages ........................................................................................160
Structure of the Amdo Basement Region .......................................................168
DISCUSSION ..........................................................................................................172
Jurassic Tectonic Evolution of the Bangong Suture at Amdo ......................173
Cretaceous Tectonic Evolution of the Bangong Suture at Amdo .................175
CONCLUSIONS .....................................................................................................179
ACKNOWLEDGMENTS ......................................................................................180
REFERENCES CITED ..........................................................................................181
FIGURE CAPTIONS .............................................................................................188
FIGURES .................................................................................................................195
TABLES ...................................................................................................................209
APPENDIX D: U-Pb geochronology of basement rocks in central Tibetan and
paleogeographic implications .......................................................................................225
Abstract ....................................................................................................................227
1. Introduction .........................................................................................................228
2. Regional Geology and Paleozoic Paleogeography ............................................230
3. Amdo Basement Geology ...................................................................................233
4. U-Pb Geochronology ...........................................................................................234
4.1. Methods ........................................................................................................235
10
TABLE OF CONTENTS - Continued
4.2. Igneous Zircon Analysis ...............................................................................239
4.2.1. JG053104-1 .........................................................................................239
4.2.2. JG061504-2 .........................................................................................240
4.2.3. PK970604-3A ......................................................................................240
4.2.4. JG060504-2 .........................................................................................240
4.2.5. JG061504-1 .........................................................................................241
4.2.6. PK970604-1B ......................................................................................241
4.2.7. JG061604-1 .........................................................................................241
4.2.8. PK970604-1A ......................................................................................242
4.2.9. JG053104-2 .........................................................................................242
4.3. Detrital Zircon Analysis ...............................................................................243
4.3.1. JG061504-4 .........................................................................................243
4.3.2. JG062504-3 .........................................................................................244
4.3.3. AP061304-A ........................................................................................244
5. Discussion .............................................................................................................244
5.1. Tibetan Paleozoic Detrital Zircon Signature ...............................................245
5.2. Regional Detrital Zircon Record .................................................................247
5.3. Regional Basement Ages ..............................................................................251
5.4. Implications for Paleogeography of the Lhasa-Qiangtang terrane ............261
6. Conclusions ..........................................................................................................265
Acknowledgements .................................................................................................267
11
TABLE OF CONTENTS - Continued
Appendix A.1 Description of Rock Samples..........................................................267
A.1.1. JG053104-1 orthogneiss ...................................................................267
A.1.2. JG061504-2 orthogneiss ....................................................................267
A.1.3. PK970604-3A orthogneiss ..................................................................268
A.1.4. JG060504-2 orthogneiss ....................................................................268
A.1.5. JG061504-1 orthogneiss ....................................................................268
A.1.6. PK970604-1B orthogneiss ..................................................................269
A.1.7. JG061604-1 orthogneiss ....................................................................269
A.1.8. PK970604-1A orthogneiss ..................................................................269
A.1.9. JG053104-2 orthogneiss ....................................................................269
A.1.10. JG061504-4 paragneiss ....................................................................269
A.1.11. JG062504-3 quartzite .......................................................................270
A.1.12. AP061304-A quartzite ......................................................................270
References ................................................................................................................270
Figure Captions .......................................................................................................290
Figures ......................................................................................................................297
Tables .......................................................................................................................309
12
ABSTRACT
The elucidation of the geologic processes that led to the creation of the Tibetan
Plateau, a large area of thick crust and high elevation, is a fundamental question in
geology. This study provides new data and insight on the geologic history of central
Tibet in the Jurassic and Cretaceous, prior to the Indo-Asian collision, as well as the
Gondwanan history of the Lhasa and Qiangtang terranes of the plateau.
This
investigation is centered on the Bangong suture zone near the town of Amdo and I
present new geochronology, thermochronology, thermobarometry and structural data of
the Amdo basement, an exposure of high-grade gneisses and intrusive granitoids. Using
a range of thermochronometers, I show there were two periods of cooling, one in the
Middle-Late Jurassic after high-grade metamorphism and a second in the Early
Cretaceous.
I attribute Middle-Late Jurassic metamorphism, magmatism, and initial
cooling of the Amdo basement to arc related tectonism that resulted in tectonic or
sedimentary burial of the magmatic arc. I propose that a second period of cooling,
nonmarine, clastic sediment deposition and thrust faulting in the Early Cretaceous is
related to the Lhasa-Qiangtang collision.
The thermochronology reveals limited
denudation between the Cretaceous and the present, indicating the existence of thickened
crust when India collided with Asia in the early Tertiary. U-Pb geochronology of the
orthogneisses and detrital zircon geochronology of metasedimentary rocks suggests that
the Lhasa and Qiangtang terrane were located farther west along Gondwanan’s northern
margin than most reconstructions depict.
13
INTRODUCTION
The Tibetan Plateau is the largest and, on average, highest orogenic feature on the
earth, with an area of approximately 5,000,000 km2 and an average elevation of 5 km
(Fielding et al., 1994). The events and processes that led to its creation are an area of
ongoing research.
A variety of end-member models have been proposed for the
formation of the plateau due to India’s collision with Asia, including distributed
shortening (Dewey and Burke, 1973), underthrusting of Indian lithosphere (Argand,
1924; Powell and Conaghan, 1973), continental injection (Zhao and Morgan, 1985),
crustal flow (Royden et al., 1997), and oblique continental subduction and sedimentary
basin infilling (Meyer et al., 1998; Tapponnier et al., 2001). However, all these models
assume that Tibet was near sea-level at the time of the Indo-Asian collision in the early
Tertiary and that the plateau is solely due to Tertiary tectonics. In addition, many of
these models are based on processes that may be currently active on Tibet’s margins but
may not have contributed to it’s earlier growth (Kapp et al., 2003a). While much of the
research concerning the geology of the plateau has focused on the time period since the
Indo-Asian collision in the Early Tertiary, much less work has been done on the prior
geologic history of Tibet and the influence of that previous tectonism on its development.
Some recent geologic investigations of Tibet’s Mesozoic history have suggested that
prior deformation played an important part in forming the plateau. These studies have
focused on the Lhasa and Qiangtang terranes which make up the central and southern
region and collided along the Bangong suture in the Early Cretaceous (Guynn et al.,
2006; Kapp et al., 2003a; Yin and Harrison, 2000). Murphy et al. (1997) documented a
14
Cretaceous thrust belt in the central Lhasa terrane near the town of Coqen and suggested
a 60-65 km thick crust in the region during the early Tertiary. Further Cretaceous and
early Tertiary shortening was documented just north of Coqen and showed that
shortening was decoupled between the upper (Cretaceous) and lower (late Paleozoic)
sedimentary rocks (Volkmer et al., accepted). Several studies have revealed thrust belts
in the northern Lhasa terrane along the Bangong suture that were active in the Cretaceous
and Early Tertiary (Kapp et al., 2003a; Kapp et al., 2005). These results imply a large
part of the Lhasa terrane has been thrust underneath the Qiangtang terrane and may be the
cause of a regional anticline in the central Qiangtang terrane (Kapp et al., 2003b; Yin and
Harrison, 2000). Finally, field studies have documented the presence of an Andean-style
retro-arc thrust belt and associated foreland basin in the southern Lhasa terrane in the
Late Cretaceous and early Tertiary (Kapp et al., submitted; Leier et al., in press).
Collectively these studies indicate significant shortening and resultant crustal thickening
throughout the southern Qiangtang and Lhasa terranes during the Cretaceous and early
Tertiary, right up until the Indo-Asian collision.
The southward propagating thrust belts of the northern and southern Lhasa terrane are
thought to be a result of the Lhasa-Qiangtang collision. At the western end of the suture,
this collision has been documented to have occurred during the Early Cretaceous, just
prior to thrust belt development (Kapp et al., 2003a; Matte et al., 1996). In the east
around the town of Amdo, however, the collision is thought to have occurred in the Late
Jurassic based on ophiolite obduction and some limited thermochronology (Dewey et al.,
1988; Girardeau et al., 1984; Xu et al., 1985).
Resolving the time of collision is
15
important for understanding thrust belt development and for relating deformation due to
the Lhasa-Qiangtang collision to tectonics associated with the Indo-Asian collision. The
Lhasa-Qiangtang collision is a poorly understood event and the Bangong suture zone has
several enigmatic aspects, including a lack of arc-related igneous rocks despite
subduction of the Meso-Tethys Ocean (Allégre et al., 1984; Dewey et al., 1988) and the
occurrence of ophiolite fragments up to 200 km across parts of the suture zone (Girardeau
et al., 1984; Matte et al., 1996). Better defining the tectonic history of the suture zone
could help our knowledge of accretionary processes.
In order to understand the evolution of the suture zone and the Cretaceous tectonics of
the Tibetan Plateau, I studied an area along the suture zone referred to as the Amdo
basement or Amdo gneiss. The Amdo basement is the only exposure of crystalline
basement along the suture zone, which makes it an ideal location for several geologic
techniques, including geochronology, thermochronology and thermobarometry.
The
gneisses within the exposure have experience high-grade metamorphism that indicates a
major tectonic event (Harris et al., 1988), but that metamorphism has been attributed to
Cambrian tectonics and not the Jurassic-Cretaceous Lhasa-Qiangtang collision (Coward
et al., 1988; Dewey et al., 1988; Xu et al., 1985). The Amdo basement is also unique
because it is the only exposure of Precambrian, crystalline basement within the central
part of the Tibetan Plateau (Dewey et al., 1988; Yin and Harrison, 2000). Thus it
provides an opportunity to study the deeper crust of the plateau and to constrain the older
geologic history of the Lhasa and Qiangtang terranes during and prior to formation of
Gondwana in the late Neoproterozoic. Despite its importance, the Amdo basement has
16
received only minor attention in past studies of the Tibetan Plateau (Coward et al., 1988;
Harris et al., 1988; Xu et al., 1985).
17
PRESENT STUDY
The research presented in this dissertation is an addition to our understanding of the
geologic history of the Tibetan Plateau prior to the Indo-Asian collision. Specifically, I
address several issues related to the Lhasa and Qiangtang terranes of Tibet, including the
tectonic evolution of the Bangong suture zone, timing and extent of shortening and
denudation related to the Lhasa-Qiangtang collision, geochronology of the gneisses that
comprise the bulk of the Amdo basement exposure and the composition and location of
the Lhasa and Qiangtang terranes prior to their rifting from Gondwana. The methods,
results and conclusions of this study are presented in the papers appended to this
dissertation and the following is a summary of the methods used and the most important
results.
Timing of Bangong suture tectonics as revealed by the Amdo basement
I applied a wide variety of thermochronometers to elucidate the cooling history of the
Amdo basement: U-Pb for zircon and titanite and 40Ar/39Ar for hornblende, mica and KFeldspar. In addition, I performed extensive dating of the granitoids that intrude the
Amdo gneisses using U-Pb analysis on zircon. These results, together with my initial
mapping, are presented in Appendix A. This initial work revealed two distinct periods of
cooling for the Amdo basement; one during the Middle-Early Jurassic (180-165 Ma) and
another during the Early Cretaceous (130-115 Ma). Furthermore, titanite U-Pb ages and
new zircon growth revealed by U-Pb analyses show that the high-grade metamorphism
occurred during the Middle-Jurassic and not the Cambrian. In addition, the ages of the
18
intruding granitoids are Middle-Early Jurassic, coeval with the metamorphism and initial
cooling, and the only documented igneous rocks along the suture that could be related to
subduction of the Meso-Tethys Ocean along the Qiangtang margin. Together, these
results demonstrate a major period of tectonism during the Middle-Early Jurassic which I
suggest is related to the development of a continental arc along the southern margin of
the Qiangtang terrane.
I further propose that this tectonism resulted in burial or
underthrusting of the arc as an explanation for the lack of subduction related igneous
rocks along the suture zone.
Finally, I propose that the second period of cooling
represents the time of Lhasa-Qiangtang collision, similar to the timing of collision to the
west, and this resulted in thrusting of the basement over unmetamorphosed sedimentary
rocks and the deformation of Cretaceous (?) nonmarine sedimentary rocks. The lowtemperature thermochronology of the K-Feldspar 40Ar/39Ar analysis implies that there
was limited denudation (< 5 km) since the mid-Cretaceous.
Metamorphism of the Amdo basement
In Appendix B, I present a detailed investigation of the timing and degree of
metamorphism of the Amdo basement.
Thermobarometry and mineral assemblages
reveal that the basement experienced peak temperatures of 700-750°C and pressures of
10-12 kbar. These conditions were experienced by the entire ~50 km x 50 km exposure
of gneisses that I mapped, which places an important constraint on the exhumation of the
basement rocks. U-Pb ages of metamorphic zircon growth show that peak conditions
occurred 185-175 Ma. Metasedimentary rocks along the southern edge of the basement
19
record slightly lower conditions, around 600°C and 8 kbar, indicating that the highergrade gneisses were thrust over them. The lithologies and the metamorphic conditions
are similar to those seen in the Coast Plutonic Complexes of the northwestern U.S. and
western Canadian Cordillera, which provides additional support for an arc-related
tectonic environment.
Jurassic and Cretaceous evolution of the Bangong suture
I spent two summers mapping and taking structural measurements of the Amdo
basement and deformed sedimentary rocks just south of the basement. I also collected
and analyzed several sedimentary rocks for detrital zircon analysis to determine
provenance and to provide constraints on depositional ages. The results of this work and
the development of a tectonic model for the Bangong suture zone in the late Mesozoic are
the subject of Appendix C. The detrital zircon record reveals marine sedimentation on
the southern margin of the Qiangtang terrane through the Jurassic, followed by
nonmarine deposition of coarse, clastic sediments in the mid-Cretaceous. Structural
measurements demonstrate southward directed thrusting within the Amdo basement,
including the Jurassic granitoids, as well as thrusts placing basement over lower-grade
metasedimentary rocks and metamorphic rocks over sedimentary rocks. I dated a granite
pluton (~117 Ma) that is cut by a thrust fault and another intrusion (~106 Ma) that cuts a
thrust fault. These demonstrate shortening in the Early Cretaceous, coeval with or just
following the period of cooling in the Amdo basement revealed by K-Feldspar 40Ar/39Ar
analysis (130-115 Ma). A simplified cross-section of the region suggests ~100 km of
20
shortening at a rate typical of many fold and thrust belts. The data suggest that the Amdo
basement region was the hinterland to the Cretaceous, southward propagating thrust belts
of the northern Lhasa terrane and lend more support to the deposition of Aptian-Albian
limestones and clastic sediments in a foreland basin rather than in a rift environment.
Geochronology of the Amdo gneisses and paleogeography of the Lhasa and
Qiangtang terranes
I performed extensive U-Pb geochronology of Amdo orthogneisses and detrital zircon
geochronology of several metasedimentary rocks associated with the orthogneisses. This
work and its implications are presented in Appendix D. Geochronology reveals two
periods of granitoid emplacement for the orthogneiss protoliths, one in the CambrianOrdovician (~530-480 Ma) and one in the early Neoproterozoic (~910-850 Ma). Detrital
zircon (DZ) geochronology of two quartzites indicates that they were deposited in the
Paleozoic, possibly the Carboniferous-Permian based on the similarity of their DZ spectra
to DZ spectra from Carboniferous-Permian Tibetan sandstones.
A paragneiss was
probably deposited in the Neoproterozoic based on its DZ spectrum. Comparison of the
DZ signature of Tibetan Paleozoic rocks, which were deposited prior to rifting of the
Lhasa and Qiangtang terranes from Gondwana, to those of Paleozoic sedimentary rocks
from the Nepalese Himalaya reveal some distinct differences.
Comparing these
signatures to others from Gondwana, I suggest that the Lhasa and Qiangtang terranes
were not directly north of India as depicted in most reconstructions, but were located
farther to the west, closer to northwest India and eastern Africa.
21
REFERENCES CITED
Allégre, C.J., Courtillot, V., Tapponnier, P., Hirn, A., Mattauer, M., Coulon, C., Jaeger,
J.J., Achache, J., Scharer, U., Marcoux, J., Burg, J.P., Girardeau, J., Armijo, R.,
Gariepy, C., Gopel, C., Li, T.D., Xiao, X.C., Chang, C.F., Li, G.Q., Lin, B.Y.,
Teng, J.W., Wang, N.W., Chen, G.M., Han, T.L., Wang, X.B., Den, W.M.,
Sheng, H.B., Cao, Y.G., Zhou, J., Qiu, H.R., Bao, P.S., Wang, S.C., Wang, B.X.,
Zhou, Y.X., and Ronghua, X., 1984, Structure and Evolution of the HimalayaTibet Orogenic Belt: Nature, v. 307, p. 17-22.
Argand, E., 1924, La tectonique de L'Asie: Proceedings of the 13th International
Geologic Conference, p. 171-372.
Coward, M.P., Kidd, W.S.F., Yun, P., Shackleton, R.M., and Hu, Z., 1988, The Structure
of the 1985 Tibet Geotraverse, Lhasa to Golmud: Philosophical Transactions of
the Royal Society of London Series A-Mathematical Physical and Engineering
Sciences, v. 327, p. 307-336.
Dewey, J.F., and Burke, K.C.A., 1973, Tibetan, Variscan, and Precambrian Basement
Reactivation - Products of Continental Collision: Journal of Geology, v. 81, p.
683-692.
Dewey, J.F., Shackleton, R.M., Chang, C.F., and Sun, Y.Y., 1988, The Tectonic
Evolution of the Tibetan Plateau: Philosophical Transactions of the Royal Society
of London Series A-Mathematical Physical and Engineering Sciences, v. 327, p.
379-413.
22
Fielding, E., Isacks, B., Barazangi, M., and Duncan, C., 1994, How flat is Tibet?:
Geology, v. 22, p. 163-167.
Girardeau, J., Marcoux, J., Allégre, C.J., Bassoullet, J.P., Tang, Y.K., Xiao, X.C., Zao,
Y.G., and Wang, X.B., 1984, Tectonic environment and geodynamic significance
of the Neo-Cimmerian Donqiao ophiolite, Bangong-Nujiang suture zone, Tibet:
Nature, v. 307, p. 27-31.
Guynn, J.H., Kapp, P., Pullen, A., Heizler, M., Gehrels, G., and Ding, L., 2006, Tibetan
basement rocks near Amdo reveal "missing" Mesozoic tectonism along the
Bangong suture, central Tibet: Geology, v. 34, p. 505-508.
Harris, N.B.W., Holland, T.J.B., and Tindle, A.G., 1988, Metamorphic Rocks of the 1985
Tibet Geotraverse, Lhasa to Golmud: Philosophical Transactions of the Royal
Society of London Series A-Mathematical Physical and Engineering Sciences, v.
327, p. 203-213.
Kapp, P., DeCelles, P.G., Leier, A., Fabijanic, J.M., He, S., Pullen, A., and Gehrels, G.,
submitted, The Gangdese Retroarc Thrust Belt Revealed: Geological Society of
America Bulletin.
Kapp, P., Murphy, M.A., Yin, A., Harrison, T.M., Ding, L., and Guo, J.H., 2003a,
Mesozoic and Cenozoic tectonic evolution of the Shiquanhe area of western
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basin development, and volcanism in central Tibet: Geological Society of
America Bulletin, v. 117, p. 865-878.
23
Kapp, P., Yin, A., Manning, C.E., Harrison, T.M., Taylor, M.H., and Ding, L., 2003b,
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Indus: Earth and Planetary Science Letters, v. 142, p. 311-316.
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Murphy, M.A., Yin, A., Harrison, T.M., Durr, S.B., Chen, Z., Ryerson, F.J., Kidd,
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24
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Zhao, W.L., and Morgan, W.J., 1985, Uplift of Tibetan Plateau: Tectonics, v. 4, p. 359369.
25
APPENDIX A: Permission for Reproduction from Geological Society of America
The following email discourse gives permission for the reproduction of the Geology
article “Tibetan basement rocks near Amdo reveal ‘missing’ Mesozoic tectonism along
the Bangong suture, central Tibet” by J.H. Guynn, P. Kapp, A. Pullen, M. Heizler, G.
Gehrels and L. Ding published in June, 2006 by the Geological Society of America
(GSA), as part of this dissertation. Email discourse is an acceptable form of permission
for UMI’s parent company Proquest. The original request for permission is included in
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Subject: RE: Geology article permissions
From: "Jeanette Hammann" <jhammann@geosociety.org>
Date: Wed, 4 Oct 2006 08:10:05 -0600
To: <jhguynn@email.arizona.edu>
Dear Mr. Guynn,
Thank you for your message. Permission is granted for your use of the
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Best regards,
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Sent: Tuesday, October 03, 2006 8:38 PM
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Subject: Geology article permissions
To whom it may concern:
I am a PhD student who will be graduating this semester and I would
like
to include a Geology article that was published in June, 2006.
the
I am
first author of the article, "Tibetan basement rocks near Amdo reveal
"missing" Mesozoic tectonism along the Bangong suture, central Tibet",
and the work presented in this paper is a significant portion of my
PhD. The University of Arizona publishes its theses through University
26
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strive for that, and not wade off into arbitrary falsehood.
George Eliot
27
APPENDIX A: Tibetan basement rocks near Amdo reveal “missing” Mesozoic
tectonism along the Bangong suture, central Tibet
Reprint from Geology
28
Tibetan basement rocks near Amdo reveal “missing”
Mesozoic tectonism along the Bangong suture, central
Tibet
Jerome H. Guynn
Paul Kapp,
Alex Pullen,
Department of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA
Matthew Heizler,
New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining
and Technology, Socorro, New Mexico, 87801, USA
George Gehrels,
Department of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA
Lin Ding
Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese
Academy of Sciences, Beijing 100029 China
ABSTRACT
U-Pb and 40Ar/39Ar studies of a unique exposure of crystalline basement along the
Jurassic - Early Cretaceous Bangong suture of central Tibet reveal previously
unrecognized records of Mesozoic metamorphism, magmatism, and exhumation. The
basement includes Cambrian and older orthogneisses that underwent amphibolite facies
29
metamorphism coeval with extensive granitoid emplacement at 185-170 Ma. The
basement cooled to ~300 °C by 165 Ma and was exhumed to upper crustal levels in the
hanging wall of a south-directed thrust system during Early Cretaceous time. We attribute
Jurassic metamorphism and magmatism to the development of a continental arc during
Bangong Ocean subduction and Early Cretaceous exhumation to northward continental
underthrusting of the Lhasa terrane beneath the Qiangtang terrane. We speculate that a
Jurassic arc extended regionally along the length of the Bangong suture but in all other
places in Tibet has been buried, either depositionally or structurally, beneath supracrustal
assemblages.
Keywords: Tibet, Bangong suture, terrane accretion, continental arcs, continental
collision
INTRODUCTION
While it is widely assumed that the high elevation and thick crust of Tibet are
largely a consequence of the Cenozoic Indo-Asian collision, the importance of older
tectonism in building the Tibetan plateau and influencing its subsequent development
must be considered (e.g., Yin and Harrison, 2000). Of particular relevance is the Jurassic
- Cretaceous collision between the Lhasa and Qiangtang terranes along the Bangong
suture in central Tibet (Fig. 1). Southward obduction of ophiolitic fragments onto the
northern margin of the Lhasa terrane during Middle to Late Jurassic time is generally
taken to mark the cessation of north-dipping oceanic subduction beneath the southern
Qiangtang terrane and the onset of Lhasa-Qiangtang collision (Girardeau et al., 1984;
30
Smith and Xu, 1988; Leeder et al., 1988; Zhou et al., 1997). The apparent absence of a
Jurassic arc and major mid-Mesozoic tectonism along the Bangong suture has contributed
to the notion that Bangong Ocean closure and subsequent Lhasa-Qiangtang collision
were relatively insignificant events in the development of central Tibet (e.g., Coward et
al., 1988; Dewey et al., 1988; Schneider et al., 2003). In contrast, thick accumulations of
northerly-derived Lower Cretaceous clastic strata in the northern Lhasa terrane (Leeder et
al., 1988; Leier et al., 2004; Zhang, 2004) together with Early Cretaceous growth of an
enormous, east-west trending structural culmination in the central Qiangtang terrane (Fig.
1) (Kapp et al., 2003, 2005) have been attributed to large-magnitude northward
underthrusting of the Lhasa terrane during Lhasa-Qiangtang collision. Furthermore,
extensive mid-Mesozoic magmatism and exhumation have been documented along a
possible extension of the Bangong suture zone in the Pamirs (Rushan-Pshart zone;
Schwab et al., 2004). In an attempt to better constrain the tectonic evolution of central
Tibet, we conducted geologic mapping and U-Pb and 40Ar/39Ar thermochronologic
studies on unique exposures of orthogneisses and cross-cutting granitoids located along
the Bangong suture near Amdo (Fig. 1).
The Amdo basement is the only established exposure of pre-Mesozoic crystalline
basement rock within the interior of Tibet. Earlier studies showed that the ~100 km long
by ~50 km wide basement exposure consists of amphibolite-facies orthogneisses and
subordinate metasedimentary rocks intruded by undeformed granitoids (Xu et al., 1985;
Harris et al., 1988b; Kidd et al., 1988; Coward et al., 1988). Conventional ID-TIMS U-Pb
dating of zircon and titanite fractions from one orthogneiss sample yielded discordant
31
ages for both minerals which were combined into a single discordia (Xu et al., 1985). The
upper intercept age (531 ± 14 Ma) was taken to represent the crystallization age of the
granitoid protolith, while the lower intercept age (171 ± 6 Ma) was interpreted to mark
the timing of low-grade metamorphism due to Lhasa-Qiangtang collision; high-grade
metamorphism was assumed to be related to the Cambrian magmatic emplacement (Xu et
al., 1985; Coward et al., 1988). Younger, intrusive granitoids (Harris et al., 1988a) were
interpreted to be Early Cretaceous based on discordant U-Pb zircon ages (140-120 Ma)
from one sample (Xu et al., 1985).
GEOLOGY
The Amdo basement is predominately composed of strongly foliated
orthogneisses which contain small but abundant mafic amphibolite gneiss “pods” with
the assemblage: amphibole + plagioclase + quartz + ilmenite ± biotite ± garnet. There are
also sporadic outcrops of sillimanite ± garnet ± K-feldspar paragneisses and migmatites.
The presence of sillimanite and K-feldspar, as well as the migmatites, are suggestive of
upper amphibolite facies metamorphic conditions. Metasedimentary rocks are exposed to
the south of the orthogneiss (Fig. 2) and include marble, quartz-mica schist, phyllite,
quartzite, and garnet-kyanite schist. The basement was intruded by widespread, generally
undeformed granitoids (Fig. 2) of variable composition (granite, monzonite, granodiorite,
quartz-syenite and diorite).
The western boundary of the Amdo basement is a west-dipping normal fault and
the northwestern boundary is a left-lateral strike-slip fault (Fig. 2), both of which cut
32
Quaternary deposits (Coward et al., 1988; Kidd et al., 1988). The Mesozoic tectonic
contacts of the northern and western exposures of the Amdo basement are buried beneath
Neogene-Quaternary basin fill. Some of the granitoids, particularly in the south, have
meters to tens of meters wide north-dipping shear zones that display mylonitic fabrics
and bookshelf microfaulting of feldspar-phenocrysts showing a top-to-the-south sense-ofshear. The gneisses and metasedimentary rocks along the southern margin are in the
hanging walls of north dipping thrust fault zones with Jurassic marine shales and
turbiditic sandstones and Cretaceous (?) red beds and conglomerates in the footwall.
GEOCHRONOLOGY
We dated eight granitoid samples and one orthogneiss sample using U-Pb laserablation multicollector inductively coupled plasma-mass spectrometry (LA-MC-ICPMS)
analyses on zircon (see footnote 1). The crystallization ages reported in this study are
based on weighted averages of concordant, clustered 206Pb*/238U ages of individual
zircons because low 207Pb concentrations in young (< 1000 Ma) granitoids result in large
uncertainties in the 207Pb*/235U and 207Pb*/206Pb* ages. The assigned uncertainties (2σ)
on the ages include all known random and systematic errors. The eight granitoid samples
define a relatively narrow range of mid-Jurassic ages from ~185 Ma to ~170 Ma (Table
1; Table DR1; Fig. DR1a-h) 1.
The zircons from the orthogneiss sample (PK97-6-4-3A) show signs of young
zircon growth due to metamorphic processes. Zircon resorption can be seen in cathodeluminescence images as bright, irregular zones only a few microns in thickness on the
33
edges of the crystals (Fig. 3a). The wide range of discordant zircon ages (Fig. 3b; Table
DR1)1 is interpreted to be due to the laser beam ablating a mix of Precambrian cores and
the younger rims. The younger ages correlate well with lower Th/U ratios (Fig. 3c), a
typical indication of zircon (re)crystallization in equilibrium with a metamorphic fluid
(Mojzsis and Harrison, 2002). The lower intercept of a discordia through the points
indicates a Mesozoic age for zircon resorption. The interpreted crystallization age of 852
± 18 Ma (Fig. 3b) provides the first direct documentation of Precambrian basement in
central Tibet.
HISTORY OF METAMORPHISM AND COOLING
We interpret the discordant analyses and metamorphic zircon rims in sample
PK97-6-4-3A to indicate that the amphibolite facies metamorphism of the Amdo
basement is Mesozoic. This interpretation is consistent with results of 40Ar/39Ar analysis
of hornblende and U-Pb analysis of titanite from orthogneiss samples. Hornblende
sample PK97-6-4-1A provides generally monotonically increasing apparent ages from
~160 Ma to ~187 Ma with an integrated age of ~180 Ma, while hornblende sample
PK97-6-4-3A yields an integrated age of ~175 Ma (Fig. DR2; Table DR2).1 We interpret
these results to indicate that the Amdo gneisses cooled to below the bulk closure
temperature for hornblende (500 ± 50 °C; McDougall and Harrison, 1999) at 180 ± 5 Ma.
U-Pb analysis on titanite from one of these samples (PK97-6-4-1A) provides a mean
206
Pb*/238U age of 179.0 ± 4.2 Ma (Fig. DR1j; Table DR1)1, which is statistically
indistinguishable from the integrated 40Ar/39Ar hornblende age from the same sample.
34
Given the higher closure temperature for titanite (~600-700 °C; see Frost et al., 2000 and
references therein), this may indicate rapid cooling of Amdo basement at ~180 Ma.
Alternatively, the titanite may have crystallized at conditions below the closure
temperature, possibly as low as 500 °C (Frost et al., 2000).
The lower temperature cooling history of Amdo basement rocks is inferred from
40
Ar/39Ar thermochronologic studies on mica and K-feldspar from the orthogneiss
samples (Fig. 2 for location) and the combined results are shown in Figure 4. Two biotite
samples (PK97-6-4-1A and PK97-6-4-3A) and one muscovite sample (PK97-6-4-2) yield
complexly varying apparent ages over the last ~80% cumulative 39Ar released, but all fall
within the 165 ± 5 Ma age range (Fig. DR3; Table DR3)1, indicating regional cooling to
below ~300 °C at this time.
The K-feldspar samples have complex spectra with age gradients that appear to be
due largely to excess argon contamination (Fig. DR4)1. Isochron analysis was used to
estimate initial 40Ar/36Ar trapped compositions (Fig. DR5)1 which were subsequently
used to calculate what we refer to as “isochron corrected” age spectra. These isochron
corrected age spectra were used to extract thermal histories using the multi-diffusion
domain model (Fig. DR6-8).1 Samples PK97-6-4-1A and PK97-6-4-3A show cooling
from ~300 °C to ~100 °C between 135 and 115 Ma, while PK97-6-4-2 shows an older
and slower episode of cooling from 155 to 125 Ma (Fig. 4). The difference in the timing
of cooling initiation could be due to internal disruption of the gneiss by Early Cretaceous
faults or shear zones that were not recognized in the field. Alternatively, PK97-6-4-2
displays an intermediate hump in the age spectra (both corrected and uncorrected) that
35
could be indicative of low-temperature recrystallization, which would negate the
significance of the calculated thermal history (Lovera et al., 2002). It is important to note
that the absolute temperatures of the thermal histories are strongly dependent on the
choice of the activation energy used, which is estimated from the step heating
experiments, and may be systematically shifted to lower or higher values. This is the
most probable explanation for the difference between timing of the 300 °C isotherm as
determined by K-feldspar and biotite for sample PK97-6-4-3A. However, the form of the
cooling histories is not affected by the choice of activation energy.
DISCUSSION AND CONCLUSIONS
Our results show that upper amphibolite-facies metamorphism of the Amdo
basement was the result of a tectonothermal event during the Early-Middle Jurassic and
not the Cambrian as previously inferred. Metamorphism and subsequent cooling of the
Amdo basement to ~500 °C were coeval with emplacement of extensive granitoids
between 185 and 170 Ma. These granitoids are the only known igneous rocks along the
Bangong suture in Tibet with ages that overlap with the timing of Bangong Ocean
subduction. Jurassic magmatism and exhumation have also been documented in the
Pamirs along and to the north of the Rushan-Pshart suture zone, a likely westward
extension of the Bangong suture zone that has been offset by the right-lateral Karakoram
fault (Schwab et al., 2004). As has been suggested for the Pamir (Schwab et al., 2004),
we attribute Early - Middle Jurassic metamorphism and magmatism to the development
of a continental arc along the southern Qiangtang terrane due to north-dipping subduction
36
of oceanic lithosphere. The opening and subsequent closing of a back-arc oceanic basin
during arc development (Fig. 5a-b) could explain the presence of ophiolitic fragments
north of the Amdo gneiss (Fig. 1) as well as extensive Early - Middle Jurassic marine
sedimentation in the southern Qiangtang terrane and Bangong suture zone, which is
apparently lacking in the Lhasa terrane (Leeder et al., 1988; Yin et al., 1988; Schneider et
al., 2003). The Amdo basement and superimposed arc remained in the mid-crust until
relatively rapid exhumation to upper crustal levels during the Early Cretaceous, most
probably due to a south-directed thrust system (Fig. 5c-d). This exhumation was coeval
with growth of the large antiformal structural culmination in the central Qiangtang
terrane (Kapp et al., 2005) and the accumulation of thick, northerly-derived, clastic
deposits in the Lhasa terrane (Leeder et al., 1988; Leier et al., 2004; Zhang, 2004) and is
therefore attributed to Lhasa-Qiangtang continental collision. Continued northward
underthrusting of the Lhasa terrane led to a southward propagation of upper crustal
deformation into the northern Lhasa terrane during the Late Cretaceous, with the
magnitude of regional shortening exceeding 40% (Murphy et al., 1997; Kapp et al., 2003)
and leading to significant crustal thickening in central Tibet (Fig. 5d). As our K-feldspar
thermochronologic results indicate minimal denudation along the Bangong suture since
the Early Cretaceous, this thick crust would have persisted until the onset of India’s
Cenozoic collision with Asia.
While the Jurassic tectonic evolution of the Bangong suture zone remains
speculative, a robust conclusion is that the Amdo region provides unambiguous evidence
for Jurassic high grade metamorphism and extensive magmatism. We suggest that the
37
Jurassic granitoids represent exhumed portions of a continental arc that paralleled the
length of the entire Bangong suture from the Amdo area to the Pamir. This arc is
"missing" in central Tibet because it was either buried depositionally beneath Upper
Jurassic and younger supracrustal assemblages or underthrust northward beneath the
Qiangtang terrane along with Lhasa terrane basement. This interpretation begs the more
general question—to what extent are entire metamorphic/magmatic belts and other
important records of tectonism “missing” beneath supracrustal assemblages in Tibet and
in other contractional orogens worldwide?
ACKNOWLEDGMENTS
Preliminary sampling of the Amdo region was undertaken in 1997 with the
assistance of A.Yin and M. Murphy. We thank R. Waldrip for assistance in the field
and J. Fox and F. Guerrero for sample preparation. The manuscript benefited from
discussions with M. Ducea and P.G. DeCelles and reviews by C. Burchfiel, L.
Ratschbacher and A.Yin. This research was supported by NSF grant EAR-0309844,
University of Arizona start-up funds, and student research grants from
ChevronTexaco and the Geological Society of America.
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42
FIGURE CAPTIONS
Figure 1. Map of Tibetan terranes and suture zones. Ophiolite exposures related to the
Bangong suture zone are shown in dark green. Note that ophiolites occur both north and
south of the Amdo basement. Traditionally, the Bangong suture is placed at the
northernmost ophiolite outcrops, but our model suggests it occurs to the south.
Abbreviations: SGFC—Songpan-Ganzi flysch complex; MBT—Main Boundary Thrust;
STD—South Tibetan Detachment.
Figure 2. Simplified geologic map of the western Amdo basement region. Compiled from
our mapping at 1:100,000 scale and the more regional geologic maps of Kidd et al.
(1988) and Pan et al. (2004).
Figure 3. U-Pb analysis of orthogneiss PK97-6-4-3A. (a) Cathode-luminescence images
of two zircons with thin, bright resorption rims. (b) U-Pb concordia diagram for
orthogneiss sample PK97-6-4-3A; error ellipses are at the 95% confidence level. Plotting
and discordia regression are from Isoplot 3.00 (Ludwig, 2003). Analyses include laser
ablation spots in the cores and at the tips of the individual crystals. The crystallization age
is based on a weighted mean of 206Pb*/238U ages for 13 concordant analyses clustered
near the upper intercept of the discordia (> 830 Ma; Fig. DR1i);1 these ellipses are shown
in black. (c) Variation in Th/U with 206Pb*/238U ratio (i.e., age), showing a systematic
decrease in Th/U values with decreasing age.
43
Figure 4. Cooling history of the Amdo basement based on thermochronologic data from
three basement samples. Ttn - titanite; Hbl - hornblende; Ms - muscovite; Bi - biotite;
Kspar - K-feldspar. The lower limit of the temperature range for titanite includes the
possibility of crystallization below the closure temperature. Total gas ages of hornblende
and mica are shown with assigned uncertainties of 2%. The thermal histories calculated
from multi-domain diffusion (MDD) modeling of K-feldspar 40Ar/39Ar age spectra
represent a 90% confidence window about the mean of at least 20 solutions.
Figure 5. Tectonic model for the evolution of the Amdo basement and Bangong suture
zone. (a) The Amdo basement rifted from the Qiangtang terrane by Early Jurassic time,
possibly due to slab rollback, creating an oceanic back-arc basin. (b) Middle Jurassic
closure of the back-arc basin resulted in high-grade metamorphism of the Amdo
basement and ophiolite obduction along the northern edge of the Bangong suture zone.
Arc magmatism is coeval with metamorphism at this time. (c) During Early Cretaceous
time, the Bangong Ocean closes and the Lhasa terrane collides with the Qiangtang
terrane. A normal fault is inferred on the north side of the Amdo gneiss to place
unmetamorphosed sedimentary rocks against the high grade Amdo basement. A foreland
basin develops south of the Amdo basement. (d) Continued convergence between the
Lhasa and Qiangtang terranes in the Cretaceous results in underthrusting of the Lhasa
terrane, exhumation of the Amdo basement to upper-crustal levels, and a fold-thrust belt
along the convergence zone.
44
1
GSA Data Repository item 2006094, Supplementary Geochronologic and
Thermochronologic Data, is available online at www.geosociety.org/pubs/ft2006.htm, or
on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140,
Boulder, CO 80301, USA.
45
FIGURES
46
47
48
49
Data Repository Item DR2006094
Supplementary Geochronologic and Thermochronologic Data
Summary of analytical methods for U-Pb analysis (University of Arizona)
Zircons and titanite were separated from 1-2 kg samples of fresh rock using standard crushing and
separation techniques, including a water table, magnetic separator and heavy liquids. Individual zircon
crystals and titanite fragments were than hand-picked under a binocular microscope to select large,
inclusion free grains. Approximately 50 grains were than mounted in epoxy and polished to approximately
2/3 of the crystal thickness. The grains were ablated with a New Wave DUV193 Excimer laser, operating
at a wavelength of 193 nm and using a spot diameter of 25, 35, or 50 microns, depending on the size of the
crystal. The ablated material is then carried in argon gas into the plasma source of a Micromass Isoprobe
multicollector ICP-MS and ionized. The Micromass Isoprobe is equipped with a flight tube of sufficient
width that U, Th, and Pb isotopes are measured simultaneously using nine Faraday collectors, an axial Daly
detector and four ion-counting channels. All measurements were made in static mode, using Faraday
detectors for
238
238
U, 232Th, and
U measurement assuming
208-206
238
Pb and an ion-counting channel for
204
235
Pb.
U is determined from the
235
U/ U = 137.88. Ion yields are ~1 mV per ppm. Each analysis consists of
one background run with 20-second integration on peaks and the laser off, 20 1-second integrations with
the laser firing, and a 30-second delay to purge for the next analysis.
The laser ablates at ~1
micron/second, resulting in an ablation pit ~20 microns in depth. A common lead correction was performed
by using the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers (1975) (with
uncertainties of 1.0, 0.3, and 2.0 for
of
204
206
Pb/204Pb,
Pb is unaffected by the presence of
subtracting any background
204
Hg and
The errors in determining
206
percent (at 2-σ level) in the
206
204
238
204
207
Pb/204Pb, and
208
Pb/204Pb, respectively). Measurement
Hg because backgrounds are measured on peaks (thereby
Pb) and because very little Hg is present in the argon gas.
Pb/ U and
206
Pb*/238U age.
Pb/204Pb result in a measurement uncertainty of several
The low concentrations of
(approximately < 1000 Ma), due to the low concentration of
235
U relative to
207
238
Pb in younger samples
U, result in a substantially
larger measurement uncertainty for 206Pb/207Pb. The 207Pb*/235U and 206Pb*/207Pb* ages for younger grains
accordingly have larger uncertainties. Inter-element fractionation of Pb/U is generally <20%, whereas
isotopic fractionation of Pb is generally <5%. These fractionations were corrected by analysis of a
standard, with a known, concordant ID-TIMS age, between every three unknown analyses. The zircon
standards are fragments of a large zircon crystal from a Sri Lankan pegmatite (e.g. Dickinson and Gehrels,
2003) with an age of 564 ± 4 Ma (2σ), while the titanite standards are fragments of a large titanite crystal
from the Bear Lake region, Canada, with an age of 1050 ± 2 Ma (2σ) (Mark Schmitz and John Aleinikoff,
50
written communication, 2003). The uncertainty resulting from the calibration correction is generally ~3%
(2σ) for both
207
Pb/206Pb and
206
Pb/238U ages. U and Th concentrations were determined by analyzing a
piece of NIST 610 glass with ~500 ppm U and Th and using the resulting measured intensities to calibrate
the sample measured U and Th intensities.
The crystallization ages reported in this paper (Table 1; Fig. 3a; Fig. DR1) are weighted averages of
individual, concordant spot analyses using the 206Pb*/238U ages, since the 207Pb*/235U and 207Pb/206Pb* ages
are less precise for these younger granitoids. The stated uncertainties (2σ) on the assigned crystallization
ages are absolute values and include contributions from all known random and systematic errors. Random
errors are included in the data tables.
Systematic errors (2σ) are as follows: PK97-6-4-3A, 2.06%;
JG062004-4, 1.00%; JG062204-1, 1.00%; JG061504-7, 0.81%; AP061504-B, 1.02%; AP061604-B, 1.00%;
AP060604-A, 1.06%; AP052904-A, 1.06%; AP062104-A, 0.96%; PK97-6-4-1A Titanite, 2.16%.
All U-Pb plots and weighted average calculations were made using Isoplot 3.00 (Ludwig, 2003).
Summary of analytical methods for mica
40
Ar/39Ar analyses (University of
California, Los Angeles).
High-purity mica mineral separates for
40
Ar/39Ar analysis were obtained using standard mineral
separation techniques. They were irradiated for 45 hours at the Ford Reactor, University of Michigan,
along with Fish Canyon sanidine (27.8 ± 0.3 Ma; Renne et al., 1994) or Taylor Creek sanidine (28.1 ± 0.3
Ma) flux monitors to calculate J-factors and K2SO4 and CaF2 salts to calculate correction factors for
interfering neutron reactions. Samples were step-heated in a Ta crucible within a double-vacuum furnace,
and
40
Ar/39Ar isotopic measurements were conducted on a VG 1200S or VG 3600 mass spectrometer at
UCLA. Apparent ages were calculated using conventional decay constants and isotopic abundances. The
uncertainties for apparent ages listed in Table A1 are at the one sigma level and do not include uncertainties
in the J factor or decay constants.
Summary of analytical methods for hornblende and feldspar
40
Ar/39Ar analyses
(New Mexico Geochronological Research Laboratory).
Feldspar and hornblende separates were obtained by standard heavy liquid and magnetic separation
methods. The separates were loaded into machined Al discs and irradiated for 75 mW hours in 5-c position
at the McMaster reactor, Hamilton, Ontario along with the Fish Canyon Tuff sanidine (FC-2) as a neutron
flux monitor. The FC-2 standard has an assigned age = 27.84 Ma (Deino and Potts, 1990) relative to
Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987).
51
The hornblendes and K-feldspars were step heated in a Mo resistance furnace, the former for 10
minutes while the latter ranged from 15 to 180 minutes. Isotopes were measured on a Mass Analyzer
Products 215-50 mass spectrometer on line with automated all-metal extraction system. Reactive gases
were removed during heating with an SAES GP-50 getter operated at ~450°C. Additional cleanup (5
minutes hornblendes; 2 minutes K-feldspar) following the step heating with two SAES GP-50 getters, one
operated at ~450°C and one at 20°C. The gas was also exposed to a W filament operated at ~2000°C.
The electron multiplier sensitivity averaged 2.87x10-16 moles/pA.
background for hornblendes was 70, 1.0, 0.4, 0.4, 0.5 x 10
-17
The total system blank and
moles for masses 40, 39, 38, 37, 36,
respectively. The total system blank and background for K-feldspars was 45, 1.2, 0.3, 0.4, 0.3 x 10-17
moles for masses 40, 39, 38, 37, 36, respectively. The J-factors were determined to a precision of ± 0.1%
by CO2 laser-fusion of 6 single crystals from each of 4 radial positions around the irradiation tray. The
Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as
follows: NM-173: (40Ar/39Ar)K = 0.02895±0.00059; (36Ar/37Ar)Ca = 0.000284±0.000006; (39Ar/37Ar)Ca =
0.00074±0.00002; (38Ar/39Ar)K = 0.0129.
K-Feldspar multi-domain diffusion modeling methods
Thermal histories are calculated from the feldspar data using the multi-domain diffusion (MDD)
model (Figs. DR6d, DR7d, DR8d). The diffusion domain distribution for each feldspar is obtained by
modeling the release of 39Ar relative to the laboratory heating schedule. In all cases, the activation energy
(E) is assumed to be constant for all diffusion domains of a given sample and seven diffusion domains were
imposed on each model (Table DR4). The model Arrhenius and log(r/ro) plots show that the measured
data can be well approximated by seven diffusion domains of varying proportions (Figs DR6b,c; DR7b,c;
DR8b,c). For each sample, two thermal histories are shown: first the measured age data that have only
been corrected for an assumed atmospheric 40Ar/36Ar initial value are modeled and secondly, apparent ages
corrected for implied trapped components from isochron analysis are modeled (Figs. DR6a,d; DR7a,d;
DR8a,d). Because the feldspar age spectra climb above the hornblende apparent ages we can be reasonably
certain that the final heating steps for each feldspar are contaminated with excess 40Ar. However, it is not
obvious that the initial age gradients, which appear to climb in a steady fashion and are younger than
coexisting hornblendes, are contaminated by excess 40Ar. These age gradients are well fit by model age
spectra that are created by cooling the samples from about 300°C at 150 Ma to about 100°C by 120 Ma
(Figs. DR6a,d; DR7a,d; DR8a,d).
The accuracy of these models depends critically upon the assumption that the apparent ages are
not influenced by excess 40Ar, however the isochron data appear to challenge this assumption (Fig. DR5).
Admittedly the isochron data define poor linear trends as revealed by high MSWD values; however we still
52
assign overall meaning to their indication of excess argon (Fig. DR5). The probable cause for the scatter
on the isochron arrays is incomplete separation of multiple trapped components (i.e. Heizler and Harrison,
1988) and potential true 40Ar* gradients related to complex thermal histories. We recognize problems with
rigorous quantitative use of the isochron data, but feel that it is more appropriate to model age spectra that
have been corrected with corresponding isochron trapped components rather then choose models that only
correct of an atmospheric trapped initial 40Ar/36Ar value. Therefore, each apparent age is calculated using
the inferred isochron trapped values (Fig. DR5) and recast as “isochron corrected” age spectra in Figures
DR6a, DR7a, and DR8a. These spectra are much flatter than the atmosphere corrected spectra and thus
produce model thermal histories that require cooling at a younger time and faster rate (Figs. DR6d, DR7d,
DR8d). The “flattening” of the age spectra is most pronounced in samples PK-97-6-4-1A and PK-97-6-43A, whereas sample PK-97-6-4-2 is less sensitive to choice of the initial trapped
40
Ar/36Ar component.
This is mainly because the apparent radiogenic yields for samples PK-97-6-4-1A and 3A are significantly
lower than that for PK-97-6-4-2 (Table DR2). As expected from the high MSWD values calculated from
the isochron arrays, the corrected age spectra show significant scatter, but are overall much flatter than the
original spectra and we assert that these isochron corrected spectra yield more accurate thermal histories.
The thermal histories determined from the isochron corrected age spectra indicate two periods of
rapid cooling. Samples PK-97-6-4-1A and 3A suggest cooling from about 300°C to 100°C occurred
between 125 and 120 Ma; whereas PK-97-6-4-2 suggests this same cooling range took place at about 140
Ma (Figs. DR6d, DR7d, DR8d). We have the least amount of confidence in sample PK-97-6-4-2 feldspar
as both versions of the age spectra reveal a significant intermediate age hump that is suggested to be caused
by post-argon closure modification of the diffusion domains (Lovera et al., 2002). For instance, Lovera et
al. (2002) showed that complex age spectra with intermediate age humps could result from relatively lowtemperature recrystallization and cautioned against assigning significance to model thermal histories in
some instances.
References Cited
Deino, A., and Potts, R., 1990, Single-Crystal 40Ar/39Ar dating of the Olorgesailie Formation,
Southern Kenya Rift: Journal of Geophysical Research, v. 95, p. 8,453-8,470.
Dickinson, W.R., and Gehrels, G.E., 2003, U-Pb ages of detrital zircons from Permian and Jurassic eolian
sandstones of the Colorado Plateau, USA: paleogeographic implications: Sedimentary Geology, v.
163, p. 29-66.
Heizler, M.T., and Harrison, T.M., 1988, Multiple trapped argon isotope components revealed by 40Ar/39Ar
analysis. Geochimica Cosmochima Acta, v. 52, p. 1,295-1,303.
Lovera, O. M., Grove, M., Harrison, T. M., 2002, Systematic analysis of K-feldspar 40Ar/39Ar step heating
results; II, Relevance of laboratory argon diffusion properties to nature: Geochimica Cosmochimica
53
Acta, v. 66, p. 1,237-1,255.
Ludgwig, K.R., 2003, Isoplot 3.00: Berkeley Geochronology Center Special Publication No. 4, 70 p.
Renne, P.R., Deino, A.L., Walter, R.C., Turrin, B.D., Swisher, C.C., Becker, T.A., Curtis, G.H., Sharp,
W.D., and Jaouni, A.R., 1994, Intercalibration of astronomical and radioisotopic time: Geology, v. 22,
p. 783-786.
Samson, S.D., and, Alexander, E.C., Jr., 1987, Calibration of the interlaboratory 40Ar/39Ar dating
standard, Mmhb-1: Chemical Geology, v. 66, p. 27-34.
Stacey, J.S., and Kramers, J.D., 1975, Approximation of Terrestrial Lead Isotope Evolution by a 2-Stage
Model: Earth and Planetary Science Letters, v. 26, p. 207-221.
54
Figures and Tables
a) AP061504-B
Zircon
0.033
190
210
Pb*/238U Age (Ma)
0.035
190
0.029
0.027
170
206
Pb*/238U
0.031
0.025
150
0.032
0.1
0.2
b) JG062004-4
Zircon
0.3
Age = 175.0 ± 3.4 Ma
MSWD = 5.5
200
200
0.030
190
180
0.028
170
0.026
206
160
0.024
150
190
180
170
160
Age = 170.7 ± 2.7 Ma
MSWD = 2.5
n = 19
0.022
0.2
150
0.4
Pb*/238U Age (Ma)
210
0.031
190
180
0.027
170
206
206
Pb*/238U
200
0.029
160
0.025
0.036
0.15
0.25
d) AP052904-A
Zircon
0.034
206Pb*/238U
0.032
180
170
160
Age = 177.7 ± 1.8 Ma
MSWD = 1.4
150
0.35
210
200
180
0.026
160
0.024
200
180
190
170
160
n = 20
0.022
0.0
0.1
n = 16
220
220
0.030
0.028
190
n = 17
150
0.023
0.05
n = 19
200
220
c) JG062204-1
Zircon
0.033
0.3
Age (Ma)
0.035
0.1
206Pb*/238U
0.0
n = 11
150
0.4
Pb*/238U Age (Ma)
0.0
Pb*/238U
160
n = 17
0.021
206
170
206
0.023
180
0.2
0.3
0.4
0.5
150
Age = 177.8 ± 4.1 Ma
MSWD = 6.8
n = 20
207Pb*/235U
Figure DR1. U-Pb concordia plots and weighted averages for Jurassic granitoids a)
AP061504-B, b) JG062004-4, c) JG062204-1, and d) AP052904-A. Error ellipses
and error bars in this and all subsequent plots are at the 2-σ level. Note that mean
age only includes random errors; see Table 1 for random & systematic errors.
55
e) AP062104-A
Zircon
0.032
200
195
0.030
206Pb*/238U
Age (Ma)
185
180
0.028
206Pb*/238U
0.026
160
0.024
140
0.022
175
165
155
145
n = 15
0.020
0.0
0.1
0.2
0.3
0.4
Age = 171.8 ± 3.8 Ma
MSWD = 4.9
135
n = 15
207Pb*/235U
0.034
f) AP060604-A
Zircon
205
210
200
Age (Ma)
195
190
0.030
180
0.028
206Pb*/238U
206Pb*/238U
0.032
170
0.026
160
0.024
150
185
175
165
Age = 179.4 ± 2.8 Ma
MSWD = 2.3
155
n = 18
0.022
0.0
0.1
0.2
0.3
n = 18
145
0.4
207Pb*/235U
0.030
195
Age (Ma)
190
180
0.028
206Pb*/238U
206Pb*/238U
205
g) AP061604-B
Zircon
200
0.032
170
0.026
160
0.024
150
185
175
165
155
n = 17
0.022
0.0
0.1
0.2
0.3
0.4
0.5
Age = 174.0 ± 3.3 Ma
MSWD = 4.1
145
n = 17
207Pb*/235U
0.0315
202
198
198
194
0.0305
190
0.0295
186
206Pb*/238U
206Pb*/238U
202
206
h) JG061504-7
Zircon
Age (Ma)
0.0325
182
0.0285
178
0.0275
0.0265
0.15
174
170
0.17
0.19
0.21
0.23
194
190
186
182
178
n = 16
174
0.25
170
Age = 182.9 ± 2.2 Ma
MSWD = 4.8
n = 16
207Pb*/235U
Figure DR1 continued. U-Pb concordia plots and weighted averages for Jurassic
granitoids e) AP062104-A, f) AP060604-A, g) AP061604-B, and h) JG061504-7
56
j) PK97-6-4-3A
Zircon
206Pb*/238U
Age (Ma)
890
870
850
830
Age = 852.0 ± 3.5 Ma
MSWD = 1.16
n = 13
810
220
i) PK97-6-4-1A
Titanite
0.032
210
0.030
Pb*/238U Age (Ma)
206Pb*/238U
200
190
180
0.028
170
206
0.026
160
n = 27
0.024
0.0
0.1
0.2
207
0.3
0.4
0.5
200
190
180
170
160
150
140
Age = 178.8 ± 1.6 Ma
MSWD = 0.69
n = 27
Pb*/235U
Figure DR1 continued. i) Weighted average of selected, concordant zircons from
orthogneiss PK97-6-4-3A to define a crystallization age and j) U-Pb concordia plot and
weighted average for Titanite from orthogneiss PK97-6-4-1A.
57
Table DR1. U-Pb data
Sample spot numbers in bold were used to calculate the weighted averages
Isotopic ratios
Sample
spot
U
(ppm)
PK97-6-4-3A Zircon
1
74
2
49
3
42
6
26
9
29
10t
66
11
34
12
67
16
103
17
23
17t
52
18
24
19
20
20
35
22t
50
25t
65
27t
41
29
26
31
18
32
76
35
24
2-1tr
24
2-2c
14
2-2tl
26
2-2tr
19
2-3c
14
2-4c
35
2-4tl
52
2-5c
27
2-5cr
25
2-5tr
39
2-6c
35
2-7c
24
2-10c
13
2-10tr
19
2-11c
24
2-11tr2
25
2-13c
26
2-13tl
43
2-14c
41
2-17tl
17
2-17tr
25
206
Pb
Pb
U/Th
204
23236
82186
64844
28827
31145
197058
119041
58200
63471
133570
49148
134179
81693
180701
68318
629740
95493
381676
64114
852773
169750
57085
8430
17008
6676
33488
20342
5798
15189
68770
2117
16064
42070
27809
6864
29165
8302
3544
4932
166873
6702
3192
207
Pb*
U
± (%)
235
3.8
3.9
3.8
4.1
32.5
10.9
3.8
31.5
4.2
5.1
31.9
4.2
3.8
3.1
6.6
5.5
12.0
4.3
12.0
6.4
4.0
4.8
3.4
3.7
3.2
4.9
2.3
10.2
2.4
4.0
10.9
2.5
4.3
2.6
4.3
1.8
6.4
3.3
7.5
3.4
5.3
11.9
1.04585
1.35389
1.31590
1.36611
0.48372
0.83463
1.33345
0.24671
0.85294
1.31926
0.28783
1.24968
1.37145
1.22229
0.71205
0.99926
0.37717
1.23819
0.82039
1.02338
1.24817
1.09325
1.30092
0.92843
0.84426
1.39342
1.28120
0.50719
1.29755
1.03891
0.50971
1.34398
1.04769
1.37472
0.64831
1.29767
0.76238
1.36904
0.71289
1.24897
0.78529
0.50105
206
Pb*
U
Apparent ages (Ma)
± (%)
238
2.28
1.50
2.37
3.82
9.15
3.53
2.38
6.32
1.53
2.51
8.27
3.61
3.22
2.19
3.77
2.79
6.80
2.40
7.25
1.45
3.68
4.86
3.46
2.70
5.01
4.73
2.89
3.30
4.10
3.37
6.73
1.86
4.48
4.98
6.30
4.60
6.10
2.60
5.21
4.40
6.08
5.95
0.11275
0.14203
0.14004
0.14298
0.05947
0.09433
0.14349
0.03185
0.08796
0.14272
0.03364
0.13489
0.14348
0.12916
0.07248
0.10830
0.04607
0.13362
0.09058
0.11115
0.13182
0.11497
0.14046
0.10475
0.09903
0.14128
0.14070
0.06164
0.14248
0.10743
0.06138
0.14398
0.11452
0.13872
0.07839
0.13613
0.08854
0.14074
0.07458
0.13447
0.09252
0.05328
2.01
0.75
0.83
0.75
5.00
3.09
1.15
1.21
1.16
0.95
1.35
1.09
1.45
0.80
1.99
2.16
2.14
0.86
3.88
0.53
0.82
1.44
0.93
1.44
1.11
1.34
0.67
2.15
0.90
1.20
2.51
0.97
3.71
1.06
2.11
1.43
5.03
0.30
4.37
2.08
1.54
2.71
error
corr
0.88
0.50
0.35
0.20
0.55
0.88
0.48
0.19
0.76
0.38
0.16
0.30
0.45
0.36
0.53
0.77
0.32
0.36
0.54
0.37
0.22
0.30
0.27
0.53
0.22
0.28
0.23
0.65
0.22
0.36
0.37
0.52
0.83
0.21
0.33
0.31
0.82
0.12
0.84
0.47
0.25
0.46
206
Pb*
U
± (Ma)
238
689
856
845
861
372
581
864
202
543
860
213
816
864
783
451
663
290
808
559
679
798
702
847
642
609
852
849
386
859
658
384
867
699
837
486
823
547
849
464
813
570
335
207
Pb*
U
± (Ma)
235
14.6
6.9
7.5
6.9
19.1
18.8
10.6
2.5
6.6
8.8
2.9
9.5
13.4
6.6
9.3
15.1
6.4
7.4
22.6
3.8
6.9
10.7
8.4
9.7
7.1
12.2
6.0
8.5
8.3
8.3
9.9
9.0
27.4
9.5
10.6
12.5
28.6
2.8
21.0
18.0
9.2
9.3
727
869
853
874
401
616
860
224
626
854
257
823
877
811
546
703
325
818
608
716
823
750
846
667
621
886
837
417
845
723
418
865
728
878
507
845
575
876
546
823
588
412
206
Pb*
Pb*
± (Ma)
207
23.9
20.3
31.2
51.6
44.0
29.4
31.7
15.7
13.2
33.1
23.9
44.9
43.9
26.8
26.9
27.9
25.7
29.7
58.7
15.0
45.6
52.5
44.7
25.2
42.1
64.8
36.9
16.9
52.6
34.9
34.2
25.0
46.6
67.2
40.7
58.8
46.2
35.5
37.0
54.3
47.4
29.8
846
903
873
908
567
747
850
459
938
839
676
844
908
888
965
835
581
844
797
831
889
898
843
751
668
973
808
592
808
932
612
859
818
982
603
903
689
944
909
849
659
875
11
13
23
39
83
18
22
69
10
24
87
36
30
21
33
18
70
23
64
14
37
48
35
24
52
46
29
27
42
32
67
16
26
50
64
45
37
26
29
40
63
55
58
Table DR1. U-Pb data
Isotopic ratios
Sample
spot
U
(ppm)
206
Pb
Pb
U/Th
204
207
Pb*
U
± (%)
235
206
Pb*
U
Apparent ages (Ma)
± (%)
238
error
corr
206
Pb*
U
± (Ma)
238
207
Pb*
U
± (Ma)
235
206
Pb*
Pb*
± (Ma)
207
AP061504-B Zircon
1R
554
1C
718
2T
998
2C
340
3R
1168
3C
835
4C
465
5
304
6
1219
7
301
8
1641
9
1082
10
294
12
363
13
1368
16
587
18
1031
6834
4756
5831
2575
2968
352
1621
3547
6951
2102
1138
3471
1879
2641
1446
4443
6906
1.4
0.7
1.8
1.7
2.2
1.0
1.2
1.2
2.0
1.1
2.1
2.2
1.1
1.3
2.5
1.9
2.3
0.21990
0.17190
0.17999
0.18240
0.18909
0.22565
0.22924
0.20124
0.19277
0.18288
0.18608
0.20390
0.15854
0.16708
0.18713
0.18269
0.18375
11.49
12.09
10.35
15.62
8.06
17.56
24.92
27.17
4.72
11.89
5.88
5.05
13.06
10.25
7.34
8.52
7.34
0.03228
0.02589
0.02764
0.02995
0.02645
0.02525
0.02653
0.02656
0.02652
0.02760
0.02671
0.02848
0.02801
0.02852
0.02737
0.02655
0.02770
0.74
2.37
0.70
4.60
1.63
3.11
4.45
3.38
1.10
1.99
1.90
0.74
3.02
2.49
1.88
1.88
1.38
0.06
0.20
0.07
0.29
0.20
0.18
0.18
0.12
0.23
0.17
0.32
0.15
0.23
0.24
0.26
0.22
0.19
205
165
176
190
168
161
169
169
169
176
170
181
178
181
174
169
176
1.5
3.9
1.2
8.6
2.7
4.9
7.4
5.6
1.8
3.4
3.2
1.3
5.3
4.5
3.2
3.1
2.4
202
161
168
170
176
207
210
186
179
171
173
188
149
157
174
170
171
21.0
18.0
16.0
24.5
13.0
32.8
47.2
46.3
7.7
18.7
9.4
8.7
18.1
14.9
11.7
13.4
11.6
167
107
61
-101
279
768
697
410
317
102
220
282
-284
-197
176
190
105
269
281
247
368
181
367
530
613
104
278
129
114
325
250
166
194
170
JG062004-4 Zircon
1R
390
1C
340
2R
241
2C
208
3T
107
3C
226
4
147
5
886
6
661
7
1514
8
320
10
361
11
258
12
262
13
693
14
788
15
788
18
1107
19
206
20
690
21
745
2558
2854
2699
1458
872
2345
937
711
2961
1884
932
2375
2021
1711
506
8207
8207
1243
2702
365
7149
1.4
2.7
1.2
1.7
0.6
0.9
0.5
1.1
1.2
1.0
2.0
1.3
0.3
1.7
0.3
1.5
1.5
1.2
0.9
0.8
1.2
0.18947
0.21016
0.18777
0.16727
0.41713
0.20325
0.20923
0.16684
0.20293
0.16104
0.20361
0.17926
0.14503
0.17450
0.18709
0.19645
0.19645
0.20630
0.19609
0.22546
0.19716
9.49
26.14
10.21
25.77
12.09
17.08
36.50
9.13
11.19
4.41
19.60
13.29
26.19
21.34
19.00
8.87
8.87
15.23
18.35
6.16
8.70
0.02673
0.02676
0.02617
0.02674
0.02939
0.02679
0.02669
0.02567
0.02784
0.02522
0.02709
0.02738
0.02673
0.02751
0.02714
0.02772
0.02772
0.02746
0.02780
0.02594
0.02858
2.80
3.35
2.49
2.08
4.01
2.20
3.33
1.19
5.07
2.01
2.13
2.06
2.23
2.47
3.50
1.61
1.61
1.59
2.52
1.37
2.72
0.29
0.13
0.24
0.08
0.33
0.13
0.09
0.13
0.45
0.45
0.11
0.16
0.09
0.12
0.18
0.18
0.18
0.10
0.14
0.22
0.31
170
170
167
170
187
170
170
163
177
161
172
174
170
175
173
176
176
175
177
165
182
4.7
5.6
4.1
3.5
7.4
3.7
5.6
1.9
8.9
3.2
3.6
3.5
3.7
4.3
6.0
2.8
2.8
2.7
4.4
2.2
4.9
176
194
175
157
354
188
193
157
188
152
188
167
138
163
174
182
182
190
182
206
183
15.3
46.1
16.4
37.5
36.2
29.3
64.2
13.3
19.2
6.2
33.7
20.5
33.7
32.2
30.4
14.8
14.8
26.5
30.6
11.5
14.5
259
490
287
-36
1677
413
486
57
323
14
392
74
-393
-2
194
259
259
391
248
709
196
209
581
227
632
211
381
828
216
227
95
441
313
689
516
437
201
201
342
421
128
192
59
Table DR1. U-Pb data
Isotopic ratios
Sample
spot
U
(ppm)
206
Pb
Pb
U/Th
204
207
Pb*
U
± (%)
235
206
Pb*
U
Apparent ages (Ma)
± (%)
238
error
corr
206
Pb*
U
± (Ma)
238
207
Pb*
U
± (Ma)
235
206
Pb*
Pb*
± (Ma)
207
JG062204-1 Zircon
1R
493
1C
303
2R
458
2C
607
3
600
4
704
7
328
8
399
9
442
10
393
11
390
12
207
13
491
14
587
17
596
21
751
24
585
7111
403
2980
2249
3654
3702
2151
3155
2110
1527
3685
2063
318
4731
8888
6720
9499
1.2
0.7
1.5
1.0
1.0
1.0
1.4
1.2
1.0
0.9
0.8
1.0
1.2
1.1
1.1
1.0
0.9
0.19904
0.19484
0.19928
0.19292
0.20976
0.18778
0.17185
0.19537
0.19302
0.22522
0.17208
0.22227
0.18751
0.19697
0.19587
0.19090
0.19223
14.43
19.18
11.21
13.48
11.06
7.18
19.42
10.10
12.23
21.27
18.75
16.10
17.04
12.22
13.15
6.83
5.80
0.02729
0.02731
0.02794
0.02829
0.02895
0.02826
0.02761
0.02852
0.02792
0.02907
0.02814
0.02886
0.02684
0.02867
0.02813
0.02719
0.02809
1.28
2.96
0.76
1.56
1.47
2.46
1.28
1.84
1.56
5.94
2.40
1.91
3.71
2.78
1.92
1.53
2.72
0.09
0.15
0.07
0.12
0.13
0.34
0.07
0.18
0.13
0.28
0.13
0.12
0.22
0.23
0.15
0.22
0.47
174
174
178
180
184
180
176
181
178
185
179
183
171
182
179
173
179
2.2
5.1
1.3
2.8
2.7
4.4
2.2
3.3
2.7
10.8
4.2
3.4
6.2
5.0
3.4
2.6
4.8
184
181
185
179
193
175
161
181
179
206
161
204
175
183
182
177
179
24.3
31.8
18.9
22.1
19.5
11.5
28.9
16.8
20.1
39.7
28.0
29.7
27.3
20.4
21.9
11.1
9.5
324
274
274
169
309
109
-48
180
201
460
-92
447
226
187
218
238
178
328
438
257
314
250
159
475
232
282
457
459
357
387
278
302
154
120
AP052904-A Zircon
1C
329
4C
369
5C
337
9C
540
11C
160
12C
722
13C
416
14C
331
15C
168
16C
251
18C
513
19C
249
20C
262
23T
283
24T
2980
25T
334
26T
394
27T
469
2497
462
4198
303
314
707
2102
4696
2073
2163
10015
490
2373
1357
960
248
361
2352
0.8
0.9
1.7
0.5
1.8
0.5
0.8
0.5
2.1
0.8
1.2
2.2
0.5
1.3
40.6
1.2
0.3
1.0
0.23719
0.21124
0.19466
0.17701
0.24206
0.21378
0.19161
0.18376
0.21815
0.20212
0.20896
0.16457
0.24066
0.21861
0.19708
0.14211
0.18736
0.19253
12.02
16.79
14.29
28.28
21.27
12.21
15.45
18.33
22.80
19.37
7.99
23.22
18.45
10.47
12.52
21.65
26.36
16.88
0.03017
0.02950
0.02850
0.02927
0.02927
0.02787
0.02781
0.02822
0.02849
0.02950
0.02832
0.02954
0.02790
0.02810
0.02878
0.02845
0.02557
0.02713
3.32
3.01
2.81
2.37
3.54
1.61
1.50
2.17
2.07
2.46
1.75
2.50
1.78
1.32
1.86
3.72
2.60
2.20
0.28
0.18
0.20
0.08
0.17
0.13
0.10
0.12
0.09
0.13
0.22
0.11
0.10
0.13
0.15
0.17
0.10
0.13
192
187
181
186
186
177
177
179
181
187
180
188
177
179
183
181
163
173
6.3
5.6
5.0
4.4
6.5
2.8
2.6
3.8
3.7
4.5
3.1
4.6
3.1
2.3
3.4
6.6
4.2
3.7
216
195
181
165
220
197
178
171
200
187
193
155
219
201
183
135
174
179
23.4
29.7
23.6
43.2
42.1
21.8
25.2
28.9
41.5
33.1
14.0
33.3
36.4
19.1
20.9
27.4
42.3
27.7
492
282
173
-118
603
437
194
60
434
181
350
-325
694
469
179
-613
335
262
256
380
328
707
459
270
359
437
512
451
176
600
395
231
290
587
604
387
60
Table DR1. U-Pb data
Isotopic ratios
Sample
spot
U
(ppm)
206
Pb
Pb
U/Th
204
207
Pb*
U
± (%)
235
206
Pb*
U
Apparent ages (Ma)
± (%)
238
error
corr
206
Pb*
U
± (Ma)
238
207
Pb*
U
± (Ma)
235
206
Pb*
Pb*
± (Ma)
207
AP062104-A
2C
3C
4C
5C
9C
10C
17C
18C
19C
21C
23C
24C
26T
27T
30T
359
763
190
369
488
210
638
472
416
704
1079
1076
888
793
1340
1381
9004
1971
1501
718
3522
1078
5203
353
378
4470
4008
286
2819
221
1.3
2.1
1.2
1.0
1.0
0.8
1.0
0.9
1.1
1.5
1.6
1.6
0.6
2.0
2.1
0.23891
0.19789
0.19702
0.18893
0.21249
0.19871
0.19988
0.21183
0.24677
0.24037
0.20290
0.18512
0.22728
0.21929
0.23693
8.83
12.14
21.09
15.89
15.85
18.41
15.65
11.19
24.64
19.56
6.56
4.58
17.25
5.54
28.76
0.02851
0.02808
0.02880
0.02715
0.02590
0.02826
0.02697
0.02604
0.02710
0.02822
0.02685
0.02661
0.02655
0.02956
0.02379
2.12
4.40
2.79
2.65
3.60
2.16
1.39
1.23
3.14
2.12
1.10
1.08
1.94
1.84
3.68
0.24
0.36
0.13
0.17
0.23
0.12
0.09
0.11
0.13
0.11
0.17
0.23
0.11
0.33
0.13
181
178
183
173
165
180
172
166
172
179
171
169
169
188
152
3.8
7.7
5.0
4.5
5.9
3.8
2.4
2.0
5.3
3.8
1.9
1.8
3.2
3.4
5.5
218
183
183
176
196
184
185
195
224
219
188
172
208
201
216
17.3
20.4
35.3
25.7
28.2
31.0
26.5
19.9
49.6
38.5
11.2
7.3
32.4
10.1
56.0
631
246
177
217
585
240
361
567
808
667
405
216
677
363
992
185
261
492
365
337
424
354
243
519
420
145
103
369
118
592
AP060604-B
1C
4C
5C
9C
11C
12C
13C
14C
15C
16C
18C
19C
20C
23T
24T
25T
26T
27T
329
369
337
540
160
722
416
331
168
251
513
249
262
283
2980
334
394
469
2497
462
4198
303
314
707
2102
4696
2073
2163
10015
490
2373
1357
960
248
361
2352
0.8
0.9
1.7
0.5
1.8
0.5
0.8
0.5
2.1
0.8
1.2
2.2
0.5
1.3
40.6
1.2
0.3
1.0
0.23719
0.21124
0.19466
0.17701
0.24206
0.21378
0.19161
0.18376
0.21815
0.20212
0.20896
0.16457
0.24066
0.21861
0.19708
0.14211
0.18736
0.19253
12.02
16.79
14.29
28.28
21.27
12.21
15.45
18.33
22.80
19.37
7.99
23.22
18.45
10.47
12.52
21.65
26.36
16.88
0.03017
0.02950
0.02850
0.02927
0.02927
0.02787
0.02781
0.02822
0.02849
0.02950
0.02832
0.02954
0.02790
0.02810
0.02878
0.02845
0.02557
0.02713
3.32
3.01
2.81
2.37
3.54
1.61
1.50
2.17
2.07
2.46
1.75
2.50
1.78
1.32
1.86
3.72
2.60
2.20
0.28
0.18
0.20
0.08
0.17
0.13
0.10
0.12
0.09
0.13
0.22
0.11
0.10
0.13
0.15
0.17
0.10
0.13
192
187
181
186
186
177
177
179
181
187
180
188
177
179
183
181
163
173
6.3
5.6
5.0
4.4
6.5
2.8
2.6
3.8
3.7
4.5
3.1
4.6
3.1
2.3
3.4
6.6
4.2
3.7
216
195
181
165
220
197
178
171
200
187
193
155
219
201
183
135
174
179
23.4
29.7
23.6
43.2
42.1
21.8
25.2
28.9
41.5
33.1
14.0
33.3
36.4
19.1
20.9
27.4
42.3
27.7
492
282
173
-118
603
437
194
60
434
181
350
-325
694
469
179
-613
335
262
256
380
328
707
459
270
359
437
512
451
176
600
395
231
290
587
604
387
61
Table DR1. U-Pb data
Isotopic ratios
Sample
spot
U
(ppm)
206
Pb
Pb
U/Th
204
207
Pb*
U
± (%)
235
206
Pb*
U
Apparent ages (Ma)
± (%)
238
error
corr
206
Pb*
U
± (Ma)
238
207
Pb*
U
± (Ma)
235
206
Pb*
Pb*
± (Ma)
207
AP061604-B
1C
3C
4C
5C
7C
10C
11C
14C
17C
18C
19T
21T
22T
23T
25T
26T
27T
371
413
450
257
367
499
238
310
234
674
244
243
524
156
273
496
520
6026
7975
5536
2739
6755
3301
2546
1551
1855
710
1378
567
3534
1485
3786
5920
8114
0.9
6.1
1.3
1.2
2.1
2.6
1.5
3.5
2.0
3.7
1.1
2.0
3.2
0.7
1.3
6.4
2.2
0.18573
0.20725
0.20042
0.19393
0.23945
0.16691
0.18991
0.19278
0.20254
0.23463
0.20722
0.27346
0.17714
0.23349
0.26487
0.19788
0.19475
8.47
6.89
13.09
20.29
9.93
10.68
23.93
15.13
22.16
26.13
14.63
24.69
15.41
15.53
12.59
7.75
8.20
0.02702
0.02922
0.02687
0.02826
0.02942
0.02752
0.02874
0.02913
0.02698
0.02618
0.02524
0.02716
0.02619
0.02708
0.02661
0.02833
0.02675
1.54
1.59
2.44
2.02
2.00
0.80
3.56
3.39
2.06
2.68
2.56
2.49
1.47
3.12
1.77
2.68
2.19
0.18
0.23
0.19
0.10
0.20
0.08
0.15
0.22
0.09
0.10
0.17
0.10
0.10
0.20
0.14
0.35
0.27
172
186
171
180
187
175
183
185
172
167
161
173
167
172
169
180
170
2.6
2.9
4.1
3.6
3.7
1.4
6.4
6.2
3.5
4.4
4.1
4.3
2.4
5.3
3.0
4.8
3.7
173
191
185
180
218
157
177
179
187
214
191
245
166
213
239
183
181
13.5
12.0
22.2
33.5
19.5
15.5
38.8
24.8
37.9
50.5
25.5
53.9
23.6
29.9
26.8
13.0
13.6
188
261
375
184
568
-111
96
99
389
774
587
1014
150
692
991
225
320
194
154
290
474
212
263
567
351
501
556
314
505
361
326
255
168
180
JG061504-7
2
3
4
5
7
9
11
14
15
17
18
19
20
22
23
24
347
1167
1256
660
1163
1142
1098
1239
1196
1240
1485
1003
1492
467
1466
1505
5165
18686
25846
8527
8317
2014
11644
4097
8106
2456
3635
2857
2653
1289
2184
3134
0.9
1.5
1.7
1.1
1.1
1.8
1.5
1.7
1.7
1.9
1.6
1.3
1.6
0.7
1.9
1.6
0.20123
0.20916
0.20197
0.18475
0.20005
0.20196
0.20230
0.20503
0.20436
0.21436
0.19602
0.20519
0.20654
0.22168
0.21214
0.21210
7.81
3.12
3.36
3.53
3.70
4.03
2.96
3.37
3.47
4.71
1.60
7.75
3.35
4.53
4.93
2.68
0.03017
0.02974
0.02912
0.02811
0.02856
0.02862
0.02949
0.02944
0.02962
0.02974
0.02768
0.02864
0.02896
0.02948
0.02902
0.02970
1.76
1.70
1.75
1.15
0.76
1.10
1.30
1.16
1.91
1.75
0.70
0.82
0.71
1.19
0.92
1.51
0.23
0.54
0.52
0.33
0.21
0.27
0.44
0.34
0.55
0.37
0.44
0.11
0.21
0.26
0.19
0.56
192
189
185
179
182
182
187
187
188
189
176
182
184
187
184
189
3.3
3.2
3.2
2.0
1.4
2.0
2.4
2.1
3.5
3.3
1.2
1.5
1.3
2.2
1.7
2.8
186
193
187
172
185
187
187
189
189
197
182
190
191
203
195
195
13.3
5.5
5.7
5.6
6.3
6.9
5.1
5.8
6.0
8.4
2.7
13.4
5.8
8.3
8.8
4.8
117
241
209
83
232
249
183
218
196
297
257
283
273
394
329
276
180
60
67
79
84
89
62
73
67
100
33
176
75
98
110
51
62
Table DR1. U-Pb data
Isotopic ratios
Sample
spot
U
(ppm)
PK97-6-4-1A Titanite
1
423
2
496
3
429
4
531
5
390
6
428
7
428
8
439
9
502
10
599
11
660
12
626
13
537
14
468
15
522
16
473
17
443
18
551
19
459
20
425
21
424
22
551
23
423
24
376
25
422
26
392
27
462
206
Pb
Pb
U/Th
204
146
204
238
269
204
256
144
213
188
232
287
215
137
194
206
203
237
251
206
153
209
154
209
221
121
172
228
207
Pb*
U
± (%)
235
2.8
3.6
3.7
5.0
4.5
5.0
3.9
3.9
3.0
6.0
7.8
4.4
7.4
5.1
3.4
6.9
3.9
3.7
4.3
7.3
4.3
4.4
3.1
3.7
4.2
3.2
5.8
0.22957
0.20024
0.18681
0.20325
0.20796
0.20349
0.28401
0.24096
0.20119
0.22083
0.22097
0.21520
0.29358
0.23347
0.20962
0.26176
0.22432
0.20137
0.24334
0.26582
0.25035
0.21052
0.23994
0.20070
0.30440
0.21224
0.21243
206
Pb*
U
Apparent ages (Ma)
± (%)
238
47.78
45.61
43.48
40.66
46.18
41.81
44.86
41.98
49.01
41.61
37.87
43.11
44.73
43.16
45.22
40.96
41.02
42.82
42.85
44.45
40.66
51.24
42.03
45.09
46.37
46.53
42.17
0.02872
0.02761
0.02743
0.02797
0.02919
0.02819
0.02971
0.02874
0.02866
0.02789
0.02824
0.02744
0.02937
0.02792
0.02797
0.02832
0.02795
0.02783
0.02865
0.02866
0.02814
0.02791
0.02917
0.02847
0.02801
0.02735
0.02791
1.81
3.86
1.71
3.64
2.55
2.35
5.50
2.53
2.58
1.20
2.17
1.85
2.42
4.41
3.63
4.00
2.26
1.51
2.78
4.12
3.62
4.10
2.74
2.81
7.85
2.88
1.92
error
corr
0.04
0.08
0.04
0.09
0.06
0.06
0.12
0.06
0.05
0.03
0.06
0.04
0.05
0.10
0.08
0.10
0.06
0.04
0.06
0.09
0.09
0.08
0.07
0.06
0.17
0.06
0.05
206
Pb*
U
± (Ma)
238
183
176
174
178
185
179
189
183
182
177
180
175
187
178
178
180
178
177
182
182
179
177
185
181
178
174
177
207
Pb*
U
± (Ma)
235
3.3
6.7
2.9
6.4
4.7
4.2
10.2
4.6
4.6
2.1
3.8
3.2
4.5
7.7
6.4
7.1
4.0
2.6
5.0
7.4
6.4
7.2
5.0
5.0
13.8
4.9
3.4
210
185
174
188
192
188
254
219
186
203
203
198
261
213
193
236
205
186
221
239
227
194
218
186
270
195
196
206
Pb*
Pb*
± (Ma)
207
86.8
74.5
67.2
67.5
77.7
69.4
96.1
79.6
80.1
73.7
67.3
74.7
98.2
79.7
76.6
82.8
73.6
70.4
81.8
90.6
79.5
86.7
79.4
73.8
104.3
79.5
72.4
529
312
167
316
271
301
909
632
237
508
482
487
1000
627
386
839
537
306
660
846
759
401
591
246
1167
464
420
523
517
508
461
529
476
459
451
565
457
418
475
454
463
506
424
448
487
458
460
427
572
455
518
453
515
470
63
250
a)
200
150
PK-97-6-4-1A Hornblende
100
0
b)
Integrated Age = 180.0 ± 0.4 Ma
Apparent age (Ma)
Apparent Age (Ma)
250
20
40
Integrated Age = 175.3 ± 0.5 Ma
200
150
PK-97-6-4-3A Hornblende
60
80
Cumulative % 39Ar Released
100
100
0
20
40
60
80
Cumulative % 39Ar Released
100
Figure DR2. Hornblende argon age and K/Ca spectra for (a) PK-97-6-4-1A and (b) PK-97-6-4-3A
180
a)
160
Apparent Age (Ma)
Apparent Age (Ma)
180
140
120
100
PK-97-6-4-2 Muscovite
80
140
120
100
PK-97-6-4-1A Biotite
80
0
180
Apparent Age (Ma)
b)
160
20
40
60
80
100
60
80
100
Cumulative % 39Ar released
0
20
40
60
80
Cumulative % 39Ar released
100
c)
160
140
120
100
PK-97-6-4-3A Biotite
80
0
20
40
Cumulative % 39Ar released
Figure DR3. 40Ar/39Ar apparent age spectra for mica from orthogneiss samples a) PK-97-6-4-2,
b) PK-97-6-4-1A and c) PK-97-6-4-3A.
64
300
100
1
0.01
300
a) PK-97-6-4-1A
0.01
b) PK-97-6-4-2
250
Apparent age (Ma)
Apparent age (Ma)
250
200
150
K/Ca
1
K/Ca
100
200
150
100
Integrated Age = 149.4 ± 0.4 Ma
100
50
Integrated Age = 163.3 ± 0.4 Ma
50
0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
50
60
70
Cumulative % 39Ar Released
80
90
100
100
Cumulative % 39Ar Released
1
300
K/Ca
100
0.01
c) PK-97-6-4-3A
Apparent age (Ma)
250
200
150
100
Integrated Age = 148.3 ± 0.3 Ma
50
0
10
20
30
40
50
60
70
80
90
100
Cumulative % 39Ar Released
Figure DR4. Feldspar age and K/Ca spectra for a) PK-97-6-4-1A, b) PK-97-6-4-2 and c) PK-97-6-4-3A.
All of the samples have relatively low K/Ca values and the ca. 1% bulk K20 estimates indicate significant
quartz and plagioclase contamination of the K-feldspar mineral separate (Table DR2). For the two samples
that have coexisting hornblende pairs (PK-97-6-4-1A and 3A), the feldspars yield apparent ages
significantly older then the hornblendes (Fig. DR2). This later observation strongly suggests excess argon
contamination.
65
0.004
P K -97-6-4-1A felds par
S teps B -E
Is ochron age = 92 ± 4 Ma
air
BC
36 Ar/40 Ar
0.003
D E
40 Ar/36 Ar
o
a)
P K -97-6-4-1A felds par
S teps F ,H,J ,L,N,P ,R ,T ,V ,X,Z,AB
air
= 312.7 ± 0.8
AH
AJ
40 Ar/36 Ar
= 355.6 ± 1.7
o
o
= 413.1 ± 0.9
MS WD = 14, n = 15
0.001
AI
H
G
P
AG J
P
AJ
J
R
R
AFAE
AD
AC
AB
0
40 Ar/36 Ar
c)
F
F
AI H
0
P K -97-6-4-1A felds par
S teps M,O,Q,S ,U,W,Y ,AA,AC -AI
Is ochron age = 117.8 ± 0.3 Ma
air
MS WD = 86, n = 12
MS WD = 2.3, n = 4
0.002
b)
Is ochron age = 123.0 ± 0.4 Ma
N
Z
I
AAL
AB
T K
YX VQ
OSM
WU
0.10 0.12 0.14 0
0.02 0.04 0.06 0.08
N
Z L
AG
AFAE
AD
AC
I
AA
T
X V
0.02 0.04 0.06 0.08
39 Ar/40 Ar
G
AH
0
0.10 0.12
K
Y Q
OSM
WU
0.02 0.04 0.06 0.08
39 Ar/40 Ar
0.10 0.12 0.14
39 Ar/40 Ar
0.004
P K -97-6-4-2 felds par
S teps B -G
air
B
C
36 Ar/40 Ar
0.003
D
AJ
B
C
AJ
F
H
G
AF
AE
AD
AC
AB
J
I
AA
PZ
TV K
NY
R
XL
W
USMQO
0
0.02 0.04 0.06 0.08
J
I
AA
PZ
TV K
NY
R
XL
W
USMQO
0.10 0.12 0.14 0
0.004
P K -97-6-4-3a felds par
S teps B -J
Is ochron age = 120.7 ± 0.5 Ma
40 Ar/36 Aro = 323.0 ± 0.9
MS WD = 59, n = 9
air
36 Ar/40 Ar
F
H
G
AF
AE
AD
AC
AB
39 Ar/40 Ar
0.003
E
AG
B
C
D
(e)
Is ochron age = 140.9 ± 0.2 Ma
40 Ar/36 Ar = 319.6 ± 1.0
o
MS WD = 44, n = 23
D
AI AH
AG
0.001
0
P K -97-6-4-2 felds par
S teps K -AG
air
E
AI AH
0.002
(d)
Is ochron age = 108.8 ± 1.1 Ma
40 Ar/36 Ar = 302.0 ± 0.8
o
MS WD = 4.7, n = 6
(f)
air
0.02 0.04 0.06 0.08 0.10 0.12 0.14
39 Ar/40 Ar
P K -97-6-4-3a felds par
S teps K -T
(g)
P K -97-6-4-3a felds par
S teps U-AI
Is ochron age = 126.6 ± 0.4 Ma
40 Ar/36 Aro = 392.2 ± 1.6
MS WD = 88, n = 15
air
Is ochron age = 130.6 ± 0.2 Ma
40 Ar/36 Aro = 320.1 ± 1.6
MS WD = 17, P rob. = 0.00, n = 10
(h)
E
0.002
AI
J
F
H
TR
AH
G
AE
AC
AF
AD
V
AB
AG AA
Z
P
0.001
I
SN
K
L
QO
M
0
0
0.02 0.04 0.06 0.08
39 Ar/40 Ar
0.10 0.12
0
0.02 0.04 0.06 0.08
39 Ar/40 Ar
0.10 0.12
0
YX
W
U
0.02 0.04 0.06 0.08
39 Ar/40 Ar
Figure DR5. Isotope correlation diagrams for selected steps from samples PK-97-6-4-1A (a-c),
PK-97-6-4-2 (d,e) and PK-97-6-4-3A (f-h).
0.10 0.12 0.14
66
-2
(a)
200
-1
-4
Measured
Modeled
Isochron corrected
Modeled
-5
2
250
-6
150
-7
E=42.0 kcal/mol
Log Do/ro2 = 4.6 /sec
-8
100
50
(b)
-3
log(D/r ) s
Apparent Age (Ma)
300
-9
0
20
40
60
39
80
100
-10
6
7
Cumulative % Ar released
2.0
9
10 11 12 13 14
10000/T(K)
400
(c)
350
o
Temperature ( C)
1.5
log(r/ro)
8
1.0
0.5
(d)
Isochron
corrected
300
250
Atmosphere
corrected
200
150
0.0
0
20
40
60
39
80
Cumulative % Ar released
100
100
50
75
100
125
150
Age (Ma)
Figure DR6. Multiple diffusion domain modeling results for sample PK-97-6-4-1A feldspar.
175
200
67
-2
(a)
-3
-1
200
-5
-6
150
-7
E=35.9 kcal/mol
Log Do/ro2 = 3.2 /sec
-8
100
50
(b)
-4
Measured
Modeled
Isochron corrected
Modeled
2
250
log(D/r ) s
Apparent Age (Ma)
300
-9
0
20
40
60
39
80
100
-10
6
7
Cumulative % Ar released
2.0
9
10 11 12 13 14
10000/T(K)
400
(c)
350
o
Temperature ( C)
1.5
log(r/ro)
8
1.0
0.5
(d)
Isochron
corrected
300
250
200
Atmosphere
corrected
150
0.0
0
20
40
60
39
80
Cumulative % Ar released
100
100
50
75
100
125
150
Age (Ma)
Figure DR7. Multiple diffusion domain modeling results for sample PK-97-6-4-2 feldspar.
175
200
68
-2
(a)
-3
-1
200
-5
-6
150
-7
E=46.52 kcal/mol
Log Do/ro2 = 5.72 /sec
-8
100
50
(b)
-4
Measured
Modeled
Isochron corrected
Modeled
2
250
log(D/r ) s
Apparent Age (Ma)
300
-9
0
20
40
60
39
80
100
-10
6
7
Cumulative % Ar released
2.0
9
10 11 12 13 14
10000/T(K)
400
(c)
350
o
Temperature ( C)
1.5
log(r/ro)
8
1.0
0.5
(d)
Isochron
corrected
300
Atmosphere
corrected
250
200
150
0.0
0
20
40
60
39
80
Cumulative % Ar released
100
100
50
75
100
125
150
Age (Ma)
Figure DR8. Multiple diffusion domain modeling results for sample PK-97-6-4-3A feldspar.
175
200
69
Table DR2. NMGRL hornblende and feldspar argon isotopic data for Amdo orthogneiss samples.
ID
Temp
Ar
Age
(%)
(Ma)
(Ma)
PK-97-6-4-3A, Hornblende, 5.51 mg, J=0.007895±0.10%, D=1.004±0.001, NM-173D, Lab#=54529-01
A
800
30.78
0.6795
60.56
4.25
0.75
42.0
8.9
B
900
14.60
0.8884
12.14
1.61
0.57
75.9
12.2
C
1000
15.76
6.768
13.63
2.85
0.075
78.1
18.2
D
1030
14.74
7.546
8.709
1.87
0.068
86.9
22.1
E
1060
13.87
6.648
4.818
14.0
0.077
93.9
51.3
F
1090
13.28
6.217
3.246
15.0
0.082
96.8
82.7
G
1120
15.05
7.545
9.700
1.15
0.068
85.2
85.1
H
1170
14.30
9.200
6.625
1.36
0.055
91.8
87.9
I
1200
14.11
6.595
4.857
2.90
0.077
93.9
93.9
J
1250
14.46
6.936
4.752
2.59
0.074
94.4
99.3
K
1300
18.67
8.930
19.68
0.313
0.057
72.9 100.0
Integrated age ± 1σ
n=11
47.9
K2O=0.42 %
175.4
151.1
167.8
174.4
176.99
174.80
174.6
178.8
179.8
185.2
185
175.13
2.1
2.9
1.8
2.6
0.54
0.50
4.3
3.6
1.7
2.0
15
0.54
PK-97-6-4-1A, Hornblende, 5.23 mg, J=0.007933±0.10%, D=1.004±0.001, NM-173D, Lab#=54530-01
A
800
30.71
3.280
64.78
5.74
0.16
38.5
7.3
B
900
13.29
0.8822
6.865
3.10
0.58
85.3
11.2
C
1000
14.32
3.637
6.216
7.35
0.14
89.4
20.6
D
1030
13.94
2.898
3.972
8.85
0.18
93.5
31.8
E
1060
14.01
3.381
2.923
18.9
0.15
96.0
55.9
F
1090
13.67
3.299
2.719
5.19
0.15
96.3
62.5
G
1120
14.47
3.668
4.244
3.12
0.14
93.6
66.5
H
1170
14.21
3.906
2.856
22.7
0.13
96.6
95.4
I
1200
15.94
5.345
8.952
3.66
0.095
86.4 100.0
Integrated age ± 1σ
n=9
78.6
K2O=0.73 %
162.1
155.2
174.58
177.51
183.09
179.3
184.2
186.59
187.3
179.78
2.0
1.7
0.78
0.66
0.42
1.0
1.9
0.41
1.5
0.42
PK-97-6-4-3A, Feldspar, 11.53 mg, J=0.007844±0.10%, D=1.0035±0.0005, NM-173D, Lab#=54527-01
B
450 344.3
0.5212
1036.0
1.80
0.98
11.1
0.4
C
450
86.62
0.4010
261.9
0.751
1.3
10.7
0.5
D
500
30.08
0.2849
68.25
1.54
1.8
33.0
0.9
E
500
24.87
0.3977
56.43
1.35
1.3
33.0
1.1
F
550
18.10
0.4053
29.31
3.44
1.3
52.3
1.9
G
550
13.78
0.4777
17.76
3.34
1.1
62.1
2.6
H
600
19.39
0.4653
29.36
9.02
1.1
55.4
4.5
I
600
10.89
0.5212
7.114
6.28
0.98
81.1
5.8
J
650
19.40
0.7211
33.16
10.1
0.71
49.7
7.9
K
650
10.38
0.6195
3.429
7.85
0.82
90.7
9.6
L
700
10.27
0.6549
2.567
8.90
0.78
93.1
11.5
M
700
10.05
0.5801
1.755
7.85
0.88
95.3
13.1
N
750
11.56
0.6557
6.491
10.3
0.78
83.8
15.3
O
750
10.29
0.5256
1.917
8.29
0.97
94.9
17.0
P
800
14.82
0.6021
16.63
11.2
0.85
67.1
19.4
Q
800
10.45
0.5525
2.361
8.63
0.92
93.7
21.2
R
850
17.52
0.5008
25.53
12.6
1.0
57.1
23.9
S
850
11.89
0.4431
6.593
9.8
1.2
83.9
25.9
T
900
18.73
0.3893
28.37
17.2
1.3
55.4
29.5
U
900
11.55
0.4197
5.436
12.1
1.2
86.4
32.1
V
950
15.22
0.6908
16.89
13.5
0.74
67.5
34.9
W
950
11.83
0.4481
6.472
7.53
1.1
84.1
36.5
X
1000
12.30
0.4347
8.177
10.3
1.2
80.6
38.7
Y
1000
12.36
0.3586
8.387
10.4
1.4
80.1
40.9
Z
1050
13.34
0.3548
11.10
12.6
1.4
75.6
43.5
AA
1050
13.70
0.3339
12.47
13.0
1.5
73.3
46.3
AB
1100
15.06
0.3854
15.49
21.4
1.3
69.8
50.8
AC
1100
15.86
0.3767
18.01
20.2
1.4
66.6
55.0
AD
1100
15.99
0.3047
17.89
28.5
1.7
67.0
61.0
AE
1100
16.23
0.2875
18.44
39.3
1.8
66.5
69.3
AF
1100
16.69
0.2636
18.68
42.0
1.9
67.0
78.1
AG
1200
15.40
0.2140
13.38
48.7
2.4
74.4
88.4
AH
1250
21.25
0.5813
29.78
32.0
0.88
58.8
95.1
AI
1350
43.17
2.669
88.52
21.1
0.19
39.9
99.6
AK
1700
9.641
0.5044
10.10
2.04
1.0
69.4 100.0
Integrated age ± 1σ
n=35
475.1
K2O=2.02 %
473
126.4
135.1
112.5
128.9
117.0
145.75
120.53
131.50
128.23
130.19
130.39
131.94
132.84
135.32
133.24
136.13
135.61
140.91
135.68
139.69
135.27
134.89
134.73
137.04
136.48
142.69
143.41
145.47
146.45
151.45
155.00
168.45
228.9
92.1
148.31
11
8.5
3.8
3.7
1.3
1.5
0.91
0.65
0.92
0.45
0.48
0.36
0.47
0.40
0.75
0.44
0.74
0.53
0.61
0.49
0.66
0.59
0.61
0.59
0.57
0.51
0.47
0.49
0.47
0.62
0.37
0.35
0.62
1.5
2.1
0.32
(°C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
(x 10-3)
39
ArK
(x 10-15 mol)
K/Ca
40
Ar*
(%)
39
±1σ
Isochron Age
(Ma)
133
28
110
92
118
110
135
118
120
127
129
130
130
132
130
133
128
134
132
129
118
127
125
124
123
121
123
121
123
123
128
138
131
120
79
70
Table DR2 continued. NMGRL hornblende and feldspar argon isotopic data for Amdo orthogneiss samples.
ID
Temp
(°C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
(x 10-3)
39
ArK
(x 10-15 mol)
K/Ca
40
Ar*
(%)
Ar
Age
(%)
(Ma)
(Ma)
(Ma)
554
233
147.5
120.8
168.7
114.6
197.6
120.02
134.06
122.79
128.16
123.51
127.05
124.07
131.1
124.17
129.01
124.03
127.57
126.64
126.49
128.10
130.70
130.45
134.91
134.39
143.73
143.26
146.77
149.78
160.51
168.14
185.53
239.65
269.7
163.33
18
13
5.2
4.1
2.2
1.8
1.6
0.81
0.89
0.57
0.64
0.40
0.74
0.62
1.1
0.53
0.95
0.55
0.72
0.54
0.57
0.44
0.51
0.42
0.41
0.49
0.76
0.62
0.54
0.41
0.69
0.54
0.61
0.97
1.4
0.44
95
80
102
88
110
39
PK-97-6-4-1A, Feldspar, 11.95 mg, J=0.007871±0.10%, D=1.0035±0.0005, NM-173D, Lab#=54528-01
B
450 711.7
0.5878
2254.1
1.77
0.87
6.4
0.4
C
450 218.7
0.3532
681.1
1.15
1.4
8.0
0.6
D
500
69.91
0.3420
200.0
1.55
1.5
15.5
1.0
E
500
51.20
0.3487
143.5
1.80
1.5
17.2
1.4
F
550
34.47
0.6782
74.62
3.99
0.75
36.1
2.2
G
550
18.17
1.254
33.58
3.32
0.41
45.9
2.9
H
600
33.11
1.767
62.75
7.62
0.29
44.4
4.6
I
600
11.99
1.683
11.41
5.18
0.30
73.0
5.7
J
650
17.71
1.495
27.12
8.07
0.34
55.4
7.4
K
650
11.11
1.058
7.554
6.82
0.48
80.7
8.9
L
700
12.15
0.7320
9.588
8.33
0.70
77.1
10.7
M
700
10.43
0.5986
4.909
6.88
0.85
86.5
12.2
N
750
13.44
0.6450
14.23
8.54
0.79
69.1
14.1
O
750
10.60
0.6648
5.374
6.41
0.77
85.5
15.4
P
800
18.50
0.8440
30.36
7.88
0.60
51.8
17.1
Q
800
10.76
0.8776
5.938
6.19
0.58
84.3
18.5
R
850
15.57
0.8168
20.96
7.47
0.62
60.6
20.1
S
850
10.46
0.6635
4.892
4.99
0.77
86.7
21.2
T
900
11.68
0.5678
8.114
6.75
0.90
79.8
22.6
U
900
10.61
0.4347
4.665
5.98
1.2
87.3
23.9
V
950
10.93
0.3792
5.791
8.06
1.3
84.6
25.7
W
950
10.80
0.3163
4.909
8.15
1.6
86.8
27.4
X
1000
11.48
0.2540
6.515
9.9
2.0
83.4
29.6
Y
1000
11.51
0.2505
6.687
11.1
2.0
83.0
32.0
Z
1050
13.15
0.2249
11.10
14.7
2.3
75.2
35.1
AA
1050
12.73
0.2026
9.784
14.0
2.5
77.4
38.2
AB
1100
14.56
0.2281
13.59
20.8
2.2
72.5
42.7
AC
1100
15.68
0.2593
17.52
17.6
2.0
67.1
46.5
AD
1100
16.44
0.2661
19.17
26.6
1.9
65.6
52.2
AE
1100
17.63
0.3083
22.44
33.5
1.7
62.5
59.5
AF
1100
18.88
0.3660
23.89
27.1
1.4
62.7
65.3
AG
1200
22.31
0.3879
33.52
46.8
1.3
55.7
75.4
AH
1250
27.23
0.6375
45.70
58.3
0.80
50.6
88.0
AI
1350
40.88
2.106
77.88
45.6
0.24
44.1
97.9
AJ
1700
39.71
3.132
66.02
9.8
0.16
51.5 100.0
Integrated age ± 1σ
n=35
462.7
K2O=1.89 %
±1σ
Isochron Age
149
119
113
119
121
116
116
116
107
115
112
117
121
120
122
121
126
121
126
119
133
116
117
115
124
117
116
123
173
71
Table DR2 continued. NMGRL hornblende and feldspar argon isotopic data for Amdo orthogneiss samples.
ID
Temp
(°C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
(x 10-3)
39
ArK
(x 10-15 mol)
K/Ca
40
Ar*
(%)
Ar
Age
(%)
(Ma)
(Ma)
(Ma)
190
163.8
111.4
114.2
116.9
109.6
128.2
124.41
130.75
134.14
135.21
138.92
139.65
138.79
141.34
140.38
142.74
142.84
145.03
144.36
145.77
142.21
140.01
140.48
141.24
142.88
140.86
143.9
144.04
145.71
148.40
166.78
215.4
316.4
242.7
149.41
12
7.6
4.3
2.3
1.9
1.2
1.1
0.96
0.76
0.76
0.54
0.51
0.45
0.40
0.41
0.33
0.27
0.24
0.25
0.22
0.33
0.32
0.41
0.41
0.55
0.59
0.73
1.0
0.85
0.75
0.90
0.90
2.2
3.2
2.0
0.36
92
131
102
108
113
107
117
39
PK-97-6-4-2, Feldspar, 11.67 mg, J=0.007937±0.10%, D=1.0035±0.0005, NM-173D, Lab#=54531-01
B
450 349.1
0.4846
1134.3
2.42
1.1
4.0
0.6
C
450 123.8
0.3424
378.5
1.40
1.5
9.7
1.0
D
500
38.16
0.3123
102.0
2.00
1.6
21.0
1.5
E
500
30.16
0.3162
74.19
1.90
1.6
27.3
2.1
F
550
20.38
0.4961
40.51
3.89
1.0
41.4
3.1
G
550
15.66
0.4724
26.34
3.09
1.1
50.5
3.9
H
600
19.84
0.5960
35.84
6.87
0.86
46.8
5.7
I
600
11.55
0.6333
8.749
4.39
0.81
78.0
6.9
J
650
12.84
0.7015
11.51
6.36
0.73
73.9
8.6
K
650
11.33
0.6411
5.523
4.98
0.80
86.0
9.9
L
700
10.94
0.6672
3.947
6.52
0.76
89.8
11.7
M
700
10.90
0.5734
2.835
6.38
0.89
92.7
13.4
N
750
11.69
0.6272
5.358
10.4
0.81
86.9
16.1
O
750
10.64
0.5489
1.973
9.16
0.93
94.9
18.6
P
800
12.56
0.5869
7.857
15.0
0.87
81.9
22.6
Q
800
10.71
0.4889
1.795
12.7
1.0
95.4
26.0
R
850
11.71
0.4414
4.580
19.4
1.2
88.7
31.1
S
850
10.94
0.3741
1.897
17.7
1.4
95.2
35.8
T
900
12.50
0.2732
6.598
37.0
1.9
84.6
45.7
U
900
11.12
0.2623
2.086
25.0
1.9
94.6
52.4
V
950
12.45
0.6659
6.363
23.2
0.77
85.3
58.5
W
950
11.12
0.3256
2.677
13.4
1.6
93.1
62.1
X
1000
11.38
0.3446
4.121
12.7
1.5
89.5
65.5
Y
1000
11.59
0.3085
4.690
11.4
1.7
88.2
68.5
Z
1050
12.34
0.3775
7.048
12.1
1.4
83.3
71.8
AA
1050
12.89
0.3659
8.510
10.0
1.4
80.7
74.4
AB
1100
15.22
0.6169
16.97
8.15
0.83
67.3
76.6
AC
1100
16.90
0.5619
21.88
7.81
0.91
62.0
78.7
AD
1100
18.09
0.6334
25.89
11.5
0.81
57.9
81.7
AE
1100
18.85
0.5873
28.02
13.6
0.87
56.3
85.4
AF
1100
19.62
0.5717
29.91
13.8
0.89
55.1
89.0
AG
1200
29.39
0.6464
58.29
19.5
0.79
41.5
94.2
AH
1250
53.30
2.262
127.0
9.02
0.23
29.9
96.6
AI
1350
83.01
6.169
201.3
6.22
0.083
29.0
98.3
AJ
1700
44.36
4.325
90.11
6.49
0.12
40.8 100.0
Integrated age ± 1σ
n=35
375.5
K2O=1.56 %
±1σ
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions.
Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 27.84 Ma.
Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties.
Integrated age calculated by quadratically combining isotopic measurements of all steps.
Integrated age error calculated by quadratically combining errors of isotopic measurements of all steps.
Plateau age is inverse-variance-weighted mean of selected steps.
Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times squareroot MSWD where MSWD>1.
Plateau and integrated ages incorporate uncertainties in interfering reaction corrections and J factors (0.1%).
Decay constants and isotopic abundances after Steiger and Jager (1977).
K20 estimated from 39Ar signal, sample weight and J-factor.
D = 1 AMU discrimination in favor of light isotopes.
Isochron age is age calculated using trapped initial 40Ar/36Ar determined from isochron analysis.
Correction factors:
(39Ar/37Ar)Ca = 0.00074 ± 2e-05
(36Ar/37Ar)Ca = 0.000284 ± 6e-06
(40Ar/39Ar)K = 0.02895 ± 0.00059
Isochron Age
132
134
138
138
138
139
140
141
142
143
144
144
141
139
139
139
140
135
137
136
137
139
148
72
Table DR3. UCLA mica argon isotopic data for Amdo orthogneiss samples.
Step
40
T (°C)
39
a
Ar/ Ar
38
39
a
Ar/ Ar
37
39
a
Ar/ Ar
36
39
a
Ar/ Ar
39
b
ArK(mol)
S39ArK (%)
40
c
Ar* (%)
40
39
d
Ar*/ ArK
± s40/39
Age ± 1
e
(Ma)
97-6-4-2 Muscovite (J=0.007344, weight = 5.8 mg) Total gas age = 166.4 ± 0.4 Ma
1
2
3
4
5
6
7
8
9
10
11
12
500
600
700
750
800
850
900
950
1000
1100
1200
1350
Measured
24.52636
0.72
0.08
0.06
0.00
0.4
26.7
6.57
0.40
85.1 ± 5.1
14.59611
0.02
0.07
0.01
0.00
0.9
76.7
11.27
0.12
143.4 ± 1.5
13.70792
0.01
0.03
0.00
0.00
2.5
92.4
12.68
0.02
160.7 ± 0.3
13.33618
0.01
0.02
0.00
0.00
5.8
95.9
12.80
0.03
162.1 ± 0.4
13.30572
0.01
0.01
0.00
0.00
8.3
96.1
12.80
0.02
162.1 ± 0.3
13.46846
0.01
0.01
0.00
0.00
12.8
96.0
12.94
0.03
163.8 ± 0.3
13.64588
0.01
0.01
0.00
0.00
22.5
95.8
13.08
0.04
165.5 ± 0.5
13.47412
0.01
0.00
0.00
0.00
40.8
98.0
13.23
0.03
167.2 ± 0.4
13.48864
0.01
0.02
0.00
0.00
50.1
97.8
13.21
0.03
167.1 ± 0.3
13.51966
0.01
0.01
0.00
0.00
68.9
97.7
13.22
0.03
167.1 ± 0.3
13.50152
0.01
0.01
0.00
0.00
99.4
98.6
13.33
0.03
168.5 ± 0.4
14.73287
0.01
0.51
0.00
0.00
100.0
90.9
13.46
0.09
170.1 ± 1.0
40
36
40
39
Ar/ Aratm = 175 ± 3.34 and Ar/ Aratm = 297.5 ± 0.5; Abundance sensitivity = 5 ppm; Date irradiated = 08-16-2000; Date analyzed = 9-29-2000
97-6-4-1A Biotite (J=0.007351, weight = 5.6 mg) Total gas age = 163.6 ± 0.3 Ma
1
600
13.60089
0.42
0.15
0.02
0.00
2.5
47.3
6.45
0.04
83.5 ± 0.5
2
700
12.79718
0.03
0.07
0.01
0.00
10.5
85.1
10.91
0.02
139.1 ± 0.2
3
750
13.45496
0.02
0.02
0.00
0.00
19.9
94.2
12.69
0.03
160.9 ± 0.3
4
800
13.59582
0.02
0.01
0.00
0.00
31.9
97.0
13.20
0.01
167.1 ± 0.2
5
850
13.7659
0.03
0.01
0.00
0.00
37.8
97.6
13.46
0.02
170.2 ± 0.2
6
900
14.06217
0.03
0.02
0.00
0.00
42.0
96.1
13.53
0.07
171.1 ± 0.8
7
950
13.90634
0.03
0.04
0.00
0.00
47.0
95.0
13.23
0.03
167.4 ± 0.3
8
1000
13.95283
0.03
0.09
0.00
0.00
51.2
95.1
13.29
0.03
168.2 ± 0.3
9
1050
13.64354
0.03
0.13
0.00
0.00
59.7
96.0
13.11
0.02
166.0 ± 0.3
10
1100
13.47398
0.03
0.23
0.00
0.00
73.1
96.9
13.08
0.03
165.6 ± 0.3
11
1200
13.69293
0.04
0.29
0.00
0.00
89.3
98.3
13.48
0.04
170.4 ± 0.5
12
1350
13.87532
0.03
0.22
0.00
0.00
100.0
98.6
13.69
0.02
173.0 ± 0.2
Measured
40
Ar/ Aratm = -36.9 ± 0.65 and 40Ar/39Aratm = 297.5 ± 0.5; Abundance sensitivity = 5 ppm; Date irradiated = 08-16-2000; Date analyzed = 9-29-2000
36
97-6-4-3A Biotite (J=0.007364, weight = 7.6 mg) Total gas age = 156.5 ± 0.4 Ma
1
2
3
4
5
6
7
8
9
10
11
12
500
600
700
780
850
900
950
1000
1050
1100
1200
1350
Measured
25.96679
1.01
0.54
0.07
0.00
0.8
15.7
4.09
0.22
53.6 ± 2.8
9.390469
0.04
0.60
0.01
0.00
2.7
67.9
6.42
0.08
83.4 ± 1.0
10.49737
0.03
0.37
0.01
0.00
11.1
85.4
8.97
0.03
115.6 ± 0.4
13.13477
0.03
0.04
0.00
0.00
25.0
95.9
12.61
0.03
160.4 ± 0.4
13.14099
0.03
0.05
0.00
0.00
35.2
96.1
12.64
0.03
160.7 ± 0.4
13.35206
0.03
0.05
0.00
0.00
42.4
95.6
12.78
0.03
162.4 ± 0.3
13.48795
0.03
0.07
0.00
0.00
49.9
95.8
12.93
0.04
164.3 ± 0.5
14.12381
0.03
0.29
0.00
0.00
54.6
93.2
13.19
0.03
167.4 ± 0.4
14.23293
0.03
0.32
0.00
0.00
60.9
92.7
13.21
0.03
167.6 ± 0.4
13.58004
0.04
0.51
0.00
0.00
74.6
96.0
13.05
0.03
165.7 ± 0.3
13.21134
0.03
1.55
0.00
0.00
88.5
97.0
12.84
0.03
163.1 ± 0.4
12.90057
0.03
0.53
0.00
0.00
100.0
96.9
12.52
0.02
159.3 ± 0.2
40
36
Ar/ Aratm = 1115 ± 1430 and 40Ar/39Aratm = 297.5 ± 0.5; Abundance sensitivity = 5 ppm; Date irradiated = 08-16-2000; Date analyzed = 9-28-2000
Total gas age error is analytical only and 1s
73
Table DR4. Arrhenius data for feldspar samples. Data shown in bold used for r0 determination.
Step
PK-97-6-4-1A
10000/T(K)
Log(D/r2)
B
C
D
E
F
G
H
I
J
K
L
M
M
O
P
Q
R
S
T
U
V
W
X
Y
Z
AA
AB
AC
AD
AE
AF
AG
AH
AI
AJ
13.83
13.83
12.94
12.94
12.15
12.15
11.45
11.45
10.83
10.83
10.28
10.28
9.78
9.78
9.32
9.32
8.90
8.90
8.53
8.53
8.18
8.18
7.86
7.86
7.56
7.56
7.28
7.28
7.28
7.28
7.28
6.79
6.57
6.16
5.07
-7.82
-7.84
-7.25
-7.28
-6.49
-6.67
-5.90
-6.18
-5.62
-5.86
-5.44
-5.71
-5.30
-5.63
-5.25
-5.59
-5.20
-5.60
-5.19
-5.46
-5.06
-5.27
-4.90
-5.07
-4.66
-4.89
-4.43
-4.71
-4.89
-5.03
-5.19
-3.53
-3.18
-2.82
-2.05
E=42.0 kcal/mol
Log(D0/r02) = 4.6 /sec
Domain
Log(D0/r2)
1
2
3
4
5
6
7
7.225
6.340
5.668
4.944
2.843
1.924
1.200
Step
B
C
D
E
F
G
H
I
J
K
L
M
M
O
P
Q
R
S
T
U
V
W
X
Y
Z
AA
AB
AC
AD
AE
AF
AG
AH
AI
AJ
PK-97-6-4-2
10000/T(K)
13.83
13.83
12.94
12.94
12.15
12.15
11.45
11.45
10.83
10.83
10.28
10.28
9.78
9.78
9.32
9.32
8.90
8.90
8.53
8.53
8.18
8.18
7.86
7.86
7.56
7.56
7.28
7.28
7.28
7.28
7.28
6.79
6.57
6.16
5.07
Log(D/r2)
-7.33
-7.44
-6.85
-7.00
-6.26
-6.48
-5.74
-6.07
-5.57
-5.85
-5.42
-5.61
-5.07
-5.31
-4.78
-5.03
-4.52
-4.75
-4.09
-4.43
-4.14
-4.58
-4.30
-4.56
-4.24
-4.53
-4.33
-4.58
-4.74
-4.90
-4.93
-3.26
-3.31
-3.23
-2.06
E=35.9 kcal/mol
Log(D0/r02) = 3.2 /sec
Volume
fraction
0.0240
0.0359
0.0399
0.0616
0.2137
0.1939
0.4310
Domain
Log(D0/r2)
1
2
3
4
5
6
7
6.481
5.640
2.857
2.800
2.699
1.871
0.710
Volume
fraction
0.0051
0.0332
0.2462
0.1658
0.1155
0.1766
0.2576
Step
B
C
D
E
F
G
H
I
J
K
L
M
M
O
P
Q
R
S
T
U
V
W
X
Y
Z
AA
AB
AC
AD
AE
AF
AG
AH
AI
AJ
PK-97-6-4-3A
10000/T(K)
13.83
13.83
12.94
12.94
12.15
12.15
11.45
11.45
10.83
10.83
10.28
10.28
9.78
9.78
9.32
9.32
8.90
8.90
8.53
8.53
8.18
8.18
7.86
7.86
7.56
7.56
7.28
7.28
7.28
7.28
7.28
6.79
6.57
6.16
5.07
Log(D/r2)
-7.80
-8.07
-7.33
-7.48
-6.64
-6.74
-5.87
-6.11
-5.52
-5.80
-5.40
-5.64
-5.20
-5.49
-5.06
-5.38
-4.91
-5.23
-4.69
-5.05
-4.71
-5.16
-4.78
-5.00
-4.63
-4.85
-4.35
-4.57
-4.78
-4.86
-4.86
-3.26
-3.10
-2.67
-2.09
E=46.5 kcal/mol
Log(D0/r02) = 5.72 /sec
2
Domain Log(D0/r )
1
2
3
4
5
6
7
7.830
6.589
5.286
4.966
3.685
2.522
2.320
Volume
fraction
0.0604
0.0613
0.0794
0.0738
0.1203
0.6047
0.0000
74
APPENDIX B:
Metamorphism and exhumation of the Amdo basement, Tibet: Implications for the
Jurassic tectonics of the Bangong suture zone
Manuscript for submission to
Journal of Metamorphic Geology
Jerome Guynn
University of Arizona
Paul Kapp
University of Arizona
George Gehrels
University of Arizona
75
ABSTRACT
Gneisses and metapelites of the Amdo basement, located along the Bangong Suture
Zone (BSZ) in central Tibet, provide a record of metamorphism and magmatism not
available elsewhere along the suture. U-Pb ages of metamorphic zircon constrain the
timing of peak metamorphism to have occurred ~185-175 Ma, prior to the Early
Cretaceous collision between the Lhasa and Qiangtang terranes along the BSZ.
Thermobarometry of mafic garnet-amphibolites and thermometry of non-garnet bearing
amphibolites indicates P-T conditions of ~11 kbar and ~650-750°C across the entire 50
km2 exposure of gneisses. Small exposures of lower grade metasedimentary rocks are in
fault contact with the gneisses along the southern edge of the basement.
Thermobarometry of garnet-kyanite schist provides peak conditions of 7-9 kbar and
~600°C and together with the gneisses record burial within a typical geotherm that was
not affected by the coeval intrusion of Jurassic (185-170 Ma) granitoids.
The metamorphism and magmatism recorded by the Amdo basement is not seen
elsewhere along the BSZ over 1200 km to the west (though it may be exposed to the
east), despite the consumption of the Meso-Tethys Ocean between the Lhasa and
Qiangtang terranes. The conditions recorded by the Amdo basement are similar to those
seen in the Coast Plutonic Complex in North America and indicate tectonism that
resulted in the removal, possibly by underthrusting, of a continental arc along the
southern edge of the Qiangtang terrane. This suggests that many major differences in
structure and metamorphism seen along extinct continental arcs and collision zones may
simply be a result of differential exhumation during later deformation.
76
INTRODUCTION
The Tibetan Plateau is composed of various terranes that accreted to the southern
Asian margin in the late Paleozoic and Mesozoic (Allégre et al., 1984; Dewey et al.,
1988; Yin & Harrison, 2000). Unraveling the geological history and deformation related
to these accretionary events is important for deciphering the subsequent deformation due
to the Indo-Asian collision during the early Tertiary and for understanding the
development of this large, high plateau, the largest orogenic feature on earth. Not only
could these accretionary collisional events have contributed to the crustal thickening of
the plateau, but the location of sutures, the distribution of lithologies, and the
tectonothermal processes related to accretion all can influence the style and nature of
magmatism and deformation as a result of India’s collision with Asia.
Tibet also
provides a laboratory for studying the nature of accretionary processes in general and
how the continental crust responds to collision. Of particular importance is the collision
of the Lhasa terrane with the Qiangtang terrane during the Cretaceous because of the
proximity of this collision to the Indo-Asian collision in both time and space and because
the resulting suture zone has several enigmatic aspects.
The Lhasa-Qiangtang collision occurred during the Early Cretaceous and resulted in
crustal shortening in the northern Lhasa and southern Qiangtang terranes from the
Cretaceous into the Tertiary (Murphy et al., 1997; Kapp et al., 2003; Kapp et al., 2005;
Guynn et al., 2006). The two terranes are separated by the Bangong suture zone (BSZ), a
broad area of Jurassic flysch and mélange, discontinuous and isolated ophiolite fragments
and Cretaceous and Tertiary nonmarine sedimentary rocks. The ophiolites fragments are
77
sparse, yet in both the west and east they occur in broad belts up to 200 km across strike
in the suture zone (Fig. 1). They represent dismembered sections of a true ophiolite
sequence and are involved in thrusts with Jurassic flysch and mélange, late Paleozoic
sedimentary rocks, Cretaceous volcanic rocks and Cretaceous-Tertiary redbeds
(Girardeau et al., 1984; Girardeau et al., 1985; Pearce & Deng, 1988). Their occurrence
over a wide north-south distance has been attributed to a large ophiolite nappe (Girardeau
et al., 1984; Coward et al., 1988), multiple sutures or oceanic basins (Pearce & Deng,
1988; Matte et al., 1996; Schneider et al., 2003) or ophiolite fragments within a mélange
involved in thrust belt propagation (Kapp et al., 2003). The oceanic crust is Middle-Late
Jurassic based on stratigraphic evidence (Girardeau et al., 1985; Pearce & Deng, 1988)
and it was obducted prior to deposition of unconformably overlying Aptian-Albian
limestones and volcanic rocks (Girardeau et al., 1985; Matte et al., 1996), ~180-175 Ma
in the Donqiao area at the northern edge of the suture zone (Zhou et al., 1997).
Igneous rocks attributable to subduction of the Meso-Tethyan Ocean between the
Lhasa and Qiangtang terranes are virtually nonexistent (Allégre et al., 1984; Dewey et al.,
1988) with the exception of Jurassic granitoids within the Amdo basement just south of
the town of Amdo (Fig. 1) (Guynn et al., 2006). While slow subduction and minimal
basin closure have been suggested to account for the lack of magmatism (Dewey et al.,
1988; Schneider et al., 2003), both require a limited ocean between the Lhasa and
Qiangtang terranes, while a significant (thousands of kilometers wide) ocean is implied
by the paleomagnetic (Li et al., 2004), stratigraphic (Leeder et al., 1988; Yin et al., 1988)
and faunal (Metcalfe, 1996) evidence. Despite the lack of an exposed igneous arc,
78
subduction is generally thought to be northward, beneath the Qiangtang terrane, based on
southward verging Cretaceous thrusts within the suture zone and the northern Lhasa
terrane (Girardeau et al., 1984; Coward et al., 1988; Kapp et al., 2003).
Deeper structural levels are generally not exposed along the BSZ in central Tibet
(Fig. 1) and a lack of apparent deformation led Coward et al. (1988) to postulate
minimum deformation associated with the collision. However, more recent work has
documented extensive shortening in the northern Lhasa terrane that is attributed to the
collision (Murphy et al., 2000; Kapp et al., 2003; Kapp et al., 2005) and it has been
suggested that shortening within deeper levels is disconnected from upper levels
(Volkmer et al., accepted). Furthermore, the high grade metamorphism of the Amdo
basement, an approximately 50 km × 100 km exposure of amphibolite-facies gneisses,
metasediments and intruding plutons within the suture zone north of Lhasa (Fig. 2), was
proposed to have occurred in the Cambrian (Xu et al., 1985; Coward et al., 1988) but
recent thermochronologic data presented by Guynn et al. (Guynn et al., 2006) revealed
that the high-grade metamorphism was in fact Middle Jurassic (~180 Ma).
The Amdo basement is one of the few exposures of high-grade metamorphic rocks in
the Tibetan Plateau (Harris et al., 1988) despite the protracted history of deformation in
the region. The majority of the basement is composed of orthogneisses with a Cambrian
or early Neoproterozoic protolith (Xu et al., 1985; Guynn et al., 2006) and it is the only
documented pre-accretionary crystalline basement in Tibet. The basement also contains
minor exposures of paragneiss, metasedimentary rocks and migmatites and it is intruded
by Jurassic granitoids (~185-170 Ma; Guynn et al., 2006). It is the only exposure of
79
metamorphic rocks along the entire BSZ to the west, over 1200 km. Recent regional
geologic maps by Chinese geologists suggest other similar exposures to the east (Fig. 1;
Pan et al., 2004), although these remain undocumented in the literature. The igneous and
metamorphic rocks make the Amdo basement an ideal and unique place along the BSZ to
study the tectonic history of the Lhasa-Qiangtang collision.
We present new geochronologic and thermobarometric data that confirm both the
presence of high-grade metamorphism and its occurrence in the Middle Jurassic (~185180 Ma). Our data reveals that the Amdo basement experienced temperatures of 650750°C and pressures over 11 kbar, indicative of a doubling of the continental crust.
These P-T conditions were experienced by the entire 50 km × 50 km of gneissic exposure
in the study area. While some lower grade rocks are present, including garnet-kyanite
schist that provides peak conditions of ~600°C at ~8 kbar, they are limited in area and
only occur along the southern boundary between the basement and the unmetamorphosed
sedimentary rocks. Thus the Bangong subduction zone experienced a major tectonothermal event prior to collision, similar to suture zones in the North American Cordillera,
and while the evidence for this tectonism is missing to the west, possible exposures to the
east indicate this may have been a widespread event along the suture zone.
We also extensively analyzed mafic amphibolites that contain garnets with
plagioclase coronas, a texture that is common in high-grade metamorphic terranes of the
Coast Plutonic Complex of the North American Cordillera. These coronas have been
attributed to decompression reactions, depletion halos, or pro-grade metamorphism. We
show that, at least for this case, these coronas developed at or near peak conditions of
80
temperature and pressure, but may be a result of decompression at slightly higher peak
pressures.
REGIONAL GEOLOGY AND PREVIOUS WORK
The Amdo basement occurs along the northern edge of the BSZ, next to the LhasaGolmud highway between the towns of Nagqu and Amdo, about 200 km north of the city
of Lhasa (Fig. 1,2). The most common rocks in the suture zone to the west, east, and
south are Jurassic flysch deposits, sometimes referred to as the “Lake Area Flysch”,
consisting of marine shales and turbidites (Leeder et al., 1988; Schneider et al., 2003).
To the north, in the Qiangtang terrane, Jurassic rocks are generally limestones or have
shallow marine facies (Leeder et al., 1988; Schneider et al., 2003). The stratigraphy is
difficult to define due to poor exposure, extensive deformation, and rapid changes in
facies and thickness (Schneider et al., 2003). The Jurassic flysch occurs with a Jurassic
mélange that includes deformed flysch, olistrosomes, Paleozoic sedimentary rocks and
scattered outcrops of ophiolite fragments. Ophiolite exposures near the Amdo basement
are located north around the town of Amdo, to the west around the town of Donqiao and
to the south near the town of Nagqu (Fig. 1).
The BSZ also includes Cretaceous
limestones, volcanic rocks, and nonmarine sandstones, occasional Paleozoic strata that
are involved in thrusts with the Jurassic mélange, and Cretaceous plutons (Girardeau et
al., 1984; Girardeau et al., 1985; Coulon et al., 1986; Coward et al., 1988).
The Amdo basement consists mostly of orthogneisses with minor outcrops of
paragneiss, amphibolite, marble, quartzite, and migmatite (Harris et al., 1988) as well as
81
marbles, quartzites, and metapelites of a lower grade along the edges (Fig. 2). The
basement is intruded by granitoids and hypabyssal dikes that account for about 30% of
the total area.
Two U-Pb zircon ages have been reported for the orthogneisses, a
Cambrian (~530) age (Xu et al., 1985) and a Neoproterozoic (~850 Ma) age (Guynn et
al., 2006). Xu et al. (1985) also obtained a slightly discordant U-Pb titanite age of ~171
Ma from the orthogneiss, but attributed this to low-grade metamorphism during the
Lhasa-Qiangtang collision. However, U-Pb titanite and zircon and 40Ar/39Ar hornblende
analyses by Guynn et al. (2006) documented high-grade metamorphism at ~185-180 Ma,
followed by cooling to ~300°C at ~165 Ma, and than resumed cooling to upper crustal
levels (< 150°C) between ~130-115 Ma. They attributed the metamorphism and initial
cooling to subduction related tectonism, possibly a minor collision, and the second period
of cooling to the Lhasa-Qiangtang collision, consistent with other estimates of collision
timing (e.g. Dewey et al., 1988; Kapp et al., 2003). The Cretaceous collision resulted in
the basement being thrust over Jurassic and Cretaceous sedimentary rocks (Coward et al.,
1988). Early Tertiary thrusting along the BSZ is undocumented but suspected (Coward et
al., 1988) and in the late Tertiary to the present eastward extrusion of central Tibet has
resulted in minor strike-slip and normal faults within the BSZ (Taylor et al., 2003), some
of which border the Amdo basement (see Fig. 2; Kidd & Molnar, 1988).
The only previous thermobarometry of the Amdo basement comes from a sillimanite
paragneiss that did not contain garnet (Harris et al., 1988). Harris et al. (1988) obtained a
minimum temperature of ~570°C based on muscovite-plagioclase equilibria, but the
presence of granitic leucosomes, K-feldspar, sillimanite, and muscovite indicate anatexis
82
at temperatures over 680°C. Guynn et al. (2006) also reported sillimanite in the gneisses
and a kyanite-bearing schist along the southwestern contact with the sedimentary rocks.
Basement rocks
Orthogneisses compose the majority of the Amdo basement and probably represent
over 90% of the metamorphic rocks exposed.
They generally have well-developed
foliation and banding due to segregation of mafic and felsic minerals, flattening of quartz,
and alignment of biotite, but they also occur as lightly foliated granitoids. Lineations are
less common and are typically a result of stretched quartz, aligned and stretched
feldspars, and aligned amphibole.
The orthogneisses are intermediate to felsic in
composition, with granite and granodiorite being the most common protoliths, and they
contain biotite, often amphibole and more rarely muscovite. Migmatites are occasionally
found within the orthogneisses and may have paragneiss protoliths. The orthogneisses do
not contain appropriate assemblages for geobarometry but the granodioritic gneisses
contain amphibole and plagioclase that can be used for geothermometry.
Paragneisses are uncommon within the Amdo basement and were only observed in
the northern part of the region, just west of the Lhasa-Golmud highway and just east of
the Nyainrong road, where they occur as small (several km to < 1 km) exposures within
the orthogneiss. Sillimanite and garnet were found in some of them, though rarely
together, and all the samples collected show retrograde reactions.
Mafic amphibolites are ubiquitous within the orthogneisses and are composed of
amphibole + plagioclase + quartz + ilmenite ± garnet ± biotite. They occur as small (< 1
83
m) pods (Fig. 3) and are usually lacking a tectonic fabric. The garnets range in size from
0.1 mm up to 2 mm and often show plagioclase coronas, from thin, discontinuous halos
to fully developed coronas (Fig. 9a). Garnets typically occur in the cores of large pods
and the garnet is absent or smaller near the edges. Mineral composition and major
element analysis from one sample indicate a probable gabbroic protolith (Table 1),
possibly as dikes within the granitic basement that have been stretched into boudins
during gneissic development. In addition, several mafic amphibolites without garnet
were found in the metasedimentary rocks in the northeast field area just east of
Nyainrong. JG062204-2 appears to be a mafic dike within marble, with a foliation that
matches the marble, and JG062204-4 is a foliated and lineated amphibolite that occurs
within a marble and schist sequence and may represent a basalt layer within the original
sedimentary protoliths.
Metasedimentary rocks that occur along the edges of the Amdo orthogneiss are
predominantly marbles but also include quartzites and metapelites. The majority of the
metapelites are monotonous schists composed of quartz + mica + plagioclase ± chlorite,
but along the southern contact garnet-kyanite and garnet-staurolite schist were
discovered. Both are a lower metamorphic grade than the orthogneiss. A small outcrop
of marble and quartzite were also found within the Amdo gneiss, near an exposure of
paragneiss, just south of Nyainrong. Detrital zircon data from two quartzites demonstrate
maximum depositional ages in the Cambro-Ordovician (unpublished data) and together
with the marbles and metapelites suggest the protoliths are Paleozoic continental shelf
84
rocks. Combined with the felsic to intermediate composition of the orthogneisses, this
establishes that the basement is continental and not an island-arc complex.
The Amdo gneisses are intruded by large, Jurassic granitoid plutons that are coeval
with the high-grade metamorphism (Guynn et al., 2006). Despite crystallization ages
similar to timing of metamorphism, the granitoids are undeformed except for narrow
mylonitic and proto-mylonitic shear zones. The bulk of the intrusions are granite, but
they also include quart monzonite, granodiorite, tonalite, and quartz-syenite. Biotite and
amphibole often show replacement by chlorite, particularly in the K-feldspar porphyritic
Jgr1.
THERMOBAROMETRY
Pressure and temperatures for rocks containing the assemblage garnet + amphibole +
plagioclase + quartz + ilmenite ± biotite were determined using the amphiboleplagioclase thermometer (AP; Blundy & Holland, 1990; Holland & Blundy, 1994), the
garnet-amphibole thermometer (GA; Graham & Powell, 1984), and the garnetamphibole-plagioclase geobarometer (GAP; Kohn & Spear, 1990).
Metapelites
containing garnet + biotite + aluminosilicate + plagioclase + quartz ± muscovite were
analyzed using the garnet-biotite thermometer (GP; Bhattacharya et al., 1992) and the
garnet-aluminosilicate-silica-plagioclase geobarometer (GASP; Ghent, 1976) with the
garnet activity model of Ganguly and Saxena (1984), the plagioclase activity model of
Holland and Powell (1992) and the data set calibrations of Holland and Powell (1985). A
few samples that lacked garnet or aluminosilicate (e.g. amphibole & plagioclase bearing
85
orthogneiss) were analyzed for peak temperatures only using the aforementioned
thermometers. All the mineral compositions are within the recommended guidelines for
each geothermometer (e.g. sufficient Ca content in garnet, moderate An content of
plagioclase, etc.).
The GA thermometer results in lower temperatures than the AP thermometer by 20120°C, or an average of 80°C (Fig. 4). Given the lack of pyroxene in the rocks and the
more realistic temperatures for inclusions, we feel the GA thermometer may be closer to
the actual metamorphic conditions. However, since we can use the AP thermometer in
non-garnet bearing rocks, we report both sets of temperatures and use the AP
thermometer to compare relative temperatures between garnet bearing and garnet absent
rocks.
The AP thermometer of Blundy and Holland (1990) uses two different
calibrations, one based on edenite-tremolite and one based on edenite-richterite, and the
latter generally results in slightly higher temperatures. As we have no reason to favor one
over the other, an average of the two is reported.
While most of the garnets in the samples show some degree of growth or diffusion
zoning, the other minerals generally lack any significant zonation of major elements.
However, extensive microprobe analysis shows that elemental compositions among the
minerals vary within samples enough to give pressure and temperature variations up to
±10%. These variations are not consistent with edges, cores, or other differences in
analysis location (see sample JG060404-1 below for an extensive discussion) and they
are probably a result of variable closure temperature of minerals to diffusion, possibly
enhanced by short duration at peak metamorphic conditions or minor retrograde reactions
86
below peak conditions. Nonetheless, the individual P-T analyses cluster in groups that
are consistent with the mineralogy of the rock. Thus, we conducted multiple analyses for
each sample, averaged the results, and report a 2-σ standard deviation based on the
variance in these analyses. In addition, calculations are made using the average values of
mineral compositions in the same area of the thin section. In the case of garnet zoning,
locations for garnet analysis were chosen based on the specific garnet profile and
generally paired with other nearby matrix mineral analyses. Only a few representative
analyses and the averages are reported in the data tables.
Microprobe analyses were conducted at the University of Arizona’s Lunar and
Planetary Laboratory (LPL) on a Cameca SX50 electron microprobe equipped with 5
LiF, PET, and TAP spectrometers. Counting times for each element were 20 s with a 10
s background at an accelerating potential of 15 kV and a beam current of 10-20 nA.
Garnet-kyanite schist thermobarometry
JG060904-13 and JG061204-2 are schists composed of garnet + kyanite + biotite +
muscovite + plagioclase + quartz + graphite that occur next to the orthogneiss in the
southwest area of the Amdo basement (Fig. 5). The two sample outcrops are about 1 km
from each other. The kyanite in JG060904-13 is generally too small to see in hand
sample, but the crystals in JG061204-2 are several mm long. In addition, kyanite crystals
several cm long, associated with massive quartz, were found in float around the outcrop
for JG061204-2. The garnets in each are subhedral to euhedral, 0.5 – 2 mm in diameter,
and generally lack inclusions aside from some quartz. Biotite and muscovite wrap
87
around the garnets and quartz and plagioclase occur as pressure shadows next to the
garnets. Schistosity is well-developed due to aligned biotite and muscovite. Biotite
occurs both in thin layers and as large porphyroblasts, while muscovite is generally
interlayered with the biotite.
Quartz and plagioclase occur as small (< 50 µm),
disseminated grains, with the exception of ~0.5 mm quartz veinlets in JG060904-13.
Graphite occurs as very thin layers between the micas.
In sample JG060904-13 garnets are almandine rich and show strong growth zoning
(Fig. 5a, 6a), indicated by the bell-shaped Mn profile and bowl-shaped Fe and Mg curves.
The outer ~100 µm rims of the garnets often have strong diffusion profiles, with a large
rise in Mn and a drop in Fe and Mg (Fig. 6a). The “shoulders” of the garnet element
curves were chosen for thermobarometric analyses as an approximation of peak
conditions. The retrograde diffusion of Fe and Mg from the garnet rims should not
greatly affect the biotite composition, as the schists are biotite rich (Bi > Grt) and the Fe
and Mg has been evenly distributed within the biotite as indicated by the lack of biotite
zoning, even next to the garnet. Kohn and Spear (2000) suggest that a Mn kick-up is a
sign of a net-transfer reaction involving the breakdown of garnet as opposed to a simple
net-exchange of cations since only garnet contains significant Mn. Such a reaction will
result in abnormally high temperatures for the garnet-biotite thermometer. We do not
think this is the case for these samples due to the euhedral shape of the garnets analyzed,
which lack any reaction textures along their edges, and because the resulting P-T
conditions are consistent with the mineralogy of the rocks. The Mn for the rim kick-up
could have come from the biotite, which contains some Mn and is volumetrically much
88
greater than the thin diffusion rims on the garnet. Plagioclase is unzoned and andesine in
composition (An39±2).
Matrix plagioclase in JG061204-2 is An39±2, while plagioclase next to the garnet is
generally more calcic (as high as An43), though it can also be less (as low as An36).
Biotite is slightly iron rich in composition, Ann52±2, with no consistent variation in
composition relative to garnet. A garnet from one thin section displays growth zoning
with no diffusion on the edges (Fig. 5c, 6b), while garnet from another thin section has
only slight growth zoning and a thin diffusion profile (Fig. 5d, 6c). In the former case,
analyses at the edge of the garnet were used in the P-T calculations, while in the latter
case, the values at the start of the diffusion profile were chosen.
The garnet measurements were paired with biotite and plagioclase adjacent to the
garnet when possible, but in many cases the biotite and plagioclase grains were not in
direct contact with the garnet. However the biotite and plagioclase do not show any
significant zoning or compositional variation related to proximity to the garnets. The
resulting cluster of 25 analyses for five different garnets in JG060904-13 record P-T
conditions of 7.8 ± 0.8 kbar and 610 ± 40°C, in the lower amphibolite facies and
consistent with the observed mineralogy. The results for JG061204-2 (13 analyses, 5
garnets) are 7.1 ± 1.0 kbar and 560 ± 50°C, similar to JG060904-13.
Mafic amphibolite thermobarometry
Sample JG060904-11 is located near the southwest edge of the Amdo basement near
the Lhasa-Golmud highway. It is a coarse grained amphibolite composed of garnet
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(~35%) + amphibole (~55%) + plagioclase (~5%) + quartz (~5%) + ilmenite (< 1%) with
garnet crystals 0.5-1.5 mm across. Few of the garnets have complete coronas and the
rims that are present are very thin. Garnets without any type of corona texture were
chosen for thermobarometry (Fig. 7). Plagioclase in this sample shows a considerable
range of compositions from An15 to An45, with most analyses in the range An20-An40
(oligoclase-andesine). The compositions show no systematic variation based on location
in the sample, such as proximity to garnet or edges versus cores, with the exception that
the few plagioclase inclusions tend to have more calcic compositions (~An40).
Amphibole is ferroan tschermakite-pargasite and shows minor compositional variations
as well, particularly with Mg and Fe where an increase in Mg is accompanied by a
decrease in Fe. Variations are generally on the order of a couple mole percent and do not
coincide with grain boundaries with other minerals. This sample does not contain any
crystals of pyroxene, but in performing traverses across amphibole with the microprobe,
several analyses adjacent to quartz (< 20 µm from the edge) had orthopyroxene
compositions (Table 2), suggesting incipient granulite facies metamorphism.
Some of the garnets show minor element variations that appear to be growth zoning
(Fig. 8a,c). Typical elemental growth profiles in garnet include a bell-shape for Mn, a
bowl-shape for Fe and Mg and a bowl-shaped core with maxima towards the edges for
Ca (e.g. Chakrabory and Ganguly, 1991; Spear, 1995). Mn, Mg, and Ca concentrations
for these garnets fit that profile, but Fe typically has a bell-shaped curve like Mn. Other
garnets have a change in composition from one end of the garnet to the other (Fig. 8a,b),
though the Mn in these cases still shows a bell-shaped growth zoning profile. The
90
departure from the average composition seen in Figure 8a appears to be due to the
proximity of an inclusion in the garnet. The garnets contain many inclusions which are
usually quartz, but a few were found that contain coexisting amphibole and plagioclase.
The garnets all have significant diffusion profiles at their edges (Fig. 8) that show
much greater variation than generally observed in the cores. As seen in Figures 8a & 8b,
the amphiboles do not have any zoning in major elements and only the one on the right
side of garnet “C” shows any sign of being perturbed by the diffusion of elements from
the garnet rims. The lack of reaction textures at the edges of the garnets, the subhedral
shape of the garnets, and the lack of zoning in the amphiboles suggests that the profiles at
the edges of the garnet are in fact due to diffusion and not net-transfer reactions involving
the breakdown of garnet. These garnets are Mn poor (XMn ~ 0.04) and the amphibole
does contain some Mn so that it could provide the Mn for the increase in the thin (25-50
µm) diffusion rim of the garnets. Given the thin diffusion rims of the garnet and the large
quantity of amphibole in the sample, the redistribution of elements from the garnet will
not have significantly changed the composition of the matrix amphibole. Thus, for
thermobarometric calculations, average values for the cores of the garnets were combined
with average values from matrix amphibole and plagioclase in the vicinity of each garnet
analyzed. Note that in this context, the core of the garnet is defined as everything inside
the diffusion rims. While the garnets show some growth zoning, the elemental variation
within individual cores is on the same order as differences between different garnets and
dividing the garnets into inner and outer cores does not result in significantly different
average values for those zones.
All the garnet cores fall within the composition
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Alm54±2Prp20±4Grs22±4Sps4±1. Inclusions were analyzed using the garnet compositions
immediately next to the plagioclase and amphibole. Garnet near the inclusions has
slightly higher Fe, Mg, and Mn and lower Ca (Alm58±2Prp22Grs15±1Sps5±1).
The pressures and temperatures calculated using the garnet cores group together
around 11 ± 0.3 kbar and 688 ± 76°C (AP: 764 ± 15°C) and indicate uppermost
amphibolite facies for this rock, consistent with the mineralogy. The four inclusions
record lower pressures and slightly lower temperatures that are similar to the garnetkyanite schist: 8.6 ± 1.2 kbar and 633 ± 51°C (AP: 823 ± 41°C). These AP temperatures
are anomalously high and further support the GA thermometer as being more accurate.
Sample JG060404-1 is also a garnet-amphibolite, composed of garnet + amphibole +
plagioclase + quartz + ilmenite + biotite, and is from a road cut beside the Lhasa-Golmud
highway at the southern edge of the Amdo basement (Fig. 1). The rock is similar to
JG060904-11 with approximate minerals abundances: 25% garnet, 55% amphibole, 10%
plagioclase, 8% quartz and 2% ilmenite (biotite is minor). The sample came from the
center of a ~1/2 m wide amphibolite pod within a granitic orthogneiss (Fig. 3). Toward
the edges of the pod, garnet is absent and the amphibolite is foliated, with the foliation
bent towards the gneissic layering.
The garnets in this sample are a similar size as those in JG060904-11 (0.5 – 1.5 mm),
but have more developed coronas composed of plagioclase with minor quartz and
amphibole (Fig. 9a). In some cases, the coronas are very thin (< 50 µm), while in others
the garnet is almost completely absent (Fig. 9d). The plagioclase in the coronas consists
of distinct, small crystal domains visible under crossed-polars, unlike the larger
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plagioclase grains in the matrix. On average, the plagioclase in the coronas tends to be
more calcic than the matrix (An40 vs. An35), especially immediately adjacent to garnet.
The difference is small, though, and there is significant variation in all the plagioclase,
such that compositions between the corona and matrix overlap. In several traverses
across the coronas around garnet “A” (Fig. 9a,c), plagioclase sometimes showed a zoning
pattern where it became less calcic away from the garnet but than increased towards the
amphibole, but in other traverses there was no zoning (Fig. 10b). Amphibole in the
coronas consists of individual, euhedral crystals or “peninsulas” from surrounding
amphibole. In the latter case, the amphibole in the corona does not appear distinct from
that of the matrix. Both forms of amphibole in the corona often occur in a bladed habit
roughly perpendicular to the perimeter of the garnet (Fig. 9c). The amphibole in the
corona shows no difference in composition from the matrix amphibole and the bladed
forms show no zoning across the corona; the overall composition of the amphibole is
ferroan tschermakite-pargasite. Similar to amphibole, garnet occasionally occurs in the
coronas as either individual grains or “peninsulas” still attached to the main garnet (Fig.
9a,b). Both the garnet and the amphibole display embayed grain boundaries. These
textures suggest that the garnet was larger and has been replaced by the plagioclase
coronas. Amphibole-amphibole contacts in the matrix are typically straight but quartz
and plagioclase in the matrix have rounded boundaries.
Growth zoning is not particularly well-displayed in the garnet cores, although there
are some compositional variations and the Mn displays a shallow, bell shaped curve. The
garnets have typical diffusion profiles at their edges. Figure 10a and 10b show elemental
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profiles across a garnet with an amphibole-plagioclase inclusion. In general, the core of
the garnet is flat, but the element concentrations are disturbed in the vicinity of the
inclusion, with an increase in Fe and Mg and a decrease in Ca. Part of the profile of
garnet “A” crosses garnet that protrudes into the corona (Fig. 9a,b). This profile shows
that the composition of the core of this peninsula is similar to the garnet core, indicating
that the diffusion profile developed after the garnet was resorbed. The diffusion profile
varies around the garnet.
The average core composition of the garnet is
Alm57±1Prp11±2Grs29±2Sps3±1., similar to JG060904-11 but with Prp content about 10 mol
% lower and Grs correspondingly higher. These basic patterns are the same even for a
very thick corona around a small garnet, as seen in Figure 10d. The core of this garnet
has the same composition as the larger garnets and the same type of diffusion profile.
Plagioclase generally shows a slight increase from An36-38 near amphibole to An42 next to
the garnet, but also deviates from this trend (Fig. 10b).
Matching average garnet cores with average matrix amphibole and plagioclase results
in a cluster of P-T conditions around 10.8 ± 0.6 kbar and 688 ± 29°C (AP: 746 ± 15°C).
This is very close to the P-T conditions recorded by sample JG060904-11. The nine
inclusions result in an average pressure and temperature of 9.4 ± 0.7 kbar and 692 ± 72°C
(AP: 775 ± 64°C), the same temperature but slightly lower pressure than the core-matrix
pairs. Temperatures are also calculated using amphibole-plagioclase pairs within the
coronas and those in the matrix. The resulting temperatures are essentially the same, with
the matrix minerals resulting in 754 ± 62°C (n=30) and the corona minerals in 758 ±
50°C (n=22).
94
At the edge of the amphibolite pod, garnet is absent, biotite is more abundant
(~15%) and there are small spots of plagioclase, amphibole and biotite that are similar to
the coronas. However, these spots are a little smaller than the garnet (0.5-1.0 mm) and
contain more amphibole and biotite, particularly in the center of the spots. Some of the
amphibole is dark green to light brown, but much of it is also light-green and blue-green,
suggesting it formed at a lower grade, though there is no epidote or chlorite. All the
amphibole has a skeletal appearance and is formed into bands between plagioclase and
quartz rich layers.
Thermometry of non-garnet bearing amphibolites
The two samples of amphibolite from east of Nyainrong were analyzed using
amphibole-plagioclase thermometry. Compositions between cores and edges of grains
are not consistently different and the results for the two were averaged. We report the
temperatures at 10 kbar assuming conditions similar to those of the garnet-amphibolites.
The exact choice of pressure is not critical since equilibrium lines for the amphiboleplagioclase thermometer are relatively shallow and a ±5 kbar difference in pressure
results in only about ± 50°C for the ed-tr thermometer and about ± 20°C for the ed-ri
thermometer.
A total of 14 analyses, a core and an edge pair for 7 different grains, were performed
for JG062204-2 (Fig. 11a) and result in an average temperature of 785 ± 45°C, consistent
with that recorded by the garnet-amphibolites.
For JG062204-4, an average of 12
95
analyses, 5 cores and 7 edges, yielded a temperature of 760 ± 60°C, similar to JG0622042.
Samples without thermobarometry
Several samples had distinctive mineral assemblages indicative of the metamorphic
grade but were not analyzed for thermobarometry. They are included on the map in
Figure 2. Samples JG062904-2 and JG060604-1 are garnet-amphibolites from within the
orthogneiss which show signs of major retrograde reactions, including mostly resorbed
garnets and recrystalized amphibole. However, their mineralogy is consistent with the
other garnet-amphibolites and is suggestive of similar P-T conditions. JG062505-2,
JG062005-4 and JG062005-7 are paragneiss samples that contain sillimanite; none of
them contain garnet, but the last does have biotite + sillimanite + plagioclase + quartz
pseudomorphs after garnet (Fig. 11b). The presence of sillimanite in these samples
indicates relatively high temperatures. Other samples located in the same area as the
other paragneisses contain garnet but no aluminosilicates. Sample JG052804-1 is a
garnet-staurolite schist (Fig. 11c) from the metapelites along the southern boundary of the
Amdo basement, east of the garnet-kyanite schist. It contains garnet + staurolite + biotite
+ muscovite + plagioclase + quartz, mineralogy indicative of lower amphibolite facies,
similar to the garnet-kyanite schist farther to the west.
Sample JG061704-3 is a paragneiss containing the mineral assemblage garnet +
biotite + muscovite + sillimanite + plagioclase + K-feldspar + quartz (Fig. 11d) from the
northwest area of the basement near the Lhasa-Golmud highway.
The amount of
96
muscovite is very minor and interlayered with biotite. The sillimanite is fibrolitic and
also occurs with biotite and the two define the foliation within the gneiss. The garnets are
generally small, usually surrounded by biotite and fibrolite that wrap around them, and
display a variety of morphologies including atoll garnets (Fig. 11e). The presence of
sillimanite and K-feldspar together with the sparse muscovite is indicative of an upperamphibolite grade rock that has undergone some degree of partial melting due to the
muscovite dehydration reaction:
muscovite + quartz → K-feldspar + Sillimanite + H2O
As seen in Figure 12, major element zoning within the garnets is complex. Fe shows
a bowl shaped profile and Ca shows a roughly bell-shaped profile with a depressed
center, both of which are typical of growth zoning. However, instead of the expected
bell-shaped Mn and bowl-shaped Mg curves, both are essentially flat, indicating that the
elements re-equilibrated at peak temperatures, eliminating the previous growth zoning.
The remaining Fe and Ca zoning suggests that the rock did not remain at peak
metamorphic conditions long enough to re-equilibrate these elements. In addition, the
atoll garnets and small diffusion profiles at the edges of some garnets are suggestive of
retrograde reactions. Furthermore, unlike the garnet-kyanite schist, this sample contains
relatively little biotite so that diffusion of elements from the garnet into the matrix biotite
could greatly affect the biotite composition. All these factors suggest that the paragneiss
minerals are not in equilibrium and this is supported by the P-T results. Pressures and
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temperatures calculated using the edges of garnets in contact with biotite and plagioclase
are in the sillimanite field but result in temperatures too low for the presence of KFeldspar. Pairing cores of the garnets with biotite and plagioclase in the matrix still
results in temperatures below the muscovite dehydration curve and gives pressures well
within the kyanite stability field.
U-Pb METAMORPHIC GEOCHRONOLOGY
U-Pb dating of zircons in the orthogneisses has revealed that metamorphic zircon
growth occurred during the Jurassic metamorphism (Guynn et al., 2006).
The
metamorphic growth observed in the previous study is too thin to date using the laser
system at the University of Arizona, but further dating of orthogneisses revealed thicker
zircon growth that allows analysis uncontaminated by older cores. The geochronology of
the Amdo orthogneisses will be presented in a separate paper and we only concentrate on
the Jurassic zircon growth here. We only provide an outline of the U-Pb dating method;
for a complete description, see Guynn et al. (2006). All analyses were conducted at the
University of Arizona Laserchron Center using the Laser-Ablation Multicollector
Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICPMS).
Apparent ages of individual zircon analyses are reported at the 1σ level in the tables,
but mean ages are reported with 2σ uncertainty (95% confidence limits).
concentration of
for
207
235
Pb*/235U and
U relative to
238
U for younger zircons results in higher uncertainties
206
Pb*/207Pb* ages relative to
are reported using the
206
The low
Pb*/238U age.
206
Pb*/238U ages, so the Jurassic ages
The stated uncertainties on the assigned
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crystallization ages are absolute values and include contributions from all known random
and systematic errors. Random errors are included in the data tables. Systematic errors
(2σ) are as follows: JG061604-1, 1.15%; JG061504-4, 1.50%; JG063004-1, 1.34%. All
U-Pb weighted average calculations and plots and probability density plots were made
using Isoplot 3.00 (Ludwig, 2003).
Metamorphic zircon growth can usually be distinguished from lead loss by high U/Th
concentrations (Mojzsis & Harrison, 2002).
Uranium can form water soluble
compounds (Faure, 1986) so that zircon which precipitates from metamorphic fluids has
U concentrations similar to igneous zircon while Th concentrations are uniformly low
(Fig. 13). A plot of U/Th versus
206
Pb*/238U age (Fig. 14) for all the analyses from the
three Amdo orthogneisses in this study and the Jurassic granitoids from Guynn et al.
(2006) shows that igneous zircons, including those from inherited grains, generally have
U/Th values less than 5, while the Jurassic zircons from the gneisses, including tips on
older cores, typically have a range of U/Th values from 5 to 80. Orthogneiss zircons with
apparent ages less than the crystallization age but older than Jurassic and with
approximately the same U/Th ratio as older zircons are interpreted as magmatic zircon
that suffered lead loss during metamorphism.
Sample JG061604-1 is a Cambro-Ordovician orthogneiss that contains Jurassic zircon
crystals as well as older zircons that appear to have suffered lead loss. The younger
zircons yielded an average
which are from zircon cores.
206
Pb*/238U age of 182 ± 4 Ma (Fig. 15a), the majority of
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Sample JG061505-4 is a paragneiss from the central part of the Amdo basement.
Many of the zircons in this metasedimentary rock have euhedral shapes with very pointed
tips, but cathode-luminescence (CL) SEM images reveals that they generally have bright,
rounded, or semi-angular cores that represent the original detrital zircon, while the tips
are Jurassic overgrowths (Fig. 16). The tips are often large enough to shoot with a 25 or
35 µm laser diameter. Most of these tip analyses yielded mid-Jurassic ages with U/Th
ratios in the range ~10-18, while the cores of grains were always older and typically have
U/Th ratios ~0.5-3 (though 8 have U/Th ratios from 3-10). The average age of all
Jurassic tip analyses is 175.2 ± 3.4 Ma (Fig. 15b).
Granodiorite orthogneiss JG063004-1 yielded very few zircons from ~3 kg of rock
and the zircons were small and subhedral. All the ages for this gneiss are Jurassic with
the exception of one Precambrian zircon. The weighted average for 19 well-defined,
concordant analyses is 181 ± 3.3 Ma (Fig. 15c), the only Jurassic age we determined for
an orthogneiss. CL-SEM images of the zircons from this sample show some complex
zoning of the zircons, but analysis of different zones did not reveal any difference in ages
with the exception of the one Precambrian core. However, the U/Th ratios for the zircons
are very high, typically in the range of 15-80, which in almost all other cases in the Amdo
basement is a sign of metamorphic zircon (Fig. 14). This suggests that these zircons are
metamorphic in origin.
Given that the other Jurassic granitoids show no signs of
deformation except for narrow mylonitic zones and that several other orthogneisses near
this sample yielded early Paleozoic ages, we tentatively regard this sample as an older
orthogneiss and the Jurassic zircon as metamorphic in origin. This interpretation requires
100
that any previous zircon was thermally reset, which is unlikely, was entirely
recrystallized, or was completely dissolved within metamorphic fluids prior to new zircon
growth. There is some evidence that zircon in high-grade garnet or hornblende bearing
metamorphic rocks is not as stable as previously thought (Fraser et al., 1997).
A relative probability plot, which sums the zircon ages as normal distributions based
on their determined age and uncertainty, is shown in Figure 15d and the combined curve
reveals a broad peak centered ~178 Ma. Sample JG061604-1 has two peaks, but the
relatively few analyses make it statistically untenable to say that these represent two
periods of zircon growth. Sample JG061504-4 has a younger age than the other two
samples, but it is located adjacent to JG061604-1 (< 1 km distance), and since the ages
overlap at the 2σ level, the age given by the combined probability density plot may be the
most accurate.
We assign an age of ~178 ± 8 Ma as a robust estimate of peak
metamorphism as recorded by the zircon growth.
DISCUSSION
Thermobarometric results
Plagioclase coronas around garnets have been attributed to retrograde decompression
reactions (Misch & Onyeagocha, 1976; Mengel & Rivers, 1991; Valley et al., 2003),
isothermal decompression (Whitney, 1992; Jamieson et al., 1995; Wernicke & Getty,
1997), incomplete prograde reactions due to a decrease in temperature (Stowell & Stein,
2005), and prograde depletion halos (Winter, 2001). A singular explanation may not
101
suffice given the differences between the coronas. For instant, in Misch and Onyeagocha
(1976), the coronas are dominantly plagioclase but still contain more amphibole and
biotite than in this study and their corona plagioclase shows a distinct increase in Ca
content, from ~An35 to ~An55. In Stowell and Stein (2005), amphibole is absent and the
garnets are very Fe rich.
At first glance, a retrograde reaction involving the breakdown of garnet to plagioclase
appears to be the simplest explanation, especially given the circular shape of the coronas
and the presence of garnet within them. However, the lack of minerals other than
plagioclase within the coronas provides a significant elemental balance problem
involving Fe and Mg disposal and Na and Si introduction:
(Fe,Mg,Ca)3Al2Si3O12 + (Si, Na) → CaAl2Si2O8 + NaAlSi3O8 + (Fe, Mg)
garnet
anorthite
albite
Modeling by Misch and Onyeagocha (1976) demonstrates that even with more
amphibole and biotite present than the coronas in this study, there is an element
imbalance. Furthermore, the corona plagioclase does not show a substantial increase in
An content as might be expected and the plagioclase and amphibole in the corona,
adjacent to the corona, and in the matrix all have essentially the same composition with
no zoning and result in the same temperatures with the amphibole-plagioclase
thermometer. This suggests that while there was a reaction from garnet to plagioclase,
the reaction involved the diffusion of elements between the corona and matrix minerals,
102
which in turn would require relatively high temperatures. This also suggests that the
minerals are in equilibrium, with the exception of the thin diffusion rims on the garnet,
and this is supported by the lack of reaction textures, other than the coronas, within the
rock. The occurrence of a diffusion profile across garnet within the corona (Fig. 9b,10a)
shows that the diffusion profiles, which develop as the garnet cools from peak
temperatures, formed after the creation of the coronas. Thus the pairing of the relatively
undisturbed garnet cores with amphibole and plagioclase in the matrix is providing an
estimate of peak P-T conditions when the rock formed, though it is possible the pressure
is less than the highest achieved.
This conclusion is supported by the P-T conditions recorded by garnets with coronas
in JG060404-1 that are equivalent to those without coronas in JG060904-11.
Interestingly, the mineralogy and garnet and plagioclase composition of JG060904-11 are
very similar to metadiabase dikes in the Grenville Front Tectonic Zone of Ontario which
yield almost exactly the same results (~750°C at ~11 kbar; Jamieson et al., 1995). Some
of the rocks in that study contain relict metamorphic clinopyroxene and garnets with
plagioclase and quartz coronas. The temperatures recorded by the amphibole-plagioclase
thermometer in other, non-garnet bearing rocks in the Amdo area are also consistent with
the garnet-amphibolite temperatures. This leaves the question, though, of what caused
the reaction from garnet to plagioclase. In some cases, the garnet is thought to have
existed prior to metamorphism (e.g. Wernicke & Getty, 1997), but this seems unlikely for
a gabbroic protolith presumably within the upper crust. The reaction could be the result
of isothermal decompression where the high temperature allowed the minerals in the rock
103
to equilibrate at the lower pressure. In the case of the Grenville Front Tectonic Zone,
additional thermobarometry on other lithologies gave convincing evidence for isothermal
decompression (Jamieson et al., 1995). This would imply pressures greater than 11 kbar,
which approaches the eclogite facies, but even if the rocks achieved those pressures a
short residence time might not allow eclogitization (Jamieson et al., 1995) and
equilibration at higher temperatures would erase any small eclogite domains.
Furthermore, the garnet-amphibolites record pressures in the kyanite stability field, yet
the paragneisses contain sillimanite. Together with the breakdown of garnet to produce
sillimanite, biotite, and plagioclase in the paragneisses, this indicates decompression at
relatively high temperatures. It is unknown why one garnet-amphibolite would have
more developed coronas than another, though it may have to do with subtle differences in
rock composition.
A summary of the P-T data is presented in Figure 17. The observed mineralogy and
rock types combined with the thermobarometry calculations reveals that the entire Amdo
basement was subject to upper-amphibolite facies metamorphism.
Plagioclase-
amphibole thermometry indicates temperatures in the 700-800°C range. The garnetamphibole thermometer suggests a lower temperature, perhaps ~650-750°C, which may
be more reasonable given the lack of pyroxene in the mafic amphibolites, but the
migmatites, K-Feldspar-sillimanite-muscovite paragneiss and pyroxene microprobe
analyses from JG060409-11 also demonstrate conditions right at the amphibolitegranulite transition.
The lack of epidote in the garnet-amphibolites also indicates
temperatures greater than ~650-700°C. Pressures are less-constrained across the region
104
due to a lack of appropriate mineralogies for geobarometry, but the two garnetamphibolites analyzed record similar values of ~11 kbar. The common grade across the
gneisses requires a minimal vertical offset within the Amdo basement, approximately less
than 5 km.
The conditions recorded by the garnet-kyanite schist are ~8 kbar and ~600°C. Both
this and the gneiss P-T conditions are consistent with a normal crustal geotherm (Fig. 17)
and crustal burial, suggesting that the intruding Jurassic granitoids did not supply extra
heat for the metamorphism. The difference in P-T between the two samples implies ~10
km of vertical offset between the two. Garnet-amphibolite sample JG060604-1 and
garnet-kyanite schist sample JG060904-11 are located ~2 km from each other across the
contact between the gneiss and metasedimentary rocks, so this requires a significant fault
or shear zone between the two samples.
Tectonic implications
The Amdo basement underwent burial to over 30 km depth in the Middle Jurassic
during closure of the Meso-Tethys Ocean between the Lhasa and Qiangtang terranes,
resulting in peak temperatures over 700°C.
These conditions are similar to those
experienced by high-grade metamorphic rocks of the Coast Plutonic Complex along the
western margin of North America, a long (> 1500 km) belt of rocks related to a
Cretaceous-early Tertiary arc. The Swakane Gneiss and Napeequa Complex, located
within the Chelan and Wenatchee blocks of the North Cascades, record pressures of 9-12
kbar and temperatures of 640-740°C along a clockwise P-T path (Valley et al., 2003),
105
while the Cascade River unit of the Chelan block reached 8-9 kbar and ~650°C (Miller et
al., 1993). The Skagit Gneiss Complex of the Chelan block was buried and heated to ~9
kbar and 700-800°C (Whitney, 1992; Wernicke & Getty, 1997) and than underwent
isothermal decompression to ~4 kbar and 650-700°C (Whitney et al., 1992). Further
north, the Central Gneiss Complex of the Coast Mountains, British Columbia, is a large
region of gneiss and amphibolite that underwent amphibolite and granulite facies
metamorphism
(Hollister
&
Andronicos,
2000).
Mineral
assemblages
and
thermobarometry have shown metamorphic conditions of 8-9 kbar and over 700°C
(Hollister, 1982; Hollister & Andronicos, 2000; Rusmore et al., 2005) for the Central
Gneiss Complex and, similar to the Amdo basement, these conditions are uniform over a
large (~50 km × 200 km) area. The region also underwent isothermal decompression
(Rusmore et al., 2005 and references therein).
The mechanism for burial and exhumation of these rocks is debated, but in general
they are thought to be related to thickening of the arc crust coeval with magmatism. In
some cases, the deformation may be related to the collision of outboard terranes accreted
to the arc, and in other cases it may be due to crustal thickening along a compressive
margin like the central Andes today. Similar to the Amdo basement, most of these areas
are intruded by plutons, with the exception of the Swakane Gneiss. Exhumation in the
Cascades had a two-stage exhumation history similar to the Amdo basement (Wernicke
& Getty, 1997; Paterson et al., 2004). In the case of the Coast Plutonic Complex, the
intruding plutons are the same age as large batholiths and volcanic sequences that are
obvious expressions of the continental arc. The similarity of the P-T-t paths and the
106
magmatic history between the Amdo basement and the high-grade rocks of the Coast
Plutonic Complex is suggestive of metamorphism in an arc setting for the Amdo
basement.
While it is unknown how long it took to bury the basement or how long it resided at
peak conditions, thermochronologic data presented by Guynn et al. (2006) and in this
study reveal relatively rapid cooling and exhumation, > 20°C/Myr. Metamorphic zircon
growth indicates peak conditions ~178 Ma, similar to the
40
Ar/39Ar hornblende age of
~180 Ma of Guynn et al. (2006). The similarity between the ages suggests that either 1)
the orthogneiss cooled very quickly, as the closure temperature for hornblende (~500°C)
is much less than zircon (> 700°C), or 2) the zircon grew at a temperature less than its
closure temperature, which would imply it may have grown at temperatures below peak
conditions (Fraser et al., 1997). Mica 40Ar/39Ar data indicates the Amdo basement was at
a mid-crustal level by ~165 Ma (Guynn et al., 2006). Whether erosion, compression
(thrust faults) or extension (normal faults) exhumed the basement to mid-crustal depths is
difficult to ascertain given the overprint of Cretaceous and Tertiary deformation and the
cover of Neogene basins. However, comparable metamorphic grade across the basement
suggests a vertical ascent similar to a dome or diapir and the presence of sedimentary
rocks just north of the Amdo basement points to a normal fault contact. The Amdo
basement has many components of “gneiss domes” in the broad sense, including a core of
high-grade orthogneisses and migmatites intruded by coeval granitoids, lower-grade
metasedimentary rocks adjacent to the gneiss (“cover rocks”), and an oval shape in map
view (Whitney et al., 2004). However, our mapping has not revealed any consistent
107
foliation in the orthogneiss dipping away from its center or any sign of normal faults
bordering the basement, though the bordering metasedimentary rocks do have foliations
paralleling their contact with the gneiss. An alternate explanation could be that the Amdo
basement is a structural culmination exhumed by duplexes or underthrusting in the lower
crust.
The lack of arc-related rocks along the BSZ remains enigmatic. Guynn et al. (2006)
proposed that the arc is buried beneath the northern Qiangtang terrane, either by
underthrusting or burial by younger sediments.
The metamorphism in the Amdo
basement may suggest the former. Alternately, the collision of an island-arc system,
suggested by geochemical signatures of some of the ophiolite fragments (Pearce & Deng,
1988), might have led to the burial of the Amdo basement as well as the arc. Either case
would be a remarkable case of subduction erosion given that there is no sign of this
process to the west. The possible basement exposures to the east, however, may be an
indication that the tectonics revealed by the Amdo basement are common to the BSZ and
were only exposed in certain regions where the Cretaceous fold and thrust belt exhumed
deeper portions of the suture zone. This idea is also supported by similar ages and styles
of metamorphism and magmatism in the Rushan-Pschart suture of the Pamirs, which has
been proposed to be an extension of the BSZ across the Karakorum fault to the west
(Schwab et al., 2004). This would suggest that some variations seen in the structure of
collisional orogens and continental arcs, such as differences between the Sierra Nevada
and the North Cascades, is a result of varying levels of exhumation rather than greatly
different tectonic histories.
108
ACKNOWLEDGMENTS
The authors thank Ken Dominik of the University of Arizona Lunar and Planetary
Laboratory for indispensable help with the electron microprobe and Alex Pullen and Ross
Waldrip for invaluable field assistance. This work benefited from conversations with
Jibamitra Ganguly, Mihai Ducea, Dave Pearson, and Steve Kidder. This work was
supported by NSF grant EAR-0309844 to P. Kapp and GSA, ChevronTexaco,
ExxonMobil and Tucson Gem and Mineral Show student grants to J. Guynn.
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Yin, J. X., Xu, J. T., Liu, C. J. & Li, H., 1988. The Tibetan Plateau - Regional
Stratigraphic Context and Previous Work. Philosophical Transactions of the
Royal Society of London Series A-Mathematical Physical and Engineering
Sciences, 327(1594), 5-52.
Zhou, M.-F., Malpas, J., Robinson, P. T. & Reynolds, P. H., 1997. The dynamothermal
aureole of the Donqiao ophiolite (northern TIbet). Canadian Journal of Earth
Science, 34, 59-65.
FIGURE CAPTIONS
Figure 1. General geological map of central and southern Tibet showing the Bangong
suture zone, defined by ophiolites and Jurassic mélange. “A.B.” is the Amdo Basement.
Based on Pan et al. (2004).
Figure 2. Generalized geological map of the central and western Amdo basement. Based
on our mapping, satellite photos, Kidd et al. (1988) and Pan et al. (2004).
Figure 3. Mafic amphibolite pods in a felsic orthogneiss of the Amdo basement in a
road-cut along the Lhasa-Golmud Highway.
117
Figure 4. Difference between calculated temperatures using the amphibole-plagioclase
(Blundy and Holland, 1990; Holland and Blundy, 1994) and the garnet-amphibole
(Graham and Powell, 1984) thermometers from garnet-amphibolites. Filled circles are
sample JG060404-1 and open triangles are sample JG060904-11. Dashed line represents
a 1:1 correlation.
Figure 5. Photomicrographs of garnet-kyanite schist. (a) JG061204-2 thin section #1,
showing major mineralogy. (b) Garnet “A” from JG060904-13. Line X-X’ is the
microprobe traverse in Figure 6a. (c) Garnet “GK1” from JG061204-2 thin section #1.
Line X-X’ is the microprobe traverse in Figure 6b. (c) Garnet “A” from JG061204-2
thin section #2. Line X-X’ is the microprobe traverse in Figure 6c. Dark area in the
lower left corner is from the marker used to number the thin section. Bt = biotite; Gr =
graphite; Grt = garnet; Ky = kyanite; Ms = muscovite; Pl = plagioclase; Qtz = quartz.
Figure 6. Profiles of element mole fractions and Fe/(Fe+Mg) across garnet from the
garnet-kyanite schist. Note the different scales for (Mg, Ca, Mn) and (Fe, Fe/(Fe+Mg)).
(a) Garnet “A” from JG060904-13. “A7g29” is a microprobe spot used in
thermobarometry calculations - see Table 2. (b) Garnet “GK1” from JG061204-2 thin
section #1. “gr_top.15” – see Table 2. (c) Garnet “A” from JG061204-2 thin section #2.
“Ag006” – see Table 2.
118
Figure 7. Photomicrographs of garnet-amphibolite JG060904-11. (a) Garnets “E1”
(top) and “E2” (bottom). Line X-X’ is a microprobe traverse across the two garnets,
intervening amphibole and amphibole next to “E2” (Fig. 8a). (b) Garnet “C”; X-X’ is a
microprobe traverse from center of garnet into neighboring amphibole (Fig. 8b). (c)
Garnet “I”; X-X’ is a traverse across the garnet (Fig. 8c). Am = amphibole; Grt = garnet;
Ill = ilmenite; Pl = plagioclase; Qtz = quartz.
Figure 8. Profiles of element mole fractions and Fe/(Fe+Mg) across garnet from garnetamphibolite JG060904-11. Note the different scales for (Mg, Ca, Mn, Na) and (Fe,
Fe/(Fe+Mg)). (a) Garnet “E1” and “E2” and associated amphibole. (b) Garnet “C”. (c)
Garnet “I”. Amph = amphibole.
Figure 9. Photomicrographs and BSE images of garnet-amphibolite JG060404-4. (a)
Garnet “A”. Insets are BSE images in (b) and (c). X-X’ and Y-Y’ are microprobe
traverses shown in Figure 10a and 10c, respectively. Lines across coronas are locations
of microprobe traverses in plagioclase, Figure 10b. (b) Peninsular garnet in plagioclase
corona. Line is the first part of X-X’. (c) Amphibole in the plagioclase corona. Line
across corona is a microprobe traverse shown in Figure 10b. (d) BSE image of garnet
“M” with a thick plagioclase corona. X-X’ and Y-Y’ are microprobe traverses across
plagioclase and garnet shown in Figure 10d. Symbols are the same as in Figures 5 and 7.
119
Figure 10. Profiles of element mole fractions and Fe/(Fe+Mg) across garnet from garnetamphibolite JG060404-1. Note the different scales for (Mg, Ca, Mn) and (Fe,
Fe/(Fe+Mg)). (a) Garnet “A”, traverse X-X’ (b) Plagioclase anorthite (An) content
across corona. (c) Garnet “A”, traverse Y-Y’. (d) Garnet “M”, traverses X-X’ and YY’. The profiles are matched up at the center of the garnet.
Figure 11. Photomicrographs of Amdo basement rocks without thermobarometry. (a)
Mafic amphibolite JG062204-2. (b) Garnet pseudomorph in paragneiss JG062005-7.
Garnet is replaced with biotite, sillimanite, plagioclase and quartz. (c) Garnet-staurolite
schist JG052804-1. (d) Garnet-sillimanite paragneiss JG061704-3 under crossed-polars
to show the tartan twinning in K-Feldspar. Sillimanite is interlayered with biotite;
muscovite is also interlayered with biotite but is too minor to show up in the image. (e)
Atoll garnets in JG061704-3. Symbols as in Figure 5 and 7, plus: Kfs = K-Feldspar; Sil
= Sillimanite; St = staurolite.
Figure 12. Profiles of element mole fractions and Fe/(Fe+Mg) across a garnet from the
garnet-sillimanite paragneiss JG061704-3. Note the different scales for (Mg, Ca, Mn)
and (Fe, Fe/(Fe+Mg)).
Fig 13. U and Th concentrations (ppm) and resulting U/Th ratios for zircons from
orthogneiss JG061604-1. U and Th concentrations are on the left y-axis, U/Th ratio on
120
the right y-axis. Zircon age is based on 206Pb*/238U. Young (~185 Ma) ages are
metamorphic zircon.
Figure 14. Zircon ages and associated U/Th ratios from three gneisses and several
Jurassic granitoids within the Amdo basement. JG061504-4 is a paragneiss so it has a
range of older ages; the range in older ages for orthogneiss JG061604-1 is interpreted as
due to lead loss. Note the log-scale on the y-axis.
Figure 15. Weighted mean age diagrams for metamorphic (Jurassic) zircons in (a)
orthogneiss JG061604-1, (b) paragneiss JG061504-4 and (c) orthogneiss JG063004-1.
(d) Probability density plots for these samples. The combined curve is all three added
together and has been arbitrarily scaled to be slightly larger than the other three for
readability.
Figure 16. Cathode-luminescence SEM images of zircons from JG061504-4. The
zircons show older, rounded or broken cores with younger (Jurassic) tips. The spot size
in all cases is 35 µm.
Figure 17. Pressure-temperature diagram summarizing the thermobarometry results.
Error bars and squares represent a 2σ uncertainty based on multiple analyses; also shown
as open symbols are P-T points based on the representative analyses reported in Table 2.
The gray shaded area represents normal geotherms; the thick, dashed gray line is the
121
approximate equilibrium line for the muscovite dehydration reaction: Muscovite +
Plagioclase + Quartz → K-Feldspar + Sillimanite + melt. Dashed, dark line is a
possible P-T path; high pressure limit could be higher if mineral assemblages equilibrated
after significant decompression. Filled hexagon represents the temperature recorded by
40
Ar/39Ar in biotite. Ages are based on geo- and thermochronology.
122
FIGURES
Figure 1. Guynn et al.
123
Figure 2. Guynn et al.
124
Figure 3. Guynn et al.
Figure 4. Guynn et al.
125
Figure 5. Guynn et al.
126
Figure 6. Guynn et al.
127
Figure 7. Guynn et al.
-
128
Figure 8. Guynn et al.
129
Figure 9. Guynn et al.
130
Figure 10.
131
Figure 11. Guynn et al.
132
Figure 12. Guynn et al.
Figure 13. Guynn et al.
133
Figure 14. Guynn et al.
134
Figure 15. Guynn et al.
135
Figure 16. Guynn et al.
136
Figure 17. Guynn et al.
137
TABLES
Table 1. Major element geochemistry of mafic amphibolite JG060404-1.
Oxide
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
P2O5
Total
Raw
48.24
2.28
14.16
14.38
0.24
6.54
9.94
2.48
0.68
0.28
99.22
Normalized
48.62
2.30
14.27
14.49
0.24
6.59
10.02
2.50
0.69
0.28
100.00
Determined by XRF analysis at the Washington State University Geoanalytical Laboratory.
138
37.84
0.16
21.25
n.a.
1.50
24.78
1.14
3.21
10.61
0.03
n.a.
100.52
Ag208
grt amph
60441
core
2.966
0.010
1.962
0.000
0.094
1.633
0.083
0.347
0.897
0.009
0.000
8.0
A ave.
grt amph
60441
core
2.971
0.009
1.972
0.000
0.072
1.661
0.167
0.287
0.856
0.005
0.000
8.0
37.50
0.15
21.11
n.a.
1.21
25.07
2.49
2.43
10.08
0.03
n.a.
100.07
Bg9
grt amph
60441
core
2.987
0.010
1.947
0.000
0.075
1.650
0.100
0.354
0.866
0.005
0.000
8.0
B ave.
grt amph
60441
core
36.97
0.02
21.36
0.01
2.97
25.34
1.48
4.40
7.32
0.03
0.01
99.91
2.962
0.002
1.976
0.000
0.096
1.523
0.147
0.600
0.694
0.000
0.000
8.0
38.18
0.03
21.60
0.00
1.65
23.48
2.24
5.19
8.34
0.00
0.00
100.71
Eg087
grt amph
6094B
core
2.919
0.001
1.988
0.001
0.177
1.673
0.099
0.518
0.619
0.005
0.001
8.0
Dg2a
grt amph
60441
incl.
2.976
0.008
1.959
0.000
0.075
1.533
0.116
0.627
0.701
0.004
0.000
8.0
E ave.
grt amph
6094B
core
Table 2. Representative and average garnet analyses.
Spot ID
Rock type
Sample
Spot Loc.
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Cations (12 oxygen basis)
Si
2.967
Ti
0.009
Al
1.964
Cr
0.000
Fe3+
0.088
Fe2+
1.625
Mn
0.076
Mg
0.375
Ca
0.891
Na
0.005
K
0.000
Sum
8.0
Ig154
grt amph
6094B
core
38.51
0.16
21.31
0.00
0.87
24.31
1.40
5.67
7.94
0.04
0.00
100.21
2.992
0.009
1.952
0.000
0.051
1.580
0.092
0.657
0.661
0.006
0.000
8.0
I ave.
grt amph
6094B
core
2.981
0.008
1.970
0.000
0.056
1.541
0.089
0.663
0.686
0.005
0.000
8.0
37.41
0.09
21.09
0.02
0.00
30.71
1.93
3.29
4.67
0.02
n.a.
99.23
A7g29
g-k schist
6094D
shoulder
36.90
0.10
21.38
n.a.
0.74
28.27
4.14
2.49
5.67
0.01
n.a.
99.70
3.002
0.005
1.995
0.001
0.000
2.061
0.131
0.394
0.402
0.003
0.000
8.0
Ag003
g-k schist
61242_2
shoulder
37.89
0.17
21.03
0.00
1.51
24.99
1.43
2.66
10.71
0.12
0.00
100.51
2.961
0.006
2.023
0.000
0.045
1.897
0.281
0.298
0.488
0.002
0.000
8.0
Ag295
g-k schist
60441
shoulder
37.72
0.03
21.20
0.03
0.13
30.77
2.29
3.17
4.84
0.04
0.00
100.22
2.980
0.010
1.950
0.000
0.089
1.644
0.095
0.312
0.902
0.018
0.000
8.0
A8g53
g-k schist
6094D
shoulder
36.85
0.08
21.49
n.a.
1.04
27.89
3.63
2.25
6.50
0.05
n.a.
99.77
3.002
0.002
1.989
0.002
0.008
2.048
0.154
0.376
0.413
0.006
0.000
8.0
Ag006
g-k schist
61242_2
shoulder
37.27
0.12
21.07
0.04
0.77
30.15
2.69
2.41
5.86
0.00
n.a.
100.38
2.953
0.005
2.030
0.000
0.062
1.869
0.246
0.269
0.558
0.008
0.000
8.0
gr_top.15
g-k schist
61242_1
edge
37.24
0.04
21.56
0.00
2.62
23.76
2.76
5.80
5.95
0.01
0.00
99.74
2.976
0.007
1.984
0.002
0.047
2.014
0.182
0.286
0.502
0.000
0.000
8.0
Gg131
grt amph
6094B
inc.
2.923
0.002
1.995
0.000
0.155
1.560
0.184
0.679
0.500
0.002
0.000
8.0
The letter in a microprobe spot ID refers to a particular garnet; for non-garnet analyses, these spots are located in the vicinity of that garnet. Average values are reported in cations only.
Rock type abbreviations: g-a amph = garnet-amphibolite; g-k schist = garnet-kyanite schist; amph = amphibolite. Sample abbreviations: 60441 = JG060404-1; 6094B = JG060409-11;
61242_1 = JG061204-2, thin section #1; 61242_2 = JG061204-2, thin section #2; 6094D = JG060904-13; 62242 = JG062204-2; 62244 = JG062204-4. Other abbreviations: incl. =
inclusion; n.a. = not analyzed. Certain elements were not analyzed only after multiple analyses on that specific thin section demonstrated negligible (< 0.05 %) quantities. Core
measurements refer to garnets without significant growth zoning and are located beyond any diffusion rims. Edge refers to an analysis on the very edge (within 10 µm) of a mineral.
Shoulder refers to garnets with growth zoning and diffusion rims and is the point on the element profiles between the two. See Figure 6 for examples. Matrix analyses are typically at
least several 100 µm away from garnet or corona edges. Microprobe measured only total FeO; FeO and Fe2O3 determined according to the program WinAX by T.J.B. Holland
(http://www.esc.cam.ac.uk/astaff/holland/ax.html).
139
Ah226
grt amph
60441
matrix
6.215
0.195
2.217
0.000
0.492
1.788
0.023
2.165
1.768
0.539
0.201
15.6
A ave.
grt amph
60441
matrix
6.343
0.213
2.103
0.002
0.316
1.902
0.024
2.176
1.804
0.490
0.210
15.6
41.70
1.86
11.73
0.02
2.76
14.96
0.19
9.60
11.07
1.66
1.08
96.63
Bh7
grt amph
60441
matrix
6.306
0.187
2.127
0.004
0.434
1.803
0.023
2.204
1.789
0.498
0.200
15.6
B ave.
grt amph
60441
matrix
45.37
0.66
9.20
n.a.
6.50
10.22
0.34
12.04
11.43
1.10
0.19
97.05
6.617
0.076
1.744
0.000
0.661
1.313
0.043
2.616
1.806
0.417
0.038
15.3
44.33
0.68
9.91
n.a.
5.88
10.52
0.34
11.76
11.29
1.44
0.20
96.35
Qh_e
amph
62244
edge
43.43
1.28
9.24
n.a.
4.39
12.39
0.33
11.26
11.43
1.39
0.84
95.98
6.708
0.073
1.604
0.000
0.723
1.264
0.043
2.653
1.811
0.315
0.036
15.2
Qh_c
amph
62244
core
42.87
1.30
9.35
n.a.
4.80
12.25
0.29
11.19
11.55
1.39
0.89
95.88
6.588
0.146
1.652
0.000
0.501
1.572
0.042
2.545
1.858
0.409
0.163
15.5
Eh_e
amph
62242
edge
42.19
1.19
13.05
0.00
6.76
9.50
0.24
11.84
11.09
1.91
0.62
98.40
6.525
0.149
1.678
0.000
0.550
1.559
0.037
2.538
1.884
0.410
0.173
15.5
Eh_c
amph
62242
core
42.79
1.67
12.50
n.a.
6.06
10.14
0.18
11.92
10.59
1.90
0.56
98.30
6.188
0.131
2.257
0.000
0.746
1.166
0.030
2.588
1.743
0.543
0.116
15.5
Gh135
grt amph
6094B
incl.
42.78
1.65
12.47
n.a.
5.79
10.36
0.17
11.98
10.81
2.02
0.52
98.55
6.270
0.184
2.159
0.000
0.668
1.244
0.022
2.602
1.663
0.539
0.105
15.5
I ave.
grt amph
6094B
matrix
42.57
1.54
12.71
n.a.
6.58
10.25
0.21
11.66
10.65
1.92
0.58
98.68
6.261
0.182
2.151
0.000
0.637
1.268
0.021
2.613
1.695
0.573
0.097
15.5
Ih172
grt amph
6094B
matrix
41.89
1.80
13.25
n.a.
7.15
9.92
0.19
11.59
10.65
1.96
0.62
99.02
6.239
0.172
2.181
0.000
0.726
1.245
0.025
2.557
1.664
0.544
0.108
15.5
E ave.
grt amph
6094B
matrix
40.00
2.16
13.23
0.00
3.69
13.61
0.13
9.79
11.20
1.78
1.26
96.85
6.120
0.198
2.282
0.000
0.786
1.212
0.024
2.524
1.667
0.555
0.116
15.5
Eh074
grt amph
6094B
matrix
6.076
0.247
2.369
0.000
0.422
1.728
0.017
2.216
1.823
0.524
0.244
15.7
Dh2a
grt amph
60441
incl.
Table 2. (Cont’d). Representative and average amphibole analyses.
Ah269
grt amph
60441
matrix
41.61
1.73
12.69
n.a.
2.99
14.84
0.18
9.65
11.18
1.84
1.13
97.84
Spot ID
Rock type
Sample
Spot Loc.
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K 2O
Total
6.252
0.196
2.248
0.000
0.338
1.865
0.023
2.161
1.800
0.536
0.217
15.6
39.93
1.78
12.83
n.a.
4.94
13.83
0.17
9.46
11.07
1.94
1.14
97.10
Cations (23 oxygen basis)
Si
6.079
Ti
0.204
Al
2.303
Cr
0.000
Fe3+
0.566
Fe2+
1.761
Mn
0.022
Mg
2.146
Ca
1.806
Na
0.573
K
0.222
Sum
15.7
140
61.00
0.04
24.77
n.a.
0.21
0.00
0.00
0.00
5.75
8.43
0.45
100.65
Ap249
grt amph
60441
matrix
57.64
0.04
27.08
n.a.
0.19
0.00
0.00
0.01
7.96
7.18
0.16
100.26
2.636
0.001
1.359
0.000
0.006
0.000
0.000
0.000
0.334
0.668
0.019
5.0
58.11
0.03
25.41
0.01
0.19
0.00
0.01
0.00
6.87
7.60
0.32
98.55
Bp5
grt amph
60441
matrix
2.575
0.001
1.426
0.000
0.006
0.000
0.000
0.001
0.381
0.622
0.009
5.0
Ap256
grt amph
60441
matrix
2.608
0.001
1.387
0.000
0.010
0.000
0.000
0.000
0.364
0.629
0.017
5.0
A,B ave.
grt amph
60441
matrix
56.36
0.00
26.49
0.00
0.66
0.00
0.01
0.00
8.28
6.78
0.29
98.87
2.632
0.000
1.371
0.000
0.008
0.000
0.000
0.000
0.330
0.665
0.009
5.0
59.20
0.01
26.16
n.a.
0.23
0.00
0.00
0.00
6.93
7.72
0.16
100.41
Ep084
grt amph
6094B
matrix
2.563
0.000
1.420
0.000
0.022
0.000
0.000
0.000
0.403
0.598
0.017
5.0
Dp2b
grt amph
60441
incl.
2.635
0.000
1.361
0.000
0.009
0.000
0.000
0.001
0.318
0.690
0.019
5.0
E ave.
grt amph
6094B
matrix
2.659
0.001
1.345
0.000
0.009
0.000
0.000
0.000
0.297
0.692
0.009
5.0
60.02
0.04
25.75
n.a.
0.27
0.00
0.00
0.00
6.26
8.06
0.16
100.56
Ip188
grt amph
6094B
matrix
2.659
0.001
1.345
0.000
0.009
0.000
0.000
0.000
0.297
0.692
0.009
5.0
I ave.
grt amph
6094B
matrix
2.521
0.001
1.465
0.000
0.020
0.000
0.001
0.000
0.434
0.581
0.006
5.0
56.87
0.04
28.02
n.a.
0.59
0.00
0.02
0.00
9.13
6.76
0.11
101.54
Gp134
grt amph
6094B
incl.
Table 2. (Cont’d). Representative and average plagioclase analyses for garnet amphibolites.
Spot ID
Rock type
Sample
Spot Loc.
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Cations (8 oxygen basis)
Si
2.700
Ti
0.001
Al
1.293
Cr
0.000
Fe3+
0.007
Fe2+
0.000
Mn
0.000
Mg
0.000
Ca
0.273
Na
0.724
K
0.025
Sum
5.0
141
A2p01
g-k schist
6094D
near
Table 2. (Cont’d). Representative and average plagioclase analyses for amphibolites and schists.
A2p46
g-k schist
6094D
near
58.07
0.05
27.18
0.02
0.18
0.00
0.03
0.01
7.78
6.98
0.20
100.50
Ap057
g-k schist
61242_2
near
58.08
0.03
26.43
0.00
0.11
0.00
0.00
0.00
7.74
7.13
0.16
99.68
2.584
0.002
1.426
0.001
0.006
0.000
0.001
0.001
0.371
0.602
0.011
5.0
GK1.7
g-k schist
61242_1
matrix
54.78
0.08
27.37
n.a.
0.17
0.00
0.00
0.00
8.87
6.89
0.05
98.21
2.605
0.001
1.397
0.000
0.004
0.000
0.000
0.000
0.372
0.620
0.009
5.0
Qp_e
amph
62244
edge
57.74
0.00
26.82
0.04
0.14
0.00
0.01
0.00
8.48
7.16
0.09
100.45
2.512
0.003
1.479
0.000
0.006
0.000
0.000
0.000
0.436
0.613
0.003
5.1
Qp_c
amph
62244
core
58.39
0.03
26.16
n.a.
0.29
0.00
0.01
0.01
7.38
7.72
0.08
100.07
2.578
0.000
1.412
0.001
0.005
0.000
0.000
0.000
0.406
0.619
0.005
5.0
Ep_e
amph
62242
edge
58.09
0.00
25.92
n.a.
0.23
0.00
0.01
0.01
7.38
7.58
0.07
99.29
2.611
0.001
1.379
0.000
0.010
0.000
0.000
0.001
0.354
0.669
0.005
5.0
Ep_c
amph
62242
core
57.44
0.02
26.74
n.a.
0.32
0.00
0.00
0.00
7.99
7.21
0.08
99.80
2.617
0.000
1.376
0.000
0.008
0.000
0.000
0.001
0.356
0.662
0.004
5.0
Spot ID
Rock type
Sample
Spot Loc.
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
2.579
0.001
1.416
0.000
0.011
0.000
0.000
0.000
0.384
0.628
0.005
5.0
57.49
0.04
25.99
n.a.
0.23
0.00
0.02
0.00
7.26
7.51
0.14
98.68
Cations (8 oxygen basis)
Si
2.607
Ti
0.001
Al
1.389
Cr
0.000
Fe3+
0.008
Fe2+
0.000
Mn
0.001
Mg
0.000
Ca
0.353
Na
0.660
K
0.008
Sum
5.0
142
Table 2. (Cont’d). Representative and average mica analyses.
A2m45
g-k schist
6094D
near
A2m47
g-k schist
6094D
near
45.99
0.48
37.82
0.05
0.24
0.79
0.00
0.64
0.00
0.78
10.02
96.81
Am060
g-k schist
61242_2
near
46.50
0.53
37.93
0.05
0.60
0.61
0.00
0.75
0.04
0.74
10.08
97.84
2.999
0.024
2.908
0.003
0.012
0.043
0.000
0.062
0.000
0.099
0.834
7.0
gr2tr.45
g-k schist
61242_1
near
44.90
0.74
37.07
0.00
0.96
0.37
0.02
0.68
0.03
0.80
9.66
95.25
3.001
0.026
2.886
0.003
0.029
0.033
0.000
0.072
0.003
0.093
0.831
7.0
A2b04
g-k schist
6094D
near
0.84
9.97
46.23
0.84
36.47
0.05
0.00
0.00
1.05
0.02
0.69
96.16
2.978
0.037
2.898
0.000
0.048
0.021
0.001
0.067
0.002
0.103
0.818
7.0
A2b42
g-k schist
6094D
near
34.99
2.41
20.12
0.03
0.00
17.52
0.19
9.25
0.00
0.10
9.52
94.14
3.038
0.034
2.825
0.001
0.000
0.058
0.000
0.082
0.003
0.107
0.837
7.0
Ab061
g-k schist
61242_2
near
35.51
2.07
19.77
0.01
0.00
17.17
0.15
9.52
0.08
0.16
8.77
93.22
2.673
0.138
1.812
0.002
0.000
1.119
0.012
1.053
0.000
0.015
0.929
7.8
GK1.8
g-k schist
61242_1
matrix
33.95
2.14
19.39
n.a.
0.73
18.02
0.13
10.06
0.03
0.15
8.66
93.26
2.720
0.119
1.785
0.001
0.000
1.100
0.010
1.087
0.007
0.024
0.858
7.7
Spot ID
Rock type
Sample
Spot Loc.
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
2.628
0.125
1.769
0.000
0.042
1.166
0.009
1.161
0.002
0.023
0.856
7.8
35.17
2.42
19.10
0.03
0.00
18.52
0.15
9.57
0.00
0.18
9.29
94.43
Cations (11 oxygen basis)
Si
2.691
Ti
0.139
Al
1.723
Cr
0.002
Fe3+
0.000
Fe2+
1.185
Mn
0.010
Mg
1.091
Ca
0.000
Na
0.027
K
0.908
Sum
7.8
143
Table 3. U-Pb geochronologic analyses for Amdo metamorphic zircon determined by Laser-Ablation Multicollector ICP
Mass Spectrometry
Apparent ages (Ma)
±
(Ma)
Isotopic ratios
Best age
(Ma)
1.9
4.4
1.6
1.5
2.9
4.8
6.6
2.1
3.7
2.6
1.3
1.4
1.4
1.3
1.4
±
(Ma)
172.9
174.7
175.0
175.1
179.2
180.0
181.1
181.5
182.0
182.1
182.2
182.8
184.8
185.7
191.1
2.8
37.2
3.5
8.1
5.0
2.9
24.4
5.4
4.1
4.8
5.6
2.5
3.4
3.2
3.6
4.3
2.6
2.8
3.5
206Pb*
207Pb*
93.7
185.1
115.0
63.3
236.7
177.9
78.5
99.7
83.5
184.2
122.8
141.4
110.2
139.0
185.0
169.4
1463.7
170.4
868.5
171.0
171.7
2490.2
172.2
172.3
173.6
173.9
174.3
174.7
175.6
178.3
178.8
179.7
179.8
180.8
±
(Ma)
184.2
124.4
96.3
177.5
133.8
78.6
161.5
77.2
150.8
-1.0
198.0
137.8
68.1
81.2
130.2
39.0
37.2
52.7
22.0
68.8
41.8
24.4
140.3
42.6
154.1
149.9
71.4
97.9
78.9
58.5
74.6
130.2
55.8
73.8
207Pb*
235U
6.7
13.0
7.7
4.6
16.5
12.6
8.2
7.0
6.8
12.1
8.9
10.0
7.6
9.6
13.4
170.2
1463.7
55.9
978.9
369.6
150.0
2490.2
337.2
74.4
312.6
424.0
239.7
264.2
189.1
217.2
36.4
121.2
273.9
301.9
±
(Ma)
173.7
171.3
169.7
175.2
176.0
173.0
179.7
174.3
179.8
169.7
183.3
179.6
176.6
178.3
186.6
3.7
19.4
4.6
8.8
7.2
3.9
32.2
11.7
4.6
12.3
13.1
5.6
7.8
6.2
5.4
6.2
9.2
4.9
6.6
206Pb*
238U
1.9
4.4
1.6
1.5
2.9
4.8
6.6
2.1
3.7
2.6
1.3
1.4
1.4
1.3
1.4
169.5
1443.2
163.0
900.2
185.1
170.3
2273.8
183.9
165.8
183.5
192.3
178.9
181.0
176.5
181.0
169.2
175.6
186.7
189.7
error
corr.
172.9
174.7
175.0
175.1
179.2
180.0
181.1
181.5
182.0
182.1
182.2
182.8
184.8
185.7
191.1
2.8
20.3
3.5
8.1
5.0
2.9
56.7
5.4
4.1
4.8
5.6
2.5
3.4
3.2
3.6
4.3
2.6
2.8
3.5
±
(%)
0.27
0.31
0.19
0.31
0.16
0.34
0.74
0.27
0.50
0.18
0.13
0.13
0.17
0.12
0.09
169.4
1429.3
170.4
868.5
171.0
171.7
2041.2
172.2
172.3
173.6
173.9
174.3
174.7
175.6
178.3
178.8
179.7
179.8
180.8
206Pb*
238U
1.1
2.6
0.9
0.9
1.7
2.7
3.7
1.2
2.0
1.4
0.7
0.8
0.8
0.7
0.7
0.71
0.63
0.69
0.68
0.70
0.70
0.91
0.46
0.80
0.38
0.44
0.43
0.42
0.48
0.63
0.62
0.26
0.54
0.52
±
(%)
0.0272
0.0275
0.0275
0.0275
0.0282
0.0283
0.0285
0.0286
0.0286
0.0287
0.0287
0.0288
0.0291
0.0292
0.0301
1.7
1.6
2.1
1.0
3.0
1.7
3.2
3.2
2.4
2.8
3.3
1.5
2.0
1.8
2.0
2.5
1.5
1.6
2.0
207Pb*
235U
4.2
8.3
4.9
2.8
10.2
7.9
5.0
4.4
4.1
7.8
5.3
6.1
4.7
5.9
7.9
0.0266
0.2482
0.0268
0.1442
0.0269
0.0270
0.3725
0.0271
0.0271
0.0273
0.0273
0.0274
0.0275
0.0276
0.0280
0.0281
0.0283
0.0283
0.0284
U/Th
0.1866
0.1837
0.1819
0.1884
0.1893
0.1857
0.1936
0.1872
0.1937
0.1819
0.1978
0.1935
0.1900
0.1919
0.2018
2.4
2.5
3.0
1.5
4.2
2.5
3.5
7.0
3.0
7.3
7.5
3.4
4.7
3.9
3.3
4.0
5.7
2.9
3.8
206Pb
204Pb
14.4
35.3
39.2
49.9
49.9
14.8
61.2
53.2
56.5
54.3
59.9
52.7
15.0
35.8
49.8
0.1816
3.1426
0.1741
1.4269
0.2000
0.1826
8.3877
0.1986
0.1774
0.1980
0.2084
0.1927
0.1952
0.1899
0.1952
0.1813
0.1888
0.2018
0.2054
U
(ppm)
Orthogneiss JG061604-1
15t
581
11864
06
311
2984
09
511
7751
04t
553
5646
16
251
5906
07
429
5706
17
775
14382
08
830
10912
25
571
14545
04c
327
4212
26c
282
10382
29
344
10588
19
468
11114
11t
427
10364
20
210
4845
17.9
1.2
16.4
0.9
14.8
16.6
2.6
11.7
15.0
12.3
16.0
10.2
11.6
14.5
11.9
10.3
9.9
10.9
13.8
Spot
Paragneiss JG061504-4
b50t1
613
9499
b50c
36
18037
b57t1
382
8634
b57c
327
41664
42t1
854
22889
b41t1
680
13003
b41c
477
27747
39t1
651
26161
b58t1
480
11872
05t1
492
21967
34t1
525
11365
31t2
750
7826
35t1
800
30468
43t1
565
35817
38t1
811
34025
16t1
615
7843
31t1
1051
7405
35t2
812
53797
10t1
808
86708
144
U
(ppm)
206Pb
204Pb
12.8
19.4
35.8
77.9
24.9
57.6
17.2
16.8
75.7
32.2
15.7
19.3
28.8
22.5
19.9
20.3
39.6
30.0
67.3
45.6
29.2
25.1
27.8
3.5
61.6
37.4
15.5
56.5
U/Th
0.1934
0.1958
0.1789
0.1920
0.1891
0.0824
0.1852
0.2865
0.2486
0.2229
0.1817
0.1889
0.1907
0.1837
0.2067
0.2927
0.1887
0.1942
0.3270
0.2018
0.1922
0.2047
0.1967
1.4639
0.1923
0.2151
0.1916
0.2482
207Pb*
235U
0.0283
0.0290
0.0281
0.0282
0.0266
0.0270
0.0275
0.0421
0.0267
0.0320
0.0277
0.0280
0.0287
0.0284
0.0296
0.0290
0.0278
0.0277
0.0271
0.0295
0.0283
0.0295
0.0287
0.1341
0.0276
0.0275
0.0276
0.0297
206Pb*
238U
Isotopic ratios
6.1
8.7
13.4
3.7
19.7
87.3
4.8
15.1
69.8
24.9
8.9
7.8
4.1
5.7
5.0
43.1
4.8
9.6
88.7
2.6
2.7
1.9
1.9
4.5
18.5
13.5
2.7
27.3
±
(%)
1.9
0.9
1.9
2.2
2.0
7.9
1.6
4.4
9.5
2.7
1.5
1.0
2.3
1.9
1.6
7.0
2.6
1.8
7.5
2.1
2.3
1.5
1.3
4.1
9.0
3.2
1.7
8.4
±
(%)
0.30
0.10
0.14
0.60
0.10
0.09
0.32
0.29
0.14
0.11
0.17
0.13
0.57
0.32
0.32
0.16
0.53
0.19
0.08
0.81
0.83
0.76
0.71
0.92
0.48
0.24
0.65
0.31
error
corr.
180.0
184.5
178.9
179.1
169.5
172.0
175.0
265.6
169.7
203.3
175.9
177.9
182.2
180.7
188.2
184.0
176.8
176.4
172.7
187.5
179.6
187.2
182.2
811.3
175.3
175.2
175.3
188.7
206Pb*
238U
3.3
1.7
3.3
3.9
3.3
13.4
2.7
11.5
15.9
5.5
2.5
1.8
4.2
3.3
3.0
12.7
4.5
3.2
12.7
3.9
4.1
2.7
2.4
31.6
15.5
5.5
3.0
15.6
±
(Ma)
10.0
14.4
20.6
6.0
31.9
67.6
7.6
34.1
142.0
46.1
13.9
12.5
6.7
9.0
8.7
99.4
7.8
15.9
225.6
4.5
4.5
3.3
3.1
27.3
30.4
24.3
4.4
55.2
±
(Ma)
Apparent ages (Ma)
207Pb*
235U
179.5
181.6
167.1
178.3
175.9
80.4
172.5
255.8
225.5
204.3
169.5
175.7
177.3
171.2
190.8
260.6
175.5
180.2
287.3
186.7
178.5
189.1
182.3
915.6
178.6
197.8
178.0
225.1
±
(Ma)
172.7 135.8
143.1 202.9
2.0 320.1
168.3
68.7
262.3 454.6
-2220.0 1277.7
138.4 106.3
167.4 337.8
856.5 1662.5
216.5 580.0
81.4 208.3
145.2 180.8
111.7
80.6
42.1 129.1
222.2 109.2
1022.0 902.2
158.6
96.0
231.1 219.1
1368.5
8.4
175.8
36.3
164.2
35.5
212.6
29.0
183.7
30.5
1176.2
35.8
221.9 377.8
477.1 290.8
213.4
47.8
625.2 569.2
206Pb*
207Pb*
Table 3. (Cont’d). U-Pb geochronologic analyses for Amdo metamorphic zircon determined by Laser-Ablation
Multicollector ICP Mass Spectrometry
Spot
Orthogneiss JG063004-1
01C
1043
14734
01R
893
11938
03
365
5190
05
3714
9099
06T
387
4393
07
53
267
08
822
6752
09
254
1075
11
49
469
12
138
1631
13
925
6520
14
799
5430
15
1345
23033
16
1029
10544
17R
815
11477
18
61
1267
19
1391
39990
22
1188
8972
2C
47
388
B1
1227
19568
B2
1386
34576
B3
946
13632
B4
1281
30474
B5
763
34610
B6
38
1470
B6T
185
1185
B7
564
25235
B9
28
3387
180.0
184.5
178.9
179.1
169.5
172.0
175.0
265.6
169.7
203.3
175.9
177.9
182.2
180.7
188.2
184.0
176.8
176.4
172.7
187.5
179.6
187.2
182.2
811.3
175.3
175.2
175.3
188.7
Best age
(Ma)
3.3
1.7
3.3
3.9
3.3
13.4
2.7
11.5
15.9
5.5
2.5
1.8
4.2
3.3
3.0
12.7
4.5
3.2
12.7
3.9
4.1
2.7
2.4
31.6
15.5
5.5
3.0
15.6
±
(Ma)
145
APPENDIX C:
Jurassic and Cretaceous tectonic evolution of the Bangong suture zone near Amdo,
central Tibet
Manuscript for submittal to
GSA Bulletin
Jerome Guynn
University of Arizona
Paul Kapp
University of Arizona
George Gehrels
University of Arizona
Lin Ding
Institute of Tibetan Plateau Research and Institute of Geology and Geophysics
146
Jurassic and Cretaceous Tectonic Evolution of the Bangong
Suture Zone near Amdo, Central Tibet
Jerome Guynn
Paul Kapp,
George Gehrels,
Department of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA
Lin Ding
Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese
Academy of Sciences, Beijing 100029 China
147
ABSTRACT
Unraveling the tectonic history of the mid-late Mesozoic Bangong suture zone, central
Tibet, is fundamentally important for understanding the development of the Tibetan
Plateau as it represents the last accretionary event in southern Asia prior to the IndoAsian collision. The Amdo basement provides a unique place to study the evolution of
the suture zone due to the exposure of high-grade metamorphic rocks and granitoids not
present elsewhere along the suture and to its location in the hinterland of Cretaceous fold
and thrust belts in the northern Lhasa terrane.
We present new geochronology,
stratigraphic data and structural analysis that shed new light on the Jurassic and Early
Cretaceous history of the suture zone. We suggest that the lack of a magmatic arc along
the southern Qiangtang terrane is due to subduction erosion, subduction beneath island
arcs in a supra-subduction zone and burial by sediments. The occurrence of ophiolite
fragments in a deformed Jurassic flysch is more consistent with an accretionary wedge
setting than a single ophiolite thrust sheet, particularly in light of obduction occurring
prior to continental collision between the Lhasa and Qiangtang terranes.
Early
Cretaceous collision resulted in the development of a south-verging basement-cored
thrust belt and corresponding foreland basin that was active at least between ~130-100
Ma. Total shortening likely exceeded 100 km. Aptian-Albian deposition of coarse
clastic sediments in front of the thrust belt was simultaneous with deposition of shallow
marine limestones to the south. Southward propagation of the northern Lhasa thrust belt
in the Late Cretaceous – Early Tertiary was coeval with northward propagation of a
148
Gangdese retro-arc thrust belt in the southern Lhasa terrane, contributing to crustal
thickening and decreased erosion prior to the Indo-Asian collision.
149
INTRODUCTION
The Bangong suture zone in central Tibet is the site of the late Mesozoic collision
between the Lhasa and Qiangtang terranes, the last of several Paleozoic-Mesozoic
accretionary events that built the southern Asian margin prior to India’s collision during
the early Tertiary (Allégre et al., 1984; Dewey et al., 1988). The suture zone is a broad
and diffuse zone defined by ophiolite fragments, Jurassic flysch and thrust related
deformation occurring over a north-south distance up to 200 km (Coward et al., 1988;
Girardeau et al., 1984; Girardeau et al., 1985; Kidd et al., 1988). The suture lacks a
magmatic arc despite the evidence for a significant ocean (the Meso-Tethys) between the
Lhasa and Qiangtang terranes. Thrust belts within the suture zone generally involve
Mesozoic sedimentary rocks and only minor Paleozoic, mostly Carboniferous and
Permian, strata (Kapp et al., 2003a; Kapp et al., 2005; Kidd et al., 1988; Pan et al., 2004),
part of the reason initial studies suggested the collision resulted in only minor
deformation (Coward et al., 1988). More recent work, however, has documented the
presence of significant crustal shortening in the northern Lhasa terrane during the
Cretaceous (Kapp et al., 2003a; Kapp et al., 2005; Murphy et al., 1997; Volkmer et al.,
accepted). Furthermore, the only exposure of metamorphic rocks along the suture zone,
the Amdo basement, was originally thought to have undergone high-grade metamorphism
in the Cambrian (Harris et al., 1988a; Xu et al., 1985), but new studies reveal Middle
Jurassic high-grade metamorphism, during oceanic subduction but prior to collision
(Guynn et al., 2006). We present new data on the timing and nature of thrust belt
150
development in the vicinity of the Amdo basement and, combined with other recent work
at Amdo, present a comprehensive model for the initial development of the suture zone in
this area. We also examine possible models for the pre-collisional history of the suture
zone as revealed by the Amdo basement.
The question of the amount of Cretaceous shortening, and resultant crustal
thickening, is important for determining the role of the Indo-Asian collision in creating
the Tibetan Plateau. Models have always assumed Tibet was at sea level at the start of
the Indio-Asian collision (e.g. Beaumont et al., 2001; Royden et al., 1997), but the
existence of initial elevation would greatly change the results of those models. The
occurrence of Aptian-Albian (100-120 Ma) limestones in the northern Lhasa terrane
indicates that portions of the terrane were below sea-level at that time (Leeder et al.,
1988; Yin et al., 1988), but thrusts that involve the limestones indicate shortening in the
Late Cretaceous (Murphy et al., 1997; Volkmer et al., accepted). Shortening in the Early
Cretaceous is more controversial and the depositional environment of the limestones, and
associated redbeds, has been proposed to be a foreland basin in response to
compressional stresses (Kapp et al., 2005; Murphy et al., 1997) or a back-arc basin as a
result of extensional tectonics (Zhang, 2004; Zhang et al., 2004). Our mapping and
observations, together with the thermochronology of Guynn et al. (2006) and
geochronology presented here, indicates hinterland thrust belt development along the
suture zone in the Early Cretaceous and the presence of a foreland basin composed of
nonmarine clastic sedimentary rocks rather than carbonates.
151
REGIONAL GEOLOGY
The northernmost Tibetan terranes are the Qilan-Qaidam terranes which collided with
Asia during the mid-Paleozoic (Sobel and Arnaud, 1999; Yin and Harrison, 2000).
Between these and the Qiangtang terrane lies the Songpan Ganzi flysch complex, a thick
accumulation of deformed Triassic and early Jurassic deep marine sediments (Yin and
Harrison, 2000).
The southern boundary of the Songpan-Ganzi terrane is the Late
Triassic-Early Jurassic Jinsha suture, which represents the collision with the Qiangtang
terrane to the south (Roger et al., 2003; Yin and Harrison, 2000). Surface geology of the
Qiangtang terrane is dominated by upper Paleozoic (Carboniferous and Permian) shallow
marine strata and Jurassic-Triassic shallow marine carbonates, fluvial deposits and
volcanic sequences, with the exception of exposures of metamorphic rocks, including
blueschists, in the central interior (Kapp et al., 2003b; Kapp et al., 2000; Yin and
Harrison, 2000). The southernmost Tibetan terrane is Lhasa, bounded by the Bangong
suture zone to the north and the Indus suture to the south. Rocks exposed in the Lhasa
terrane consist of upper Paleozoic shelf strata, Mesozoic marine and fluvial rocks, and,
especially to the south, Cretaceous-Tertiary volcanic rocks and plutons (Yin and
Harrison, 2000). The voluminous upper Cretaceous-lower Tertiary volcanic and plutonic
rocks of the southern Lhasa terrane are related to the subduction of the Neo-Tethys Ocean
beneath Tibet as India moved northward. The Indus suture is the sight of the early
Tertiary collision of India with Asia and the Himalayan mountain chain marks the
southern boundary of Tibet.
152
Paleomagnetic data for the Qiangtang and Lhasa terranes are sparse, partly due to a
lack of study and partly due to Mesozoic and Cenozoic remagnetizations, but the
available data are consistent with the timing of rifting events and collisions. A summary
by Li et al. (2004) indicates the end of northward Qiangtang movement and separation of
Lhasa from Gondwana during the Late Triassic and a coeval latitude for Lhasa and
Qiangtang by the end of the Early Cretaceous, with a maximum separation of ~40°
latitude during the Late Triassic. Thus approximately 4000 km of ocean crust had to be
subducted between the Lhasa and Qiantang terranes in the Mesozoic.
The Bangong suture zone is represented by a wide region of Jurassic-Cretaceous
marine flysch, scattered and discontinuous Jurassic ophiolite fragments and CretaceousTertiary nonmarine clastic rocks (Pan et al., 2004; Yin et al., 1988). Arc-related igneous
rocks are largely absent along the suture (Allégre et al., 1984; Dewey et al., 1988) but
subduction of the Meso-Tethys is thought to have been northward based on southdirected thrusts within the suture zone (Coward et al., 1988; Girardeau et al., 1984; Kapp
et al., 2003a). Timing of ophiolite obduction is Middle-Late Jurassic based on fossil
assemblages in overlying sedimentary rocks (Girardeau et al., 1984; Girardeau et al.,
1985) and 40Ar/39Ar dating of a metamorphic sole (Zhou et al., 1997). In western Tibet,
two separate occurrences of ophiolites and associated mélange ~70 km apart have been
interpreted as either the result of underthrusting beneath the Lhasa terrane (Kapp et al.,
2003a) or of two collision zones (Matte et al., 1996). Ophiolite fragments in the central
part of the Bangong suture are restricted to the southern edge of the Qiangtang terrane,
but in the eastern section between Siling Lake and the Lhasa-Golmud highway, ophiolites
153
again occur across a wide region, up to ~200 km south of the presumed southern edge of
the Qiangtang terrane (Kidd et al., 1988; Pan et al., 2004). Here the scattered exposures
have been explained by a southward obducted, originally continuous ophiolite thrust
sheet that has been tectonically dissected and partially eroded (Coward et al., 1988;
Girardeau et al., 1984), a transtensional rift basin that did not evolve to a full ocean
(Schneider et al., 2003) or again, as multiple suture zones, possibly involving an island
arc and back-arc basin (Pearce and Deng, 1988; Pearce and Mei, 1988).
Another
enigmatic aspect of the Bangong suture is the apparent lack of subduction-related
magmatism along almost its entire length, as indicated by the lack of Triassic-Jurassic
igneous rocks (Fig. 1), with the only documented exception being Jurassic granitoids that
intrude the Amdo basement (Guynn et al., 2006). Guynn et al. (2006) interpret the midJurassic granitoids to be related to a magmatic arc which has been buried, either
tectonically or depositionally, beneath the southern Qiangtang margin.
The Amdo basement, located southeast of the town of Amdo (Fig. 1), is a large
exposure of amphibolite-grade orthogneisses, with subordinate metasedimentary rocks,
amphibolites and migmatites, that have been intruded by extensive Jurassic granitoids
(Coward et al., 1988; Guynn et al., 2006; Harris et al., 1988a; Schärer et al., 1986). It is
generally considered part of the Lhasa terrane due to its occurrence south of ophiolite
exposures at the town of Amdo (Coward et al., 1988), but if during the mid-Jurassic the
southern Qiangtang margin was active and the northern Lhasa margin passive, the Middle
Jurassic timing of metamorphism implies a Qiangtang affinity. Ophiolite fragments are
located north of the basement, in the town of Amdo, and south of the basement near
154
Nagqu (Girardeau et al., 1984; Pearce and Deng, 1988). The western and northern
boundaries of the basement are recently active normal and sinistral strike-slip faults,
respectively (Kidd and Molnar, 1988). Contacts between the basement and rocks on the
opposite sides of the faults are obscured by Neogene basin fill and alluvium. To the
south, the basement appears to be thrust over unmetamorphosed sedimentary rocks that
have been interpreted to be Jurassic and Paleozoic (Coward et al., 1988; Kidd et al.,
1988), Triassic (Pan et al., 2004) or Jurassic and Cretaceous (Guynn et al., 2006). The
eastern exposure of the Amdo basement was not investigated in this study because of a
lack of access, but regional geologic maps indicate a fault contact with Jurassic mélange
(Pan et al., 2004).
The Lhasa-Qiangtang collision is considered to be diachronous (Yin and Harrison,
2000), occurring during the Late Jurassic in the east around Amdo (Dewey et al., 1988;
Girardeau et al., 1984) and during the Early Cretaceous to the west around Bangong Lake
(Kapp et al., 2003a; Matte et al., 1996), though recent thermochronology of the Amdo
basement suggests an Early Cretaceous collision in the east as well (Guynn et al., 2006).
Coward et al. (1988) suggested minimal deformation due to the collision, but more recent
work has documented extensive Cretaceous shortening in the northern Lhasa terrane
(Kapp et al., 2003a; Murphy et al., 1997; Volkmer et al., accepted). Despite the presence
of Cretaceous thrusts, Zhang et al (2004) argue that the northern Lhasa terrane was a
back-arc basin of the Gangdese arc in the Aptian-Albian based on sedimentalogical
provenance and geochemistry of the clastic sedimentary rocks.
155
GEOCHRONOLOGY
In order to provide provenance and age constraints on sedimentary units and crosscutting igneous rocks, we analyzed U-Pb ages of detrital zircons from several different
sedimentary rocks and of zircons from several Cretaceous granitoids and hypabyssal
intrusions, some of which provide age constraints on faulting. We only provide an
outline of the U-Pb dating method here; for a complete description, see Guynn et al.
(2006). All analyses were conducted at the University of Arizona LaserChron Center
using the Laser-Ablation Multicollector Inductively Coupled Plasma Mass Spectrometer
(LA-MC-ICPMS).
Typically, ~25 analyses (1 analysis per zircon grain) were conducted for igneous
zircons and ~100 for detrital samples. Multiple igneous analyses allow the resulting
useable ages to be averaged to provide a higher accuracy than a single LA-MC-ICPMS
zircon analysis. At least 60 useable ages are desirable for detrital samples to adequately
characterize major peaks of the age spectrum (Dodson et al., 1988) and the additional
analyses compensate for ages that are unusable due to low U, high common Pb, high
discordance or large age uncertainty. Detrital zircon analyses are rejected if they have
more than 10% uncertainty, 30% discordance or 5% reverse discordance. The low
concentration of
206
235
U relative to
238
U results in higher uncertainties for
Pb*/235U and
Pb*/207Pb* ages relative to 206Pb*/238U ages for younger samples. Therefore ages less
than ~1000 Ma are reported using the
206
207
Pb*/207Pb* ages.
206
Pb*/238U age and older ages are reported using
156
All analyses are plotted with 2σ or 95% confidence limits, while apparent ages of
individual zircon analyses are reported at the 1σ level (Table 2). Maximum depositional
ages are based on at least three ages that overlap at the 2σ level and are reported at the 2σ
level.
Means for igneous samples are weighted by the uncertainty but means for
maximum depositional ages are not.
The stated uncertainties (2σ) on the assigned
crystallization ages are absolute values and include contributions from all known random
and systematic errors. Random errors are included in the data tables. Systematic errors
(2σ) are as follows: JG052704-1, 0.84%; AP060504-B, 1.33%; AP062004-B, 0.96%;
JG062504-1, 1.00%; JG061805-1, 1.38%; JG061805-2, 1.90%; JG062305-1, 1.57%;
JG063005-1, 1.27%; AP052504-B, 1.15%; JG070605-1, 2.28%. Systematic errors are
not factored in the detrital zircon age spectra but are included in calculations of maximum
depositional age. Each samples location and age data is summarized in Table 1 and all
zircon U-Pb analyses are reported in Table 2.
All U-Pb plots, weighted average
calculations and discordia regressions were made using Isoplot 3.00 (Ludwig, 2003).
Igneous Zircon Analysis
Sample JG052704-1 is a biotite bearing quartz-monzonite with K-Feldspar
porphyroclasts to 1 cm across that intrudes the Jurassic mélange and low-grade
metasedimentary rocks at the southern edge of the Amdo basement. A cluster of 15
concordant zircons gives an average age of 117.4 ± 2.2 Ma (Fig. 2). A quartz-diorite,
AP062004-B, intrudes the Jgr1 granite (JG062004-1, 170.7 ± 3.2 Ma; (Guynn et al.,
2006)) in the central part of the Amdo basement and a cluster of 25 concordant zircons
157
results in an age of 111.6 ± 1.6 Ma (Fig. 2).
Small hypabyssal intrusions occur
throughout the Amdo basement as well as in some of the sedimentary rocks that surround
it. AP060504-B intrudes metapelites at the central-southern edge of the Amdo basement
and 16 zircons yield an age of 116.2 ± 2.0 Ma (Fig. 2). At the southern edge of the Amdo
basement on the west side of the highway, a series of hypabyssal dikes (JG062504-1)
cuts both Jurassic marine shale and overlying marble and a cluster of 12 zircons produced
an average age of 105.9 ± 2.2 Ma (Fig 2).
Detrital Zircon Analysis
Concordia diagrams for six detrital zircon samples are shown in Figures 3 and 4,
probability density functions (PDFs) are shown in Figure 5, and PDFs for the minimum
age groups are shown in Figure 6. The PDFs are only plotted for ages less than 1200 Ma
because there are fewer older ages and they do not typically occur in distinct groups, so
that the peaks are negligible. Included with the PDFs is an age spectrum based on our UPb dating of rocks within the Amdo basement, including orthogneisses (Guynn et al.,
unpublished), Jurassic granitoids (Guynn et al., 2006) and Cretaceous intrusions (this
study). The broad spread of ages around the Cambrian and earliest Neoproterozoic peaks
is a result of lead loss and young metamorphic growth in the upper-amphibolite facies
orthogneisses, which also results in the subdued peaks compared to those from the
younger igneous rocks. The maximum depositional ages and details of the age spectra
are presented here; lithology and stratigraphic significance will be discussed in a later
section.
158
Jurassic flysch
Sample JG061805-2, a sandstone from a sequence of thin turbidites (Fig. 9e,f),
yielded only a few very small zircons. Only 73 zircons were analyzed, 8 of which were
poor analyses and not included in the PDF; of the remaining 65, 26 were analyzed with a
25 µm laser spot diameter and the other 39 had to be analyzed with a 15 µm spot size.
The youngest are a group of 3 zircons that yield a mean age of 171.2 ± 14.5 (Fig. 6).
Another flysch sample, JG062305-1, comes from a strongly cleaved sequence of
sandstone and mudstone to the east along the road from Nagqu to Chamdo (just beyond
the mapped area). The youngest zircons in this sandstone are a group ~150 Ma; the
youngest 8 yielded a mean age of 148.5 ± 10.6 Ma (Fig. 6), substantially younger than
the other flysch sample. Both samples have a large group of zircons less than 300 Ma
and a large spread of ages between 300 Ma and 900 Ma that do not define specific peaks.
Red arenites and shale
Sample JG061805-1 is a red arenite dominated by a large, ~500 Ma peak which
encompasses 77% of the zircon ages. There is a small group at ~800 Ma and a group of
three zircons defines the maximum depositional age as 178.0 ± 16.2 Ma (Fig. 6). There
are two younger ages, ~154 and ~79 Ma, but these are single zircon ages and may be
affected by lead loss. Red arenite sample JG063005-1 has a much broader, subdued
group of ages from 400-520 Ma, but two sharp peaks at ~165 Ma and ~115 Ma; there is
also a small group of Triassic ages. The younger peak, represented by seven zircon ages,
159
provides a maximum age estimate of 115.7 ± 9.0 Ma (Fig. 6), similar to the ages
determined for the Cretaceous igneous intrusions. There is a single zircon with an age of
~11 Ma, but this zircon is small and has a U concentration over 3000 ppm, which
suggests that the young age is a result of lead loss due to radiation damage in the crystal.
This sandstone also contains a greater percentage of older zircons (43% over 900 Ma)
than the other samples, with groups around 850-1250 Ma and 1800-2000 Ma and a ~2500
Ma peak.
Sample AP052504-B has many similarities with both other red arenite
samples. It has a large Cambro-Ordovician peak and a significant ~175 Ma peak. It also
contains a small group ~800 Ma, a few Permo-Triassic ages and three zircon ages from
~105-115 Ma. The last group yields a maximum depositional age estimate of 108.4 ± 6.2
Ma (Fig. 6), also coeval with the Cretaceous magmatism. In general the age spectrum is
comparable to JG063005-1.
Two samples of thin sandstone interbeded with thicker shales were also collected, but
one did not yield any zircons and the second (JG070605-1) only yielded 26 small grains.
Two analyses resulted in young, overlapping ages, ~21 and ~24 Ma. However, both of
these grains have high U concentrations (1200 and 1680 ppm) and combined with their
small size (~30-35 µm) they are likely to have suffered radiation damage that resulted in
lead loss. The next two youngest ages are ~106 and ~116 Ma and when averaged provide
a maximum depositional age of 111 ± 8 Ma, but given the low number of analyses, this is
a very tentative constraint. The sample also contains two ages ~150 Ma, a few PermoTriassic ages, a ~480 Ma peak and a broad, ~750 Ma peak. There are two mid-Jurassic
ages, but these also have high U concentrations. In general, the age spectrum is similar to
160
that of the Cretaceous redbeds and suggests a similar source provenance and depositional
timing, though there are two few analyses for a definitive conclusion.
GEOLOGY OF THE AMDO AREA
Rock Types and Ages
The oldest rocks in the Amdo area are the gneisses and metasedimentary units of the
Amdo basement. The orthogneisses are Cambrian or older (Guynn et al., 2006; Xu et al.,
1985) and have Proterozoic Nd model ages (Harris et al., 1988b). The majority of the
basement consists of amphibolite facies orthogneiss with minor outcrops of paragneiss,
mafic garnet-amphibolite and migmatite (Fig. 8a), but there are also lower grade
metasedimentary units along the edges, which in the area mapped consist of marble,
amphibolite, schist, and quartzite. U-Pb analysis of detrital zircons from two quartzites
(AP061304-1 & JG062305-3) and a paragneiss (JG061504-4) indicates a Paleozoic
(maximum Ordovician) depositional age for the former and a possible Neoproterozoic
age for the latter (Guynn et al., unpublished data). The association of carbonates clastic
and pelitic rocks, together with the maximum defined ages, suggests that the protoliths of
the metasedimentary rocks are Paleozoic shelf deposits that are common throughout the
Lhasa and Qiangtang terranes and not Triassic-Jurassic forearc assemblages.
It is
difficult to estimate their thickness given the high degree of folding and deformation,
some of which probably occurred during Middle Jurassic burial and exhumation.
The Amdo gneisses are intruded by extensive granitoids that are ~185-170 Ma
(Guynn et al., 2006) and make up about 30% of the Amdo basement exposure (Fig. 7).
161
In addition, we have mapped small granitoid and hypabyssal intrusions within the
basement, some of which intrude the Jurassic granitoids (Fig. 7), and several of these we
dated as mid-Cretaceous. The hypabyssal intrusions are widespread but small, with
individual outcrops no more than several square meters in size. They consist of a finegrained groundmass with phenocrysts of plagioclase, hornblende and biotite, indicating a
shallow level of emplacement, and vary in composition from rhyolite to andesite. All the
hypabyssal dikes in the area are presumed to be mid-Cretaceous in age based on our
dating.
The Middle-Late Jurassic ophiolites exposed in the vicinities of the towns of Amdo
and Nagqu are assumed to be slices of the oceanic “basement” on which much of the
suture zone’s Jurassic flysch and mélange was deposited (Pearce and Deng, 1988). The
outcrops are only small pieces of an ophiolite sequence, generally associated with the
Jurassic flysch, and geochemical analysis suggests they represent different oceanic
environments, including a supra-subduction zone (Pearce and Deng, 1988). The only
outcrop examined by us consisted of completely serpentinized ultramafic rock.
The mélange and flysch in the map area consist of turbidites, alternating layers of
dark shale and fine-grained sandstone (Fig. 9e,f), or massive, dark shale. In some cases,
the sedimentary layering is well-preserved, but in general the rocks are transposed (Fig.
9g), strongly cleaved and poorly exposed, making it difficult to assign a thickness to the
unit. Other studies have indicated the flysch may be as thick as 5-6 km in places, but the
thickness and lithology are highly variable (Schneider et al., 2003; Yin et al., 1988).
These and similar units throughout northern Lhasa have sometimes been lumped together
162
as the “J-K”, indicating a poorly defined age that includes Jurassic and possibly
Cretaceous sedimentary rocks (Kidd et al., 1988; Schneider et al., 2003). Recent Chinese
regional geological maps have divided these rocks into multiple units of Triassic-Jurassic
age, including a Triassic-Jurassic and Jurassic mélange south of the Amdo basement
exposure (Pan et al., 2004). Our detrital zircon analysis of two samples indicates a
maximum Middle-Late Jurassic depositional age (~149 Ma and ~171 Ma) and suggests
they are not as young as Cretaceous nor as old as Triassic. These are younger than a “JK” maximum depositional age of ~270 Ma from the Lunpola basin just to the west
(Leier, 2005) but older than an Early Cretaceous maximum depositional age of ~125 Ma
for a “J-K” marine turbidite from ~300 km to the west near Nima (Kapp et al.,
unpublished). The PDFs for these two latter samples are shown with the samples from
the Amdo area in Fig. 10.
The 200-300 Ma ages suggest a provenance to the north, as Leier (2005) proposed
for the Lunpola unit based on provenance and southward paleocurrent data. Granitoids of
these ages are found in the Kunlun batholith (Harris et al., 1988b), along the Jinsha suture
(Roger et al., 2003), and in the Qiangtang terrane itself (Kapp et al., 2003b). Sandstones
of the Songpan-Ganzi flysch complex also contain abundant detrital zircons with 200-300
Ma ages (Weislogel et al., 2006). The cluster of ages around 170 Ma for JG061805-2 is
consistent with the ages of the Jurassic granitoids intruding the Amdo basement. There
are a few ages similar to those of the Jurassic granitoids in JG062505-1 & the Lunpola
sample, but not a distinct peak, even though the maximum depositional age for the former
(~149 Ma) is younger than JG061805-2. Igneous rocks with ages ~150 Ma are reported
163
from the central Qiangtang terrane (Kapp et al., 2003a; Kapp et al., 2005) and central
Lhasa terrane (Murphy et al., 1997; Volkmer et al., accepted), though the latter is
precluded as a source if the marine sandstones were deposited on the Qiangtang margin
during the Jurassic. The group of Cretaceous ages (~125 ma) in the Nima sample
indicates depositional and structural variations along strike in the Bangong suture zone.
The abundance of older zircons in the marine sandstones and the many peaks (a result of
only a few zircons per peak) may indicate significant recycling of sedimentary rocks as a
source for the Jurassic marine rocks. In a more detailed study of the Jurassic flysch by
Schneider et al. (2003), they concluded that the Middle-Late Jurassic Qiangtang margin
was steep, with olistrosomes and braided rivers feeding deltas on top of carbonate
platforms, while the Lhasa margin had a shallow continental slope.
A wide variety of conglomerates, red sandstones and multi-colored shales crop out on
the south side of the Amdo basement. It is difficult to put these in a stratigraphic context
due to the poor exposure and limited extent of each unit. No complete, undeformed
section is exposed within the mapped area. Detrital zircon analysis of several different
samples reveals that the units are no older than mid-Cretaceous, but the data are not
sufficient to define the relative ages of the units.
Clast-supported conglomerates are common along the southern edge of the basement
exposure (Fig. 7) and we divided these into two separate units. One, Kc1, is composed of
rounded to sub-rounded clasts of limestone, marble and carbonate breccia, typically
pebble to cobble sized, intercalated with thin beds of coarse, pebbly sandstone. The
sandstone and conglomerate matrix contains coarse grains of quartz. Another section of
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this unit also contains quartzite and occasional red arenite and granitoid clasts. These
clasts are also rounded to sub-rounded, typically cobble-sized, but also with pebbles and
a few small boulders. The second unit, Kc2, is well-exposed in a stream-cut just east of
the Lhasa-Golmud highway (Fig. 9a). It consists of alternating meter thick layers of
conglomerate with rounded to sub-rounded, cobble and boulder sized clasts and several
centimeter thick sandstone layers with sub-angular and very-coarse grains.
Clast
composition consists of almost every rock type in the Amdo area: marble, limestone,
quartzite, gneiss, mafic amphibolite, granitoid, volcanic-hypabyssal rock, red arenite and
dark shale. Distinctly missing are mafic or ultramafic clasts of ophiolite origin. Welldefined clast imbrication indicates a southward flow direction, assuming the beds are not
overturned (Fig. 9b). While we have no direct age control, the presence of clasts of
hypabyssal intrusive rocks and red arenites suggests a maximum age of ~115-105 Ma for
Kc2.
One particular section of note, located just west of the road to Nyainrong, is a hill
with four different, thin layers of north-dipping limestone interbedded with Kc2 (Fig.
11a). The limestone forms ~2-5 m thick layers, whose thickness varies along strike
(Fig.11b), and is separated by conglomerate layers ~200 m thick. The entire outcrop is
~2.5 km wide. The limestone is cracked and brecciated and does not appear to contain
any fossils. The association of the limestone with a coarse conglomerate suggests a
landslide deposit, but the continuity and thin nature of the beds does not support that
hypothesis. Instead, we cautiously interpret these conglomerates as alluvial fan deposits
at the edge of a warm sea, where changes in eustatic sea level, subsidence or depositional
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rate resulted in occasional transgressions that deposited thin carbonate beds.
The
irregular thickness of the units could be due to a change in water depth between fan lobes
(Fig. 11b). In particular, given that the detrital zircon ages from the red sandstones
suggest deposition in the mid-Cretaceous, we propose that these carbonates are the
northern-most expression of the Aptian-Albian limestones that occur as much thicker
deposits further south and west of Amdo and are conspicuously missing in this area.
Thus, they may mark the northern part of a foreland basin formed by the advancing fold
and thrust belt resulting from the Lhasa-Qiangtang collision.
The red sandstones crop out widely across the southern region of the map area west
of the road to Nyainrong (Fig. 7). Some exposures consist of red or occasionally tan,
medium to coarse-grained sandstone interbedded with red, brown and grey shale and
siltstone. In some cases, the sandstone beds are thin (< 10 cm) and intercalated with
thicker (to 20 cm) mudstone beds, but in general sandstone is dominant (Fig. 9c). The
sandstone includes small-scale crossbeds, channel fills and fining upwards sequences.
There are also pebbly layers with sub-angular to rounded, matrix-supported clasts with
lithologies that include granite, gneiss, red sandstone and quartzite. More common
exposures are red and occasionally tan, thickly bedded (10-50 cm), medium-coarse
grained and pebbly sandstone with only occasional, thin siltstone beds. The pebbles are
matrix supported, sub-angular to sub-rounded, and include granite, gneiss, marble and
quartzite. Crossbeds are very common (Fig. 9d). While the coarser, sandier lithologies
may represent a different time of deposition, mapping indicates the two occur in close
proximity, and without any control on relative stratigraphy, we group them together as
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unit Kr. Without any control on the relative stratigraphy of the different sandstone and
siltstone exposures, we group them all together as unit Kr.
Adjacent to the best exposure of Kc2 is a thin section of alternating red sandstone and
red siltstone (Fig. 9c). The beds are subvertical to steeply south-dipping and crossbeds,
channels, fining upwards sequences and mudcracks all indicate stratigraphic up to the
north. Sample JG061805-1 from this outcrop was analyzed for detrital zircon ages. A
single zircon yielded a ~154 Ma age, but a more robust maximum depositional age is
~177 Ma based on three grains. This sample has a predominant ~500 Ma peak indicative
of the Cambrian Amdo orthogneisses, which suggests they were exposed at the time of
deposition. A sample of red sandstone (AP052504-B) collected along strike to the east
from a cross-bedded section without substantial mudstones also has the ~500 Ma peak
and a well-constrained minimum age of ~108 Ma. Just south of the Lhasa-Golmud
highway, another outcrop of red sandstone consists of medium-coarse grained,
occasionally pebbly and K-feldspar rich, layers contain cross-beds up to ~1/2 m in height.
The beds are south-dipping but overturned and up-section (to the north) they grade into
coarse-grained sandstone and sub-angular pebble conglomerates and than to
conglomerates with sub-rounded, cobble (3-6 cm) size clasts.
Clast composition is
limestone, quartzite, marble and red sandstone. Sample JG063005-1 from this section
yielded a robust maximum depositional age of ~115 Ma. It does not have a well-defined
peak at ~500 Ma, though there are some zircons around that age.
In the southwestern part of the mapped area, visible just north of the Lhasa-Golmud
highway, there is a section of light-colored shales (Ks). The shales are predominantly
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white and light-tan, though there are also red, purple and green beds. In the lower part of
the section, the shale layers (~1/2 – 2 m thick) are interrupted by thin (few cms), tabular
beds of fine-grained gray sandstone.
The number, thickness and coarseness of the
sandstone layers increases up-section and near the top there are matrix-supported
conglomerates with pebble to cobble sized clasts filling in channels. Clast composition
includes limestone, marble, quartzite, red sandstone and conglomerate. From a distance
the section appears homogeneously tilted to the north, but closer inspection reveals tight,
recumbent folds within the unit (Fig. 9h). Detrital zircon analysis did not provide a
reliable maximum depositional age for the unit, but suggests a similar age as the red
sandstones, which is also supported by the comparable clast composition in the channel
fills.
The coarse nature of the clastic sedimentary rocks, including conglomerates
indicative of an alluvial fan, and the terrestrial deposition suggests a foreland basin
setting for their deposition. In that context, the simplest interpretation of the stratigraphy
is a coarsening upward sequence, from the shales and siltstones interbedded with
sandstones, to the coarse-grained redbeds and than to the conglomerates. The different
conglomerates could than be interpreted as representing an unroofing sequence of the
Amdo basement, Kc1 as the oldest and Kc2 as the youngest, with the Kc1 unit containing
quartzite, granitoid and red arenite clasts representing a transition between the two.
However, it is unlikely the carbonate clasts of Kc1 indicate the Amdo gneiss was
regionally unexposed, since granite and gneiss pebbles appear in the red arenites. The
overall lithology of these Cretaceous sedimentary rocks is quite similar to that of the
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Early Cretaceous rocks of the Duba formation ~200 km west-southwest. The Duba
formation contains abundant red sandstone interbedded with subordinate colored shale
and also has abundant conglomerates with clasts of limestone, quartzite, granite and
volcanic rocks (Schneider et al., 2003).
The Duba formation fines upward, with the
number and thickness of shale beds increasing, and the upper part consists of dark shales
that may signal the beginning of the Aptian-Albian sea transgression (Schneider et al.,
2003).
There are two tufa deposits associated with active hot springs just south of the Amdo
basement (Fig. 7) that indicate active faults, though no clear sign of recent faulting was
observed. North of Nyainrong, a thin (< 10 m) carbonate conglomerate rests on top of a
Jurassic granitoid and probably represents Neogene basin fill in a small graben created by
a south-dipping normal fault at the northern edge of the Amdo basement.
Structure of the Amdo Basement Region
The entire Amdo orthogneiss exposure has experienced upper-amphibolite facies
metamorphism with regional temperatures of ~700°C and pressures ~9-11 kbar (Guynn
et al., Appendix B). Thermochronologic data along the Lhasa-Golmud highway reveal
that peak metamorphism occurred ~180-185 Ma and that the entire basement had cooled
to ~300°C at ~165 Ma (Guynn et al., 2006). Along the southwest contact, garnet-kyanite
schist adjacent to the gneiss provides P-T conditions of 7-9 kbar at ~600°C and garnetstaurolite-biotite-muscovite schist farther to the east suggests a similar grade (Guynn et
al., Appendix B). The abrupt change in grade across the contact indicates a fault between
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the gneiss and the metapelites. Furthermore, the limited outcrop of metasedimentary
rocks, representing slivers of lower-amphibolite or upper-greenschist facies rather than a
full Barrovian sequence, requires that much of the stratigraphic section has been
tectonically thinned or cut out.
The gneisses in the Amdo basement display a widely varying degree and orientation
of foliation and lineation development. Most often they have a well-defined foliation and
banding due to separate felsic and mafic mineral assemblages, but occasionally are only
slightly foliated. S-tectonites are most common. In general, the strike of the foliations
and the trend of the lineations are approximately northward, but there is not a consistent
pattern and they can change considerably over short distances. Detailed measurements of
foliations and small-scale fold axes in two different areas in the interior ( stereonets (a)
and (b) in Fig. 7) show that the D1 flattening fabric has been folded in a phase of D2
deformation. In the western outcrop (a), the dip of the foliation is generally steep and the
calculated fold-axis plunges slightly to the north.
At the eastern outcrop (b) the
orthogneiss is highly folded and the fold-axes plunge moderately to the south and a
calculated fold-axis also plunges 45° to the south. At an outcrop south of (a), the
calculated fold axis trends to the east of north with a moderate plunge (stereonet (c), Fig.
7), but here foliations may be affected by the contact with the marble. The short-distance
variability in fold-axes, foliations and lineations suggests a third phase, D3, of ductile
deformation.
Near the southern boundaries with the metamorphic and sedimentary rocks,
particularly in the southwest, the foliation strike tends to align itself with the contact.
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The foliation in both the gneiss and metasedimentary rocks at the contact is typically
steep and fold axes plunge moderately to the northwest (stereonet (d), Fig. 7). The
garnet-kyanite schist dips steeply to the northeast and contains microfabrics indicating
top to the southeast sense-of-shear (Fig. 12e-g). At an outcrop along the Lhasa-Golmud
highway (stereonet (e), Fig. 7), foliation dip and lineation trend is generally to the
northwest, as is the fault dip and striation trend of small faults within the orthogneiss (Fig
8b,c).
Small-scale asymmetric folds and offset dikes indicate top to the southeast.
Further to the east, the orthogneiss is in contact with quart-mica-chlorite schist along an
E-W striking fault that dips 62° to the north with mineral slip fibers indicating top to the
south motion. In the next drainage valley to the east, folded schist layers define a fold
axis with a shallow plunge to the east (stereonet (i) in Fig. 7; 76°→33°) and mica folding
and cleavage in the schist indicate a top to the north sense of shear (Fig. 12h). Schist and
marble along the northeast contact with the Jurassic granite also parallel the contact and
define a northwest plunging fold axis (stereonet (h) in Fig. 7; 305°→30°). Overall the
fabric and structures indicate southward directed thrust movement along north dipping
faults (Fig. 11d).
The intrusion of the granitoids into the gneisses was coeval with the high-grade
metamorphism of the gneisses. However, the granitoids do not display any gneissic
fabric or foliations with the exception of narrow shear zones. This indicates that either
the high-temperature metamorphism and fabric development began some time before the
granitoids intruded the gneisses or that much of the gneissic fabric was developed prior to
the Jurassic metamorphism, possibly during Cambro-Ordovician orogenesis associated
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with magma emplacement (Coward et al., 1988). In the southeast corner of the study
area, the Jgr1 granitoid varies from undeformed to mylonitic zones 100s of meters wide.
S-C fabrics are not well-developed and no sense of shear was obtained. Further north
along the road to Nyainrong, a zone several tens of meters wide in the same Jgr1
granitoid has a proto-mylonitic development with flattened and stretched quartz grains
and bookshelf-faulted K-feldspar phenocrysts that indicate a top-to-the-north sense of
shear (Fig.12a-c). In addition, an otherwise undeformed Jurassic granitoid near the shear
zone contains recrystallized quartz grains (Fig. 12d).
The conglomerates and sandstones at the southern edge of the Amdo basement
generally dip northward at moderately steep angles. Redbeds farther south in the vicinity
of the Jurassic (?) limestone dip steeply to the south and are overturned. Folds in the
shales and sandstones west of the Lhasa-Golmud highway have east-west fold axes with
shallow plunges (stereonet (j); Fig. 7), indicating north-south compression. With the
exception of the southern orthogneiss outcrop by the Lhasa-Golmud highway, fault
surfaces were not observed due to the lack of exposure, but the relationships suggest that
gneisses and metasediments are thrust southward over the Jurassic and Cretaceous
sedimentary rocks. Marble is structurally above sedimentary rocks along the southern
area and windows of dark shale, probably the Jurassic mélange, occur beneath the marble
in the western mapped area. To the west of the Lhasa-Golmud highway, Cretaceous
hypabyssal intrusions (JG062504-1) cut through the shale and the overlying marble,
indicating the thrust was inactive by ~106 Ma. Along the road to Nyainrong, marble
overlies a Cretaceous granitoid (~117 Ma; JG052704-1) and we interpret this as a fault
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contact based on 1) the flat nature of the contact (Fig. 11c), 2) the lack of a metamorphic
aureole in the limestone, and 3) the occurrence of brecciated marble float near the contact
between the two. There is also brecciated and chloritized orthogneiss along the contact
between the granitoid and the orthogneiss in the same area. Between the Lhasa-Golmud
highway and the road to Nyainrong, Jurassic (?) limestone sits above the red arenites
along flat or south-dipping contacts and at one locality a conglomerate composed of
subangular-subrounded limestone and limestone breccia clasts cropped out between the
two. We interpret the contact between the two units as a thrust fault.
DISCUSSION
Following Guynn et al. (2006), we regard the age of Lhasa-Qiangtang collision as
Early Cretaceous and not related to the Middle-Jurassic metamorphism. This conclusion
is based on 1) the Early Cretaceous cooling history, 2) the development of Cretaceous
thrust belts in northern Lhasa, including Amdo, as a proxy for collision, 3) the
continuation of marine sedimentation during the Late Jurassic and even Early Cretaceous
near Nima, 4) the change to nonmarine sedimentation in the Early Cretaceous, and 5) the
paleomagnetic data which indicate Lhasa traveled northward during the Late Jurassic and
Early Cretaceous.
Obduction for some of the ophiolite exposures is Middle-Late
Jurassic, but obduction often precedes collision (e.g. the Troodos and Oman ophiolites
which have preceded African-Eurasian collision).
We also adopt the view of Coward et al. (1988) that subduction of the Meso-Tethys
was northward based on southward directing thrusting, though this is more tenuous.
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Furthermore, there may have been additional arcs and back-arc basins that had
subduction zones with southward subduction.
Given northward subduction at the
southern Qiangtang margin, we also regard the Amdo basement as part of the Qiangtang
terrane prior to Lhasa-Qiangtang collision since it was metamorphosed at the Bangong
subduction zone in the Middle Jurassic.
Jurassic Tectonic Evolution of the Bangong Suture at Amdo
The presence of transposed and highly deformed turbidites and deep marine
sedimentary rocks and their association with olistrosomes and small, dismembered slices
of ophiolites in a “Cordilleran” style of obduction (e.g. Moores, 1982; Moores et al.,
2000) are indicative of an accretionary wedge complex, possibly in a back-arc setting,
that involved offscraping or underplating of oceanic crust and its incorporation into the
wedge complex (Gray and Foster, 1998; Spaggiari et al., 2004). This precludes a single,
ophiolite thrust sheet (e.g. Girardeau et al., 1984), a conclusion reinforced by the lack of
any mafic or ultramafic rocks in the Cretaceous clastic deposits. Some of the Jurassic
mélange and ophiolite fragments show the development of oxidized and silicified
horizons followed by marine transgressions, demonstrating that they were above sealevel in the Late Jurassic and than subsided again in the latest Jurassic (Girardeau et al.,
1984; Leeder et al., 1988), indicating sufficient shortening for the accretionary wedge to
become subaerial at times. The ophiolite fragments themselves have a geochemistry
suggestive of a supra-subduction zone (SSZ) setting (Pearce and Deng, 1988), a young,
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hot environment that would also explain the presence of a metamorphic sole and the lack
of an accreted island arc (Hawkins, 2003; Zhou et al., 1997).
The specific tectonic event that led to the burial, metamorphism and initial
exhumation of the Amdo basement is difficult to assess given the overprinting of later
tectonics and basin development. One possibility is that the Amdo basement was a piece
of the Qiangtang terrane that rifted off, creating a back-arc basin, and then collided with
the Qiangtang margin, possibly as a result of an island arc arriving at the trench, closing
the back-arc basin and burying the Amdo basement (Fig. 13a). This scenario provides a
supra-subduction zone setting for the generation of the different types of ophiolite within
the Bangong suture zone (Pearce and Deng, 1988). The island arc complex itself would
have been underthrust and tectonically eroded or buried by subsequent marine
sedimentation, as would much of the Amdo basement and any continental arc that
developed on the basement (Fig. 13b).
This collision also would have resulted in
ophiolite “obduction” and incorporation into the Jurassic mélange and overthrusting of
the mélange onto the Amdo basement and Qiangtang terrane. Regional geologic maps
indicate large areas of Triassic sedimentary rocks just north of the Bangong suture (Pan et
al., 2004) which may represent areas of elevated topography and erosion on the southern
Qiangtang margin due to this collision. Subsequent exhumation, possibly by extension,
would have thinned the thickened crust and dropped unmetamorphosed sedimentary
rocks, including the mélange and ophiolites, against the Amdo basement (Fig. 13c). A
new convergent boundary and accretionary wedge would than be re-established (Fig.
13d). During the Early Cretaceous, continental collision with the Lhasa terrane resulted
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in the Jurassic and Early Cretaceous marine rocks being thrust onto the Lhasa terrane’s
passive margin (Fig. 13e). The Late Jurassic – Early Cretaceous Meso-Tethys Ocean
also may have contained additional small island arc complexes, rifted continental
fragments, and back-arc basins that could account for the widespread Jurassic flysch and
ophiolite fragments to the south of the Amdo basement, the rapid changes in thickness
and lithology of the Jurassic flysch (Schneider et al., 2003), the continuation of marine
sedimentation into the Cretaceous near Nima, and the lack of Late Jurassic arc
magmatism. The closure of the Meso-Tethys in this region may be analogous, on a
smaller scale, to final closure of the Mediterranean Sea, resulting in multiple sutures,
ophiolites of different ages and formational settings, and a variety of sedimentary basins
within a wide “suture” zone.
Cretaceous Tectonic Evolution of the Bangong Suture at Amdo
A detailed cross-section across the area is difficult to construct due to the limited
exposure, poor age constraints on the sedimentary units involved, and late Tertiary strikeslip and normal faulting. In addition the change from flat to steeply dipping contacts and
different lithologies along strike indicates corresponding changes in structure. However,
a reasonable regional model can be developed by taking into account the following
constraints:
1) Regional ophiolite obduction during the Middle-Late Jurassic, the occurrence of
ophiolite fragments to the north and south of the Amdo basement, and continued marine
deposition in the Late Jurassic.
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2) Exhumation of the Amdo basement to upper crustal levels (< 4 km) from ~130 Ma
to ~115 Ma.
3) Undeformed Jurassic granitoids except for shear zones that indicate thrusting at
mid-crustal levels.
4) The presence of high-grade rocks (> 600°C) across the entire Amdo basement.
5) Deformed red sandstones as young as ~108 Ma and the presence of deformed
alluvial fan conglomerates approximately the same age.
6) A ~117 Ma granite cut by a southern fault and a southern fault cut by ~106 Ma
intrusions.
7) The following contact relationships from south to north: Metamorphic rocks thrust
over unmetamorphosed sedimentary rocks; upper amphibolite facies gneisses thrust over
lower-middle amphibolite facies metapelites; 50 km width of the gneisses;
unmetamorphosed sedimentary rocks only a few kilometers north of the gneisses (and
possibly in contact with the gneisses in an unmapped region of the Amdo basement).
8) Unmetamorphosed Paleozoic sedimentary rocks are not involved in the thrust belt
south of the Amdo basement.
9) The Amdo basement is thrust over Cretaceous nonmarine sediments in the west,
but to the east these rocks are “missing” and the gneisses are in contact with Jurassic
marine rocks.
In Figure 14a, we start with an idealized stratigraphy composed of basement, a thin
cover of metamorphosed sedimentary rocks, a thin section of Paleozoic and Triassic
sedimentary rocks and a thick section of Jurassic flysch and mélange. The northern end
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of this undeformed cross-section could be significantly different but not affect the
resulting model, as long as the basement is buried to > 10 km. Also, we do not include
the initial thickening of the basement due to thrust faulting as indicated by the granitoid
mylonites. The initiation of the thrust belt places a long section of basement on top of the
metasedimentary rocks in a flat-on-flat relationship (Fig. 14b). The second step involves
thrusting the basement over the rest of the section and on top of the Jurassic strata, taking
a horse of the metasedimentary rocks with it (Fig. 14c). We also show the development
of a foreland basin, the erosion of the frontal part of the thrust sheet and the start of the
thrust sheet overriding the foreland basin. This step of thrust belt development occurs
~130-115 Ma based on the K-feldspar 40Ar/39Ar cooling data for the Amdo basement and
establishes the long flat of basement rocks. The initiation of a thrust ramp within the
weak Jurassic unit results in the northward tilting of the gneiss-metasediment flat-on-flat
thrust, thrusting of the Jurassic flysch over the Cretaceous clastic sedimentary rocks of
the foreland basin and folding and thrusting within the Cretaceous rocks (Fig. 14d). This
is essentially the relationship seen in the eastern part of the mapped area (Fig. 7).
Another ramp in the Jurassic could result in a fault-bend anticline that structurally
elevates the Jurassic-over-Cretaceous fault, exposing the Cretaceous sedimentary rocks
(Fig. 14e), as seen in the eastern part of the mapped area (Fig. 7).
The total amount of shortening in this model is ~115 km. Assuming a thrust belt
development from ~130 Ma to ~100 Ma, this is a shortening rate of ~3.8 km/Myr, or 3.8
mm/yr, a typical rate for thrust belts. This model does not take into account intrabasement thickening or additional folds and thrusts within the Jurassic and Cretaceous
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sedimentary units, which would increase the amount of shortening. The shortening
calculation is only an approximate value based on a simplified model of the Amdo
basement area, but does suggest a typical fold and thrust belt setting for exhumation of
the Amdo basement with a reasonable amount of deformation.
The Amdo basement region provides a record of continuous Early Cretaceous
shortening in the Bangong suture zone. The basement cooling, thrust faulting, and
folding of nonmarine sedimentary rocks in the Aptian-Albian does not support an
extensional setting for the Bangong suture zone at this time as proposed by Zhang et al.
(2004) and Zhang (2004). We view the basement, which is located at the northernmost
edge of the suture zone, as a hinterland to the northern Lhasa fold and thrust belt which
developed in the Late Cretaceous and Early Tertiary farther south (Murphy et al., 1997;
Volkmer et al., accepted).
The formation of a foreland basin south of the thrust belt
(e.g. Kapp et al., 2005; Murphy et al., 1997) resulted in a mid-Cretaceous seaway and the
deposition of Aptian-Albian limestone in deeper areas and nonmarine clastic sediments in
the north near the leading edge of the thrust belt. The continued development of the
thrust belt eventually incorporated the early foreland basin, including the Aptian-Albian
limestone. Thus, shortening in the Lhasa terrane occurred throughout the Cretaceous,
leading to significant underthrusting of the Lhasa terrane beneath the Qiangtang terrane
(Kapp et al., 2003a).
The Lhasa terrane appears to have experienced simultaneous foreland basin
development in the north and south during the mid-Cretacous, the latter due to the LhasaQiangtang collision and the former due to a Gangdese retroarc fold and thrust belt (Kapp
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et al., submitted; Leier et al., in press). Two facing mountain belts that “met” in the
middle of the Lhasa terrane in the Late Cretaceous would create a significant area of
thickened crust with a decreased topographic gradient towards the center, which would
reduce the erosion rate and provide an intermontane setting to accumulate sediment,
resulting in an elevated, low relief region prior to the Indo-Asian collision. The lack of
denudation after Cretaceous crustal thickening is supported by the Early Cretaceous age
of the K-Feldspar 40Ar/39Ar data (Guynn et al., 2006). This scenario for the Lhasa terrane
is similar to that proposed by Tapponnier et al. (2001) for the Tibetan Plateau, except that
they have the Lhasa terrane rising in the Eocene, after the Indo-Asian collision. Our
study contributes to the recent work promoting substantial crustal thickness in Tibet prior
to the Indo-Asian collision and underthrusting of the Lhasa terrane beneath the Qiangtang
terrane.
CONCLUSIONS
The Bangong suture zone is the sight of protracted orogenesis and deformation
throughout the mid-late Mesozoic as revealed by the Amdo basement and associated
sedimentary rocks. The lack of an arc due to Meso-Tethys subduction is likely a result of
tectonic underthrusting and subduction erosion, intra-oceanic island arcs and sediment
burial prior to the Lhasa-Qiangtang collision.
Accretionary wedges and mélange
complexes probably developed in different settings at different times, resulting in
widespread, extended and varied Jurassic deposition, ophiolite obduction, and
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deformation. Marine deposition in the Amdo area continued into the latest Jurassic as
revealed by detrital zircon age groups of ~171 Ma and ~149 Ma in marine sandstones.
The Cretaceous collision between the Lhasa and Qiangtang terranes exhumed the
Amdo basement to upper crustal levels along southward verging thrust faults. Initial
shortening in the Early Cretaceous thickened the basement itself as revealed by mylonitic
and proto-mylonitic shear zones, though the displacements were not substantial enough to
juxtapose distinctly different metamorphic grades within the basement.
Nonmarine
deposition of coarse clastic sediments occurred in the mid-Cretaceous as revealed by
detrital zircon populations of 105-115 Ma, contemporaneous with marine limestone
deposition to the south and southwest. Thrust faulting at the southern edge of the Amdo
basement continued until at least the latest Early Cretaceous based on a ~117 Ma
granitoid cut by a thrust fault and the deformation of the mid-Cretaceous sedimentary
rocks. A simplified regional cross-sections indicates moderate shortening on the order of
~100 km over ~30 Myr; this suggests ~100 km of the lower crust of the Lhasa terrane
being thrust beneath the Qiangtang terrane.
The lack of denudation of the Amdo
basement after mid-Cretaceous shortening indicates that most of the crustal thickening
generated during the Early Cretaceous has persisted until the present day.
ACKNOWLEDGMENTS
This work was supported by NSF grant EAR-0309844 to P. Kapp and student grants
to J. Guynn sponsored by The Geological Society of America, ChevronTexaco,
ExxonMobil and the Peter J. Coney Fellowship. We would like to thank Alex Pullen and
181
Ross Waldrip for assistance in the field and Jen Fox for lab work. The manuscript
benefited from conversations with Pete DeCelles, Ross Waldrip, Alex Pullen and John
Volkmer.
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FIGURE CAPTIONS
Figure 1. Regional geologic map of southern and central Tibet based on Kapp et al.
(2005). Inset shows location of map relative to India and China. Thrusts shown in red
are related to the Lhasa-Qiangtang (L-Q) collision; those in blue are part of the Gangdese
retro-arc fold and thrust belt. GCT = Great Counter Thrust.
Figure 2. Pb/U concordia plots, weighted averages and probability density plots (PDFs)
for Amdo Cretaceous igneous samples. Ellipses in gray on the concordia plot are not
used in the calculation of average ages. Rel. Prob. = Relative Probability. The PDFs are
scaled to all have the same height. The concordia for AP062004-B does not show spot
20C (very high uncertainty) or 16C (slightly discordant & Ordovician) and the concordia
for JG062504-1 does not show spot 1 (discordant Paleoproterozoic) or 4C (discordant
Paleozoic).
189
Figure 3. Pb/U concordia plot of detrital zircon analyses of the three red sandstone
samples. Ages less than ~1000 Ma are based on 206Pb*/238U apparent ages, those older
than ~1000 Ma are 206Pb*/207Pb* apparent ages.
Figure 4. Pb/U concordia plot of detrital zircon analyses of the two Jurassic flysch
samples and a sandstone from the Ks shale unit. Ages less than ~1000 Ma are based on
206
Pb*/238U apparent ages, those older are 206Pb*/207Pb* apparent ages.
Figure 5. Probability density for all the Amdo detrital zircon samples. The curves are
only shown to 1200 Ma; older ages are sparse and generally do not occur in groups so
that the peaks are negligible compared to the younger ages. “n” is the number of zircons
used in the curves; in parenthesis is the total number of good analyses. The curves have
been normalized so that each has the same area.
Figure 6. Probability density functions for the zircon analyses of each sedimentary
sample used in the calculation of the maximum depositional age. Sample JG070605-1 is
not shown because the number of ages is too small for a robust determination. Average
ages and uncertainty are not weighted.
Figure 7. Geologic map of the Amdo basement and southern sedimentary rocks.
Structural measurements are plotted on lower hemisphere equal-area stereonets. Areas
outside of the mapping limit are based on observations from a distance, satellite photos
190
and the geological map of Pan et al. (2004). Sample numbers are those mentioned in the
text; for locations of dated Jurassic granitoids, see Guynn et al. (2006).
Figure 8. a) Layered Amdo orthogneiss at a road cut on the Lhasa-Golmud highway
(location of samples PK97-6-4-1A and PK97-6-4-1B). Dark rocks within the orthogneiss
are garnet-amphibolite pods. A hammer for scale is located between the two lower
amphibolite pods. b) Small faults within the orthogneiss at the same location; see
stereonet plot on map (stereonet (e), Fig. 7). c) Close-up of a fault surface.
Figure 9. (a) Kc2 conglomerate near sample location JG061805-1. Beds are dipping 55°
to the north (left in the photo). Hammer circled for scale. (b) Imbrication in the Kc2
conglomerate at the same location. Photo is taken at an angle such that the top and
bottom edges are parallel to bedding; “D” is down and “N” is north (horizontal). Flow
direction is to the right which is south after removing tilt and assuming the beds are not
overturned. (c) Sequence of sandstone and siltstone just south of the conglomerate;
location for detrital zircon sample JG061805-1. North, toward the conglomerate, is left.
(d) Cross-bedding in red sandstone just north of the Lhasa-Golmud highway. (e)
Sequence of Jurassic turbidites next to the Lhasa-Golmud highway; location of detrital
zircon sample JG061805-2. (f) Close-up of the thinly bedded sandstone and shale. (g)
Transposed bedding in the Jurassic flysch. (h) Tight fold in the shale unit Ks.
191
Figure 10. Probability density plot for Jurassic flysch samples from Amdo, the Lunpola
basin, and Nima. The Lunpola curve is sample LNPLA from Leier (2005) and the Nima
curve is sample 7-11-05-2 from Kapp et al. (unpublished). “n” is the number of zircons
used in the curves. The curves have been normalized so that each has the same area.
Figure 11. (a) A view of Kc2 conglomerate just west of the road to Nyainrong showing
the thin beds of limestone located within the conglomerate. View is toward the northeast
and beds are dipping north ~64°. Houses and yaks in lower left for scale. Pm = marble;
Ps = schist and other metasedimentary rock. (b) Close-up view of one of the limestone
beds. Line shows the curved boundary between the limestone and the conglomerate.
Knobs at the top of the hill in the background are also limestone. Alex Pullen to left in
foreground for scale. (c) View looking south at marble (Pm) overlying Cretaceous
granite (Kgr) along a flat fault contact. Location is just west of the road to Nyainrong.
(d) Orthogneiss (gn) thrust south over schist (Ps) along the southern basement boundary.
View is to the northwest. Picture is taken standing on Jurassic flysch (Jm). Location is
~10 km west of the edge of the map in Fig. X along the road east from Nagchu (road is in
foreground).
Figure 12. (a, b & c) Bookshelf faulting of K-feldspar in the Jgr1 granitoid in a shear
zone along the road to Nyainrong (see Fig. 7). South is to the right. (d) Crossed-polars
photomicrograph of quartz recrystallization in an otherwise undeformed granodiorite next
to the shear zone. (e) Pressure shadows around garnet in a crossed-polars
192
photomicrograph of garnet-kyanite schist JG060904-13 indicating top to right sense of
shear. Dip direction (northeast) is to the left. (f) Plain light photomicrograph of another
garnet in JG060904-13; same orientation. (g) Plain light photomicrograph of microfolds
in JG061904-13 outlined by graphite. (h) Plain light photomicrograph of quartzmuscovite schist from metasedimentary rocks on the southeast border of the gneiss.
Microfolds are outlined by quartz and muscovite and indicate top to the right sense of
shear. Dip direction (north) is to the left.
Figure 13. One possibility for the evolution of the southern Qiangtang margin during the
Mesozoic. Not drawn to scale. (a) As the Meso-Tethys ocean develops during the
Triassic to Early Jurassic, rifting of the Amdo basement from southern Qiangtang creates
a back-arc ocean basin. Another back-arc basin to the south of Amdo involves a small
island arc and the creation of oceanic crust in a supra-subduction zone (SSZ). Fore-arc
flysch is deposited on the slopes of the island arc and the Amdo basement. (b) During
the Middle Jurassic, compressive stresses cause the Amdo basement to collide with the
Qiangtang terrane. Compressive stresses may be a result of island arc collision or
shallowing of the downgoing oceanic slab, which may also enhance tectonic erosion.
The Amdo basement and the associated continental arc are tectonically buried and
metamorphosed. Jurassic flysch, including mélange with ophiolite fragments, is
obducted onto the margin of Qiangtang and the Amdo basement. (c) The Amdo
basement is exhumed during the Late Jurassic, coeval with magma emplacement. In this
scenario, exhumation is accomplished via normal faulting across the newly accreted
193
Qiangtang margin, possibly in response to a thickened crust and a steepening slab. A
normal fault drops down accreted mélange and Qiangtang supracrustal assemblages
against the Amdo basement. The accretionary wedge at the convergent boundary is reestablished. (d) During the Late Jurassic – Early Cretaceous, the remaining MesoTethys Ocean is consumed. Additional small island arcs, back-arc basins or small
terranes (not shown) may exist and contribute to the additional ophiolite mélange and the
lack of arc magmatism south of the Amdo area. (e) During the Early Cretaceous, the
Lhasa terrane collides with and is underthrust beneath the southern Qiangtang margin,
resulting in obduction of Jurassic flysch and mélange onto the northern Lhasa terrane.
AB = Amdo basement; LT = Lhasa Terrane; QT = Qiangtang terrane; SSZ = Suprasubduction zone.
Figure 14. Generalized cross-section of the Amdo basement and the Cretaceous thrust
belt based on the major features of the geologic map and on the available data. No
vertical exaggeration. Jurassic mélange may have been thickened prior to LhasaQiangtang collision as part of an accretionary wedge and incorporates Jurassic and
possibly Paleozoic sedimentary rocks and ophiolite fragments. The basement involved in
the thrust was also probably thickened by thrusting prior to emplacement to upper crustal
levels. The cross-section assumes a tectonic contact between the high-grade metapelites
and low-grade Paleozoic sedimentary layers. (a) Initial layers. Unit thicknesses are:
basement thrust sheet, 6 km; metamorphosed sedimentary rocks, 2 km; Paleozoic
sedimentary rocks, 2 km; Jurassic flysch and mélange, 6 km. (b) Initial fault places a
194
long sheet of basement on metapelites in a flat-on-flat relationship. (c) Subsequent
faulting cuts a horse of metapelites and creates a long flat-on-flat thrust between the
metamorphic rocks and the Jurassic sedimentary rocks. Syn-orogenic (foreland basin)
sediments start to be overthrust. Note that erosion is assumed to have removed the
leading part of the thrust sheet of Jurassic and Paleozoic sedimentary rocks. (d) A long
thrust forms in the weak Jurassic units, which are then thrust over the foreland basin
sedimentary rocks. The resulting ramp in the Jurassic rocks exhumes the metamorphic
and basement rocks to surface levels. The northernmost edge of the mid-Cretaceous
Aptian-Albian seaway laps against the syn-orogenic conglomerates at the front of the
thrust belt. The erosion level depicts the geology of the eastern part of the geologic map,
where foreland basin sedimentary rocks are not exposed. (e) Another ramp and thrust in
the Jurassic units creates a fault-bend fold that forms an anticline in the overlying thrust
sheet and exposes the overthrust foreland basin sedimentary rocks, as seen in the western
part of the geologic map. A normal fault to the north drops unmetamorphosed
sedimentary rocks against the high-grade basement; this fault was likely active during
step (d) as well. Total shortening is 115 km.
195
FIGURES
Fig. 1 Guynn et al.
196
Figure 2. Guynn et al.
197
Figure 3. Guynn et al.
198
Figure 4. Guynn et al.
199
Figure 5. Guynn et al.
200
Figure 6. Guynn et al.
201
91°40' E
92°20' E
92°00' E
Q
Amdo
Tcg ?
Tcg ?
Tcg ?
Jgr
Tcg ?
Q
Q
Q
Q
Q
gn
Jgr
Tcg ?
Tcg ?
Tcg ?
Q
5
Jgr ?
?
75
66
85
PK97-6-4-3A
Q
Jgr
70
63
42
?
Jgr ?
Tcg
Pm ?
75
65
Jgr
Nyainrong
(h)
Tcg
Q
Pm
56
70
gn
Q
Pq
gn
48
58
25
Q
Fold Axis
305 → 30
28
gn
gn
Pm/Ps
47
Poles to Foliation
68
Q
Pq ?
?
Jgr
45
Tcg ?
Tcg ?
gn
77
Jgr
Q
Pm
Pq ?
Q
Jgr
Jgr
80 Pm/Ps
30
39
Jgr
18
21
Pm
Jgr
21
33
Jgr ?
Q
Jgr ?
Jgr ?
40
5
44
Jgr
gn
?
gn
35
57
Pq
Q
gn
Nag
Lake
Poles to Foliation
Jgr1 ?
PK97-6-4-2
(a)
Jgr ?
Fold Axis
355 → 10
gn
Pm
Poles to Foliation
Lineation
Jgr ?
Q
gn
68
gn
Jgr
46
56
Q
Jgr1
(c)
Jgr
Jgr1
32°00' N
gn
Fold Axis
45 → 40
Jgr
Poles to Foliation
Fold Axes
20
gn
gn
gn
gn
gn
gn
33
40
Jgr1
gn
48
47
gn
83
36
Q
gn
74
gn
Q
41
48
(d)
36
9
31
gn
gn
36
78
31
(e)
Q
AP062004-B
gn
Pm
gn
Pm
Q
Q
gn
Jgr1
Jgr ?
Poles to Foliation
Fold Axes
40
60
Jgr1
Kgr
gr
75
75
PK97-6-4-1A
57
Striations
Lineations
31 31
Q
gn
gn
Gray: Faults
Black: Foliations
24
Fold Axis
191 → 43
16
46
gn
64
(b)
gn
Fold Axis
325 → 45
60
gn
gn
Pm
65
gn
43
48
Jgr ?
43
Q
gn
gn
34
Jgr1
Jgr ?
61
gn
gn
Q
80
gn
20
44
31
gn
Q
48
27
Fold Axes
27
gn
(f)
Pm
21
26
33
Jgr ?
64
65
Jm ?
40
45
Pm
Poles to Foliation
gn
84
36
85
22
?
78
74
5
25
Pm
Kc1 62
44
46
Pm
27
Kc2
Kc2
Q
Kl
Kc1
25
45
32°00' N
44
Figure 7.
Ps
56
33
34
Ps
87
Qt
JG052704-1
Poles to Foliation
Jm
Pm
Q
AP060504-B
Jm
Kc1
Jm
AP052504-B
Kc1
Qt
Jl
Kr
Kr
72
JG063005-1
55
Kr
Jl
Kr
53
44
Jl
Jm
(i)
Kr
59
Fold Axis
76 → 33
0
72
Jl
1
2
3
4
5
10 km
Kgr ?
Jm
Jm
Jm
Jm
Jm
Jm
oph
JG061805-2
Jl ?
Kr
Jm
Depositional or intrusive contact
Jm ?
Jm
Quaternary alluvium and basin fill
Map Symbols
Mapping limit
Kr
Explanation of Units
Town
Road
30
J. Guynn, A. Pullen and R. Waldrip
Q
Jm
30
Kgr ?
45
Kc2
64
Kl ?
64
Geologic Map of the Amdo Basement,
Bangong Suture, Central Tibet
Pm
33
87
25
Kr
Kc1 ?
14
Qt
82
Jm
Jgr1
Ps
Kc1
Kr
Jm
Ps
Pm
Q
Ks
21
60
Kc2
Kl
Kl
Kl
Jm ?
56
Kr
60
34
14
16
40
40 39
43
31
45
Ps
72
Pm
Jm ?
Kc2
Kr ?
24
30
75
Kr
Kl
42
16
Ks
45
Kgr
60
JG061805-1
Kr
(j)
38
Kgr
Q
?
Pm
55 Kc2
Kr ?
Kl
Kr
?
?
62
Kl
Kr
60
?
Pm
45
JG070605-1
Fold Axis
265 → 20
?
Pm
Kc1
Pm
gn
Jm
70
32
40
37
64
54
Kgr ?
Jm
Kr
5
54
Pq
Ps
72
Jm ?
Fold Axis
330 → 43
83
52
12
62
Q
Kgr
JG062504-1
Pm
Q
gn
gn
29
Pm
Jm ?
40
Poles to Foliation
84
Ps
34
(g)
30
70
35
85
Q
29
43
gn
71
Pm
Ps
Q
gn
87
76
83
62
Ps
5
gn 60
gn
Pm
Jm
Fault contact
Inferred fault contact
Normal fault (ball on foot-wall)
Thrust fault (triangle of hanging-wall)
Qt
Quaternary tufa associated with hot springs
Ng
Neogene limestone pebble conglomerate
Kc2
Cretaceous conglomerate with clasts of multiple rock types, including gn, gr,
Pm, Ps, Pq and Kr
Kgr
Cretaceous granitoids
Kc1
Cretaceous conglomerate composed of largely limestone, Pm, Pq and some
Kr clasts or exclusively limestone and Pm
Jgr
Jurassic granitoids
Kr
Cretaceous medium-coarse grained sandstone interbedded with siltstone;
cross-bedded, coarse-grained and pebbly red arenite
Jgr2
Jurassic porphyritic granitoid with K-feldspar megacrysts and usually chloritized biotite
Kl
Cretaceous (?) limestone that occurs as blocks in the valley, often including
breccia clasts
oph
Jurassic ophiolite fragment, serpentinized
Ks
Cretaceous tan, red and green shale with interbedded sandstone layers,
locally coarse and pebbly
Pm
Paleozoic (?) marble; includes some metapelites in the NE
Jm
Jurassic turbidites, marine sandstones, dark shales; often transposed
Ps
Paleozoic (?) schists
Jl
Jurassic (?) massive, dark gray limestone
Pq
Paleozoic (?) quartzite
gn
Cambrian and Precambrian gneiss; mostly orthogneiss with some paragneiss & migmatites;
includes small outcrops of metasediments and granitoid intrusions in places.
Strike-slip fault
Cretaceous hypabyssal intrusions
43
55
16
Foliation & lineation
Orientation & plunge of fold-axis
64
Strike & dip
21
Overturned bed
JG063005-1
Sample number
202
Figure 8. Guynn et al.
203
Figure 9. Guynn et al.
204
Figure 10. Guynn et al.
205
Figure 11. Guynn et al.
206
Figure 12. Guynn et al.
207
Figure 13. Guynn et al.
208
Figure 14. Guynn et al.
209
TABLES
Table 1. Sample location and summary of U-Pb age results
Zircon
Sample
Lat
Lon
Description
Age (Ma)
AP060504-B
31.946
92.138 Hypabyssal intrusion
116.2 ± 2.0
JG052704-1
31.766
92.073 Quartz-monzonite
115.4 ± 1.6
AP062004-B
31.934
92.224 Quartz-diorite
111.6 ± 1.5
JG062504-1
31.750
91.780 Hypabyssal intrusion
105.9 ± 2.5
JG061805-1
31.744
91.867 Red sandstone
176.2 ± 21.6
JG061805-2
31.616
91.738 Turbidite S.S. (J-K)
169.4 ± 9.6
JG062305-1
31.734
92.485 Turbidite S.S. (J-K)
145.9 ± 11.2
JG063005-1
31.671
91.857 Red sandstone
114.5 ± 9.0
AP052504-B
31.735
92.028 Red sandstone
108.4 ± 5.7
JG070605-1
31.680
91.777 Tan sandstone in white shale ~110 ?
Shaded samples are detrital zircon maximum depositional ages
210
Isotopic ratios
1.8
5.4
2.2
2.9
2.0
1.9
7.8
4.4
2.1
1.4
2.2
2.3
3.0
1.1
2.0
1.2
1.5
6.1
1.6
2.4
4.1
1.3
3.2
1.9
Apparent ages (Ma)
206Pb*
207Pb*
±
(Ma)
Best age
(Ma)
±
(Ma)
Table 2. U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector ICP Mass
Spectrometry
0.01817
0.01968
0.01748
0.01745
0.01839
0.01830
0.01876
0.02068
0.02654
0.02685
0.01859
0.01827
0.01811
0.01763
0.01872
0.01824
0.02990
0.02046
0.01801
0.01842
0.01827
0.02628
0.02670
0.01784
±
(Ma)
2.1
6.7
2.5
3.2
2.3
2.2
9.3
5.7
3.6
2.4
2.6
2.6
3.5
1.2
2.3
1.4
2.7
7.9
1.9
2.8
4.7
2.2
5.3
2.2
207Pb*
235U
116.1
125.6
111.7
111.5
117.5
116.9
119.8
131.9
168.9
170.8
118.7
116.7
115.7
112.7
119.6
116.5
190.0
130.5
115.1
117.7
116.7
167.2
169.9
114.0
±
(Ma)
372.9
338.9
353.5
332.7
449.6
252.9
685.1
506.3
367.8
218.4
441.1
580.4
570.8
236.8
376.8
200.9
121.1
790.4
288.5
285.1
256.4
138.7
281.0
187.1
1.8
1.3
2.1
2.8
2.0
1.5
4.7
1.8
2.1
1.1
1.1
206Pb*
238U
291.4
556.9
174.4
298.6
161.5
718.1
340.6
490.2
644.6
307.7
515.5
17.0
-456.5
457.9
291.4
113.7
228.5
551.5
327.2
161.4
481.6
148.9
112.0
204.9
116.6
119.2
118.5
119.1
113.9
112.6
118.1
118.5
117.9
115.7
114.7
error
corr.
19.2
22.8
16.5
16.8
21.6
16.8
37.7
32.8
31.8
16.0
26.2
25.5
19.2
13.0
19.9
9.4
9.6
51.4
15.1
14.0
15.6
9.3
18.8
9.2
58.0
158.3
142.2
94.4
498.9
54.3
126.1
88.2
108.0
59.0
49.6
±
(%)
124.6
150.1
114.6
120.4
119.6
150.5
131.1
152.8
205.1
180.4
139.9
112.2
93.0
129.9
128.2
116.4
192.9
155.3
125.5
119.7
135.6
166.0
166.0
118.2
43.8
256.6
-39.0
102.6
399.0
48.7
62.8
-88.1
77.0
34.1
35.4
206Pb*
238U
2.1
6.7
2.5
3.2
2.3
2.2
9.3
5.7
3.6
2.4
2.6
2.6
3.5
1.2
2.3
1.4
2.7
7.9
1.9
2.8
4.7
2.2
5.3
2.2
3.1
8.3
6.4
5.2
26.6
2.8
7.2
4.0
5.4
2.8
2.4
35 ⎠ m spot size.
±
(%)
116.1
125.6
111.7
111.5
117.5
116.9
119.8
131.9
168.9
170.8
118.7
116.7
115.7
112.7
119.6
116.5
190.0
130.5
115.1
117.7
116.7
167.2
169.9
114.0
113.3
126.0
111.3
118.4
128.1
109.8
115.5
109.2
116.0
112.0
111.1
207Pb*
235U
0.11
0.33
0.15
0.19
0.10
0.16
0.25
0.19
0.13
0.15
0.11
0.09
0.14
0.10
0.12
0.14
0.27
0.17
0.13
0.19
0.33
0.22
0.26
0.23
1.8
1.3
2.1
2.8
2.0
1.5
4.7
1.8
2.1
1.1
1.1
U/Th
Spot
16.3
16.4
15.2
14.8
19.2
12.0
30.7
23.1
17.1
9.7
20.0
24.0
21.6
10.7
16.5
8.6
5.4
35.7
12.8
12.4
12.3
6.1
12.3
8.3
116.6
119.2
118.5
119.1
113.9
112.6
118.1
118.5
117.9
115.7
114.7
206Pb
204Pb
JG052704-1 granitoid: igneous zircon
1
380
1458
1.8
0.13059
2
252
524
1.3
0.15933
3
282
2317
0.7
0.11943
4c
427
3611
0.2
0.12586
6
265
1097
0.6
0.12499
7
372
349
0.7
0.15972
9
202
1747
1.1
0.13778
10
204
762
0.6
0.16241
11c
518
1023
0.2
0.22380
11t
263
10006
0.9
0.19443
13
408
1772
1.5
0.14769
13c
330
2323
0.8
0.11679
14
131
3336
0.2
0.09590
15
297
613
0.6
0.13646
16
361
4563
1.2
0.13459
17
350
2368
1.1
0.12146
18
433
6322
1.1
0.20917
19
240
229
1.0
0.16523
20
346
5154
0.8
0.13153
21
277
6772
0.9
0.12516
22
267
9019
0.4
0.14293
23
454
12998
1.3
0.17763
23c
258
9635
2.1
0.17766
24
546
10208
0.8
0.12349
0.53
0.16
0.29
0.51
0.08
0.51
0.60
0.39
0.37
0.38
0.43
U
(ppm)
50 ⎠ m spot size.
AP060504-B hypabyssal intrusion: igneous zircon
1
421
5380
0.5
0.11802
2.9
0.01825
1.5
2
611
1803
0.8
0.13210
7.0
0.01866
1.1
3
279
6199
1.4
0.11589
6.1
0.01855
1.8
4
540
4291
0.7
0.12363
4.6
0.01865
2.4
5
474
809
0.3
0.13442
22.1
0.01783
1.8
6
652
6678
0.4
0.11415
2.6
0.01762
1.4
7
225
2725
0.5
0.12050
6.6
0.01849
4.0
8
300
3236
0.5
0.11355
3.9
0.01855
1.5
9
329
3362
0.7
0.12106
4.9
0.01846
1.8
10
512
5650
0.6
0.11663
2.7
0.01811
1.0
11
575
7562
0.7
0.11564
2.3
0.01795
1.0
211
0.11725
0.11843
0.11917
0.15953
0.12089
0.12434
0.01786
0.01868
0.01811
0.02463
0.01860
0.01802
1.2
1.4
1.6
2.1
2.2
1.9
2.5
3.2
3.3
3.9
7.5
1.4
2.0
1.0
2.9
2.8
5.7
3.6
5.6
3.5
5.3
1.3
1.8
1.3
31.2
2.5
1.4
1.8
1.5
1.5
2.7
0.9
1.0
1.2
2.4
2.1
2.0
1.6
1.8
1.9
50 ⎠ m spot size.
0.02661
0.01737
0.02656
0.01773
0.01871
0.01711
0.01736
0.01603
0.01892
0.01778
0.02859
0.01578
0.01798
0.01737
0.07240
0.01796
0.01721
0.01677
0.03051
0.01765
0.01703
0.01785
0.01756
0.01794
0.01806
0.01743
0.01702
0.01771
0.01789
0.01765
0.01747
0.01742
0.01841
0.01802
0.13
0.07
0.42
0.13
0.17
0.06
0.08
0.07
0.11
0.09
0.26
0.15
0.21
0.18
0.71
0.10
0.03
0.07
0.67
0.09
0.07
0.11
0.11
0.11
0.19
0.10
0.05
0.13
0.08
0.11
0.10
0.10
0.15
0.18
0.41
0.51
0.53
0.53
0.70
0.09
169.3
111.0
169.0
113.3
119.5
109.4
110.9
102.5
120.8
113.6
181.7
100.9
114.9
111.0
450.6
114.7
110.0
107.2
193.8
112.8
108.8
114.0
112.2
114.6
115.4
111.4
108.8
113.2
114.3
112.8
111.6
111.3
117.6
115.1
114.1
119.3
115.7
156.9
118.8
115.1
4.2
3.5
5.4
4.3
8.9
1.5
2.2
1.0
3.5
3.1
10.1
3.6
6.4
3.9
22.9
1.5
2.0
1.3
59.6
2.8
1.5
2.0
1.7
1.8
3.1
1.0
1.1
1.4
2.7
2.4
2.3
1.8
2.1
2.1
1.4
1.7
1.8
3.2
2.5
2.1
176.2
111.0
179.5
142.8
123.1
105.5
113.7
118.4
189.4
110.6
213.6
120.7
223.2
139.9
495.9
104.1
130.7
122.8
305.2
94.8
116.4
125.8
115.3
135.5
109.8
116.7
141.1
139.1
107.8
111.0
133.9
122.6
143.0
149.6
112.6
113.7
114.3
150.3
115.9
119.0
31.5
47.2
12.9
38.7
52.2
23.7
28.4
16.4
45.7
33.6
42.1
26.6
55.0
26.2
29.2
13.5
65.0
21.8
124.3
26.2
22.0
19.0
15.2
18.3
14.8
9.9
26.7
12.3
31.6
20.9
24.8
18.7
16.5
14.3
3.1
3.1
3.2
5.5
3.4
22.4
80.5
-3.4
85.5
47.9
55.9
196.9
63.6
59.4
60.5
79.2
52.0
466.0
269.9 445.8
111.7 1106.1
320.6 161.9
666.5 627.8
192.7 1080.0
18.2 574.5
171.7 624.2
450.6 325.9
1161.6 529.3
47.0 780.1
581.5 463.5
530.9 512.2
1608.7 509.8
663.6 427.1
710.8 112.1
-132.5 337.5
526.8 1245.3
435.8 421.4
1277.4 704.7
-338.0 755.2
274.2 460.4
354.6 362.9
178.9 324.2
518.9 315.8
-9.3 339.2
226.2 206.7
725.7 433.8
607.6 203.6
-34.5 764.2
73.3 474.3
549.8 433.8
346.9 365.7
587.8 266.4
737.7 215.0
169.3
111.0
169.0
113.3
119.5
109.4
110.9
102.5
120.8
113.6
181.7
100.9
114.9
111.0
450.6
114.7
110.0
107.2
193.8
112.8
108.8
114.0
112.2
114.6
115.4
111.4
108.8
113.2
114.3
112.8
111.6
111.3
117.6
115.1
114.1
119.3
115.7
156.9
118.8
115.1
4.2
3.5
5.4
4.3
8.9
1.5
2.2
1.0
3.5
3.1
10.1
3.6
6.4
3.9
22.9
1.5
2.0
1.3
59.6
2.8
1.5
2.0
1.7
1.8
3.1
1.0
1.1
1.4
2.7
2.4
2.3
1.8
2.1
2.1
1.4
1.7
1.8
3.2
2.5
2.1
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
0.6
0.7
0.7
0.7
0.6
0.7
2.9
2.9
3.0
3.9
3.1
20.0
4268
8182
6466
6902
4611
727
13
14
15
16
17
18
19.4
44.8
7.8
29.0
45.0
23.7
26.4
14.6
26.4
32.0
21.9
23.4
27.4
20.1
7.5
13.7
52.9
18.8
46.9
28.9
20.0
16.1
13.9
14.4
14.2
9.0
20.3
9.5
30.9
19.9
19.8
16.2
12.4
10.3
420
562
433
328
528
166
AP062004-B quartz-diorite: igneous zircon
1C
511
3161
0.8
0.18952
3C
292
903
0.8
0.11554
4C
503
5979
0.7
0.19337
5C
194
1362
0.7
0.15101
6C
111
445
0.7
0.12888
7C
487
3713
0.8
0.10944
8C
488
2925
0.8
0.11848
9C
539
2551
0.4
0.12369
10C
155
1231
0.8
0.20503
11C
175
1858
0.5
0.11510
12C
316
1445
1.0
0.23410
13C
924
270
1.2
0.12626
14C
210
1292
0.8
0.24588
15C
306
1845
0.7
0.14777
16C
352
6876
0.9
0.62965
17C
664
6390
0.7
0.10798
18C
222
1590
0.7
0.13742
19C
451
4809
0.5
0.12854
20C
184
1287
1.4
0.35065
21C
313
3198
0.9
0.09786
22T
416
3429
1.1
0.12147
24T
443
4117
1.0
0.13191
25T
412
4872
0.7
0.12022
26T
491
3784
1.0
0.14274
27T
632
3331
1.1
0.11419
28T
526
3318
1.0
0.12177
29T
536
2369
0.5
0.14911
30T
429
4290
0.9
0.14681
31T
347
1328
0.9
0.11196
32T
315
909
0.9
0.11555
33T
334
1496
0.8
0.14097
34T
589
4317
1.0
0.12834
35T
421
3080
0.9
0.15122
36T
464
2671
0.7
0.15870
212
3.0
1.9
10.0
3.6
1.8
6.3
1.8
5.7
1.0
4.5
3.6
1.8
3.5
4.3
3.2
1.8
5.8
3.9
5.0
0.94
0.22
0.96
0.13
0.12
0.07
0.06
0.07
0.40
0.19
0.14
0.16
0.13
0.26
0.14
0.56
0.64
0.20
0.07
0.08
0.19
0.22
0.64
0.36
0.63
0.32
0.61
0.96
0.72
0.89
0.68
0.79
0.83
0.52
0.29
0.66
0.86
0.52
0.88
1538.1
103.1
243.3
107.3
106.8
107.7
149.4
99.6
93.8
102.7
105.0
107.8
114.3
114.3
104.1
87.1
170.9
109.1
149.6
104.5
78.7
154.3
170.8
171.2
178.7
184.1
307.5
399.7
413.7
422.2
429.1
429.6
440.3
444.8
457.0
461.2
461.7
462.5
463.5
26.4
1.0
22.8
1.6
0.9
1.4
1.0
2.7
2.8
1.8
2.1
1.6
2.1
6.8
2.0
5.3
19.6
2.9
3.1
4.0
2.3
2.9
16.8
6.0
3.2
11.5
5.4
22.1
4.0
18.5
15.1
7.6
15.1
18.3
14.3
8.0
25.6
17.4
22.3
1972.9
106.9
258.2
110.6
114.1
118.4
147.0
69.6
97.8
118.0
114.9
111.3
112.0
95.5
100.8
106.4
167.5
128.1
130.2
132.9
89.9
136.4
193.4
184.2
169.5
338.0
301.2
406.0
423.6
442.1
432.9
443.6
460.0
481.5
534.4
494.6
469.3
496.8
468.6
17.9
4.3
22.8
12.6
7.4
20.6
14.9
25.3
7.0
10.0
15.4
9.6
15.2
20.8
13.4
11.1
28.1
16.4
38.2
59.2
13.6
10.9
27.5
16.6
4.4
57.6
7.7
20.0
4.8
18.3
18.9
8.3
15.7
31.4
46.1
10.6
25.3
29.6
21.2
Bolded sample numbers used a 25 ⎠ m spot size; the rest were 35 ⎠ m.
0.01229
0.02422
0.02685
0.02691
0.02811
0.02897
0.04886
0.06396
0.06628
0.06768
0.06883
0.06892
0.07070
0.07144
0.07346
0.07416
0.07424
0.07437
0.07454
2465.6
11.8
190.8
95.2
395.1
65.1
182.8 277.8
269.1 157.1
339.3 418.9
108.0 257.5
-870.3 1110.2
197.8 160.3
439.9 195.4
325.0 319.6
188.4 209.6
62.0 340.5
-352.6 575.3
22.8 334.3
563.9 198.5
119.4 331.5
497.0 296.1
-210.3 799.5
677.4 1069.5
78.7
154.3
170.8
171.2
178.7
184.1
307.5
399.7
413.7
422.2
429.1
429.6
440.3
444.8
457.0
461.2
461.7
462.5
463.5
2465.6
103.1
243.3
107.3
106.8
107.7
149.4
99.6
93.8
102.7
105.0
107.8
114.3
114.3
104.1
87.1
170.9
109.1
149.6
104.5
2.3
2.9
16.8
6.0
3.2
11.5
5.4
22.1
4.0
18.5
15.1
7.6
15.1
18.3
14.3
8.0
25.6
17.4
22.3
11.8
1.0
22.8
1.6
0.9
1.4
1.0
2.7
2.8
1.8
2.1
1.6
2.1
6.8
2.0
5.3
19.6
2.9
3.1
4.0
397.8
-165.9
478.9
355.2
42.8
1602.8
253.0
442.1
477.9
547.0
453.0
516.8
559.4
660.2
880.4
652.5
506.7
658.1
493.9
350.4
207.2
266.3
208.3
52.5
357.2
53.7
38.8
21.5
51.9
87.6
31.0
51.0
150.1
219.7
43.6
76.4
138.4
58.6
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
15.8
8.5
15.6
9.9
2.8
20.0
3.0
6.0
1.4
5.1
5.4
2.3
4.2
8.2
11.1
2.7
6.7
7.5
5.6
50 ⎠ m spot size.
JG062504-1 hypabyssal intrusion: igneous zircon
1
417
68024
1.3
5.97954
2.1
0.26946
1.9
3
1629
12953
1.5
0.11097
4.2
0.01613
0.9
4c
1397
15504
3.0
0.28951
10.0
0.03847
9.6
6
589
4504
1.1
0.11506
12.0
0.01678
1.5
7
913
1363
1.3
0.11894
6.9
0.01671
0.8
9
772
3276
0.5
0.12367
18.4
0.01685
1.3
10
652
11487
5.2
0.15575
10.9
0.02345
0.7
11
1140
814
0.4
0.07095
37.6
0.01557
2.7
12
1291
2944
0.6
0.10112
7.5
0.01465
3.0
13
1032
1227
1.1
0.12328
8.9
0.01606
1.7
14
523
6312
2.0
0.11979
14.2
0.01642
2.0
15
1337
9093
1.2
0.11590
9.1
0.01686
1.5
17
461
2848
0.3
0.11657
14.3
0.01789
1.8
19
286
3186
0.5
0.09861
22.9
0.01789
6.0
20
624
2200
0.5
0.10437
14.0
0.01628
2.0
21
740
757
0.5
0.11045
11.0
0.01360
6.1
22
408
3641
2.6
0.17933
18.2
0.02687
11.6
24
561
2200
0.9
0.13447
13.6
0.01707
2.7
25
222
4424
0.7
0.13680
31.3
0.02347
2.1
26
455
787
0.8
0.13989
47.4
0.01634
3.9
JG061805-1 red arenite: detrital zircon
595
1255
1.1
0.09261
102
77
187
3730
1.4
0.14372
48
1136
11600
17.0
0.20981
88
511
4598
0.6
0.19894
92
503
6921
0.7
0.18170
105
902
349
0.9
0.39493
22
503
18657
2.0
0.34537
84
245
7320
0.8
0.49159
96
568
19532
1.6
0.51767
6
682
41642
1.4
0.54552
56
210
8737
1.0
0.53161
93
731
10470
1.6
0.54792
60
571
12448
1.7
0.57309
67
314
15323
0.8
0.60676
65
545
2225
0.7
0.69275
17
685
11819
1.3
0.62758
87
169
8359
1.9
0.58755
95
1224
4691
1.9
0.63108
18
1054
39391
2.5
0.58647
213
575
506
500
783
612
512
284
619
318
746
418
839
412
592
319
1550
249
679
637
436
338
581
635
139
397
1744
395
406
411
178
149
328
400
524
334
803
186
442
119
175
458
1888
35622
29313
36764
34303
13929
6718
29514
6753
16880
9780
76165
12669
17437
4209
54709
16849
13164
30120
38438
17058
63978
49733
8331
24997
63927
23055
20955
14790
12715
26642
15751
60468
16997
18865
41043
10125
6370
8427
11943
10422
2.0
1.6
0.9
1.5
4.0
0.9
1.5
1.7
1.3
0.9
1.2
1.5
0.6
1.8
1.9
1.4
1.2
1.7
2.6
1.3
6.7
1.3
1.1
1.1
2.1
1.2
1.0
1.7
1.8
1.3
1.3
1.5
2.2
3.7
1.6
1.1
1.4
1.6
2.3
1.6
1.4
0.61692
0.58575
0.60929
0.60551
0.62068
0.62958
0.62210
0.62138
0.59626
0.60292
0.60431
0.61888
0.62517
0.61102
0.62907
0.62092
0.61613
0.62878
0.61977
0.63172
0.63088
0.64296
0.63543
0.64914
0.65367
0.68750
0.62611
0.61718
0.63248
0.63395
0.61728
0.62294
0.63720
0.63752
0.64583
0.63515
0.63842
0.62822
0.62071
0.64171
0.65090
10.5
2.9
3.7
2.6
2.1
5.5
3.6
6.4
4.2
6.1
3.3
2.2
3.6
2.7
9.2
4.1
2.8
4.1
2.0
3.0
5.8
3.3
3.5
5.4
3.8
4.3
4.1
3.8
3.6
2.7
7.1
2.6
3.9
2.2
2.2
6.5
3.3
4.5
4.1
2.7
5.2
0.07455
0.07534
0.07584
0.07612
0.07658
0.07662
0.07677
0.07719
0.07720
0.07739
0.07742
0.07755
0.07769
0.07802
0.07821
0.07821
0.07826
0.07832
0.07838
0.07848
0.07889
0.07896
0.07905
0.07907
0.07909
0.07953
0.07972
0.07980
0.07984
0.08003
0.08018
0.08029
0.08030
0.08049
0.08055
0.08091
0.08096
0.08100
0.08107
0.08121
0.08135
9.2
2.4
2.7
1.9
1.1
1.4
1.8
3.6
2.2
5.7
2.6
1.0
2.3
1.4
3.9
3.3
2.5
2.9
1.8
1.7
5.4
2.5
1.0
4.8
1.6
2.0
3.8
2.6
2.3
1.8
6.3
1.9
1.6
1.3
1.6
6.1
1.3
2.1
2.8
1.8
3.4
0.88
0.84
0.72
0.73
0.50
0.26
0.50
0.57
0.53
0.94
0.78
0.45
0.65
0.52
0.42
0.82
0.87
0.71
0.90
0.57
0.92
0.76
0.30
0.89
0.43
0.48
0.93
0.68
0.65
0.66
0.88
0.74
0.41
0.60
0.73
0.93
0.40
0.47
0.68
0.66
0.65
463.5
468.2
471.2
472.9
475.7
475.9
476.8
479.3
479.4
480.5
480.7
481.5
482.3
484.3
485.4
485.4
485.7
486.1
486.5
487.0
489.5
489.9
490.5
490.6
490.7
493.3
494.5
494.9
495.2
496.3
497.2
497.8
497.9
499.1
499.4
501.5
501.8
502.1
502.5
503.4
504.2
41.4
11.0
12.1
8.8
4.9
6.6
8.3
16.7
10.4
26.5
12.0
4.6
10.8
6.5
18.3
15.5
11.6
13.8
8.3
7.9
25.2
11.8
4.9
22.5
7.8
9.6
18.2
12.4
11.1
8.5
30.0
9.2
7.6
6.4
7.7
29.4
6.3
10.3
13.3
8.6
16.6
487.9
468.2
483.1
480.7
490.3
495.8
491.2
490.7
474.9
479.1
480.0
489.1
493.1
484.2
495.5
490.4
487.4
495.3
489.7
497.2
496.6
504.1
499.5
508.0
510.7
531.3
493.7
488.1
497.6
498.6
488.1
491.7
500.6
500.8
505.9
499.3
501.3
495.0
490.3
503.4
509.0
40.5
10.9
14.3
10.1
8.3
21.6
14.2
24.8
16.0
23.3
12.7
8.7
13.9
10.3
36.2
15.8
11.0
16.2
7.7
11.8
22.8
13.1
13.7
21.4
15.3
17.6
16.0
14.8
14.2
10.6
27.7
10.1
15.4
8.7
8.7
25.8
13.1
17.7
15.8
10.6
21.0
604.1
467.7
539.9
518.0
558.9
588.8
558.7
544.3
453.0
472.2
476.5
525.1
543.4
483.7
542.4
513.8
495.3
538.4
504.9
544.1
529.6
569.1
540.9
586.9
601.5
698.0
490.0
456.0
509.0
508.9
445.8
463.1
512.8
508.5
535.4
489.0
499.0
462.1
433.6
503.4
530.9
105.3
35.4
56.7
39.3
40.3
115.1
68.9
115.0
79.5
46.5
46.0
43.7
58.9
50.3
183.4
51.7
31.1
63.6
19.0
54.0
49.6
46.4
72.7
53.5
74.7
79.8
32.0
61.9
60.3
44.7
76.0
38.8
78.3
38.4
32.5
52.4
67.0
88.0
66.2
44.0
86.9
463.5
468.2
471.2
472.9
475.7
475.9
476.8
479.3
479.4
480.5
480.7
481.5
482.3
484.3
485.4
485.4
485.7
486.1
486.5
487.0
489.5
489.9
490.5
490.6
490.7
493.3
494.5
494.9
495.2
496.3
497.2
497.8
497.9
499.1
499.4
501.5
501.8
502.1
502.5
503.4
504.2
41.4
11.0
12.1
8.8
4.9
6.6
8.3
16.7
10.4
26.5
12.0
4.6
10.8
6.5
18.3
15.5
11.6
13.8
8.3
7.9
25.2
11.8
4.9
22.5
7.8
9.6
18.2
12.4
11.1
8.5
30.0
9.2
7.6
6.4
7.7
29.4
6.3
10.3
13.3
8.6
16.6
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
90
64
15
57
21
72
75
52
46
26
50
8
66
58
51
7
70
89
91
27
104
4
13
69
20
29
106
23
30
109
62
99
1
40
24
78
76
32
86
107
11
214
640
303
847
339
390
203
278
387
345
211
253
455
583
206
852
260
400
215
189
1292
356
95
258
342
649
454
427
1791
90
487
336
187
897
542
102
392
321
381
35164
15407
19406
27322
12218
9691
15675
4205
14478
12618
7661
23425
62426
2151
32747
24240
5273
21768
4906
2481
17870
13934
18430
5495
26870
42782
16866
81378
19706
108314
25523
47614
116152
15089
32148
83096
69862
194227
1.3
1.0
1.5
1.2
1.4
1.4
1.4
1.7
1.1
1.6
1.2
1.2
1.1
1.4
1.4
0.9
2.0
0.6
1.4
0.9
0.9
1.0
1.8
0.9
1.1
1.5
1.1
1.2
1.8
4.1
1.1
1.5
6.0
3.4
1.7
1.7
1.5
2.1
0.64762
0.63308
0.65835
0.65454
0.65115
0.63630
0.64054
0.62429
0.65903
0.65469
0.67707
0.66268
0.64231
0.72558
0.67025
0.66403
0.99364
0.95494
0.86913
1.14702
1.21321
1.15343
1.19086
1.12583
1.21682
1.22408
1.20485
1.25203
1.36163
1.46671
1.65707
3.26000
4.73500
5.23409
6.83665
10.09685
13.93991
19.43282
2.2
2.8
2.8
2.5
4.3
3.3
2.1
6.7
5.8
3.9
4.8
3.7
2.6
4.7
4.0
3.0
6.0
4.3
4.1
19.4
7.8
3.5
4.0
7.0
2.7
1.7
2.6
5.6
6.5
3.3
2.9
2.7
4.9
3.2
2.7
3.1
6.2
5.3
0.08144
0.08157
0.08158
0.08161
0.08196
0.08199
0.08220
0.08240
0.08288
0.08301
0.08364
0.08366
0.08389
0.08468
0.08471
0.08549
0.10359
0.10592
0.10778
0.11185
0.12345
0.12487
0.12492
0.12648
0.13038
0.13266
0.13317
0.13916
0.14223
0.15345
0.16883
0.25581
0.30019
0.32728
0.36940
0.45765
0.53842
0.54586
1.5
2.4
1.5
1.6
3.4
1.6
1.2
3.2
3.7
3.0
3.8
3.4
1.9
2.7
3.5
2.6
2.0
3.6
3.3
5.2
6.9
2.6
2.5
4.7
2.5
1.2
1.9
4.3
2.3
2.9
1.5
1.8
4.2
1.1
2.0
2.2
6.0
3.1
0.69
0.85
0.56
0.67
0.79
0.48
0.58
0.48
0.63
0.77
0.80
0.92
0.73
0.57
0.89
0.86
0.34
0.84
0.80
0.27
0.88
0.73
0.63
0.67
0.91
0.70
0.73
0.77
0.36
0.89
0.52
0.68
0.87
0.35
0.73
0.72
0.97
0.59
504.7
505.5
505.6
505.7
507.8
508.0
509.3
510.5
513.3
514.0
517.8
517.9
519.3
524.0
524.2
528.8
635.4
649.0
659.8
683.5
750.4
758.5
758.8
767.7
790.0
803.0
805.9
839.9
857.3
920.3
1005.7
1468.4
1692.2
1825.2
2026.6
2429.2
2776.8
2807.9
7.2
11.5
7.5
8.0
16.7
7.8
5.8
15.8
18.0
14.8
19.1
17.1
9.4
13.4
17.7
13.2
12.2
22.3
20.8
33.6
48.5
18.6
17.9
34.2
18.4
8.9
14.5
33.7
18.5
24.9
14.3
23.6
62.5
18.0
34.3
45.0
135.7
70.9
507.0
498.0
513.6
511.3
509.2
500.0
502.6
492.5
514.0
511.4
525.0
516.3
503.7
554.0
520.9
517.1
700.6
680.7
635.1
775.8
806.7
778.9
796.4
765.8
808.3
811.6
802.8
824.3
872.6
916.8
992.3
1471.6
1773.5
1858.2
2090.5
2443.7
2745.6
3063.6
8.6
11.0
11.1
9.9
17.3
13.0
8.2
26.3
23.5
15.7
19.7
15.2
10.3
20.1
16.1
12.3
30.4
21.4
19.6
105.4
43.5
19.3
21.9
37.7
15.1
9.4
14.4
31.4
37.9
19.7
18.7
20.7
40.7
27.7
24.0
28.5
58.6
51.2
517.4
463.7
549.5
536.1
515.3
463.7
472.6
410.0
517.3
499.4
556.3
508.7
433.8
679.0
506.4
465.5
915.5
786.9
548.0
1051.6
965.3
837.6
902.9
760.0
859.0
835.3
794.2
782.5
911.6
908.4
962.7
1476.2
1870.5
1895.3
2154.0
2455.8
2722.8
3235.8
34.1
32.7
50.1
40.2
57.5
63.7
37.3
132.2
99.4
55.2
63.3
33.0
39.8
82.7
39.9
34.4
116.3
49.5
54.0
379.0
76.3
50.2
63.7
109.3
22.7
25.0
37.1
74.4
124.7
30.9
51.5
37.2
43.8
54.8
32.5
36.0
23.1
67.7
504.7
505.5
505.6
505.7
507.8
508.0
509.3
510.5
513.3
514.0
517.8
517.9
519.3
524.0
524.2
528.8
635.4
649.0
659.8
683.5
750.4
758.5
758.8
767.7
790.0
803.0
805.9
839.9
857.3
920.3
1005.7
1476.2
1870.5
1895.3
2154.0
2455.8
2722.8
3235.8
7.2
11.5
7.5
8.0
16.7
7.8
5.8
15.8
18.0
14.8
19.1
17.1
9.4
13.4
17.7
13.2
12.2
22.3
20.8
33.6
48.5
18.6
17.9
34.2
18.4
8.9
14.5
33.7
18.5
24.9
14.3
37.2
43.8
54.8
32.5
36.0
23.1
67.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
16
97
83
85
31
101
80
36
49
39
43
79
9
94
53
61
19
63
45
38
98
14
3
28
42
100
54
59
34
2
55
33
41
12
71
44
25
5
215
3.4
6.7
4.7
20.4
3.8
3.5
7.1
1.7
3.4
4.7
15.8
12.1
6.8
4.0
2.0
5.0
10.9
8.7
15.0
5.5
4.2
4.7
8.6
4.2
6.2
3.1
4.5
2.6
6.2
4.5
6.8
3.0
4.5
3.3
2.9
5.7
4.3
4.5
3.3
6.3
2.5
5.3
3.1
7.8
1.8
2.9
5.4
1.3
2.5
3.8
10.1
4.6
2.4
2.1
1.0
4.0
4.9
7.1
1.4
4.4
3.9
3.7
8.2
2.6
3.6
2.4
1.9
1.9
6.0
3.5
6.2
2.5
4.2
2.6
2.1
5.1
2.8
4.2
1.8
3.0
0.73
0.78
0.66
0.38
0.48
0.84
0.76
0.75
0.73
0.82
0.64
0.38
0.35
0.53
0.49
0.80
0.45
0.81
0.09
0.79
0.93
0.79
0.95
0.61
0.58
0.78
0.42
0.72
0.97
0.78
0.92
0.84
0.94
0.78
0.73
0.91
0.66
0.94
0.57
0.48
165.5
173.5
174.5
199.8
207.7
208.7
210.6
239.4
253.0
253.3
256.9
263.8
270.8
271.4
297.2
327.1
333.9
349.2
364.7
376.8
394.6
447.3
448.1
480.1
553.1
560.0
563.8
594.7
633.5
651.4
716.0
725.7
742.0
776.4
787.6
826.8
841.7
916.9
951.9
971.0
4.1
9.0
5.3
15.2
3.8
6.0
11.2
3.1
6.2
9.5
25.5
11.8
6.4
5.6
2.9
12.8
16.0
24.1
4.8
16.0
14.9
15.9
35.4
11.9
19.0
12.9
10.2
10.5
36.4
21.9
42.1
17.4
29.6
19.2
15.5
39.9
22.1
36.0
16.4
27.1
165.0
176.2
181.6
149.6
213.1
213.0
249.8
243.3
254.9
259.0
249.8
340.4
263.3
284.8
298.4
322.9
353.4
354.1
450.8
388.1
414.7
449.6
458.8
479.9
532.1
569.0
564.5
607.0
640.7
714.5
692.9
779.0
767.1
821.8
770.5
830.4
928.0
907.2
975.7
957.3
Bolded samples used a 15 ⎠ m spot size; the rest were 25 ⎠ m.
0.02600
0.02729
0.02744
0.03147
0.03274
0.03291
0.03320
0.03783
0.04003
0.04007
0.04066
0.04177
0.04290
0.04299
0.04718
0.05205
0.05317
0.05567
0.05821
0.06019
0.06313
0.07185
0.07198
0.07732
0.08958
0.09076
0.09139
0.09665
0.10327
0.10633
0.11748
0.11916
0.12200
0.12799
0.12995
0.13684
0.13947
0.15285
0.15912
0.16257
5.2
10.9
7.8
28.4
7.3
6.7
15.7
3.8
7.7
10.7
35.1
34.9
15.8
10.0
5.2
13.9
32.6
26.1
54.7
17.8
14.3
16.9
31.6
16.2
25.7
13.5
19.5
11.7
29.5
23.3
33.9
16.5
24.1
18.9
15.4
32.1
25.9
26.8
20.4
39.1
158.0
212.4
275.2
-586.8
273.9
260.8
636.9
282.0
272.4
311.5
182.9
903.2
197.5
396.3
308.2
293.3
483.4
385.8
917.4
456.3
527.6
461.2
513.0
478.9
443.1
605.2
567.6
652.9
666.0
918.1
618.6
934.7
840.9
946.9
721.3
840.0
1139.0
883.5
1029.7
925.7
55.1
96.6
81.1
515.3
76.7
43.9
99.2
26.4
54.1
60.1
284.4
230.5
147.9
76.6
40.2
69.3
215.8
114.3
309.1
74.9
34.8
63.6
56.1
74.1
112.3
42.3
88.9
38.2
32.2
58.3
57.4
34.0
31.4
42.6
41.3
48.7
63.9
31.0
54.2
113.7
165.5
173.5
174.5
199.8
207.7
208.7
210.6
239.4
253.0
253.3
256.9
263.8
270.8
271.4
297.2
327.1
333.9
349.2
364.7
376.8
394.6
447.3
448.1
480.1
553.1
560.0
563.8
594.7
633.5
651.4
716.0
725.7
742.0
776.4
787.6
826.8
841.7
916.9
951.9
971.0
4.1
9.0
5.3
15.2
3.8
6.0
11.2
3.1
6.2
9.5
25.5
11.8
6.4
5.6
2.9
12.8
16.0
24.1
4.8
16.0
14.9
15.9
35.4
11.9
19.0
12.9
10.2
10.5
36.4
21.9
42.1
17.4
29.6
19.2
15.5
39.9
22.1
36.0
16.4
27.1
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
JG061805-2 turbidite sandstone: detrital zircon
32
649
6464
2.9
0.17646
33
499
1956
2.2
0.18954
59
1092 241823
2.4
0.19584
70
112
2440
0.7
0.15877
24
479
8591
1.9
0.23358
48
1209 146018
0.7
0.23346
80
745
13478
1.3
0.27894
50
969
89277
2.7
0.27081
57
1625 164400
24.7
0.28541
66
817
19781
0.8
0.29060
25
188
5962
1.1
0.27887
54
297
41879
1.7
0.39823
7
207
8579
1.7
0.29606
76
427
8408
1.8
0.32375
68
749
20483
1.3
0.34165
45
341
37625
0.8
0.37445
40
138
54244
0.8
0.41632
6
228
5200
6.1
0.41721
44
1613
82571
3.2
0.55885
39
1847
36954
34.8
0.46555
79
337
5317
1.3
0.50438
75
969
18667
2.5
0.55697
74
318
8633
0.6
0.57127
58
436 367328
2.8
0.60425
10
112
5246
0.7
0.68882
37
608
51052
4.0
0.75138
36
629
68523
4.0
0.74365
52
992 459094
4.3
0.81806
64
233
9352
1.6
0.87947
5
545
34508
3.5
1.02113
47
148
81750
1.1
0.97869
2
1093
11939
1.2
1.15370
31
269
12882
1.0
1.12870
13
1045
62796
1.3
1.24654
17
141
17458
0.8
1.13582
27
1453 190375
1.9
1.26545
35
407
2297
0.6
1.49402
19
500
32944
3.9
1.44346
65
612
51360
2.2
1.61416
28
35
5239
1.2
1.56707
216
0.18840
0.17376
0.17509
0.21328
0.26532
0.25634
0.26260
0.31338
0.34467
0.36226
0.34161
0.33362
0.30731
0.32519
0.32632
0.27218
0.29309
0.30646
0.40785
0.40544
0.46499
0.44725
0.44571
0.51595
0.64514
0.12778
0.35150
0.01722
0.01512
0.03929
0.09573
0.11673
0.04646
4.1
2.1
3.3
1.4
7.7
4.3
3.0
1.6
2.4
1.4
1.0
2.6
6.1
1.0
2.3
8.3
3.2
9.5
1.9
3.4
1.0
2.6
1.9
1.5
9.9
4.5
1.3
2.8
7.2
7.2
4.1
6.6
7.8
5.0
1.9
4.0
1.0
5.3
1.2
25 ⎠ m spot size.
0.02163
0.02251
0.02334
0.02343
0.02350
0.02381
0.24
0.55
0.51
0.37
0.40
0.12
0.88
0.91
0.90
0.67
0.90
0.91
0.78
0.73
0.69
0.67
0.40
0.70
0.94
0.36
0.67
0.60
0.85
0.93
0.94
0.94
0.56
0.72
0.79
0.30
0.94
0.75
0.39
0.18
0.29
0.57
0.59
0.76
0.46
137.9
143.5
148.7
149.3
149.8
151.7
1112.7
1032.8
1040.1
1246.3
1517.0
1471.1
1503.1
1757.3
1909.1
1992.9
1894.4
1855.9
1727.5
1815.0
1820.5
1551.9
1657.0
1723.3
2205.1
2194.0
2461.6
2383.0
2376.1
2682.0
3209.2
775.2
1941.7
110.0
96.7
248.4
589.3
711.7
292.8
6.8
2.7
5.9
1.5
7.8
1.7
41.7
20.2
31.9
15.6
103.9
56.2
40.0
25.0
39.4
24.2
16.4
41.9
92.2
15.8
37.2
114.5
46.9
143.5
35.1
62.8
20.5
52.0
37.8
32.7
249.9
32.8
22.3
3.1
6.9
17.6
23.4
44.7
22.2
177.0
145.9
140.4
151.6
149.5
177.7
1132.7
1089.1
1142.4
1289.2
1519.3
1499.9
1582.7
1799.8
1896.3
1950.1
1903.2
1887.7
1823.9
1875.4
1936.1
1789.8
1913.2
2025.6
2330.0
2357.9
2502.1
2485.7
2529.5
2732.3
3428.1
1135.4
1886.7
186.8
269.5
377.5
738.2
795.4
567.7
33.3
4.7
10.4
3.8
18.5
16.0
31.7
15.6
25.2
14.9
67.4
36.6
30.7
18.7
29.8
18.4
21.3
32.0
54.7
23.8
30.2
117.8
32.4
89.9
18.3
32.9
16.6
33.7
22.4
46.4
103.5
40.8
29.5
27.6
58.1
39.8
36.7
48.1
73.2
736.8
184.6
1.5
188.1
144.7
539.5
1171.4
1203.5
1342.1
1361.5
1522.6
1540.8
1690.5
1849.3
1882.3
1905.0
1912.7
1922.9
1935.9
1943.0
2062.1
2079.6
2203.5
2349.5
2441.3
2502.8
2535.2
2570.7
2654.9
2769.7
3558.7
1909.2
1826.6
1317.5
2297.2
1272.2
1221.0
1037.7
1909.6
424.5
66.8
164.4
58.3
288.0
214.2
43.8
19.3
30.5
29.1
70.4
35.5
44.1
27.4
45.4
28.4
40.7
48.1
38.8
46.5
45.5
197.8
33.9
65.1
11.7
20.3
24.8
42.1
24.3
76.7
55.5
70.2
57.5
310.5
409.3
202.2
110.6
113.9
269.0
137.9
143.5
148.7
149.3
149.8
151.7
1171.4
1203.5
1342.1
1361.5
1522.6
1540.8
1690.5
1849.3
1882.3
1905.0
1912.7
1922.9
1935.9
1943.0
2062.1
2079.6
2203.5
2349.5
2441.3
2502.8
2535.2
2570.7
2654.9
2769.7
3558.7
1909.2
1826.6
110.0
2297.2
248.4
589.3
711.7
1909.6
6.8
2.7
5.9
1.5
7.8
1.7
43.8
19.3
30.5
29.1
70.4
35.5
44.1
27.4
45.4
28.4
40.7
48.1
38.8
46.5
45.5
197.8
33.9
65.1
11.7
20.3
24.8
42.1
24.3
76.7
55.5
70.2
57.5
3.1
409.3
17.6
23.4
44.7
269.0
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
2.05136
1.92293
2.08046
2.55980
3.46528
3.38055
3.75291
4.88557
5.47249
5.82501
5.51649
5.41811
5.02732
5.34091
5.73135
4.82818
5.58090
6.35145
8.92177
9.19766
10.75454
10.56535
11.07553
13.74538
28.25904
2.05937
5.41154
0.20198
0.30399
0.45032
1.06892
1.18885
0.74904
4.6
2.3
3.7
2.0
8.5
4.7
3.8
2.2
3.5
2.1
2.5
3.7
6.5
2.8
3.5
13.9
3.8
10.2
2.0
3.6
1.8
3.6
2.4
4.9
10.5
6.0
3.4
16.2
24.5
12.6
7.0
8.7
16.8
1.0
1.5
4.6
0.9
2.2
0.8
1.3
0.6
3.1
4.9
0.8
0.8
2.1
1.9
1.2
2.9
1.7
2.4
5.7
1.4
2.6
2.6
1.1
1.1
1.1
0.9
2.2
1.0
1.8
1.3
1.4
1.2
1.1
23
1
69
46
14
30
73
78
63
38
72
77
15
60
42
11
49
53
26
12
61
71
8
51
34
41
55
62
67
22
4
18
20
20.5
3.4
7.9
2.7
13.3
9.8
189
26862
655
35505
1011
10386
317 313500
171
27286
271
12724
347
9219
126
14498
296
98171
195 216687
171
9444
90
47051
147
5086
832 412781
180 867878
437
33388
961 121709
147 168088
631 104852
660
13554
959 585676
293
57142
269
16829
212 260562
147
3519
890
26962
614 1568192
250
2738
725
589
406
772
76
1743
154
2679
135
323
JG062305-1 marine sandstone: detrital zircon
67
430
1639
0.7
0.19042
108
1006
14982
1.0
0.15451
64
302
4297
1.9
0.14828
34
936
5809
1.2
0.16107
5
174
7105
0.9
0.15858
27
2112
2602
2.5
0.19130
217
223
727
889
434
465
729
889
1684
1140
99
308
208
850
68
546
870
429
92
185
322
483
546
980
153
614
736
113
367
923
759
434
231
546
160
231
198
197
168
431
129
150
2335
1789
14875
5461
7556
6106
17878
34468
9168
1912
9894
7519
21080
1032
11970
2709
9319
1936
3525
6270
6678
9991
8905
3907
11321
18267
3374
9627
9165
18147
10740
4439
8594
2034
2663
5826
9490
6404
11648
4330
6846
1.0
1.9
1.6
2.5
1.6
0.6
1.9
2.5
0.4
1.3
1.1
1.5
1.7
1.5
1.6
2.1
1.7
1.9
1.7
1.2
1.7
1.4
1.9
1.2
1.6
1.0
1.3
2.7
1.6
1.6
1.7
2.7
1.3
1.2
0.6
1.4
1.1
1.6
3.3
1.4
0.5
0.15410
0.14763
0.17048
0.17539
0.20283
0.20039
0.21746
0.21043
0.23376
0.23645
0.22396
0.24289
0.24806
0.33719
0.27786
0.32909
0.27118
0.24848
0.32596
0.28451
0.30119
0.31335
0.31607
0.29877
0.30920
0.33605
0.33839
0.46039
0.49552
0.50225
0.51905
0.51992
0.56235
0.65171
0.63770
0.56024
0.56724
0.59394
0.66382
0.62230
0.66686
10.2
15.2
3.4
8.2
9.3
2.7
3.2
2.6
7.6
10.6
3.9
8.0
4.7
17.6
6.2
13.5
4.1
12.0
10.9
5.6
5.4
4.8
5.4
6.9
4.5
2.2
7.6
6.4
5.5
2.9
6.3
4.9
7.6
12.1
7.0
5.0
3.4
6.9
5.6
8.1
3.8
0.02398
0.02417
0.02503
0.02635
0.02680
0.02901
0.03018
0.03080
0.03243
0.03411
0.03444
0.03544
0.03575
0.03833
0.03857
0.03901
0.03917
0.03928
0.03935
0.03970
0.04195
0.04308
0.04349
0.04360
0.04381
0.04667
0.05089
0.06158
0.06384
0.06419
0.06542
0.06566
0.06796
0.07129
0.07200
0.07224
0.07498
0.07536
0.07953
0.08235
0.08364
2.7
2.2
2.9
4.2
6.5
1.7
2.5
1.0
3.1
4.6
2.2
2.0
4.5
7.6
5.3
4.0
2.3
5.2
5.6
2.3
3.7
3.3
4.6
3.4
2.0
1.7
3.9
5.6
5.1
2.1
4.6
2.1
4.0
1.8
5.9
2.0
2.0
5.2
1.6
2.5
2.8
0.27
0.15
0.85
0.50
0.70
0.63
0.77
0.39
0.41
0.43
0.57
0.25
0.96
0.43
0.84
0.30
0.55
0.43
0.51
0.41
0.69
0.68
0.85
0.50
0.45
0.76
0.51
0.87
0.92
0.72
0.73
0.43
0.52
0.15
0.84
0.41
0.59
0.76
0.29
0.30
0.73
152.8
154.0
159.4
167.7
170.5
184.3
191.7
195.5
205.7
216.2
218.3
224.5
226.4
242.5
244.0
246.7
247.7
248.4
248.8
251.0
264.9
271.9
274.4
275.1
276.4
294.1
320.0
385.2
398.9
401.1
408.5
409.9
423.8
443.9
448.2
449.6
466.1
468.4
493.3
510.1
517.8
4.1
3.4
4.6
6.9
11.0
3.1
4.7
1.9
6.4
9.7
4.8
4.4
10.0
18.2
12.6
9.7
5.5
12.7
13.6
5.7
9.5
8.7
12.4
9.3
5.5
4.9
12.1
20.8
19.6
8.1
18.2
8.5
16.4
7.7
25.4
8.7
9.0
23.7
7.7
12.1
13.7
145.5
139.8
159.8
164.1
187.5
185.5
199.8
193.9
213.3
215.5
205.2
220.8
225.0
295.0
249.0
288.9
243.6
225.3
286.5
254.2
267.3
276.8
278.9
265.4
273.6
294.2
295.9
384.5
408.7
413.2
424.5
425.1
453.1
509.5
500.9
451.7
456.2
473.4
516.9
491.3
518.8
13.8
19.9
5.0
12.5
16.0
4.6
5.9
4.5
14.6
20.6
7.3
15.9
9.4
45.1
13.8
33.9
8.8
24.3
27.1
12.5
12.6
11.6
13.1
16.0
10.9
5.7
19.6
20.5
18.5
9.8
21.8
17.1
27.8
48.4
27.6
18.1
12.6
26.2
22.8
31.6
15.4
29.1
-94.3
166.7
112.5
408.0
199.7
296.6
174.1
297.9
207.8
57.4
181.1
210.1
735.1
296.3
645.7
204.5
-8.6
606.3
284.2
288.8
318.0
316.1
180.7
249.4
295.1
110.4
380.2
463.9
481.7
512.5
508.1
604.1
815.8
749.5
462.1
406.9
497.8
622.7
404.4
523.1
236.4
370.9
41.5
168.1
149.2
49.5
47.4
54.9
157.8
221.8
77.0
181.0
29.8
337.8
76.6
277.3
79.0
261.9
201.6
115.9
88.8
80.0
63.7
138.5
93.6
33.1
155.6
71.2
46.8
44.8
93.9
97.8
140.4
250.3
80.0
100.6
62.0
99.7
116.4
173.2
57.1
152.8
154.0
159.4
167.7
170.5
184.3
191.7
195.5
205.7
216.2
218.3
224.5
226.4
242.5
244.0
246.7
247.7
248.4
248.8
251.0
264.9
271.9
274.4
275.1
276.4
294.1
320.0
385.2
398.9
401.1
408.5
409.9
423.8
443.9
448.2
449.6
466.1
468.4
493.3
510.1
517.8
4.1
3.4
4.6
6.9
11.0
3.1
4.7
1.9
6.4
9.7
4.8
4.4
10.0
18.2
12.6
9.7
5.5
12.7
13.6
5.7
9.5
8.7
12.4
9.3
5.5
4.9
12.1
20.8
19.6
8.1
18.2
8.5
16.4
7.7
25.4
8.7
9.0
23.7
7.7
12.1
13.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
89
26
60
50
97
88
51
74
105
91
99
24
56
110
102
12
28
87
77
43
23
62
79
104
84
29
32
90
98
55
72
35
39
85
101
49
94
41
54
93
20
218
492
434
408
467
30
95
12
681
156
1706
283
879
95
430
198
452
230
541
318
891
555
417
345
583
189
139
246
842
136
216
248
316
208
86
363
206
902
333
166
537
162
21359
16544
27054
50316
668
10445
1047
29017
2297
45467
8309
59649
9858
35947
14458
28835
18932
49226
47188
71671
48185
46486
30163
33359
23955
15910
22400
41890
20488
131594
69525
76374
22052
10848
49399
14882
105572
56308
72860
118565
25569
4.6
2.0
10.3
2.3
400.8
0.9
2.5
1.1
1.3
9.4
0.8
1.2
1.0
2.4
2.0
1.2
1.6
2.6
22.9
4.2
2.3
1.8
2.0
4.4
2.5
0.6
1.0
1.4
1.4
1.7
2.9
2.9
1.0
1.2
1.4
1.1
1.7
2.3
0.6
4.2
2.1
0.76138
0.70644
0.78383
0.79953
0.63560
0.99409
0.65177
1.17482
1.11337
1.28124
1.36404
1.29522
1.26904
1.52387
1.50262
1.51736
1.50966
1.55910
1.53553
1.58182
1.66623
1.70993
1.67184
1.74111
1.81871
2.06356
2.15708
1.96926
2.39482
2.63841
3.35271
3.13417
2.90580
3.74805
3.49561
4.21388
5.18629
7.63960
8.44318
10.07169
12.69661
5.9
6.4
3.2
3.6
15.1
4.2
21.0
3.7
3.4
9.0
3.5
5.1
6.6
6.2
2.8
2.7
2.9
2.7
2.6
2.8
1.7
3.8
2.3
8.2
3.9
2.0
8.0
2.6
4.7
4.5
7.3
7.9
7.7
6.4
3.4
10.4
2.6
2.3
12.2
3.2
4.2
0.08420
0.08506
0.09534
0.09936
0.11088
0.11893
0.12202
0.12867
0.13023
0.13052
0.13812
0.13903
0.14506
0.15066
0.15320
0.15402
0.15548
0.15789
0.15890
0.16273
0.16743
0.16939
0.16396
0.16887
0.17424
0.19232
0.19999
0.18213
0.20849
0.22582
0.26147
0.24429
0.22640
0.27440
0.24809
0.26493
0.32038
0.39873
0.38585
0.43686
0.51325
5.7
6.1
2.8
3.1
8.8
2.5
7.5
3.2
2.5
8.7
2.6
4.8
5.5
4.4
1.5
2.3
2.5
1.9
2.3
2.1
1.2
2.4
2.0
8.1
3.1
1.4
7.7
2.0
4.2
3.8
6.7
7.4
7.3
6.2
2.5
10.1
1.0
1.4
11.6
3.1
3.4
0.97
0.95
0.88
0.87
0.58
0.59
0.36
0.87
0.74
0.96
0.73
0.95
0.85
0.71
0.56
0.83
0.85
0.72
0.88
0.74
0.68
0.62
0.85
0.99
0.81
0.73
0.96
0.77
0.88
0.86
0.92
0.93
0.96
0.97
0.73
0.97
0.39
0.60
0.95
0.96
0.81
521.1
526.3
587.1
610.7
677.9
724.4
742.2
780.3
789.2
790.8
834.0
839.2
873.2
904.7
918.9
923.5
931.6
945.0
950.7
971.9
998.0
1008.7
978.7
1005.9
1035.4
1133.9
1175.3
1078.6
1220.8
1312.6
1497.4
1408.9
1315.6
1563.1
1428.6
1515.0
1791.6
2163.2
2103.6
2336.6
2670.5
28.7
30.6
15.7
17.9
56.5
16.8
52.4
23.4
18.9
64.5
20.1
37.9
45.3
37.0
13.1
19.6
21.5
16.9
20.2
18.7
11.0
22.2
17.7
75.2
29.9
15.0
82.2
20.0
46.2
45.7
89.3
93.8
87.3
86.4
32.3
136.0
15.6
25.2
208.2
61.1
74.3
574.8
542.6
587.7
596.6
499.6
700.8
509.6
788.9
759.8
837.4
873.6
843.6
832.0
940.0
931.4
937.4
934.3
954.1
944.7
963.1
995.7
1012.3
997.9
1023.9
1052.2
1136.8
1167.3
1105.1
1241.1
1311.4
1493.4
1441.1
1383.4
1581.7
1526.2
1676.7
1850.4
2189.5
2279.8
2441.4
2657.4
26.0
26.8
14.2
16.1
59.8
21.0
84.4
20.1
18.3
51.5
20.6
29.0
37.3
37.8
16.8
16.8
17.9
16.5
16.0
17.4
11.0
24.6
14.6
52.8
25.3
13.6
55.4
17.6
33.7
33.1
57.2
61.2
57.9
51.7
27.2
85.7
21.7
20.5
110.9
29.9
39.3
793.1
611.9
590.0
543.3
-252.3
625.9
-434.3
813.3
674.2
963.0
975.3
855.3
723.3
1023.8
961.3
970.3
940.6
975.1
930.8
943.0
990.9
1019.9
1040.2
1062.5
1087.2
1142.3
1152.6
1157.5
1276.4
1309.5
1487.8
1488.9
1489.6
1606.6
1664.2
1885.5
1917.1
2214.3
2441.9
2529.9
2647.5
31.2
42.3
32.4
39.0
312.7
72.2
520.3
38.4
49.2
51.9
48.8
32.3
74.4
87.7
46.7
31.3
32.2
37.8
25.5
38.5
25.7
61.0
24.3
26.8
45.7
27.1
45.2
33.2
43.0
44.8
55.5
54.0
41.5
31.0
43.3
47.8
42.2
31.7
62.5
14.3
40.2
521.1
526.3
587.1
610.7
677.9
724.4
742.2
780.3
789.2
790.8
834.0
839.2
873.2
904.7
918.9
923.5
931.6
945.0
950.7
971.9
998.0
1019.9
1040.2
1062.5
1087.2
1142.3
1152.6
1157.5
1276.4
1309.5
1487.8
1488.9
1489.6
1606.6
1664.2
1885.5
1917.1
2214.3
2441.9
2529.9
2647.5
28.7
30.6
15.7
17.9
56.5
16.8
52.4
23.4
18.9
64.5
20.1
37.9
45.3
37.0
13.1
19.6
21.5
16.9
20.2
18.7
11.0
61.0
24.3
26.8
45.7
27.1
45.2
33.2
43.0
44.8
55.5
54.0
41.5
31.0
43.3
47.8
42.2
31.7
62.5
14.3
40.2
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
65
57
92
76
103
11
36
14
100
15
68
38
44
69
73
47
40
13
96
82
71
59
52
37
78
109
31
106
81
7
10
58
80
18
86
66
45
19
9
16
61
219
12.87657
14.13823
14.86146
0.22551
1.73945
3.61048
5.43768
2.33124
0.49603
0.53764
0.54085
0.02130
0.16709
0.27905
0.35576
0.12901
8.3
12.3
2.3
8.9
8.8
4.5
5.6
9.2
2.9
2.8
7.3
2.1
1.7
3.5
2.2
3.7
6.0
2.7
1.5
2.4
3.4
2.5
2.8
2.2
2.1
7.3
2.5
8.2
6.6
2.6
4.5
4.4
1.3
1.8
2.2
2.6
1.7
3.2
1.6
25 ⎠ m spot size.
0.00169
0.01728
0.01782
0.01785
0.01817
0.01834
0.01857
0.01876
0.02101
0.02702
0.02705
0.02745
0.02748
0.02813
0.02938
0.03503
0.03684
0.03836
0.03932
0.04392
0.05100
0.06655
0.06708
0.06876
0.06902
0.07015
0.07115
0.07189
0.07190
0.07526
0.07582
0.31
0.26
0.82
0.58
0.32
0.30
0.39
0.51
0.18
0.33
0.52
0.44
0.31
0.29
0.19
0.08
0.29
0.62
0.40
0.32
0.29
0.31
0.95
0.53
0.60
0.57
0.64
0.15
0.44
0.73
0.24
0.99
0.98
0.73
0.36
0.85
0.88
0.91
0.96
10.9
110.4
113.9
114.1
116.1
117.1
118.6
119.8
134.1
171.9
172.1
174.6
174.7
178.8
186.6
221.9
233.2
242.7
248.6
277.1
320.7
415.4
418.5
428.7
430.2
437.1
443.1
447.5
447.6
467.8
471.1
2596.7
2773.6
2787.0
135.8
996.1
1586.6
1962.0
782.2
0.3
3.0
8.2
2.4
2.0
4.0
2.6
4.3
7.9
4.6
2.5
4.2
5.8
4.4
5.1
4.8
4.8
17.5
6.2
22.3
20.6
10.3
18.3
18.4
5.2
7.7
9.6
11.4
7.4
14.5
7.4
177.8
277.3
52.6
11.9
81.5
63.7
94.5
67.5
10.6
113.8
111.7
111.0
115.6
121.2
121.1
121.5
168.5
186.3
169.1
172.4
176.3
182.8
205.1
286.4
242.3
272.6
237.5
346.2
375.2
476.5
419.2
499.7
423.8
437.9
449.2
528.8
411.5
469.9
514.5
2670.7
2759.0
2806.4
206.5
1023.3
1551.8
1890.8
1221.9
1.0
11.6
9.4
3.9
5.8
13.3
6.3
8.2
52.9
14.1
4.4
8.7
17.7
14.5
27.0
69.6
15.6
28.1
13.3
74.9
71.8
31.6
16.3
33.2
7.3
11.4
12.8
70.6
13.2
16.5
27.0
79.4
119.2
30.1
45.5
67.1
40.8
52.7
67.7
-57.8
184.8
65.0
46.6
105.7
201.3
170.7
154.2
684.7
373.1
127.1
142.8
197.1
233.6
422.8
851.9
331.7
538.2
129.1
840.9
727.4
782.6
423.2
839.5
388.7
442.1
480.4
896.7
213.9
480.3
712.2
2727.1
2748.4
2820.4
1115.9
1081.9
1504.8
1813.5
2112.1
216.6
243.3
122.9
72.7
118.1
258.3
118.6
144.1
734.4
176.9
57.4
115.9
242.5
191.8
320.5
587.8
156.9
201.6
135.0
508.8
469.4
166.3
32.5
148.7
37.6
58.2
59.9
351.3
81.1
66.4
138.2
21.1
38.5
35.1
458.7
110.3
45.3
46.2
45.3
10.9
110.4
113.9
114.1
116.1
117.1
118.6
119.8
134.1
171.9
172.1
174.6
174.7
178.8
186.6
221.9
233.2
242.7
248.6
277.1
320.7
415.4
418.5
428.7
430.2
437.1
443.1
447.5
447.6
467.8
471.1
2727.1
2748.4
2820.4
135.8
1081.9
1504.8
1813.5
2112.1
0.3
3.0
8.2
2.4
2.0
4.0
2.6
4.3
7.9
4.6
2.5
4.2
5.8
4.4
5.1
4.8
4.8
17.5
6.2
22.3
20.6
10.3
18.3
18.4
5.2
7.7
9.6
11.4
7.4
14.5
7.4
21.1
38.5
35.1
11.9
110.3
45.3
46.2
45.3
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
1.5
2.6
1.2
0.8
3.8
0.8
2.4
5.6
8.4
12.5
3.2
24.4
10.4
5.1
6.1
9.5
47448
34250
23522
4409
32107
17097
33761
2750
70
46
83
4
8
22
25
17
9.3
10.8
8.9
3.7
5.3
11.6
5.5
7.2
34.0
8.3
2.9
5.5
10.9
8.7
14.6
27.8
7.2
11.8
6.3
25.5
22.9
8.3
4.7
8.4
2.1
3.2
3.5
17.1
3.9
4.4
6.7
171
551
59
719
63
116
234
1257
JG063005-1 red arenite: detrital zircon
72
3172
5753
1.4
0.01050
4
313
1797
0.9
0.11860
63
794
10310
1.3
0.11624
60
825
4707
1.6
0.11555
27
550
5741
2.4
0.12060
77
494
3520
1.8
0.12675
32
347
3663
1.6
0.12668
51
540
5887
1.3
0.12707
83
304
1828
2.2
0.18052
25
838
8165
2.0
0.20138
84
862
11165
1.0
0.18115
66
584
13479
0.7
0.18506
12
449
7699
0.7
0.18961
80
829
6813
2.2
0.19720
46
1072
3666
1.1
0.22382
1
1468
1419
5.4
0.32580
53
588
5933
4.2
0.26954
105
712
2578
0.9
0.30794
79
322
6227
0.9
0.26356
7
584
1550
1.0
0.40635
20
128
1391
1.1
0.44708
106
673
2817
1.6
0.59882
30
1791
43363
97.1
0.51118
89
1751
2273
5.1
0.63572
98
354
34326
1.4
0.51790
49
819
36129
7.4
0.53919
74
533
46761
2.3
0.55637
16
1000
1930
5.7
0.68328
47
184
6340
0.9
0.49975
15
230
14285
1.1
0.58849
99
678
2725
2.3
0.65988
220
592
395
648
729
621
881
1591
794
60
458
102
220
272
291
317
336
467
261
327
490
178
306
489
417
258
282
1906
291
219
581
288
723
2224
182
459
350
309
358
381
930
380
20339
17917
1332
12291
7886
49040
4007
13004
2892
1837
2081
13154
6979
3931
16168
17656
50566
27048
37911
50926
11752
3581
19342
56569
35750
30681
2377
20112
18247
2837
68290
130233
8362
31018
19559
78589
48583
56427
17793
9799
94760
1.9
0.9
0.7
1.2
1.3
3.1
5.2
1.3
0.8
0.5
0.7
1.2
0.9
2.8
1.2
2.9
3.7
9.1
1.9
2.0
0.9
0.6
1.0
2.9
1.6
1.5
0.9
3.9
0.7
1.3
2.0
2.3
2.3
2.0
7.0
1.9
2.0
2.0
3.2
0.7
1.6
0.63271
0.61566
0.77670
0.67932
0.69684
0.79773
0.88665
1.01786
0.97158
1.14381
1.16317
1.07710
1.18583
1.36417
1.17479
1.41213
1.49248
1.54822
1.70209
1.74827
1.71465
1.65363
1.97190
2.09089
2.24460
2.61196
2.13119
3.58019
4.70952
3.87132
5.05362
5.17397
4.64995
5.83792
5.45946
6.21672
6.03167
6.35430
5.84563
6.15590
7.72111
1.6
2.6
18.1
3.8
7.3
1.5
6.5
3.6
7.1
4.9
21.5
2.4
7.0
19.4
3.6
2.8
5.5
2.6
4.9
2.1
2.7
4.8
3.5
3.5
2.5
2.0
8.6
4.8
2.6
9.0
2.1
3.1
3.2
3.0
4.0
1.3
3.6
6.6
11.7
6.2
2.7
0.07910
0.07930
0.08298
0.08340
0.08362
0.09668
0.09687
0.10969
0.11357
0.11747
0.11864
0.11981
0.12739
0.13013
0.13274
0.14637
0.15025
0.15951
0.16893
0.17233
0.16819
0.16211
0.18570
0.19528
0.20520
0.22382
0.17396
0.25937
0.31470
0.25572
0.32528
0.32707
0.29273
0.35282
0.32612
0.36772
0.35218
0.37085
0.33286
0.33944
0.38448
1.2
1.3
7.0
1.8
2.1
1.0
5.0
2.2
2.4
3.3
3.8
1.9
6.7
3.7
1.7
2.3
4.3
2.4
3.5
1.9
2.4
1.8
3.4
2.8
2.1
1.0
7.4
3.5
1.8
3.2
1.7
2.7
2.1
2.1
1.8
1.0
3.0
6.1
6.0
5.7
1.8
0.73
0.50
0.39
0.47
0.28
0.68
0.78
0.60
0.34
0.68
0.18
0.79
0.96
0.19
0.48
0.83
0.79
0.93
0.71
0.87
0.89
0.37
0.96
0.80
0.83
0.49
0.86
0.72
0.69
0.36
0.81
0.85
0.66
0.70
0.46
0.79
0.84
0.92
0.51
0.92
0.68
490.7
491.9
513.9
516.4
517.7
594.9
596.1
670.9
693.5
716.0
722.7
729.5
773.0
788.6
803.5
880.6
902.4
954.0
1006.2
1024.9
1002.1
968.5
1098.0
1149.9
1203.2
1302.0
1033.9
1486.6
1763.8
1467.9
1815.5
1824.2
1655.1
1948.0
1819.5
2018.7
1945.0
2033.4
1852.2
1884.0
2097.2
5.7
6.3
34.6
8.8
10.2
5.7
28.6
13.8
15.8
22.6
26.2
12.9
48.8
27.7
13.0
19.3
36.5
21.3
32.4
17.6
22.2
16.0
34.1
29.4
22.6
11.8
71.1
46.2
27.9
42.6
26.9
42.5
30.9
35.1
28.9
17.3
51.0
106.8
96.7
92.8
32.4
497.8
487.1
583.6
526.4
536.9
595.5
644.6
712.8
689.3
774.3
783.4
742.2
794.0
873.7
788.9
894.1
927.3
949.8
1009.3
1026.5
1014.0
990.9
1106.0
1145.8
1195.1
1304.0
1159.0
1545.1
1768.9
1607.7
1828.4
1848.3
1758.3
1952.1
1894.2
2006.8
1980.4
2026.0
1953.2
1998.2
2199.1
6.4
10.2
80.5
15.5
30.5
6.7
30.8
18.6
35.7
26.5
118.0
12.4
38.5
114.3
19.7
16.9
33.4
15.9
31.5
13.9
17.2
30.4
23.8
23.9
17.4
14.8
59.6
38.2
21.8
73.1
17.8
26.8
26.8
25.9
34.1
11.1
31.5
58.4
101.8
54.0
24.0
530.2
464.5
865.0
569.9
619.4
597.8
818.4
847.1
675.6
946.5
960.5
780.8
853.6
1095.9
747.8
927.6
987.1
939.9
1016.1
1029.9
1039.8
1041.0
1121.5
1138.1
1180.5
1307.3
1401.0
1626.1
1775.0
1796.1
1843.0
1875.6
1883.1
1956.3
1977.1
1994.6
2017.6
2018.4
2062.0
2118.5
2295.4
24.5
50.6
348.5
72.7
151.6
23.6
84.8
60.6
144.0
73.3
437.2
30.2
41.2
384.9
66.6
32.8
68.8
18.8
70.4
21.7
24.1
89.9
20.5
41.2
27.2
34.1
82.9
61.7
34.3
154.0
22.5
29.9
43.2
38.3
62.9
13.9
34.6
45.8
177.7
42.5
33.7
490.7
491.9
513.9
516.4
517.7
594.9
596.1
670.9
693.5
716.0
722.7
729.5
773.0
788.6
803.5
880.6
902.4
954.0
1016.1
1029.9
1039.8
1041.0
1121.5
1138.1
1180.5
1307.3
1401.0
1626.1
1775.0
1796.1
1843.0
1875.6
1883.1
1956.3
1977.1
1994.6
2017.6
2018.4
2062.0
2118.5
2295.4
5.7
6.3
34.6
8.8
10.2
5.7
28.6
13.8
15.8
22.6
26.2
12.9
48.8
27.7
13.0
19.3
36.5
21.3
70.4
21.7
24.1
89.9
20.5
41.2
27.2
34.1
82.9
61.7
34.3
154.0
22.5
29.9
43.2
38.3
62.9
13.9
34.6
45.8
177.7
42.5
33.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
2
55
40
57
45
93
38
8
91
48
102
92
95
26
33
58
90
81
13
9
65
42
39
19
85
104
29
36
37
24
11
54
5
73
100
10
14
62
68
69
70
221
0.31560
0.42563
0.46254
0.49901
0.47414
0.46765
0.44206
0.43841
0.40352
0.49367
0.60900
0.01658
0.02524
0.02531
0.02783
0.06221
0.06899
0.09464
0.20865
0.15712
0.13207
0.12832
0.12188
0.37113
0.52060
3.8
4.7
6.7
3.3
2.4
1.0
4.4
6.9
4.0
2.0
1.3
5.1
6.5
4.0
9.8
5.1
3.0
1.9
2.7
3.9
3.8
4.2
10.2
3.1
3.7
3.1
1.5
2.9
1.7
2.2
2.9
1.4
1.6
2.5
5.3
2.2
1.4
1.6
2.4
35 ⎠ m spot size.
0.01667
0.01680
0.01738
0.01972
0.02571
0.02577
0.02622
0.02655
0.02685
0.02754
0.02808
0.02824
0.03431
0.03617
0.63
0.52
0.52
0.51
0.17
0.69
0.47
0.39
0.69
0.30
0.64
0.71
0.43
0.39
0.84
0.91
0.99
0.90
0.73
0.62
0.86
0.82
0.65
0.87
0.65
0.20
0.19
0.36
0.21
0.56
0.26
0.06
0.69
0.49
0.25
0.29
0.39
0.90
0.95
106.6
107.4
111.1
125.9
163.7
164.0
166.8
168.9
170.8
175.2
178.5
179.5
217.5
229.0
1768.2
2286.0
2450.8
2609.5
2501.7
2473.2
2359.9
2343.5
2185.2
2586.5
3066.0
106.0
160.7
161.1
177.0
389.1
430.1
582.9
1221.6
940.8
799.7
778.3
741.4
2034.7
2701.7
3.3
1.6
3.2
2.2
3.5
4.6
2.3
2.7
4.1
9.1
3.9
2.5
3.3
5.3
58.1
91.0
136.4
71.3
49.2
20.5
87.9
135.2
73.6
43.5
31.2
5.4
10.3
6.4
17.1
19.2
12.5
10.7
30.1
34.2
28.9
30.5
71.6
53.8
81.0
110.1
101.7
111.5
124.2
192.6
162.4
166.0
171.9
180.7
219.8
182.7
179.7
214.8
229.3
2077.1
2385.0
2479.7
2550.5
2509.0
2502.7
2472.7
2498.9
2450.1
2703.7
3112.4
179.3
230.0
256.2
260.2
509.4
623.7
770.6
1185.7
997.5
938.5
972.2
1011.6
1946.3
2567.3
5.2
2.8
6.0
4.0
21.9
6.2
4.6
6.6
5.9
34.1
5.8
3.3
7.0
12.5
39.5
47.9
62.6
34.4
30.3
15.0
47.9
77.7
56.7
22.2
18.9
43.1
69.8
25.1
105.3
36.1
54.6
162.8
27.4
50.4
94.5
88.6
170.2
29.8
36.0
185.7
-30.7
120.3
91.4
564.0
138.5
153.6
213.4
312.5
729.8
237.0
182.3
186.0
232.0
2399.1
2470.7
2503.6
2503.9
2515.0
2526.6
2566.8
2627.7
2677.9
2792.5
3142.5
1304.2
1015.4
1251.6
1097.7
1093.8
1407.7
1361.9
1120.8
1124.4
1280.4
1440.6
1654.3
1853.4
2462.8
89.9
60.8
113.9
68.9
268.8
69.9
62.7
89.3
59.3
350.7
60.9
32.6
76.4
129.9
41.2
36.8
14.8
26.9
37.5
21.3
43.9
78.7
77.3
19.2
23.5
506.8
691.9
202.9
941.6
149.1
217.2
588.1
56.4
137.8
291.9
258.4
456.5
27.5
19.4
106.6
107.4
111.1
125.9
163.7
164.0
166.8
168.9
170.8
175.2
178.5
179.5
217.5
229.0
2399.1
2470.7
2503.6
2503.9
2515.0
2526.6
2566.8
2627.7
2677.9
2792.5
3142.5
106.0
160.7
161.1
177.0
389.1
430.1
582.9
1120.8
1124.4
1280.4
1440.6
1654.3
1853.4
2462.8
3.3
1.6
3.2
2.2
3.5
4.6
2.3
2.7
4.1
9.1
3.9
2.5
3.3
5.3
41.2
36.8
14.8
26.9
37.5
21.3
43.9
78.7
77.3
19.2
23.5
5.4
10.3
6.4
17.1
19.2
12.5
10.7
56.4
137.8
291.9
258.4
456.5
27.5
19.4
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
4.5
6.73396
2.4
9.47385
2.2 10.49799
1.2 11.32814
3.1 10.83446
1.0 10.76060
0.8 10.41879
1.0 10.71715
1.3 10.16718
1.1 13.33572
3.2 20.43944
0.9
0.19317
1.3
0.25424
1.3
0.28703
2.8
0.29203
1.5
0.65151
0.8
0.84819
5.2
1.13602
1.1
2.21479
1.1
1.67085
1.9
1.52014
1.4
1.60500
0.7
1.70810
2.0
5.79914
2.4 11.53360
4.5
5.2
6.7
3.7
3.3
1.6
5.2
8.4
6.1
2.4
2.0
26.2
33.9
11.1
45.7
9.0
11.7
29.9
3.9
7.9
15.4
14.1
26.3
3.4
3.8
32309
22774
52832
44600
27316
45387
3617
1946
3222
44890
81502
366
1538
601
4076
1352
662
763
13268
4882
3718
757
739
31960
86198
17
61
28
64
35
87
59
50
34
31
22
56
23
82
21
75
6
88
67
86
18
97
76
44
71
5.0
2.9
5.7
3.4
12.5
4.1
3.0
4.2
3.6
17.3
3.4
2.0
3.6
6.1
1575
462
501
419
52
173
118
148
228
176
161
347
1151
2208
965
926
1087
446
146
209
300
151
349
429
232
AP052504-B red arenite: detrital zircon
5
767
7614
1.2
0.11448
86
483
4595
2.1
0.10535
23
751
5387
1.1
0.11608
43
796
7848
1.6
0.13007
95
1265
1180
1.5
0.20888
83
725
6409
0.3
0.17342
89
886
10013
1.3
0.17756
8
580
5870
0.7
0.18453
91
412
1393
0.6
0.19481
18
322
1314
0.4
0.24169
60
903
8083
0.7
0.19711
36
1050
11014
0.8
0.19360
7
333
8448
4.4
0.23563
3
114
3473
1.3
0.25334
222
757
369
1888
899
1504
721
44
416
68
974
737
569
169
968
224
698
696
504
560
319
542
861
371
451
653
423
453
412
465
249
522
1030
385
797
1238
657
323
835
278
609
857
17984
10509
6338
3756
1574
2600
1886
9753
2850
12126
15855
7585
6179
33026
16229
29604
23124
8892
16673
9220
33329
19681
22464
26864
15598
7662
13461
20201
22822
9953
19604
35103
17999
31278
64487
20349
5552
23398
13541
21010
54593
0.8
2.1
6.1
0.7
1.9
1.4
1.5
1.5
1.1
1.0
1.6
1.8
1.4
1.8
1.1
1.0
1.5
1.8
1.2
1.4
2.0
1.2
7.1
2.4
2.2
1.2
1.4
1.0
1.7
1.8
1.2
2.7
1.3
1.5
2.3
1.5
1.6
1.7
1.5
1.6
3.1
0.28395
0.30736
0.49580
0.56572
0.61144
0.60935
0.52689
0.55558
0.49269
0.59156
0.57728
0.61109
0.54981
0.58450
0.60615
0.59013
0.60960
0.62378
0.61292
0.57388
0.59852
0.60032
0.66640
0.61098
0.63546
0.62624
0.62170
0.61329
0.61906
0.63357
0.61988
0.62971
0.62432
0.63561
0.63353
0.65303
0.69359
0.64995
0.64319
0.65090
0.64126
2.5
7.5
5.8
7.0
5.2
8.0
6.8
2.5
9.3
5.3
1.8
3.4
5.0
6.4
2.0
2.0
2.9
4.1
2.8
2.7
1.8
7.8
9.0
2.5
4.3
2.8
10.5
2.8
4.1
7.0
1.4
1.4
3.3
2.8
2.2
9.6
7.4
1.5
4.2
3.5
2.9
0.04091
0.04262
0.05980
0.06519
0.06695
0.06910
0.07115
0.07140
0.07206
0.07234
0.07286
0.07476
0.07515
0.07524
0.07551
0.07565
0.07604
0.07624
0.07629
0.07633
0.07639
0.07660
0.07675
0.07684
0.07726
0.07726
0.07729
0.07758
0.07777
0.07820
0.07845
0.07934
0.07959
0.07982
0.07994
0.08025
0.08047
0.08090
0.08090
0.08104
0.08126
2.1
2.3
5.5
6.4
2.9
6.8
2.5
1.9
1.9
2.4
1.0
1.7
4.5
5.6
1.3
1.6
2.7
2.3
1.8
2.0
1.4
7.5
8.7
1.8
3.0
2.0
10.1
2.3
2.4
3.3
1.0
1.0
3.1
1.8
1.0
9.0
1.9
1.1
4.0
3.0
2.4
0.85
0.30
0.95
0.92
0.55
0.85
0.36
0.75
0.20
0.44
0.57
0.49
0.90
0.88
0.65
0.83
0.95
0.57
0.63
0.74
0.80
0.96
0.97
0.74
0.70
0.70
0.96
0.84
0.58
0.47
0.73
0.70
0.94
0.62
0.46
0.93
0.26
0.69
0.95
0.86
0.82
258.5
269.0
374.4
407.1
417.8
430.7
443.1
444.6
448.5
450.2
453.3
464.8
467.1
467.7
469.2
470.1
472.5
473.6
473.9
474.2
474.5
475.8
476.7
477.2
479.8
479.8
479.9
481.7
482.8
485.4
486.9
492.2
493.7
495.1
495.8
497.6
498.9
501.5
501.5
502.3
503.6
5.3
5.9
19.9
25.4
11.5
28.2
10.5
8.0
8.2
10.3
4.4
7.5
20.4
25.4
5.8
7.4
12.4
10.6
8.1
9.1
6.5
34.5
40.1
8.4
13.8
9.2
46.7
10.7
11.1
15.3
4.7
4.7
14.6
8.4
4.8
43.0
9.3
5.1
19.1
14.7
11.7
253.8
272.1
408.9
455.2
484.5
483.2
429.7
448.6
406.7
471.9
462.7
484.2
444.9
467.4
481.1
471.0
483.3
492.2
485.4
460.5
476.3
477.4
518.5
484.2
499.5
493.8
490.9
485.6
489.3
498.3
489.8
495.9
492.5
499.6
498.3
510.3
535.0
508.4
504.3
509.0
503.1
5.5
17.9
19.5
25.7
20.0
30.8
23.9
9.1
31.1
20.0
6.6
13.2
18.1
23.9
7.5
7.5
11.1
15.9
10.8
10.0
6.7
29.8
36.6
9.5
16.9
11.1
41.0
10.6
15.9
27.5
5.3
5.6
12.8
11.2
8.6
38.7
30.8
6.1
16.6
14.2
11.6
210.6
298.8
608.4
706.2
813.7
740.1
359.0
469.6
175.8
578.4
509.5
577.4
331.5
465.7
538.1
475.0
535.1
579.6
539.8
392.9
484.8
485.3
707.4
517.2
590.9
559.1
542.5
504.4
519.5
558.3
503.2
513.3
487.3
520.3
509.8
567.7
691.7
539.9
516.9
539.3
500.5
30.3
163.1
41.0
57.9
90.9
90.2
143.7
36.9
212.0
103.3
32.1
64.8
49.6
66.2
32.6
24.5
20.7
72.6
47.2
40.4
23.6
47.9
47.1
36.0
66.3
43.8
66.3
33.4
73.2
134.5
20.6
22.1
25.6
49.2
42.7
77.1
152.4
23.9
29.9
39.2
36.7
258.5
269.0
374.4
407.1
417.8
430.7
443.1
444.6
448.5
450.2
453.3
464.8
467.1
467.7
469.2
470.1
472.5
473.6
473.9
474.2
474.5
475.8
476.7
477.2
479.8
479.8
479.9
481.7
482.8
485.4
486.9
492.2
493.7
495.1
495.8
497.6
498.9
501.5
501.5
502.3
503.6
5.3
5.9
19.9
25.4
11.5
28.2
10.5
8.0
8.2
10.3
4.4
7.5
20.4
25.4
5.8
7.4
12.4
10.6
8.1
9.1
6.5
34.5
40.1
8.4
13.8
9.2
46.7
10.7
11.1
15.3
4.7
4.7
14.6
8.4
4.8
43.0
9.3
5.1
19.1
14.7
11.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
45
48
22
2
59
26
76
50
77
20
94
54
25
85
30
32
33
70
39
19
21
14
88
15
62
74
57
53
84
35
65
80
55
98
16
81
34
58
69
90
13
223
105
226
236
473
950
149
523
282
338
641
276
231
296
331
90
559
329
288
287
666
741
948
365
730
186
351
643
169
258
539
359
260
722
429
43
335
150
582
216
120
746
3526
11475
12890
18206
38687
11113
33638
14632
12820
32450
17532
10469
23311
23007
3213
31270
21590
11988
23929
63544
67957
57936
9933
72866
3695
14954
18269
14317
26779
31472
58900
61612
126249
87858
10069
47035
38654
64113
79095
22208
151982
1.4
2.1
1.4
1.3
1.4
1.7
1.6
1.6
1.0
2.0
2.1
1.2
1.1
2.0
1.1
1.1
1.6
1.0
1.8
1.9
1.3
1.7
1.6
1.2
2.4
1.3
2.0
0.5
1.0
3.8
2.5
4.2
3.3
4.6
0.7
6.5
1.7
2.4
1.9
2.4
9.2
0.67638
0.62950
0.65297
0.65377
0.64411
0.64679
0.65773
0.64890
0.64547
0.69482
0.73401
0.99206
1.04474
1.14495
1.16256
1.18359
1.17925
1.19961
1.17399
1.20714
1.22043
1.22920
1.27854
1.24449
1.30831
1.28011
1.46368
1.67729
1.78208
2.24368
2.95173
5.27478
5.28108
5.54841
5.28102
5.61310
6.80141
6.40116
11.64913
13.07006
12.62547
11.2
1.9
2.2
2.1
2.1
2.0
2.3
4.4
1.9
3.5
2.5
3.3
3.2
3.8
8.5
3.3
1.9
2.7
1.8
2.6
2.2
1.4
3.6
1.1
9.1
4.1
2.1
8.4
2.1
8.5
6.2
2.1
2.3
4.1
2.2
3.5
2.5
6.1
2.0
2.7
2.2
0.08171
0.08185
0.08186
0.08187
0.08200
0.08204
0.08209
0.08261
0.08401
0.08486
0.09226
0.11367
0.11616
0.12517
0.12646
0.12650
0.12699
0.12882
0.12896
0.13032
0.13142
0.13305
0.13428
0.13572
0.13770
0.13986
0.15049
0.16931
0.17378
0.18632
0.21296
0.33816
0.33574
0.35216
0.33327
0.35058
0.38168
0.30175
0.48036
0.52112
0.48794
4.0
1.3
1.1
1.4
1.6
1.3
2.0
4.1
1.5
3.1
2.4
2.7
2.8
3.3
2.2
3.1
1.7
2.1
1.5
2.4
1.0
1.0
2.9
1.0
1.6
3.3
1.2
8.1
1.7
8.3
5.4
1.9
1.9
3.5
1.2
3.1
1.1
5.7
1.8
2.5
1.7
0.36
0.67
0.50
0.67
0.79
0.64
0.86
0.93
0.80
0.87
0.95
0.79
0.85
0.88
0.26
0.92
0.87
0.76
0.87
0.92
0.45
0.71
0.80
0.89
0.18
0.79
0.56
0.97
0.78
0.97
0.87
0.91
0.83
0.87
0.57
0.91
0.45
0.94
0.89
0.94
0.79
506.3
507.2
507.2
507.3
508.0
508.3
508.6
511.7
520.0
525.1
568.9
694.0
708.4
760.3
767.7
767.8
770.7
781.1
781.9
789.7
796.0
805.2
812.3
820.4
831.6
843.9
903.7
1008.3
1032.9
1101.4
1244.6
1877.8
1866.2
1944.9
1854.2
1937.4
2084.1
1700.0
2528.8
2703.9
2561.8
19.4
6.1
5.3
6.9
8.0
6.2
9.6
20.3
7.6
15.4
13.1
17.5
18.5
24.0
16.1
22.3
12.1
15.1
11.4
17.5
7.5
7.6
21.9
7.7
12.6
25.7
10.0
76.0
15.8
83.7
60.8
31.6
30.5
59.3
20.1
52.6
19.9
85.4
37.0
56.1
36.3
524.6
495.8
510.3
510.8
504.8
506.5
513.2
507.8
505.7
535.7
558.9
699.8
726.3
774.9
783.2
793.0
791.0
800.4
788.5
803.9
810.0
814.0
836.2
820.9
849.4
836.9
915.5
1000.0
1038.9
1194.8
1395.3
1864.8
1865.8
1908.1
1865.8
1918.1
2085.9
2032.4
2576.6
2684.7
2652.1
46.0
7.3
8.9
8.5
8.2
8.0
9.1
17.7
7.6
14.6
10.9
16.9
16.8
20.7
46.5
18.3
10.5
15.1
9.7
14.3
12.5
7.8
20.6
6.3
52.2
23.5
12.8
53.3
13.8
59.7
47.1
18.2
19.3
34.9
18.7
29.9
22.2
53.4
18.6
25.4
20.4
605.0
443.5
524.1
526.6
490.4
498.4
534.0
490.4
441.4
581.1
518.4
718.2
781.8
817.1
827.5
864.3
848.6
854.4
807.1
843.3
848.6
837.9
900.4
822.2
896.0
818.5
944.2
981.6
1051.6
1367.9
1633.8
1850.3
1865.4
1868.4
1878.7
1897.3
2087.6
2389.2
2614.4
2670.3
2721.8
227.6
30.7
42.1
34.6
28.3
33.9
25.1
34.7
25.5
37.6
17.6
43.0
35.3
38.2
171.8
26.5
19.3
37.0
18.0
21.5
41.6
20.5
45.1
10.6
184.2
52.6
36.0
38.6
26.9
37.6
57.6
16.3
22.6
36.1
32.4
26.1
39.4
35.4
15.2
15.0
21.8
506.3
507.2
507.2
507.3
508.0
508.3
508.6
511.7
520.0
525.1
568.9
694.0
708.4
760.3
767.7
767.8
770.7
781.1
781.9
789.7
796.0
805.2
812.3
820.4
831.6
843.9
903.7
1008.3
1051.6
1367.9
1633.8
1850.3
1865.4
1868.4
1878.7
1897.3
2087.6
2389.2
2614.4
2670.3
2721.8
19.4
6.1
5.3
6.9
8.0
6.2
9.6
20.3
7.6
15.4
13.1
17.5
18.5
24.0
16.1
22.3
12.1
15.1
11.4
17.5
7.5
7.6
21.9
7.7
12.6
25.7
10.0
76.0
26.9
37.6
57.6
16.3
22.6
36.1
32.4
26.1
39.4
35.4
15.2
15.0
21.8
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
1
46
64
61
72
42
28
75
87
96
66
93
11
78
73
100
37
6
27
49
92
97
56
41
82
99
17
40
4
71
29
44
24
9
47
52
68
10
51
79
67
224
2642.2
436.9
47.6
24.2
2995.3
582.5
23.0
125.7
3241.8
1200.6
15.0
555.1
21.1
23.7
105.6
116.3
147.0
148.9
169.8
178.6
213.5
237.5
266.3
442.3
484.2
489.1
490.2
493.3
577.0
636.7
704.2
748.8
749.1
756.5
922.3
1094.0
1125.2
1827.0
3241.8
436.9
0.9
1.5
3.5
4.2
3.5
5.3
4.9
6.8
13.6
7.4
5.9
17.4
6.2
15.2
39.9
12.0
11.4
18.5
36.6
15.3
35.8
18.4
27.1
82.1
99.7
56.0
15.0
24.2
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector
ICP Mass Spectrometry
0.92
0.20
252.1
387.9
126.3
401.5
192.2
151.6
98.0
39.8
116.3
125.7
90.9
39.5
23.8
235.3
97.4
32.7
89.9
99.0
35.1
31.6
80.9
81.6
30.7
82.1
99.7
56.0
2.2
5.7
-11.9
286.3
-29.6
493.7
504.8
370.7
330.1
170.8
170.8
296.7
254.5
516.1
478.8
877.2
713.7
483.6
852.6
771.6
780.6
759.8
882.1
786.5
964.5
1094.0
1125.2
1827.0
0.50663
0.07012
2.3
4.7
5.9
23.5
14.2
11.5
8.6
6.9
15.5
13.7
10.6
16.3
6.6
51.0
39.8
11.4
22.6
27.4
29.7
14.0
34.9
25.0
21.3
31.7
41.5
30.1
2.4
28.2
20.8
26.4
100.1
135.9
170.1
162.9
181.0
178.1
210.0
243.0
265.1
454.4
483.3
563.6
531.6
491.6
636.2
667.2
722.7
751.5
783.3
764.2
934.9
1118.8
1074.3
1558.9
18.10500
0.77483
0.9
1.5
3.5
4.2
3.5
5.3
4.9
6.8
13.6
7.4
5.9
17.4
6.2
15.2
39.9
12.0
11.4
18.5
36.6
15.3
35.8
18.4
27.1
23.3
36.6
26.9
1.5
1.7
21.1
23.7
105.6
116.3
147.0
148.9
169.8
178.6
213.5
237.5
266.3
442.3
484.2
489.1
490.2
493.3
577.0
636.7
704.2
748.8
749.1
756.5
922.3
1131.6
1049.4
1368.7
20139
584
12
63
0.38
0.36
0.54
0.20
0.26
0.47
0.56
0.92
0.79
0.50
0.50
0.91
0.78
0.27
0.88
0.86
0.43
0.54
0.96
0.82
0.79
0.55
0.90
0.48
0.60
0.58
192
308
25 ⎠ m spot size.
JG070605-1 sandstone (white shale): detrital zircon
7
1199
2268
0.6
0.02070
11.2
0.00328
4.2
22
1682
2143
0.7
0.02638
18.0
0.00368
6.4
13
454
4259
1.1
0.10362
6.2
0.01652
3.3
28
644
7098
1.0
0.14323
18.4
0.01821
3.6
31
645
3631
2.2
0.18240
9.0
0.02307
2.4
32
550
3984
0.8
0.17399
7.6
0.02337
3.6
2
1993
14859
126.6
0.19519
5.2
0.02669
2.9
26
1126
12460
1.7
0.19168
4.2
0.02809
3.9
8
334
5833
1.2
0.22977
8.2
0.03368
6.5
23
647
9901
2.3
0.27042
6.4
0.03753
3.2
21
582
11314
1.2
0.29839
4.6
0.04218
2.3
20
475
14789
6.3
0.56439
4.4
0.07101
4.1
10
511
42432
1.7
0.60958
1.7
0.07801
1.3
24
114
3418
1.2
0.74208
11.8
0.07882
3.2
16
377
8258
1.9
0.68806
9.6
0.07901
8.4
9
290
9029
1.1
0.62273
2.9
0.07952
2.5
25
288
3138
2.8
0.87120
4.8
0.09364
2.1
17
186
4828
2.4
0.92917
5.6
0.10381
3.0
11
382
10849
1.3
1.03751
5.7
0.11542
5.5
15
310
18247
2.9
1.09622
2.6
0.12316
2.2
30
219
13927
1.1
1.16294
6.4
0.12323
5.1
19
47
4472
2.3
1.12248
4.7
0.12452
2.6
4
743
37611
2.3
1.51102
3.5
0.15382
3.2
1
39
4975
0.7
2.00972
4.7
0.19189
2.2
29
54
4395
0.8
1.88075
6.3
0.17680
3.8
33
360
15969
1.4
3.64259
3.8
0.23655
2.2
Igneous analyses in italics were not used in the average age calculation
Detrital analyses in italics were not plotted in the probability density plots because they 1) are more than 30% discordant, 2) are more than 5% reverse
discordant, or 3) have greater than 10% uncertainty
Shaded values were used in the maximum depositional age calculations
225
APPENDIX D:
U-Pb geochronology of basement rocks in central Tibet and paleogeographic
implications
Manuscript for submittal to
Journal of Asian Earth Sciences
Jerome Guynn
University of Arizona
Paul Kapp
University of Arizona
George Gehrels
University of Arizona
Lin Ding
Institute of Tibetan Plateau Research and Institute of Geology and Geophysics
226
U-Pb Geochronology of Basement Rocks in Central Tibet
and Paleogeographic Implications
Jerome Guynn
Paul Kapp,
George Gehrels,
Department of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA
Lin Ding
Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese
Academy of Sciences, Beijing 100029 China
227
Abstract
U-Pb geochronologic data from basement exposures in central Tibet provide new
constraints on the Gondwanan paleogeography of the Lhasa and Qiangtang terranes. The
Amdo basement is composed largely of granitic orthogneisses with subordinate
paragneisses and metasedimentary rocks. The intermediate-felsic compositions of the
orthogneisses suggest a continental arc or orogenic origin for the granitoid protoliths.
The orthogneisses have a bimodal distribution of Middle-Late Proterozoic (ca. 850-910
Ma) and Ordovician-Cambrian (ca. 480-530 Ma) crystallization ages.
The former
correspond in time with Rodinia post-assembly magmatism while the latter granitoid ages
are common throughout Gondwana, including the Himalaya, and indicate postGondwana assembly tectonics.
In addition, detrital zircon analysis of three
metasedimentary rocks from the Amdo basement reveals an abundance of Middle-Late
Proterozoic and Cambrian ages with strong peaks at ~550 Ma and ~950 Ma, scattered
early Middle-Early Proterozoic ages, a distinctive 2500 Ma peak and very few Archean
ages. The detrital zircon age spectra are distinct from those determined for Tethyan and
Greater Himalaya rocks from the northern margin of India, which have a much stronger
“Grenville” signature (1000-1300 Ma ages) indicating the two were not as proximal as
indicated in most reconstructions. Considering the geochronologic data, the necessity of
Greater India in addition to the current Indian continent, the constraints of southeast
Asian terranes that were rifted from northwest Australia, and the age and distribution of
cratons and orogenic belts in Gondwana, we propose a Gondwana location for the
combined Lhasa and Qiangtang terranes that is farther west than in most reconstructions,
228
closer to Iran, Afghanistan, Arabia and northwest India than to northeast India and
western Australia. We also suggest that the source for the Nepalese Himalayan rocks is
to the east near western Australia, eastern India and Antarctica.
1. Introduction
The Phanerozoic evolution of Asia is characterized by rifting of continental terranes
off of Gondwana, their travel across the Tethys and their accretion to a Eurasian core
(e.g. Metcalfe, 1996; Şengör, 1979; 1984; see Fig. 1). The delineation and timing of
rifting and accretion of these terranes has important consequences for the formation and
subsequent breakup of supercontinents and the process of accretion. The location of
these terranes in time also has implications for the timing and causes of orogenesis along
the margins of Gondwana in the Paleozoic and in Asia during the Mesozoic and
Cenozoic. However, the reconstruction of these terranes along Gondwana’s margin is
hampered by a scarcity of good paleomagnetic data due to the lack of appropriate
lithologies, difficult access and remagnetization as a result of Mesozoic and Cenozoic
tectonics. The latter has also resulted in the modification and overprinting of rifting and
collisional events by younger deformation. As a result, there is still a great deal of debate
concerning the position of peri-Gondwanan terranes in the Paleozoic and even less
constraint on pre-Gondwana locations. Here we present new geochronologic data from
Tibet that provides information on the Neoproterozoic-Cambrian history and Paleozoic
location of two peri-Gondwana terranes that comprise the bulk of the Tibetan Plateau.
229
The Tibetan Plateau is the largest orogenic feature on Earth, encompassing an area
circa 5,000,000 km3 with an average elevation around 5 km. The primary terranes that
compose the Tibetan Plateau (Fig. 1) accreted to the southern margin of Asia throughout
the late Paleozoic and Mesozoic (Allégre et al., 1984; Chang and Zheng, 1973; Dewey et
al., 1988; Şengör and Natal'in, 1996; Yin and Harrison, 2000; Yin and Nie, 1996) and,
from north to south, are the Kunlun-Qaidam, Songpan-Ganzi, Qiangtang and Lhasa; this
paper focuses on the last two. The Tibetan Plateau has been the focus of many recent
geologic and geophysical investigations but due to its large size, high elevation and
remoteness, there is still a great deal unknown about the geology of the plateau,
particularly the interior. Furthermore, the older geologic history of the terranes that make
up Tibet is obscured by the paucity of basement exposures and the predominance of
supercrustal assemblages that are late Paleozoic or younger.
We present new
geochronologic data from the Amdo gneisses, the only confirmed crystalline basement in
central Tibet, that gives information on the Precambrian and early Cambrian history of
the Lhasa and Qiangtang terranes and, combined with detrital zircon ages of Paleozoic
sedimentary rocks, provides new constraints on their position along the margin of
Gondwana. We propose that the primary source for Tibetan sediments in the Paleozoic
was northwest India and east Africa and consequently we suggest a location for the
terranes further west, towards northwest Indian and northeast African, than most
reconstructions indicate. Furthermore we show that the Tibetan Paleozoic detrital zircon
signature is distinct from that of the Himalaya and suggest that the sedimentary source for
230
the early Paleozoic Himalayan sediments is western Australia, eastern Indian and
Antarctica and not northwest Africa as previously proposed (DeCelles et al., 2000).
2. Regional Geology and Paleozoic Paleogeography
In this paper, in the context of the Lhasa and Qiangtang terranes, the Tethys Ocean is
subdivided into three oceanic regions after Şengör (1984) and Metcalfe (1996): the PaleoTethys, the ocean between Qiangtang and Songpan-Ganzi (Eurasia); the Meso-Tethys,
the ocean between Lhasa and Qiangtang (Eurasia); and the Neo-Tethys, the ocean
between India and Lhasa (Eurasia). Note that these three terms do not necessarily
indicate separate, unconnected oceans, but rather designate ocean crust that was created
and consumed between distinct continental terranes. Also, we use the terms terrane and
block interchangeably to indicate a fragment of continental or ocean island crust that is
significantly smaller than a continent (e.g. India). While authors have referred to the
North and South China “blocks”, we will use the alternative designation of “craton”,
since both contain Archean crust and are not much smaller than India.
The northernmost Tibetan terrane is the Kunlun-Qaidam terrane, a composite of Late
Paleozoic and Triassic arcs, Proterozoic gneisses and Paleozoic to Triassic sedimentary
rocks (Gehrels et al., 2003c; Yin and Harrison, 2000), followed by the Songpan-Ganzi
flysch complex, a thick accumulation of Triassic and early Jurassic deep marine
sediments thought to have come from the Triassic Qinling-Dabie orogeny which resulted
from the collision between the North China and South China cratons (Hacker et al., 1996;
Weislogel et al., 2006; Yin and Nie, 1996; Zhou and Graham, 1996). The Qiangtang
231
terrane is separated from the Songpan-Ganzi by the Jinsha suture and is dominated by
upper Paleozoic (Carboniferous and Permian), Jurassic and Triassic shallow marine
strata, fluvial deposits and volcanic sequences (Yin and Harrison, 2000). The southernmost Tibetan terrane is Lhasa, bounded by the Bangong suture zone to the north and the
Indus-Yarlung suture to the south. The Bangong suture zone is defined by a broad and
discontinuous series of ophiolites and is a result of Meso-Tethys Ocean closure and
subsequent collision between the Lhasa and Qiangtang terranes in the Early Cretaceous
(Dewey et al., 1988; Girardeau et al., 1984; Guynn et al., 2006; Kapp et al., 2003a).
Rocks exposed in the Lhasa terrane consist of minor exposures of Paleozoic shelf
sequences, Mesozoic marine and fluvial rocks, and, especially to the south, CretaceousTertiary volcanic sequences and plutons (Yin and Harrison, 2000). The Indus suture is
the sight of the collision of India with Asia in the early Tertiary and rocks to the south of
the suture have an Indian affinity (Hodges, 2000).
The Lhasa and Qiangtang terranes are typically placed at the northern edge of India in
most Gondwana reconstructions (e.g. Metcalfe, 1996, 1999; Scotese, 2001; Torsvik and
Smethurst, 1999 - see Fig. 2) in essentially the same configuration they have today. The
two terranes are considered to have been one prior to Permian-Triassic rifting on the basis
of similar lithologies, including the presence of Carboniferous-Permian diamictites that
are also found in northern Arabia, northern India and Australia (Leeder et al., 1988), and
shared Gondwanan flora and fauna (Dewey et al., 1988; Metcalfe, 1996). The timing of
rifting from Gondwana and between each other is largely based on Permo-Triassic
basalts, syn-rift sedimentation and normal faulting recorded in the Lhasa terrane,
232
Qiangtang terrane and northern India (Gaetani and Garzanti, 1991; Leeder et al., 1988;
Pearce and Mei, 1988; Yin and Harrison, 2000). Unfortunately, the roughly east-west
orientation of the northern edge of Gondwana in the late Paleozoic (see Fig. 2; Scotese,
2001) precludes the use of paleomagnetic data to constrain the lateral position of the
Lhasa-Qiangtang terrane and there is little data in the early Paleozoic. A summary of the
limited available paleomagnetic data by Li et al. (2004) suggests that the two terranes
were at approximately the same latitude until the Qiangtang terrane started drifting
northward in the late Permian and their ensuing latitudinal paleopositions are broadly
consistent with the timing of collisions along their bounding sutures.
The Qiangtang terrane is thought to have been part of a possibly continuous strip of
continent, the Cimmerian continent, that rifted off of northern Gondwana in the Permian
and drifted northward, closing the Paleo-Tethys and opening the Meso-Tethys (Metcalfe,
1996; Şengör, 1979, 1984). The Cimmerian continent is composed, from west to east, of
Turkey, Iran, Qiangtang and Sibumasu (Sino-Burma-Malaya-Sumatra) and its existence
is based on similarities in paleolatitudes, flora, fauna and age of accretionary deformation
between the different terranes. The longitudinal location of the terranes is largely based
on their current position and their exact arrangement in the Paleozoic is debatable.
Within this framework, in the Paleozoic the Iranian blocks (Yazd, Tabas, Lut) are
typically placed next to the northern margin of the Arabian plate, Afghan blocks (Farah,
Helmand) are tucked in a triangular region between Arabia and India, the Lhasa and
Qiangtang blocks are located along northern India and Sibumasu and West Burma sit
against northwestern Australia (Fig. 2) (e.g. Metcalfe, 1996). The South China craton
233
and Indochina block were likely located outboard of Arabia and India in the early
Paleozoic and than drifted to a position closer to Australia in the late Paleozoic (Fig. 2).
An important consideration in positioning terranes on the northern edge of India that
is often ignored is the space occupied by Greater India, that portion of the Indian
continent whose lower crust has been underthrust beneath Asia (Veevers et al., 1975) and
whose upper crust was composed of the three tectono-stratigraphic units that make up the
Himalaya, the Lesser Himalayan Sequence, the Greater Himalayan Sequence and the
Tethyan Himalaya (e.g. DeCelles et al., 2002). Shortening estimates suggest that at least
600-700 km of Indian lower crust has been thrust beneath Tibet (DeCelles et al., 2002
and references therein). The extra ~600 km on the northern edge of India, combined with
the known location of Indo-China terranes to the northwest of Australia, leaves much less
room for the placement of the Lhasa-Qiangtang terrane directly north of India. The room
problem is exacerbated by shortening in the Lhasa and Qiangtang terranes which plate
reconstruction models do not typically take into account. Geologic and geodetic data
suggest at least 50% shortening of the Lhasa and Qiangtang terranes due to Mesozoic and
Cenozoic deformation as a result of accretionary tectonics and the Indo-Asian collision
(Murphy et al., 1997; Yin and Harrison, 2000).
3. Amdo Basement Geology
In general, exposures of rocks within the Tibetan terranes consist of supracrustal
assemblages: low-grade sedimentary rocks, volcanic sequences and igneous plutons of
Phanerozoic age. Within the Qiangtang and Lhasa terranes, the only known Precambrian
234
basement exposure occurs near the town of Amdo along the Bangong suture zone
(Dewey et al., 1988; Guynn et al., 2006; Harris et al., 1988b; Xu et al., 1985). The Amdo
basement is composed of high-grade orthogneisses with subordinate metasedimentary
rocks, mafic amphibolites and migmatites that have been intruded by extensive Jurassic
granitoids (see Fig. 3; Coward et al., 1988; Guynn et al., 2006; Harris et al., 1988a;
Schärer et al., 1986). The Amdo basement has generally been considered part of the
Lhasa terrane because of ophiolite exposures at the town of Amdo north of the basement
(Coward et al., 1988), but Jurassic metamorphism of the basement indicates a Qiangtang
affinity since Lhasa did not arrive at the Eurasian margin until the Cretaceous and MesoTethys subduction is considered to have been northward (Dewey et al., 1988; Girardeau
et al., 1984; Guynn et al., 2006). However, since the evidence indicates the two terranes
were one until Permo-Triassic rifting, the differentiation between them is not critical for
Paleozoic paleogeography and the Amdo basement can be considered to have a combined
Lhasa-Qiangtang affinity and we will refer to the two terranes together for the rest of the
paper.
4. U-Pb Geochronology
We analyzed nine samples of orthogneisses from the Amdo basement using U-Pb
zircon geochronology to determine crystallization ages. The igneous composition of the
gneisses consists largely of granite and granodiorite, with smaller outcrops of monzonite,
tonalite, and quartz-diorite, and most contain biotite and/or hornblende. The majority of
the gneisses are strongly banded, though some show only a slight foliation fabric. A
235
more complete description of each dated sample is given in Appendix A. Migmatites,
mafic garnet amphibolites and the presence of sillimanite indicate upper amphibolite
facies conditions. U-Pb ages were also obtained for detrital zircons from two quartzites
and a paragneiss that occur in the basement. Locations of all samples are shown in
Figure 3 and sample coordinates and ages are listed in Table 1. The zircons were
analyzed using the Laser Ablation-MultiCollector-Inductively Coupled Plasma Mass
Spectrometer (LA-MC-ICPMS) facility at the University of Arizona.
4.1. Methods
Zircons were separated from samples of fresh rock using standard crushing and
separation techniques.
For magmatic analyses (including orthogneiss samples),
individual zircon crystals were hand-picked using a binocular microscope. All sizes and
morphologies were selected for analysis, though an attempt was made to choose crystals
that were free of inclusions and cracks. Approximately 50 grains were than mounted in
epoxy. For detrital analyses, zircons were poured in a layer onto the mount, generally
resulting in several hundred grains per sample.
While the addition of other heavy
minerals on the mount is generally not a problem, any pyrite was removed from the
mount as the mineral generally results in lead contamination. The mounts are than
polished to approximately 2/3 of the crystal thickness. If possible, magmatic zircons
were imaged using cathode-luminescence on the SEM prior to analysis. As a rule, the
number of grains analyzed is: 25 for a simple magmatic sample, 50 for a more complex
236
magmatic (such as lead loss, young zircon growth or abundant inheritance) and 100 for a
detrital sample.
The grains were ablated with a New Wave DUV193 Excimer laser, operating at a
wavelength of 193 nm and using a spot diameter of 25, 35, or 50 microns. Larger laser
spot sizes are more accurate, due to the larger volume of material, but small grain sizes or
metamorphic overgrowth may necessitate a smaller spot size. The ablated material is
carried via argon gas into the plasma source of a Micromass Isoprobe multicollector ICPMS and ionized. The Micromass Isoprobe is equipped with a flight tube of sufficient
width such that U, Th, and Pb isotopes are measured simultaneously using nine Faraday
collectors, an axial Daly detector and four ion-counting channels. All measurements
were made in static mode, using Faraday detectors for
ion-counting channel for 204Pb.
238
U, 232Th, and
208-206
Pb and an
235
U is determined from the 238U measurement assuming
238
U/235U = 137.88. Ion yields are ~1 mV per ppm. Each analysis consists of one
background run with 20-second integration on peaks and the laser off, 20 1-second
integrations with the laser firing, and a 30-second delay to purge for the next analysis.
The laser ablates at ~1 micron/second, resulting in an ablation pit ~20 microns in depth.
A common lead correction was performed by using the measured 204Pb and assuming an
initial Pb composition from Stacey and Kramers (1975) (with uncertainties of 1.0, 0.3,
and 2.0 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively). Measurement of 204Pb
is unaffected by the presence of
204
Hg because backgrounds are measured on peaks
(thereby subtracting any background
present in the argon gas.
204
Hg and
204
Pb) and because very little Hg is
237
Isotopic fractionation of Pb is generally <5%, while inter-element fractionation of
Pb/U is typically <20%. These fractionations were corrected by analysis of a standard
with a known, concordant ID-TIMS (Isotope Dilution – Thermal Ion Mass Spectromety)
age; standard analyses were conducted after every three unknowns for magmatic samples
and after every five unknowns for detrital samples. The zircon standards are fragments
of a large zircon crystal from a Sri Lankan pegmatite (e.g. Dickinson and Gehrels, 2003)
with an age of 564.0 ± 4.0 Ma (2-σ). The uncertainty resulting from the calibration
correction is generally ~3% (2-σ) for both
207
Pb/206Pb and
206
Pb/238U ages. U and Th
concentrations were determined by analyzing a piece of NIST 610 glass with known U
and Th concentrations of approximately 500 ppm and using the resulting measured
intensities to calibrate the zircon U and Th intensities.
206
Pb/238U and
σ) in the
The errors in determining
206
Pb/204Pb typically leads to several percent measurement uncertainty (2-
206
Pb*/238U age. Analyses which have very low
206
P/204Pb ratios, high
204
Pb
intensities or very low U concentrations are discarded due to the impractically high
uncertainty.
The low concentration of 235U relative to 238U produces low concentrations of 207Pb in
younger (approximately < 1000 Ma) samples which leads to substantially larger
measurement uncertainty for
206
Pb/207Pb.
Consequently, the
207
Pb*/235U and
206
Pb*/207Pb* ages for younger grains have larger uncertainties and are less precise than
the
206
Pb*/238U ages. Therefore, if a magmatic sample contains a cluster of concordant
ages and the age is less than ~1000 Ma, the reported crystallization age is based on a
weighted average of the individual, concordant spot analyses using the
206
Pb*/238U ages.
238
For most of the orthogneisses, however, the high-grade metamorphism resulted in lead
loss and metamorphic overgrowth so that many of the samples do not have clear clusters
of crystallization ages. In these cases, the reported crystallization age is based on a
discordia regression through the zircon analyses or on averages of
206
Pb*/207Pb* ages.
These crystallization ages are accordingly less precise and for a couple of samples an age
with reasonable uncertainty simply cannot be defined. If Jurassic ages were obtained for
some zircons or tips of the crystals, a weighted average of those ages was used to fix the
lower intercept of the discordia since the high-grade metamorphism is known to be EarlyMiddle Jurassic in age (Guynn et al., 2006), while for samples lacking at least two young,
concordant ages, the lower intercept was fixed at 180 ± 10 Ma. The exact choice of a
lower intercept (within ± 50 Ma) is actually not that important as it has a negligible effect
on the upper intercept. In order to best define the discordia, those analyses with large
error or discordance were not used in the regression; these analyses are shown with gray
error ellipses in the concordia plots. For the quartzite detrital samples, the zircon age
spectra are based on
otherwise.
206
Pb*/238U ages if less than 1000 Ma and
206
Pb*/207Pb* ages
However, in the paragneiss, lead loss and magmatic zircon growth is
significant enough that we feel more confident using only the
206
Pb*/207Pb* ages despite
the large uncertainty in those ages. In addition, detrital zircon analyses are rejected if
they have more than 10% uncertainty, 30% discordance or 5% reverse discordance.
All analyses are plotted with 2σ or 95% confidence limits, while apparent ages of
individual zircon analyses in Table 2 are reported at the 1σ level.
The stated
uncertainties (2σ) on the assigned crystallization ages are absolute values and include
239
contributions from all known random and systematic errors. Random errors are included
in the data tables. Systematic errors (2σ) are as follows: JG053104-1, 1.18%; JG0531041, 0.94%; JG060504-2, 1.31%; JG061504-1, 0.81%; JG061504-2, 1.24%; JG061604-1,
1.15%; JG063004-1, 1.34%; PK97-6-4-1A, 1.30%; PK97-6-4-1B, 1.30%.
All U-Pb plots, weighted average calculations and discordia regressions were made
using Isoplot 3.00 (Ludwig, 2003).
4.2. Igneous Zircon Analysis
Despite zircon lead loss and metamorphic growth, dating of individual crystals has
allowed us to broadly define a bimodal distribution of orthogneiss ages for the Amdo
basement (Table 1). The younger age group is Cambrian-Ordovician (~450-550 Ma) and
the older is Middle-Late Proterozoic (~800-950 Ma). The latter are the first documented
Precambrian basement ages in the Lhasa and Qiangtang terranes.
Most of the
orthogneisses do not have inherited zircons, with the possible exception of a few
discordant analyses and sample JG053104-2.
4.2.1. JG053104-1
The zircons in this sample exhibit significant lead loss and most analyses are
discordant (Fig. 4). A discordia anchored at 180 ± 10 Ma gives an upper intercept age of
910 ± 16 Ma and an average of
206
Pb*/207Pb* ages for the least discordant analyses
results in an overlapping age of 915 ± 14 Ma; the latter is reported as the crystallization
age.
240
4.2.2. JG061504-2
Some of these zircons appear to have suffered lead loss (Fig. 5). One zircon tip
yielded an age of 185.6 ± 1.5 Ma and though the U/Th ratio is only 5.6, it is significantly
higher than the rest of the analyses (U/Th ~ 0.8). A discordia line anchored at 180 ± 10
Ma gives an upper intercept of 878 ± 15 Ma.
4.2.3. PK970604-3A
The age of this sample was previously reported in Guynn et al. (2006) as 852 ± 18
Ma. The crystallization age is based on a distinct cluster of older, concordant zircon
analyses.
4.2.4. JG060504-2
The majority of zircon analyses in this sample are discordant, though there are some
concordant analyses both in the Precambrian and the Jurassic, and several analyses have
large uncertainties (Fig. 6). Two concordant zircon analyses from the tips of zircons with
older cores have Jurassic ages of 175.2 ± 2.1 Ma and 178.8 ± 5.6 Ma and moderate U/Th
ratios (U/Th ~ 8). Other tip analyses yielded older ages. Using an average age of 177 ± 6
Ma for the lower intercept of an anchored discordia line and only using analysis with low
uncertainty, the upper intercept of the regression is 838 ± 23 Ma. Given the lack of
concordant analyses of that age and the large uncertainties relative to the regression line,
241
we regard this sample as having an earliest Late Proterozoic crystallization age of ~840
Ma.
4.2.5. JG061504-1
Many of the zircons in this sample had a significant common lead component (i.e.
very high
204
Pb concentrations) and therefore did not yield reliable ages, resulting in
fewer analyses shown than most samples (Fig. 7). The two Proterozoic ages probably
represent inherited zircon grains. One zircon gave a
206
Pb*/238U age of 187.4 ± 2.1 Ma
which we interpret as young, metamorphic zircon growth (U/Th = 114.2). A small
cluster of concordant ages around ~530 Ma was used to define a
260
Pb*/238U
crystallization age of 532 ± 7 Ma.
4.2.6. PK970604-1B
This sample yielded many discordant analyses with large uncertainties (Fig. 8).
There are too few precise ages and the ages are too young to define a meaningful
discordia. A cluster of concordant
206
Pb*/238U ages provides a weighted average age of
501 ± 12 Ma.
4.2.7. JG061604-1
The older analyses are mostly discordant without a clear cluster of ages (Fig. 9).
There is a distinct cluster of Jurassic ages with a mean 206Pb*/238U age of 181 ± 3 Ma, the
majority of which are from zircon cores. Although there is no distinct cluster of older
242
ages and the Jurassic ages are clustered, we interpret the crystallization age to be
Precambrian and the Jurassic age to be metamorphic growth based on the high U/Th
ratios (15-60) of the Jurassic zircon analyses and the discordant nature and wide range of
the older ages. A discordia regression yields a poorly defined upper intercept of ~470
Ma, consistent with the other early Paleozoic crystallization ages, and we interpret it
simply to have a Cambro-Ordovician protolith.
4.2.8. PK970604-1A
Three of these zircons yield young ages, one discordant, but the majority cluster
around 500 Ma (Fig. 10). An average of clustered 206Pb*/238U ages yields an interpreted
crystallization age of 483 ± 15 Ma, statistically equivalent to the age of PK970604-1B
which was collected from approximately the same location.
4.2.9. JG053104-2
The analyses for this sample yielded two clusters of ages, a large group around 900
Ma and a smaller group around 500 Ma, some of which have large uncertainties (Fig. 11).
While the population of ~500 Ma zircons is much smaller, their concordance and the lack
of intermediate ages (500-900 Ma) makes lead loss an unlikely reason for these younger
ages. As a result, we interpret the younger ages as representing crystallization age and
the older ages as inherited, though the large group suggests a significant component of
crustal melting of the Precambrian gneiss in the generation of this sample. A weighted
average of 206Pb*/238U ages was calculated for four concordant, low uncertainty analyses
243
around 500 Ma and yielded an age of 482 ± 18 Ma. A weighted 206Pb*/238U average was
also calculated for the older population. Similarly, only four analyses were used for this
population as well since there appears to be minor lead loss that pulled many of the ages
down. The resulting age is 839 ± 21. An average of all 206Pb*/207Pb* ages between 800
and 900 Ma yielded 866 ± 22 Ma. Considering the uncertainty and discordance, we only
confidently report a Cambro-Ordovician crystallization age with significant earliest
Neoproterozoic inheritance.
4.3. Detrital Zircon Analysis
U-Pb detrital zircon analysis was performed on three samples of metasedimentary
rocks from the Amdo basement. The concordia diagrams of all three samples are shown
in Figure 12 and the probability density functions are shown in Figure 13.
4.3.1. JG061504-4
This sample is a paragneiss from the central portion of the Amdo basement (Fig. 3).
The zircons in this sample have a wide range of morphologies and while some of the
grains are well-rounded, others have very well-defined crystal shapes with sharp points.
However, CL-SEM images reveal that the latter generally have rounded or angular cores
separated from the tips by bright zones (Fig. 14). Many of the tips were large enough to
analyze with a 25 or 35 µm laser spot size and most of these analyses yielded midJurassic ages, revealing that the majority of the euhedral zircon shapes are due to
metamorphic growth. The high-grade metamorphism also appears to have resulted in
244
lead loss for some of the grains so we prefer to rely on the
206
Pb*/207Pb* ages for all of
the grains in the probability density function, despite the larger uncertainty. There were a
total of 161 analyses, 16 of which were Jurassic tips and 135 were used in the age
spectra. The resulting age spectra has large overlapping peaks at ~850 and ~950 Ma,
small peaks at ~1150 and ~1430 Ma, and large peaks at ~1600, ~1700 and ~2500 Ma.
Given the lead loss and metamorphic overgrowth, we assign a conservative estimate of
the maximum depositional age as ~750 Ma.
4.3.2. JG062504-3
All 99 zircon grains analyzed from this quartzite sample are of sufficient quality to
include in the age spectrum. The maximum depositional age is 493 ± 64 Ma, a weighted
average of the youngest 11 zircons, all of which are concordant and overlap at the 2σ
confidence level.
4.3.3. AP061304-A
A total of 92 zircons from this quartzite were analyzed and 85 of these passed the
minimum constraints to be used in the age spectrum. The two youngest concordant ages
combined give a maximum depositional age of 447 ± 30 Ma; the next youngest group of
ages (n=4) gives a maximum age of ~510 Ma.
5. Discussion
245
Unfortunately there are no other reported exposures of crystalline basement rock in
the Lhasa or Qiangtang terranes to compare with the Amdo basement ages. Instead we
will emphasize a comparison of basement and detrital zircon ages to regional Gondwanan
continents and terranes. First we will generate a “Tibetan” Paleozoic detrital zircon
signature from several samples and compare these to regional detrital zircon records.
Second, we will discuss both the Amdo basement and “Tibetan” detrital zircon ages
together in the framework of Gondwanan tectonics and magmatic provinces.
5.1. Tibetan Paleozoic Detrital Zircon Signature
Based on the youngest zircons, the Amdo quartzites had to be deposited no earlier
than the Cambro-Ordovician, but it must be emphasized that this is only a maximum
depositional age. If northern Gondwana developed a passive margin (Stern, 1994), the
early Paleozoic magmatism would have been the last magmatic event until Gondwana
started to break up in the late Paleozoic. Thus Cambro-Ordovician zircons could easily
be the youngest in a Carboniferous sandstone or quartzite and in fact there is only one
zircon age younger than Ordovician in two Tibetan samples that are known to be
Carboniferous in age (see below). Carboniferous sandstones and quartzites are very
common in the Lhasa and Qiangtang terranes and there is a high probability that the
Amdo quartzites are in fact Carboniferous.
The abundance of Cambro-Ordovician
magmatism in Gondwana, including Amdo and the Himalaya, does make it highly
246
unlikely, however, that the Amdo paragneiss is Paleozoic in age and therefore we regard
this sample as Neoproterozic.
The three Amdo detrital zircon samples share some significant characteristics,
notably a group of 800-1000 Ma Proterozoic ages and a 2500 Ma peak at the ArcheanProterozoic boundary, as well as various peaks in the late Paleoproterozic and early
Mesoproterozoic (1100-2000 Ma) and few early Paleoproterozoic (2000-2400 Ma) or
Archean ages. The quartzites also share a Cambro-Ordovician group. In order to provide
a broader context for comparing a Tibetan Paleozoic detrital zircon signature to
magmatism and basement ages in Gondwanan continents, we combine the Amdo
quartzite age spectra with three additional detrital zircon age spectra from Tibet in order
to create a “Tibetan” Paleozoic detrital zircon signature. PK052402-2 is a Carboniferous
sandstone from northern Qiangtang and PK061302-1 is a Carboniferous sandstone from
the Nyainqentanglha mountains of the Lhasa terrane, about 150 km south of Amdo; both
are unpublished data.
NAMCO and DMXNG are Paleozoic sandstones, probably
Carboniferous, also from near the Nyainqentanglha mountains (Leier, accepted).
PK970619-1 is a quartzite from the Qiangtang blueschist belt in central Qiangtang (Kapp
et al., 2003b).
Mfab 4LL7 is an Eocene redbed also located in the Nyainqentanglha
Mountains, also unpublished. While the sample is Eocene in age, over 80% of the grains
yield ages older than 500 Ma and given its immature nature and the lack of preCretaceous igneous rocks in the area, it likely represents a proxy of the large exposures of
Carboniferous sedimentary rocks in the region. The post-Cambro-Ordovician Paleozoic
represents a stable time period for the Lhasa-Qiangtang terrane, between Cambro-
247
Ordovician Gondwana assembly and Permo-Triassic rifting, such that even if the Amdo
quartzites are Ordovician they should record similar ages as the Carboniferous samples.
The Amdo paragneiss sample is kept as an individual spectrum.
The probability density curves for all nine samples are shown in Fig. 13 and they
share many commonalities, including Cambro-Ordovician peaks, abundant 550-1000 Ma
ages, scattered Middle and Early Proterozic ages and peaks around 2500 Ma. Despite the
geographical and possible age differences, the Tibetan samples show remarkable
similarities, even for samples separated by several 100s kms, and the shared
characteristics increase our confidence in combining the six samples into a common
Paleozoic Tibetan signature.
5.2. Regional Detrital Zircon Record
We compare the Paleozoic Tibetan age spectra to detrital zircon (DZ) populations
from other Gondwanan terranes, in particular those from the Nepalese Himalaya. The
Tibetan terranes are generally placed along the northern edge of India in Paleozoic
reconstructions, so it might be expected that Tibetan and northern Indian sediments
should have some similarities in their detrital zircon signatures.
In addition to the
Nepalese sedimentary rocks, we also compare our ages to Cambrian sandstones of the
Spiti region in the northwestern Indian Himalaya, approximately 700 km west-northwest
of Nepal (Myrow et al., 2003), Permian sandstones of the Paleozoic Perth Basin of
southwestern Australia (Cawood and Nemchin, 2000) and Cambrian-Ordovician
sandstones from Israel and Jordan (Kolodner et al., 2006). The probability density
248
function of each group is shown in Figure 15, together with the Amdo orthogneiss
signature based on the ages defined by our U-Pb zircon geochronology, and ages of
magmatism and tectonics from Gondwana which are discussed in detail in the next
section.
The early Paleozoic Tethyan sedimentary rocks have a signature that is very similar
to the GHS and the only significant difference between them is the Cambro-Ordovician
peak in the Tethyan rocks, a result of Cambro-Ordovician granites in Greater India and
the Neoproterozic-Cambrian depositional age of the GHS (DeCelles et al., 2000). The
two are dominated by broad peaks of ~900-1300 Ma ages, centered ~1050 Ma, and also
contain peaks ~2500 Ma. Both are very different from the LHS sequence which is
characterized by a dominant group between 1800-2100 Ma with a peak ~1900 Ma. The
Lesser Himalayan Sequence (LHS) of metasedimentary rocks is presumed to have been
sourced from the Indian continent in the Paleoproterozoic, while the metamorphosed
Greater Himalayan Sequence (GHS) and the Tethyan sedimentary rocks appear to have
been derived from a source external to India (DeCelles et al., 2004; DeCelles et al., 2000;
Gehrels et al., 2003a).
The Tibetan rocks, including the Amdo paragneiss, have a DZ signature that is quite
different from the Himalayan rocks. The Paleozoic rocks have a very wide spread in
zircon ages, resulting in small peaks or regions that have very low wavelength peaks.
The dominant age groups are ~500-550, ~900-1000 and ~2450-2550 Ma, with smaller
and broader peaks ~800-900, ~1050-1250 and ~1700-1900 Ma. The paragneiss has three
main groups, ~750-1000, ~1550-1800 and ~2450-2550 Ma, that overlap with the
249
Paleozoic age groups. The Tibetan rocks do not contain the late Mesoproterozic ages of
the Tethyan and GHS and the latter are missing the 800-900 Ma ages of the Tibetan
rocks. Interestingly, the DZ signature of the Indian Himalayan rocks from farther west in
the Himalaya have an age spectrum that contains elements of both Tibetan and Nepalese
Himalayan rocks, including abundant 800-1000 Ma ages similar to the former and some
1000-1100 Ma ages and a lack of Paleoproterozoic ages like the latter. These rocks also
contain peaks at ~575 and ~750 Ma, as well as a Cambrian peak. The Indian Himalaya
and Tibetan samples contain ages similar to those in the Amdo basement, while the only
commonality between the Amdo basement and the Himalayan rocks is the CambroOrdovician peak in the Tethyan sedimentary units.
The DZ age spectrum that most closely matches the Nepalese Tethyan and GHS
distributions is that from the Perth Basin in western Australia. It contains a dominant
peak ~1100 Ma, within a large group from 850-1250 Ma, and a significant CambroOrdovician peak. The primary difference is a ~2600 Ma Archean peak instead of the
~2500 Ma peak observed in the Himalayan and Tibetan spectra. Cawood and Nemchin
(2000) also analyzed a single Ordovician sandstone sample from the Perth Basin and it is
broadly similar to the Permian sandstones, though it has less 1100-1250 Ma ages.
The Cambo-Ordovician sandstones from Jordan and Israel were deposited on the
edge of the Arabian-Nubian Shield (ANS) and the dominance of ~550-750 Ma ages is a
result of being largely derived from the ANS to the south (Avigad et al., 2003; Kolodner
et al., 2006). The units also contain a population of 750-1100 Ma ages and a few ages in
the mid-Paleozoic and late Archean. The age spectrum is very distinct from the others
250
and like the LHS is dominated by one large group of locally derived zircons. This
signature is similar to that from the immature, clastic-rich late-Neoproterozoic
Hammamat Group from eastern Egypt, which shows a large group of ages from 600-800
Ma, with peaks ~650 and ~750 Ma, a few 1800-2000 Ma grains a few 2400-2600 Ma
grains (Wilde and Youssef, 2002).
All the rocks contain limited Archean grains, with the exception of ubiquitous peaks
in the 2450-2650 Ma age range. Even in the LHS, which was sourced from cratonic
India, only about 20% of the ages are Archean and half of those are between 2500 and
2600 Ma.
In summary, the Tibetan rocks have a detrital zircon age signature that is distinct
from that of the Nepalese Himalaya, indicating a difference in source region and
suggesting a more distal location than generally assumed. Furthermore, the variation in
the Indian and Nepalese detrital zircon signature may indicate a lateral change in
Himalayan sources, though the limited number of ages in the Indian distribution makes
this a tentative conclusion. Finally, DeCelles et al. (2000) suggest that the source of the
800-1000 Ma ages in the Tethyan and GHS rocks was the Pan-African orogen,
specifically the ANS, but the Jordan and Israel units have a week 800-1100 Ma signature
and the Himalayan samples do not show the strong 600-800 Ma age group as seen in the
Middle Eastern sandstones. In addition, the 900-1100 Ma ages in the Jordanian and
Israeli samples are not known in the ANS and are proposed to have come from a
southern, distal source unless there are unexposed rocks of that age in the ANS (Avigad
et al., 2003). In light of the striking similarity between the Nepalese Himalayan ages and
251
those of western Australia, we propose an alternate source in eastern India, western
Australia and Antarctica.
5.3. Regional Basement Ages
By the end of the Cambrian, Gondwana had been assembled and the continent of
India was surrounded by (clockwise order) Australia, Antarctica, Africa and the newly
created Arabian-Nubian Shield (Fig. 2), a configuration that remained until the initiation
of its break-up in the Jurassic. The South China Craton also appears to have been located
near Arabia and India in the early Paleozoic, but than drifted further north and east to a
position north of Australia by the middle Paleozoic (Fig. 2). These continents and
possibly some of the peri-Gondwanan terranes are the possible sources for Tibetan
sediments.
Our discussion of source areas is concentrated on regions proximal to
northern India, such as northeastern Africa, western Australia and “east” Antarctica (~30
E-120 E). While it has been shown that sediments can be dispersed thousands of kms
from there source regions (e.g. Dickinson and Gehrels, 2003), the Neoproterozoic and
Cambrian tectonics surrounding India resulted in mountain ranges that decrease the
likelihood of sediments being derived from the interiors of the large continents (Fig. 16).
Archean rocks occur in cratons throughout all the major continents of Gondwana,
particularly in India and western Australia, but Archean ages are not well represented in
any of the detrital zircon signatures, with the exception of ages in the 2500-2700 Ma
range. The lack of Archean ages may be due to a lack of exposed Archean material, to
the interior location of most Archean cratons, or to Archean zircons not surviving
transport as well, possibly due to long-term radiation damage. The ~2500 Ma peak is
252
common to most of the detrital zircon records and may indicate a common crust-forming
event at that time (Condie et al., 2005). The Aravalli Craton in western India contains
abundant granitoids with ~2500 Ma ages, as well as 2600-2700 Ma and 2900-3300 Ma
(Choudhary et al., 1984; Pandit et al., 2003; Roy and Kroner, 1996; Wiedenbeck et al.,
1996) and modern sands of northwest India show a peak of 2500 Ma ages as well
(Condie et al., 2005). The Bundelkhand Massif in central India also contains basement
with ages of ~2500 Ma, ~2700 Ma, and ~3300 Ma and may be part of the Aravalli Craton
(Mondal et al., 2002). Metavolcanics of the northern Indian Himalaya have Nd depleted
mantle model ages of 2400-2600 Ma (Miller, 2000), suggestive of underlying ~2500 Ma
basement. The Dharwar craton of southern India also has significant 2500-2550 Ma
magmatism (Chadwick et al., 2000; Collins et al., 2003; Fitzsimons and Hulscher, 2005;
Jayananda et al., 2000) as does the Rayner complex in Antarctica (Cox et al., 2004)
which would have been next to the Dharwar craton in Gondwana. In addition, Dharwar
contains appreciable basement ages of ~2650 and 3000-3400 Ma (Collins et al., 2003;
Fitzsimons and Hulscher, 2005; Jayananda et al., 2000) and detrital zircon data from
southeast India include peaks at ~2500 Ma, ~2650 Ma and ~2950 Ma (Cox et al., 2004).
Ages of ~2500 Ma are also common in Madagascar, in both bedrock and detrital
sediments, as well as ages of ~2700 Ma and ~3200 Ma (Collins et al., 2003; Cox et al.,
2004; Fitzsimons and Hulscher, 2005; Tucker et al., 1999); these ages are also common
in Tanzania and Zimbabwe, as well as 3000-3100 Ma (Fitzsimons and Hulscher, 2005;
Johnson et al., 2003). Inherited zircons and Neodymium model ages suggest there may
be 2400-2600 Ma basement within the much younger Neoproterozoic crust of the
253
Arabian-Nubian Shield, including Somalia and Ethiopia (Johnson and Woldehaimanot,
2003; Whitehouse et al., 1998). The cratons of central and western Australia contain
abundant Archean ages (2600-3200 Ma), including 2600-2850 Ma from the Yilgarn
Craton and 2100-2700 Ma and ~3200 Ma in the Gawler, but generally lack significant
~2500 Ma crust (Veevers, 2004; Veevers et al., 2005). Some Archean ages have been
documented in the South China Craton in the Kongling region of the north-central
Yangtze craton (Qiu et al., 2000), but much of the craton is covered in Meso- and
Neoproterozoic sedimentary rocks (Goodwin, 1996) that indicate it was not well exposed
in the Paleozoic either.
Paleoproterozoic and early Mesoproterozoic ages are also sparse in the Gondwanan
DZ spectra, with the exception of a 1550-1700 Ma group in the Amdo paragneiss. The
Indian and Nepalase Himalaya contain Middle Proterozoic gneisses with ages ~18001900 Ma, generally from within the LHS (DiPietro and Isachsen, 2001; Le Fort and Rai,
1999; Miller et al., 2000; Zeitler et al., 1989). The Singhbhum craton contains 21002400 Ma granitoids and orthogneisses while the nearby Bhandara craton contains ~2200
Ma magmatism (Goodwin, 1996) and the Aravalli Craton contains sources with 15001700 Ma and 1900-2000 Ma ages (Choudhary et al., 1984; Deb et al., 2001). The lack of
1700-2100 Ma ages in India is surprising given the very strong peak in the LHS, but this
could be due to a lack of exposure, including the large extent of Paleoproterozoic
sedimentary rocks which may themselves be sources, and to a paucity of geochronologic
data. The ANS also has indications of 1600-1800 Ma inherited crust (Johnson and
Woldehaimanot, 2003) and ~1400 Ma and 1700-2500 Ma crust are indicated in Somalia
254
(Kroner and Sassi, 1996; Stern, 1994). Detrital zircon data from the Rayner complex in
Antarctica has significant ages in the 1250-1400 Ma range (Cox et al., 2004). In Western
Australia there are various Early and Middle Proterozoic basement rocks, particularly
1500-1800 Ma and ~2000 Ma (Veevers, 2004; Veevers et al., 2005) and the 1600-1900
Ma Capricorn Orogeny at the northern edge of the Yilgarn Craton (Bruguier et al., 1999).
These ages are also scarce in the South China block, though Chen, 2003 suggest 18001900 Ma ages.
The late Mesoproterozoic is dominated by extensive orogenies that are thought to
mark the amalgamation of most of the continents into a supercontinent termed “Rodinia”
(see Weil, 1998 for some references). The time period of most intense activity is ~10001300 Ma and herein termed the “Grenville” based on the North American orogenic
province of the time period. A Grenville signature is most apparent in the Nepalese
Himalaya and the Perth Basin samples, though these ages are also present in the Tibetan,
Indian Himalaya and Jordan/Israel sandstones.
Magmatism and crustal genesis
associated with Grenville orogenies is limited in eastern Africa, Madagascar and western
India (Kroner & Cordani, 2003). The nearest Rodinia-related orogenic belts are the
Kibaran, Irumide and Zambezi belts located south and west of the Tanzania Craton in
Tanzania, Malawi, Zambia and Zaire. The Neoproterozoic Molo quartzite in Madagascar
contains abundant zircons of that age (Cox et al., 2004) which probably came from these
belts and magmatism of that age is also reported in the Mozambique belt of Tanzania and
Malawi (Kroner, 2001) to the west of these belts. The Eastern Ghats of India contain
Grenville age metamorphism and magmatism and may related to 1000-1100 Ma
255
magmatism in the Darling Mobile belt of northwestern Australia and 900-1100 Ma
magmatism in the Rayner and Napier complexes and northern Prince Charles Mountains
of Antarctica (Boger et al., 2000; Bruguier et al., 1999; Collins and Pisarevsky, 2005;
Dobmeier and Raith, 2003; Meert and Torsvik, 2003; Mezger and Cosca, 1999).
The
Albany-Fraser belt of southwestern Australia and the Windmill and Bunger Hills regions
of Antarctica are probably related to a Grenvillian collision between a proto-India and
proto-Antarctica and contain ages between 1150-1350 Ma (Clark et al., 2000; Condie,
2003; Meert, 2003; Veevers et al., 2005). Magmatism ~1100 Ma is also seen in the
Leeuwin block along the southwest coast of Australia (Cawood and Nemchin, 2000) and
there is some 1000-1100 Ma magmatism in the South China craton (Chen et al., 2003).
The most significant ages in the Tibetan and Indian Himalaya spectra are the 8001000 Ma group because this age range, between the amalgamation and break-up of
Rodinia, is not especially common. These ages appear in the Israeli/Jordanian and Perth
Basin sandstones, yet there is no currently local source for either (Cawood and Nemchin,
2000; Kolodner et al., 2006). It is also the presence of these ages in the Nepalese
Himalayan units that prompted DeCelles et al. (2000) to suggest a northwest African
source for these sediments. Magmatism of this age occurs in the South China Craton,
though it is unlikely to have contributed to the Perth Basin sediments. Along the northern
edge of the Yangtze Craton (part of the South China Craton) in the Qinling Mountains,
825-880 and 900-960 Ma granites may be related to subduction and Rodinia assembly
(Chen et al., 2006; Ling et al., 2003; Xue et al., 2006) while in the central and southern
Yangtze Craton, 800-900 Ma magmatism is proposed to be related to subduction and the
256
subsequent collision with the Huanan block during the Jinning orogeny (Li, 1999; Wang
et al., 2006; Wu et al., 2006) or to an arriving plume prior to rifting (Li et al., 2003a; Li,
1999; Li et al., 1999; Li et al., 2003b). Magmatism in the Irumide belt of Mozambique
and Malawi is 950-1050 Ma (De Waele et al., 2003). As discussed earlier, magmatism in
the Eastern Ghats continued past the “Grenville” time period to as young as ~900 Ma and
there is evidence of an 800-1000 tectonothermal event in the Sausar Mobile belt of the
Central Indian Tectonic Zone, though the extent of magmatism is unclear (Roy et al.,
2006). The most extensive area of magmatism this age is in the Aravalli Craton, which
includes 800-900 Ma granitoids in the Aravalli-Delhi orogenic belt (Choudhary et al.,
1984; Rathore et al., 1999) and volcanics, granitoids and orthogneisses of the AmajiSendra belt, a possible arc terrane (Deb et al., 2001; Pandit et al., 2003). Orthogneisses in
the Indian Himalaya northeast of the Aravalli Craton have also been dated at ~825 Ma
(DiPietro and Isachsen, 2001; Singh et al., 2002). Another possible source is Antarctica,
where 900-1020 Ma granitoids have been reported in the Prince Charles Mountains,
which would have been opposite the Eastern Ghats in the Paleozoic (Boger et al., 2000;
Fitzsimons, 2003). There are also ~825 Ma and 920-930 Ma plutons in the QaidamQiling terrane of northern Tibet, though this terrane may have been much farther east and
north in the Paleozoic (Gehrels et al., 2003b; Gehrels et al., 2003c). Finally, it is
important to recognize that the Amdo basement itself is a possible source of 800-925 Ma
ages and if the Amdo paragneiss was sourced locally, the Tibetan basement could contain
plutons from 750-1000 Ma.
257
The earliest magmatism in the Arabian-Nubian shield, including parts of Ethiopia and
Somalia, is ~870 Ma and related to initial rifting (Grenne et al., 2003; Pallister et al.,
1988; Stern, 1994), though inherited zircons and Neodymium model ages indicate the
presence of older (800-1200 Ma and older) crust (Grenne et al., 2003; Teklay et al.,
1998). Rifting was closely followed by seafloor spreading, calc-alkaline arc-related
magmatism and terrane accretion between 600-870 Ma (Ayalew et al., 1990; Grenne et
al., 2003; Johnson et al., 2004; Kroner and Sassi, 1996; Pallister et al., 1988; Stern, 1994;
Teklay et al., 1998), with higher volumes of pluton production around 620-660 Ma and
720-780 Ma (Johnson and Woldehaimanot, 2003). The ANS had amalgamated by ~600
Ma, possibly by collision of older crustal material to the east (Windley et al., 1996), and
younger activity is post-orogenic (Johnson and Woldehaimanot, 2003; Stern, 1994). This
magmatic history matches fairly well with the distribution of ages in the Jordanian and
Israeli Cambrian sandstones, which show a peak ~630 Ma, though the low number of
700-850 Ma ages is surprising. The 850-1250 Ma ages in this sample could represent
older basement buried beneath Phanerozoic sedimentary rocks or may simply represent
more distal sources (Kolodner et al., 2006).
Magmatism of 600-800 Ma age is fairly common in other locations around India,
though these ages are scarce in the Perth Basin and Nepalese Himalayan units and are
only a minor component of Tibetan distributions. Igneous activity between 700 and 850
Ma in the Yangtze block is thought to represent the break-up of Rodinia (Chen et al.,
2006; Li et al., 2003b), the Jinning orogeny which resulted from the collision of the
Huanan block with the Yangtze block (Chen et al., 2003; Wang et al., 2006) or
258
subduction and back-arc spreading along the Yangtze continental margin (Druschke et
al., 2006). The primary age of the Milani complex in the Aravalli Craton is ~750 Ma
(Choudhary et al., 1984; Rathore et al., 1999; Torsvik et al., 2001b) and is proposed to be
related to 700-800 Ma magmatism in the Seychelles (Tucker et al., 2001) and
Madagascar (Collins et al., 2003; Handke et al., 1999; Tucker et al., 1999), with the three
areas forming a Neoproterozoic subduction zone (Handke et al., 1999; Torsvik et al.,
2001a). Magmatism in the 600-700 Ma range is also found in Madagascar (Kroner,
2001; Tucker et al., 1999). The Leeuwin block in western Australia contains abundant
plutons between 680 and 800 Ma (Cawood and Nemchin, 2000; Collins, 2003) and 700800 Ma magmatism occurs in the Yilgarn and Pilbara Cratons (Wingate and Giddings,
2000).
Latest Neoproterozoic and early Paleozoic (450-600 Ma) magmatism is almost
exclusively Gondwanan in origin (Veevers, 2004) and granitoids of this age are common
along the margins of Gondwanan continents.
Cambrian-Ordovician magmatism in
Gondwana (e.g. Himalayan events in northern India (Gehrels et al., 2003a) and the RossDelamerian Orogeny in eastern Australia-eastern Antarctica (Boger and Miller, 2004))
may be related to a reorganization of plate boundaries following late Neoproterozoic
Gondwana amalgamation (Boger and Miller, 2004). Around India, late Neoproterozoic
and early Paleozoic orogenic events include the Mozambique-Malagasy, Kuunga and
Pinjarra orogenies (Collins and Pisarevsky, 2005; Veevers, 2004). All the Paleozoic
detrital zircon distributions show peaks or age groups of ~550 Ma, which could be
represented by plutonism in the ANS of Arabia and Ethiopia (Ayalew et al., 1990;
259
Johnson and Woldehaimanot, 2003), the Malagasy-Mozambique orogeny between West
and East Gondwana (Boger and Miller, 2004; Collins and Pisarevsky, 2005; Kroner,
2001; Tucker et al., 1999), the Kuunga-Pinjarra orogenies in western Australia, eastern
India and Antarctica (Boger and Miller, 2004; Cawood and Nemchin, 2000; Collins,
2003; Fitzsimons, 2003; Mezger and Cosca, 1999), and magmatism in Iran (Ramezani
and Tucker, 2003).
Cambro-Ordovician plutonism is also represented in Himalaya and Tibet. Half the
Amdo orthogneiss ages presented here are of this age and Kapp et al. (2000) reported a
Cambrian-Ordovician garnet-amphibolite gneiss within a blueschist-bearing mélange in
the central Qiangtang terrane which they suggest was derived from the northern
Qiangtang basement, an interpretation consistent with the presence of the Amdo CambroOrdivician orthogneisses. Inherited ~550 Ma (and ~2500 Ma) zircons have been reported
in the Cretaceous-Tertiary granitoids and orthogneisses of the Nyainqentanglha
mountains of the Lhasa terrane (Kapp et al., 2005). Cambro-Ordovician orthogneisses
are also widespread in the GHS of the Himalaya (DeCelles et al., 1998; Gehrels et al.,
2003a; Godin et al., 2001; Le Fort, 1986; Schärer and Allègre, 1983) and may be related
to early Paleozoic collisional tectonics (Gehrels et al., 2003a and references therein). In
the Tethyan Himalaya, north of the GHS and the high Himalayan peaks, orthogneisses
are exposed in the cores of several gneiss domes. An initial U-Pb zircon date of the
Kangmar dome orthogneiss yielded an age of ~565 Ma (Schärer et al., 1986), but more
recent U-Pb zircon work yielded two well-constrained ages of ~510 Ma (Lee et al.,
2000).
In the Malashan dome further west, Aoya et al. (2005) concluded that the
260
crystallization age of the orthogneiss is Miocene and older ages are due to inherited
xenocrysts, though it may be as likely that some of the older cores are magmatic and the
young rims are a result of Miocene metamorphism. Early Paleozoic orthogneiss exposed
in the Kangmar and possibly other gneiss domes may be related to the orthogneisses of
the GHS (Gehrels et al., 2003a). The Tibetan and Himalayan magmatism is slightly
younger (~480-510 Ma) than the main peaks seen in most of the DZ signatures (~550
Ma), suggesting that the local magmatism may not have contributed a large signal to the
age spectra. The ~550 Ma ages are representative of final Gondwana assembly and postcollison magmatism represented by the Mozambique-Malagasy-Kuunga orogenies
around the margins of India and could have come from East Africa, Madagascar, western
Australia, Antarctica or peri-Gondwanan terranes which are not well dated.
Alternatively, this slightly older magmatism may exist in Tibet and the Himalaya but has
not been exposed or dated.
The Cambrian tectonics may have resulted in an early episode of metamorphism and
deformation of the Amdo orthogneisses (Coward et al., 1988; Xu et al., 1985), which
could explain why the Jurassic granitoids that intrude the gneisses are undeformed
despite being coeval with Jurassic metamorphism. The region may have than developed
into a passive continental-shelf margin, generating sandstones, pelites and carbonates that
eventually became the quartzites, schists and marbles seen in the Amdo basement as well
as the Carboniferous-Permian shales, sandstones, carbonates and diamictites that are
common throughout the Lhasa and Qiangtang
261
5.4. Implications for Paleogeography of the Lhasa-Qiangtang terrane
The Tibetan Paleozoic sedimentary rocks have a DZ age spectrum that is significantly
different from those of the northern margin of India, indicating that they did not have the
same spatial relationship in the Paleozoic that they do today. The Nepalese Himalayan
samples are surprisingly similar to age spectra from Permian sandstones of the Perth
basin, suggesting common source terranes. The most significant group of ages in the
Nepalese Himalayan samples is Grenville in age. Grenville age mountain belts are
lacking in eastern Africa and western India but common in western Australia, eastern
India and Antarctica (Fig. 16), making this region a likely source for the Perth basin
(Cawood and Nemchin, 2000) and Nepalese Himalayan sedimentary rocks. The Tibetan
samples have much fewer Grenville ages with a more significant peak in the 900-1000
Ma range.
Of all the Gondwanan source regions, northwestern India is the most likely for the
Tibetan signatures, as well as Madagascar and northwestern Africa, while southern and
central Africa, western Australia and much of Antarctica seem the least likely. The
former areas have significant crust of 2500 Ma age, orogenic events from 750-1000 Ma
and little 1000-1300 Ma orogenesis. In particular, the Aravalli Craton of India and the
island of Madagascar contain significant 2500 Ma rocks, the Aravalli Craton and
northwestern India contain the uncommon 900-1000 Ma ages as well as other
Neoproterozoic ages and Madagascar and east Africa are sources for 500-750 Ma ages.
This would place Lhasa-Qiangtang farther east (Fig. 16), closer to the Helmand block
(Afghanistan) and Karakorum (Pakistan) which have many similarities to the Tibetan
262
terranes (Le Fort et al., 1994) and also to Iran, whose blocks include Cambrian arc
magmatism (Ramezani and Tucker, 2003). The South China craton also contains many
of the appropriate ages but could not have contributed to late Paleozoic sediments as it
was distally located from the Africa and Indian margins at that time (see Fig. 2; Cocks
and Torsvik, 2002; Li et al., 2004; Torsvik and Cocks, 2004).
The peaks in the paragneiss are surprisingly similar to magmatic and metamorphic
ages in the Aravalli-Delhi fold belt, the Aravalli Craton and Madagascar. The AravalliDelhi orogeny produced 1500-1700 Ma magmatism and this area also contains 800-900
Ma granitoids (Choudhary et al., 1984); the Amaji-Sendra belt, a possible arc terrane,
contains ~986 Ma rhyolites, ~970 Ma granitoids and ~820-840 Ma orthogneisses and
granitoids (Deb et al., 2001; Pandit et al., 2003); and the Malani complex contains ~730850 granitoids and volcanics (Deb et al., 2001; Pandit et al., 2003; Rathore et al., 1999;
Torsvik et al., 2001b). The early Neoproterozoic magmatism appears to extend to both
the northeast, where ~825 Ma granitoids occur in the Himalaya (DiPietro and Isachsen,
2001; Singh et al., 2002), and to the southwest, with 750-800 Ma magmatism in
Madagascar (Collins et al., 2003; Handke et al., 1999; Tucker et al., 1999) and 750-810
Ma magmatism in the Seychelles (Torsvik et al., 2001a; Tucker et al., 2001), which are
proposed to have formed a continuous northern Madagascar-Seychelles-Malani arc in the
Neoproterozoic (Handke et al., 1999; Torsvik et al., 2001a; Tucker et al., 1999).
Druschke et al. (2006) even proposed that ~700-850 South China magmatism is related to
this postulated continental arc in and is a result of rifting of continental fragments from
263
Rodinia, creation of back-arc basins and the re-establishment of continental subduction
zones.
One speculative possibility is that the ~840-920 Ma Amdo orthogneisses and the
Neoproterozoic paragneiss are related to the ~850-1000 Ma magmatism in the Aravalli
Craton, the Indian Himalaya and the northern Yangtze craton of South China and part of
this continental arc system. The South China Craton is sometimes placed to the east of
Australia in ~750 Ma Rodinia reconstructions (e.g. Li et al., 1995; Meert and Torsvik,
2003), but the paleomagnetic poles also allow it to be placed further east near northwest
Australia (e.g. Evans et al., 2000; Macouin et al., 2004). India may also have been
located at this approximate latitude and position (Meert and Torsvik, 2003; Torsvik et al.,
2001b), but others place it further north (Evans et al., 2000; Yang et al., 2004). If India
and the South China Craton were adjacent to one another, they could have shared a
subduction zone along their margins; alternatively, an ocean could have been consumed
between them, resulting in coeval but spatially distinct arcs. Regardless, a continental arc
margin may have stretched from Madagascar and the Seychelles, across Malani and the
Aravalli Craton, through northwestern India and into the Lhasa-Qiangtang terranes
between at least 750-950 Ma. Paleomagnetic data for these terranes and this time period
is relatively sparse and more work, including more extensive geochronology, is necessary
to test the validity of this hypothesis.
The proposed paleogeography and source areas implies that Himalayan rocks further
to the west should have a signature more similar to Tibet, with a northwestern Indian
influence and less western Australia-Antarctica input, and that is indeed the case for
264
Cambrian sandstones from the northwestern Indian Himalaya. They contain smaller
Grenville populations, a large group of ages between 900-1000 Ma and peaks between
600-900 Ma that could be a result of the East African Orogen. The occurrence of
Grenville ages in the Indian Himalaya and the Tibetan samples and the presence of 9001000 Ma zircons in the Nepalese Himalaya suggest that there was some mixing along the
northern Gondwana margin between the different source areas.
One possible problem with northeastern Africa and western India as a source region
are the limited number of 650-800 Ma ages in the Tibetan detrital zircon record despite
the protracted history of the East African Orogeny. One possibility is that if the ocean
containing the arc terranes of the ANS closed as a result of continental fragments, such as
Turkey, Iran and Lhasa-Qiangtang, colliding with Africa (Fig. 2; 16), the collision
produced a mountain belt along the northeast edge of the ANS that hindered sediment
transport in that direction. In modern times, Himalayan foreland basin rocks and modern
Neogene sands record mostly Proterozoic ages with few zircons of Himalaya orogenesis
or Gangdese arc age (DeCelles et al., 2000; DeCelles et al., 1998). The Tibetan signature
does show a small ~650 Ma peak similar to an age peak in the East African orogen that is
seen in the Jordan/Israel DZ ages (Kolodner et al., 2006; Meert, 2003). Another issue is
the very few Archean ages ≥ 3000 Ma in the detrital zircon signature, despite the
association of crust that age with the ~2500 Ma crust. It is difficult to assess the meaning
of the lack of these ages, particularly given that the LHS, which is understood to be
derived from cratonic India, also has very few older (>2600 Ma) Archean ages. Finally,
while the age associations discussed suggest a general source region, the exact area may
265
be unknown due to a lack of data.
Many of the possible source regions, such as
northwest Arabia, the Horn of Africa and northern India are extensively covered by
younger deposits of sedimentary and volcanic rocks (e.g. the Deccan traps and the
Himalayan foreland basin in India) or have not been adequately dated. Basement ages of
800-1000 Ma and ~2500 Ma may also be more common in Ethiopia, Somalia, Arabia and
northwest India. Indeed, Condie et al. (2005) propose a widespread episode of juvenile
crust creation at ~2500 Ma based on U-Pb ages and Hf isotopic compositions of detrital
zircons from Brazil, Australia, India and the Ukraine, which suggests that crust of this
age is presently disproportionately exposed.
6. Conclusions
Our basement and detrital zircon geochronologic data, combined with detrital zircon
data from other Tibetan Paleozoic sandstones, suggest a location of the combined LhasaQiangtang terrane that is farther west along the Gondwanan margin than most current
models indicate. The presence of post-Grenville and Pan-African ages, a strong and
consistent 2500 Ma peak and a lack of Grenville ages is more consistent with a Paleozoic
location near northeastern African and northwestern India than a location towards eastern
India and Australia. Likewise the available data indicate an eastern source for sediments
of the Late Proterozoic and early Paleozoic rocks of the Nepalese Himalaya which have
less early Pan-African ages and more Grenville ages, indicating a smaller input from
Africa and western India and a larger input from western and southern Australia, eastern
India and Antarctica.
266
While a limited exposure, the Amdo basement suggests extensive Neoproterozoic and
early Paleozoic magmatism and reworking of a Proterozoic Lhasa-Qiangtang crust,
consistent with a location along the margin of Rodinia and than Gondwana.
The
Neoproterozoic magmatism may be related to a continental arc in eastern India and even
possibly the South China craton. The Cambro-Ordovician orthogneisses are part of an
extensive region of magmatism and metamorphism along the margins of Gondwana
following its final assembly, which occurred as the Indian continent collided with East
Africa and Australia/Antarctica in the latest Neoproterozoic (Boger and Miller, 2004;
Collins and Pisarevsky, 2005; Meert, 2003). The continued tectonism along the margin
of Gondwana into the Paleozoic is likely a result of the initiation of subduction along its
edges (Boger and Miller, 2004) and the accretion of additional terranes, such as Iran or
the South China craton.
Our model predicts similarities in Cambrian detrital zircon records between the
Nepalese Himlaya, western Australia and western Sibumasu, similarities between the
Iranian, Afghan and Lhasa-Qiangtang terranes and specific changes in Paleozoic detrital
zircon age populations along the northern margin of Gondwana, from the west
(Afghanistan, Lhasa-Qiangtang, northwest Himalaya), to central (Nepalese and
Bhutanese Himalaya, South China) to the east (east Indian Himalaya, Indochina,
Sibumasu). The Cambrian sandstones from the Indian Himalaya hint at this change in
provenance along strike but further analyses are needed. The model preferred here is
geologically permissible, but not unique, and requires more data to resolve.
267
Recent work has resulted in many proposed redefinitions of Gondwana assembly, the
discovery of previously unknown magmatic provinces and new suggestions for
reconstructions between the breakup of Rodinia and the assembly of Gondwana. Most of
these new reconstructions or definitions include, and are the result of, additional
recognized tectonic complexity. Detrital zircon geochronology is one method that is
being used to resolve the location of continents and terranes in the absence of
paleomagnetic data and has the potential to further elucidate some of the questions
concerning paleogeographic reconstructions.
Acknowledgments
We thank Jen Fox, Alex Pullen, Jenn Pullen and Kelley Stair for sample preparation,
zircon analysis and SEM imaging. We also thank Paul Myrow and especially Peter
Cawood for providing copies of their data. Alex Pullen also provided invaluable field
assistance. This manuscript benefited from discussions with Pete DeCelles.
Appendix A.1 Description of Rock Samples
A.1.1 JG053104-1 orthogneiss
This sample is a felsic gneiss composed of quartz, plagioclase and microline with
minor biotite. It is located in the south-central part of the gneiss about 10 km from the
southern boundary.
A.1.2. JG061504-2 orthogneiss
268
This granodiorite orthogneiss has a well-developed foliation defined by biotite, with
only minor chlorite alteration of the biotite, and contains stretched quartz grains. It was
collected ~10 km west of the Lhasa-Golmud highway. This gneiss and JG061504-1 were
interlayered within the same outcrop, along with garnet amphibolite.
A.1.3. PK970604-3A orthogneiss
This is a medium-grained, hornblende-biotite granite orthogneiss located about
halfway across the Amdo basement along the Lhasa-Golmud highway.
Both the
hornblende and biotite display chlorite alteration. Many of the zircons in this sample
display thin (< 10 µm) bright rims in CL-SEM images that were interpreted as Jurassic
zircon resorption by Guynn et al. (2006).
A.1.4. JG060504-2 orthogneiss
This is a mylonitic orthogneiss of granite composition from the central part of the
Amdo basement with stretched quartz grains, quartz and feldspar porphroblasts, and finegrained biotite defining the foliation.
A.1.5. JG061504-1 orthogneiss
This sample is a weakly foliated, felsic orthogneiss that contains abundant accessory
magnetite with some crystals up to several mm across. It was collected from the same
outcrop as JG061504-1 and is interlayered with that orthogneiss.
269
A.1.6. PK970604-1B orthogneiss
This is a felsic granite orthogneiss from the southern boundary of the Amdo basement
along the Lhasa-Golmud highway. It has a weak foliation defined by flattened quartz
grains and highly chloritized biotite.
A.1.7. JG061604-1 orthogneiss
Located along the Lhasa-Golmud highway just south of the northern basement
contact, this fine-grained granitic orthogneiss has a very weak biotite foliation.
A.1.8. PK970604-1A orthogneiss
Though collected from the same area as PK970604-1B, this sample is a hornblendebiotite granodiorite with a weak foliation defined by aligned hornblende.
A.1.9 JG053104-2 orthogneiss
This is a biotite-granite orthogneiss located close to JG053104-1. It is compose
largely of plagioclase with quartz and minor biotite and K-feldspar. The biotite displays
some chlorite alteration. The orthogneiss has a well-developed foliation defined by
layers of felsic minerals and aligned biotite.
A.1.10 JG061504-4 paragneiss
This dark, banded gneiss is located along the Lhasa-Golmud highway about halfway
across the Amdo basement. It is largely composed of quartz, plagioclase and biotite that
270
has some chlorite alteration. It has a well-developed banding defined by layers of
coarser, more felsic minerals and aligned biotite rich layers. It also contains minor
epidote, muscovite and garnet and the biotite has some chlorite alteration.
A.1.11. JG062504-3 quartzite
Quartzite sample JG062504-3 comes from a small outcrop located within the gneisses
near the northern edge of the Amdo basement. This quartzite is very clean with only
minor mica.
A.1.12. AP061304-A quartzite
This is a quartzite from a narrow outcrop located at the southwest edge of the
orthogneiss. The quartzite is associated with a garnet-kyanite schist that is a lower grade
(lower amphibolite facies) than the orthogneisses. It is also a very clean quartzite with
only minor muscovite and biotite.
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neighboring regions., in Yin, A., and Harrison, M.T., eds., The tectonic evolution
of Asia: New York, Cambridge University Press, p. 442-485.
Zeitler, P.K., Sutter, J.F., Williams, I.S., Zartman, R.E., and Tahirkheli, R.A.K., 1989,
Geochronoogy and temperature history of the Naga Parbat-Haramosh massif,
Pakistan, in Malinconico, L.L., and Lillie, R.J., eds., Tectonics of the Western
Himalayas, Volume 232, Special Paper Geolocial Society of America.
Zhou, D., and Graham, S.A., 1996, The Songpan-Ganzi complex of the western Qinling
Shan as a Triassic remnant ocean basin, in Yin, A., and Harrison, M.T., eds., The
tectonic evolution of Asia: New York, Cambridge University Press, p. 281-299.
Figure Captions
Figure 1. Location of terranes and continental blocks within southeast Asia. All the
terranes shown were originally part of East Gondwana with the exception of Northeast
China and the Kazakh Block. The questions marks highlight an area that is poorly
defined due to Cenozoic deformation and poor access; the Qiangtang terrane and
Sibumasu are generally considered to have been joined, but the relationship is unclear.
Likewise, the Qiangtang and Lhasa terranes may continue into the Pamirs. The
Himalayan mountains are made up of Indian upper-crustal rocks whose lower crust is
291
presumed to have been underthrust northward beneath Tibet (“Greater India”). AB is the
location of the Amdo Basement.
Figure 2. One view of Gondwana and other continents in the Ordovician and
Carboniferous from Scotese (2001; http;//www.scotese.com/newpage1.htm). The main
differences in the Carboniferous are 1) the assembly of Laurasia and Gondwana to form
Pangea, 2) the counter-clockwise rotation of Gondwana and 3) the rifting and relocation
of the South China Craton and Indochina. With the exception of South China and
Indochina, the configuration of Gondwana is essentially the same from the late Cambrian
through the early Permian. Note that India does not include Greater India, the part of the
continent underthrust beneath Tibet in the Cenozoic Indo-Asian collision. The Sibumasu
and West Burma terranes are part of Malaya, near northwest Australia. Af = Africa; Ar =
Arabia; Au = Australia; Av = Avalonia; Ba = Baltica; Gr = Greenland; IC = Indochina; In
= India; Kz = Khazakstania; La = Laurentia; LQ = Lhasa-Qiangtang; Ma = Malaya; Md =
Madagascar; NC = North China; SA = South America; Si = Siberia; SC = South China.
Figure 3 Simplified geologic map of the Amdo basement and surrounding sedimentary
rocks.
Figure 4. Pb/U concordia plot of zircon analyses of orthogneiss JG053104-1. The
regression line is anchored at 180 ± 10 Ma and is based on all analyses. Inset shows the
weighted average of clustered 207Pb*/206Pb* ages, which are the black error ellipses in the
292
concordia plot. Error ellipses in gray were not used in the discordia regression. In this
and all following concordia and weighted average figures, error ellipses and error bars are
at the 2-σ (95% confidence) level. Plots, regressions and averages were made using the
programs of Ludwig (2003).
Figure 5. Pb/U concordia plot of zircon analyses of orthogneiss JG061504-2. The
regression line is anchored at 180 ± 10 Ma and is based on all analyses.
Figure 6. Pb/U concordia plot of zircon analyses for orthogneiss JG060504-2. Ellipses
in gray were not used in the regression. Regression is anchored at 177 ± 6 Ma, the mean
of the two concordant, overlapping Jurassic 206Pb*/238U ages.
Figure 7. Pb/U concordia plot of zircon analyses of orthogneiss JG061504-1. Inset
shows the weighted average of clustered 206Pb*/238U ages, which are the black error
ellipses in the concordia plot.
Figure 8. Pb/U concordia plot of zircon analyses of orthogneiss PK970604-1B. Inset
shows the weighted average of clustered 206Pb*/238U ages, which are the black error
ellipses in the concordia plot.
293
Figure 9. Pb/U concordia plot of zircon analyses for orthogneiss JG061604-1. Ellipses
in gray were not used in the regression. Regression is anchored at 181 ± 3 Ma, the mean
of the concordant, overlapping Jurassic 206Pb*/238U ages.
Figure 10. Pb/U concordia plot of zircon analyses of orthogneiss PK970604-1A. Inset
shows the weighted average of clustered 206Pb*/238U ages, which are the black error
ellipses in the concordia plot.
Figure 11. Pb/U concordia plot of zircon analyses from orthogneiss JG053104-2. Insets
show the most concordant analyses with the least uncertainty for the old and young
cluster of ages and the weighted 206Pb*/238U average of each.
Figure 12. Pb/U concordia plots for the Amdo metasedimentary detrital zircon samples.
Analyses with gray error ellipses are not included in the probability density functions due
to discordance, large uncertainty or metamorphic overgrowth. Ages less than 1000 Ma
are based on 206Pb*/238U apparent ages, while ages older than 1000 Ma are based on the
207
Pb*/206Pb* age. The dashed lines in sample JG061504-4 show interpreted lead loss
paths as a result of the Jurassic metamorphic event.
Figure 13. Probability density functions of detrital zircon ages for the three Amdo
metasedimentary samples and other Paleozoic Tibetan sedimentary samples. The number
of individual zircon analyses in each curve is given beside the group name. The curves
294
have been normalized so that each has the same area. Spikes in the curves are caused by
relatively high precision analyses, generally by an ion-microprobe, while lower precision
analyses form broader peaks.
Figure 14. Catholde-luminescence SEM images of JG061504-4 zircons showing rounded
or irregular cores with sharp tips. The cores are older, detrital zircon grains and the tips
represent Jurassic metamorphic growth, as revealed by the U-Pb dating, shown for two of
the zircons.
295
Figure 15. Probability density plots for Gondwanan detrital age data and Amdo
orthogneisses (upper panel) compared to periods of magmatism and orogenesis in
Gondwanan terranes (lower panel). In the upper panel, the number in parentheses is the
number of individual zircon analyses in each curve. The curves have been normalized so
that each has the same area. As in Figure 13, the spikes are due to high precision TIMS
(Thermal-Ion Mass Spectrometry) or ion-microprobe analyses. Perth Basin from
Cawood and Nemchin (2000), Jordan/Israel from Kolodner et al. (2006), Nepalese
Himalaya from Gehrels et al. (2003a) and DeCelles et al. (2004), Indian Himalaya from
Myrow et al. (2003) and Tibetan data from this study and unpublished data. In the lower
panel, dark bars represent significant, concentrated magmatism; medium bars are broader
times of orogenesis based on magmatic and metamorphic thermochronology; light bars
are based on sparse geochronology, inherited zircon and isotopic model ages. Sources
for Gondwana orogenesis discussed in text. LHS = Lesser Himalaya Sequence; GHS =
Greater Himalaya Sequence; ANS = Arabian-Nubian shield and includes parts of
Ethiopia and Somalia.
296
Figure 16. Our interpretation of the position of the Lhasa-Qiangtang terrane in the
Cambrian, ~500 Ma. The map is based on Boger et al., 2004 and shows major
continents, cratons terranes and orogenic belts of Gondwana. The location of detrital
zircon samples are: 1) Amdo basement, 2) Nepalese Himalaya, 3) NW Indian Himlaya,
4) Perth Basin and 5) Jordan/Israel. The Cambro-Ordovician orogenic belt across
northern India is only generally defined due to the overprint by rifting and the Himalayan
thrust belt. The Lhasa-Qiangtang terrane is based on the current boundaries with 50%
extension and rotation at the ends to account for Mesozoic and Cenozoic deformation; the
internal line represents the Bangong suture between the Lhasa and Qiangtang terranes.
Other peri-Gondwana terranes are only generally defined. Extent of Greater India is
based on Ali and Aitchison (2005). AD = Aravalli-Delhi craton; AF = Africa; AG =
Afghanistan blocks; ANS = Arabian-Nubian shield; AN = Antarctica; AU = Australia;
BH-LC = Bunger Hills-Leeuwin Complex; BK = Bundelkhand craton; DW = Dharwar
craton; EG = Eastern Ghats; GI = Greater India; GL = Gawler craton; HB = Huanan
block; IC = Indo-China; IR = Iranian blocks; L-Q = Lhasa-Qiangtang terranes; MD =
Madagascar; NC = Northampton Complex; NPCM; Northern Prince Charles Mountains;
PC = Pilbara craton; SA = South America; SB = Singhbhum craton; SC = South China
craton; SEY = Seychelles; SPCM = Southern Prince Charles Mountains; SWB =
Sibumasu-West Burma; TB = Tarim Block; TK = Turkey; YB = Yangtze block; YC =
Yilgarn craton.
297
Figures
Figure 1. Guynn et al.
298
Figure 2. Guynn et al.
299
Figure 3. Guynn et al.
300
Figure 4. Guynn et al.
Figure 5. Guynn et al.
301
Figure 6. Guynn et al.
Figure 7. Guynn et al.
302
Figure 8. Guynn et al.
Figure 9. Guynn et al.
303
Figure 10. Guynn et al.
Figure 11. Guynn et al.
304
Figure 12. Guynn et al.
305
Figure 13. Guynn et al.
306
Figure 14. Guynn et al.
307
Figure 15. Guynn et al.
308
Figure 16. Guynn et al.
309
Tables
Table 1. Location and ages of Amdo basement samples
Sample
JG053104-1
JG061504-2
PK970604-3A
JG060504-2
JG061504-1
PK970604-1B
JG061604-1
PK970604-1A
JG053104-2
JG063004-1
JG061504-4
""
JG062505-3
AP061304-A
Lat
31.884
31.932
32.124
31.946
31.932
31.881
32.130
31.881
31.885
31.817
32.123
""
32.048
31.794
Lon
92.048
91.838
91.711
92.136
91.838
91.699
91.716
91.699
92.060
91.752
91.711
""
92.209
91.773
Description
Granitie gneiss
Granodiorite gneiss
Granite gneiss
Granodiorite gneiss
Granite gneiss
Granite gneiss
Granite gneiss
Granodiorite gneiss
Granite gneiss
Quartz-diorite gneiss
Paragneiss
""
Quartzite
Quartzite
Zircon
Age (Ma)
915 ± 14
878 ± 15
852 ± 18
~ 840
532 ± 7
501 ± 12
~ 470
483 ± 15
482 ± 18
?
Other Zircon Ages
Type
Age (Ma)
meta.
177 ± 6
meta.
181 ± 3
inherited
meta.
max. depo.
meta.
max. depo.
max. depo.
~ 840
180 ± 3
~750
177 ± 3
493 ± 64
447 ± 30
meta. = metamorphic age; max. depo. = maximum age of deposition; Maximum depositional ages based
on clusters of the youngest, overlapping 206Pb*/238U ages. Uncertainty is at the 2σ level
310
Apparent ages (Ma)
±
(Ma)
Table 2. Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass Spectrometry
Isotopic ratios
Best age
(Ma)
25.9
5.8
9.4
12.9
12.0
5.7
13.0
22.2
30.4
5.4
16.9
13.9
34.1
10.1
5.7
5.6
5.6
11.1
7.8
5.8
8.8
7.8
6.7
5.6
5.5
39.6
27.0
6.1
±
(Ma)
783.3
834.8
847.0
831.0
868.8
865.8
787.0
729.4
819.4
825.1
862.5
697.7
717.5
811.1
868.6
850.0
851.0
760.9
438.2
813.0
822.4
840.8
810.4
838.9
830.7
647.8
627.5
854.4
5.9
6.6
5.1
5.6
5.7
206Pb*
207Pb*
46.5
15.2
14.5
14.4
14.4
14.7
14.5
62.0
14.5
14.4
14.4
22.4
14.9
14.7
15.4
17.1
14.6
16.9
32.5
15.1
16.3
14.5
14.6
14.8
15.3
26.9
25.9
14.5
897.8
857.2
774.4
860.3
871.2
±
(Ma)
934.8
915.2
927.9
923.9
924.2
911.3
900.2
935.1
905.9
905.2
923.9
962.3
855.8
861.0
906.3
864.5
871.2
852.9
792.0
881.6
878.3
880.6
871.1
890.6
886.6
773.1
847.9
894.5
21.7
25.3
75.4
31.9
33.3
207Pb*
235U
23.6
6.1
8.1
10.4
9.7
5.9
10.6
24.1
23.0
5.7
13.0
12.7
26.9
8.5
6.0
6.2
5.7
9.5
9.5
6.0
7.9
7.0
6.4
5.8
5.9
32.3
23.1
6.0
859.4
888.4
904.0
872.1
858.0
±
(Ma)
823.8
857.1
869.7
856.8
884.6
878.7
817.2
782.0
843.1
847.1
879.9
763.8
751.9
824.6
879.3
854.1
856.6
784.6
499.7
831.6
837.7
851.8
826.8
853.2
846.1
676.4
677.6
865.6
7.4
8.6
20.8
9.8
10.2
206Pb*
238U
25.9
5.8
9.4
12.9
12.0
5.7
13.0
22.2
30.4
5.4
16.9
13.9
34.1
10.1
5.7
5.6
5.6
11.1
7.8
5.8
8.8
7.8
6.7
5.6
5.5
39.6
27.0
6.1
886.8
866.0
808.6
863.6
867.5
error
corr.
783.3
834.8
847.0
831.0
868.8
865.8
787.0
729.4
819.4
825.1
862.5
697.7
717.5
811.1
868.6
850.0
851.0
760.9
438.2
813.0
822.4
840.8
810.4
838.9
830.7
647.8
627.5
854.4
5.9
6.6
5.1
5.6
5.7
±
(%)
0.84
0.71
0.86
0.92
0.90
0.70
0.93
0.73
0.98
0.71
0.95
0.89
0.99
0.88
0.68
0.65
0.71
0.88
0.77
0.72
0.82
0.82
0.78
0.70
0.69
0.98
0.96
0.73
897.8
857.2
774.4
860.3
871.2
206Pb*
238U
3.5
0.7
1.2
1.7
1.5
0.7
1.8
3.2
4.0
0.7
2.1
2.1
5.0
1.3
0.7
0.7
0.7
1.5
1.9
0.8
1.1
1.0
0.9
0.7
0.7
6.4
4.5
0.8
0.56
0.56
0.19
0.41
0.40
±
(%)
0.1292
0.1382
0.1404
0.1376
0.1443
0.1438
0.1298
0.1198
0.1355
0.1365
0.1432
0.1143
0.1177
0.1341
0.1442
0.1409
0.1411
0.1253
0.0703
0.1344
0.1361
0.1393
0.1340
0.1390
0.1375
0.1057
0.1022
0.1417
0.7
0.8
0.7
0.7
0.7
207Pb*
235U
4.2
1.0
1.4
1.8
1.6
1.0
1.9
4.4
4.0
1.0
2.2
2.4
5.1
1.5
1.0
1.1
1.0
1.7
2.4
1.1
1.4
1.2
1.1
1.0
1.0
6.5
4.7
1.0
0.1494
0.1422
0.1276
0.1428
0.1447
U/Th
1.2509
1.3258
1.3549
1.3251
1.3898
1.3759
1.2362
1.1601
1.2940
1.3031
1.3787
1.1216
1.0971
1.2526
1.3773
1.3190
1.3249
1.1657
0.6358
1.2683
1.2819
1.3138
1.2576
1.3170
1.3008
0.9468
0.9490
1.3456
1.3
1.5
3.7
1.7
1.7
206Pb
204Pb
1.7
1.2
0.9
1.4
1.2
0.9
1.1
1.6
1.1
1.5
1.1
1.3
1.2
1.8
1.0
1.1
1.7
1.5
0.9
1.3
1.2
1.1
1.6
1.2
1.2
3.1
1.6
1.0
1.3950
1.3464
1.2174
1.3409
1.3498
U
(ppm)
Orthogneiss JG053104-1
04
2018
6305
07
1402
15363
11
1045
134293
12
1430
84931
13
1300
105755
14
679
53443
17
1473
112261
18
2054
2530
21
1272
82698
22
1951
91430
24
760
97904
25
2579
1628
28
1939
47177
32
1755
72528
34
1374
12596
36
1385
60243
38
1218
31672
40
1289
93209
44
1929
1808
48
1531
14646
52
1314
44169
53
2023
66468
54
1396
48616
57
1904
18262
58
2406
9441
59
1034
24812
60
1054
4206
63
1230
91586
0.7
0.8
0.7
0.7
0.5
Spot
ID
Orthogneiss JG061504-2
01c
272
18248
01t
271
27949
02
197
4769
03
341
14268
04
252
15451
311
298
388
400
679
325
490
425
596
610
297
522
204
388
266
440
288
2313
303
353
454
408
395
1266
455
U
(ppm)
26227
11405
16652
27528
19822
7216
46859
23906
57193
34382
30514
7534
20690
29916
6778
34587
13363
19830
31912
43190
29624
33655
5249
43389
206Pb
204Pb
Isotopic ratios
Apparent ages (Ma)
880.9
865.6
809.6
870.3
783.3
751.2
812.3
838.5
822.3
871.8
826.3
629.2
859.6
957.9
847.0
844.6
852.1
804.8
843.1
896.6
851.2
838.0
185.6
917.8
34.4
21.4
27.4
2.1
38.1
27.4
29.5
15.7
12.0
19.9
11.3
5.7
7.7
5.7
8.0
5.1
5.3
12.1
5.4
24.9
9.5
24.8
17.5
61.2
12.4
5.5
6.9
15.1
18.9
32.0
6.2
5.5
1.5
14.6
±
(Ma)
28.0
26.7
19.9
15.3
29.6
31.6
29.1
20.3
14.5
25.1
18.2
86.3
20.5
121.9
27.0
24.7
15.0
28.1
21.3
15.8
20.0
14.8
71.3
16.3
734.4
678.4
335.0
175.2
658.5
772.6
466.8
383.9
688.5
563.3
Best age
(Ma)
873.6
882.8
879.8
882.3
850.2
882.6
859.3
874.8
878.9
849.3
874.9
805.1
869.4
981.3
893.3
838.6
880.7
865.5
854.8
892.7
879.4
859.4
324.2
872.8
56.3
81.7
59.5
40.9
95.7
26.8
91.8
34.4
38.9
38.2
±
(Ma)
11.3
8.6
7.9
6.0
9.9
9.2
8.9
10.5
5.7
19.1
8.6
28.6
13.9
56.8
11.8
7.9
6.5
13.6
14.9
23.2
7.2
5.7
5.8
11.3
762.4
885.2
632.0
181.6
998.4
858.5
887.0
550.3
781.4
692.8
206Pb*
207Pb*
878.9
870.4
828.6
873.7
800.9
785.1
825.0
848.5
837.8
865.5
839.6
668.9
862.4
965.0
859.9
842.9
860.1
821.1
846.3
895.5
859.1
843.9
196.1
904.6
29.7
26.8
27.7
3.4
40.5
22.0
33.4
15.1
13.3
18.4
±
(Ma)
11.3
5.7
7.7
5.7
8.0
5.1
5.3
12.1
5.4
24.9
9.5
24.8
17.5
61.2
12.4
5.5
6.9
15.1
18.9
32.0
6.2
5.5
1.5
14.6
741.4
728.4
375.2
175.6
740.8
795.1
545.0
408.6
710.7
589.7
207Pb*
235U
880.9
865.6
809.6
870.3
783.3
751.2
812.3
838.5
822.3
871.8
826.3
629.2
859.6
957.9
847.0
844.6
852.1
804.8
843.1
896.6
851.2
838.0
185.6
917.8
34.4
21.4
27.4
2.1
38.1
27.4
29.5
15.7
12.0
19.9
±
(Ma)
0.71
0.48
0.72
0.69
0.61
0.42
0.45
0.84
0.71
0.93
0.81
0.71
0.91
0.75
0.77
0.51
0.76
0.83
0.92
0.98
0.63
0.70
0.26
0.91
734.4
678.4
335.0
175.2
658.5
772.6
466.8
383.9
688.5
563.3
206Pb*
238U
1.4
0.7
1.0
0.7
1.1
0.7
0.7
1.5
0.7
3.1
1.2
4.1
2.2
6.9
1.6
0.7
0.9
2.0
2.4
3.8
0.8
0.7
0.8
1.7
0.88
0.64
0.95
0.57
0.79
0.95
0.83
0.94
0.70
0.90
error
corr.
0.1464
0.1437
0.1338
0.1445
0.1292
0.1236
0.1343
0.1389
0.1361
0.1448
0.1368
0.1025
0.1426
0.1602
0.1404
0.1400
0.1413
0.1330
0.1397
0.1492
0.1412
0.1388
0.0292
0.1530
5.0
3.3
8.4
1.2
6.1
3.8
6.6
4.2
1.8
3.7
±
(%)
1.9
1.5
1.4
1.0
1.8
1.7
1.6
1.8
1.0
3.3
1.5
5.8
2.4
9.1
2.0
1.4
1.1
2.4
2.6
3.9
1.2
1.0
3.3
1.9
0.1207
0.1110
0.0533
0.0275
0.1075
0.1273
0.0751
0.0614
0.1127
0.0913
206Pb*
238U
1.3763
1.3567
1.2614
1.3642
1.2008
1.1667
1.2535
1.3064
1.2821
1.3451
1.2862
0.9324
1.3380
1.5867
1.3323
1.2937
1.3327
1.2449
1.3014
1.4154
1.3304
1.2958
0.2130
1.4374
5.6
5.2
8.8
2.1
7.7
4.0
7.9
4.5
2.6
4.1
±
(%)
0.7
0.8
0.7
0.6
0.8
0.6
0.9
1.6
0.7
0.8
0.7
1.5
0.8
0.9
0.5
0.7
1.0
0.7
1.0
0.8
0.6
0.7
5.6
0.9
1.0754
1.0489
0.4471
0.1888
1.0742
1.1881
0.7104
0.4954
1.0137
0.7874
207Pb*
235U
0.8
1.3
0.4
8.7
0.4
1.1
0.9
2.6
2.8
2.2
U/Th
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
06
07
08
09
10
11
13
14
15
16
17
18
19
20
21
22
23
24
25
25c
26
27
29t
30
Orthogneiss JG060504-2
02C
774
35290
04C
531
14026
05C
2673
11665
05T
1537
30736
06C
1121
3587
06T
1315
34851
07C
618
4268
07T
1705
7691
08C
390
36365
09
1424
48798
312
727
1331
1189
1138
1827
391
866
1126
925
178
1211
555
148
490
436
521
874
1322
893
245
879
1361
475
778
986
1438
1276
1375
U
(ppm)
5710
31741
20107
6590
23787
2572
22114
26909
19172
14251
3035
22277
11050
19643
41116
29236
3326
13605
65606
22289
77814
25624
40628
21358
61327
75909
3425
55224
206Pb
204Pb
Isotopic ratios
Apparent ages (Ma)
450.8
638.8
210.2
580.8
518.0
576.8
824.7
178.8
576.0
652.9
590.7
716.8
616.5
556.0
786.6
561.6
300.9
754.4
716.8
505.9
768.0
360.4
809.7
704.9
771.7
745.1
505.3
506.5
14.5
3.6
5.5
2.1
11.8
4.5
34.0
36.2
14.0
29.6
41.5
29.2
11.5
5.6
15.8
21.6
45.4
18.1
39.7
21.1
9.1
10.5
15.9
31.3
14.7
45.2
12.6
12.1
7.6
38.7
16.0
10.2
25.1
29.1
±
(Ma)
167.4
62.7
28.5
59.7
19.3
87.7
29.3
43.3
38.5
22.6
166.9
25.3
78.3
22.2
26.2
23.0
31.8
28.7
27.9
72.7
19.0
35.3
35.4
55.3
25.8
14.2
85.6
20.3
775.1
533.1
503.3
187.4
540.2
529.1
Best age
(Ma)
941.5
756.7
251.7
913.9
742.7
989.7
895.4
181.8
818.0
752.5
1077.2
804.4
830.3
773.6
814.1
753.2
773.9
845.9
800.7
719.6
824.2
645.8
846.9
845.5
842.4
826.7
958.8
752.1
49.9
20.0
130.3
47.8
28.6
34.3
±
(Ma)
47.4
32.5
13.3
29.2
36.2
33.4
11.8
6.0
15.9
17.9
58.6
15.4
37.7
18.6
9.7
10.1
17.1
25.0
13.4
41.9
10.7
12.7
11.1
33.5
13.9
8.6
30.0
25.8
795.7
590.4
591.2
216.5
565.0
556.2
206Pb*
207Pb*
541.0
665.4
213.7
653.7
561.6
668.1
844.1
179.0
627.5
675.7
702.1
738.4
664.3
600.8
793.8
601.2
361.8
777.9
737.4
546.5
782.6
401.6
819.7
739.4
790.1
765.8
596.2
553.5
16.9
4.9
24.8
4.1
11.1
7.5
±
(Ma)
34.0
36.2
14.0
29.6
41.5
29.2
11.5
5.6
15.8
21.6
45.4
18.1
39.7
21.1
9.1
10.5
15.9
31.3
14.7
45.2
12.6
12.1
7.6
38.7
16.0
10.2
25.1
29.1
780.4
544.1
519.5
189.6
545.0
534.2
207Pb*
235U
450.8
638.8
210.2
580.8
518.0
576.8
824.7
178.8
576.0
652.9
590.7
716.8
616.5
556.0
786.6
561.6
300.9
754.4
716.8
505.9
768.0
360.4
809.7
704.9
771.7
745.1
505.3
506.5
14.5
3.6
5.5
2.1
11.8
4.5
±
(Ma)
0.69
0.89
0.98
0.88
0.99
0.78
0.72
0.86
0.84
0.96
0.70
0.91
0.87
0.97
0.70
0.87
0.96
0.95
0.85
0.94
0.89
0.90
0.51
0.91
0.87
0.91
0.78
0.99
775.1
533.1
503.3
187.4
540.2
529.1
206Pb*
238U
7.8
6.0
6.8
5.3
8.3
5.3
1.5
3.2
2.9
3.5
8.0
2.7
6.7
4.0
1.2
2.0
5.4
4.4
2.2
9.3
1.7
3.5
1.0
5.8
2.2
1.5
5.2
6.0
0.64
0.61
0.19
0.49
0.87
0.49
error
corr.
0.0724
0.1042
0.0331
0.0943
0.0837
0.0936
0.1365
0.0281
0.0935
0.1066
0.0960
0.1176
0.1004
0.0901
0.1298
0.0910
0.0478
0.1242
0.1176
0.0816
0.1265
0.0575
0.1338
0.1155
0.1272
0.1225
0.0815
0.0817
2.0
0.7
1.1
1.2
2.3
0.9
±
(%)
11.3
6.7
6.9
6.1
8.4
6.8
2.1
3.7
3.4
3.6
11.6
2.9
7.7
4.1
1.8
2.2
5.6
4.6
2.5
9.9
2.0
3.8
2.0
6.4
2.5
1.6
6.7
6.0
0.1278
0.0862
0.0812
0.0295
0.0874
0.0855
206Pb*
238U
0.7037
0.9258
0.2342
0.9036
0.7387
0.9310
1.2963
0.1928
0.8553
0.9455
0.9967
1.0692
0.9237
0.8070
1.1853
0.8077
0.4281
1.1515
1.0673
0.7129
1.1613
0.4851
1.2419
1.0714
1.1775
1.1260
0.7988
0.7249
3.1
1.2
6.1
2.4
2.6
1.8
±
(%)
0.6
1.5
5.6
1.0
1.4
1.4
0.7
7.7
1.0
1.3
1.0
1.2
1.5
0.6
1.9
1.2
2.4
0.8
1.2
4.2
1.5
1.7
1.4
0.9
1.4
1.1
1.2
1.7
1.1567
0.7089
0.6680
0.2053
0.7104
0.6924
207Pb*
235U
1.4
2.3
0.3
114.2
1.7
2.4
U/Th
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
10
11
12T
13C
14C
15C
16
17T
18
19C
20C
21C
22C
23B
23C
24C
25B
25C
26
27C
27T
28
29
30
32T
33T
34
36
Orthogneiss JG061504-1
01
251
23740
03
1063
10816
04
300
2755
06
1362
13880
07t
768
18708
07t2
738
8804
313
502
466
544
356
416
507
711
698
289
564
U
(ppm)
12778
10641
46871
24705
21498
21896
18372
27793
9378
2073
206Pb
204Pb
Isotopic ratios
Apparent ages (Ma)
827.8
574.0
535.1
531.2
489.1
522.5
501.4
381.4
537.8
331.4
27.7
16.6
8.0
11.1
24.8
11.7
18.1
15.9
13.5
10.9
28.7
7.7
5.9
24.8
8.7
17.1
13.1
22.1
14.9
12.7
8.8
5.6
50.7
9.0
4.5
22.8
4.1
7.0
18.9
3.6
9.9
±
(Ma)
31.4
60.0
29.4
33.2
45.5
26.8
96.9
45.5
76.1
351.0
494.0
332.9
411.4
480.6
441.9
494.8
451.5
517.4
473.9
481.9
474.8
499.9
500.6
554.7
513.8
351.2
509.8
463.8
310.1
529.4
503.5
Best age
(Ma)
863.2
751.9
551.7
518.5
486.2
468.3
608.9
429.4
509.1
694.1
53.8
77.3
26.5
16.8
67.0
49.2
191.0
43.4
162.1
21.8
37.9
35.2
81.9
66.2
63.2
151.5
119.6
63.2
260.2
60.6
47.1
±
(Ma)
9.6
44.4
9.2
7.2
20.4
5.9
19.2
17.7
14.6
52.9
507.2
484.3
475.0
500.0
730.9
496.6
1208.5
405.1
550.5
501.8
474.4
466.6
545.3
467.7
511.9
741.1
756.9
541.1
1156.3
516.5
452.7
206Pb*
207Pb*
837.4
611.3
538.3
528.8
488.6
512.5
521.2
388.3
532.4
380.9
24.8
18.5
8.0
9.7
25.8
13.0
47.7
14.7
30.9
9.9
24.6
8.8
15.8
23.2
13.6
29.3
26.9
21.8
49.3
15.3
10.9
±
(Ma)
5.6
50.7
9.0
4.5
22.8
4.1
7.0
18.9
3.6
9.9
496.4
352.6
421.1
484.0
491.7
495.2
599.6
497.2
487.3
485.4
474.7
494.0
508.7
538.0
513.4
407.6
557.4
477.0
433.4
527.0
494.5
207Pb*
235U
827.8
574.0
535.1
531.2
489.1
522.5
501.4
381.4
537.8
331.4
27.7
16.6
8.0
11.1
24.8
11.7
18.1
15.9
13.5
10.9
28.7
7.7
5.9
24.8
8.7
17.1
13.1
22.1
14.9
12.7
8.8
±
(Ma)
0.43
0.96
0.79
0.50
0.92
0.56
0.31
0.93
0.20
0.18
494.0
332.9
411.4
480.6
441.9
494.8
451.5
517.4
473.9
481.9
474.8
499.9
500.6
554.7
513.8
351.2
509.8
463.8
310.1
529.4
503.5
206Pb*
238U
0.7
9.2
1.8
0.9
4.8
0.8
1.5
5.1
0.7
3.1
0.92
0.83
0.86
0.95
0.88
0.74
0.39
0.86
0.37
0.92
0.96
0.71
0.31
0.84
0.52
0.57
0.43
0.86
0.35
0.67
0.65
error
corr.
0.1370
0.0931
0.0866
0.0859
0.0788
0.0844
0.0809
0.0609
0.0870
0.0527
5.8
5.1
2.0
2.4
5.8
2.5
4.1
3.2
2.9
2.4
6.3
1.6
1.2
4.7
1.8
5.0
2.7
4.9
4.9
2.5
1.8
±
(%)
1.7
9.7
2.2
1.7
5.3
1.5
4.7
5.5
3.5
16.7
0.0796
0.0530
0.0659
0.0774
0.0710
0.0798
0.0726
0.0836
0.0763
0.0776
0.0764
0.0806
0.0808
0.0899
0.0830
0.0560
0.0823
0.0746
0.0493
0.0856
0.0812
206Pb*
238U
1.2813
0.8258
0.6992
0.6834
0.6180
0.6566
0.6708
0.4658
0.6893
0.4551
6.3
6.2
2.3
2.5
6.6
3.3
10.5
3.7
8.0
2.6
6.5
2.3
3.9
5.5
3.4
8.7
6.3
5.7
14.0
3.7
2.8
±
(%)
1.5
2.0
3.5
0.4
0.6
0.5
0.6
3.3
0.4
21.2
0.6304
0.4152
0.5140
0.6106
0.6230
0.6285
0.8050
0.6317
0.6159
0.6129
0.5961
0.6266
0.6504
0.6987
0.6581
0.4940
0.7314
0.5997
0.5324
0.6804
0.6274
207Pb*
235U
1.8
2.2
1.6
1.9
1.6
1.5
2.0
1.8
1.8
2.0
1.8
1.8
1.4
1.7
1.6
0.6
1.5
1.5
2.6
1.7
1.6
U/Th
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
09c
10
11
12
14
16
17
21
22
28
Orthogneiss PK97-6-4-1B
01
710
46403
03
349
8546
04
696
36334
05
826
37512
06
1211
3629
07
348
19334
08
125
767
09
190
14772
10C
473
27629
10T
639
41913
11
549
21325
12
227
19250
13
383
18759
14
528
12746
15
170
14553
16C
2912
3110
16T
667
3995
17
544
10925
18
1427
1027
19
298
14191
20
208
18732
314
U
(ppm)
206Pb
204Pb
0.7
0.6
1.0
54.3
49.9
1.1
35.3
14.8
53.2
39.2
0.9
0.7
1.0
35.8
0.6
1.0
1.5
0.8
1.0
14.4
49.9
61.2
0.8
15.0
49.8
0.9
0.8
0.9
2.5
0.9
56.5
59.9
0.8
0.7
52.7
U/Th
0.5780
0.5878
0.4075
0.1819
0.1884
0.5494
0.1837
0.1857
0.1872
0.1819
0.4168
0.6520
0.5514
0.1919
0.4397
0.5050
0.5082
0.5950
0.3449
0.1866
0.1893
0.1936
0.6389
0.1900
0.2018
0.5893
0.5272
0.4603
0.3167
0.7629
0.1937
0.1978
0.7447
0.6383
0.1935
207Pb*
235U
0.0788
0.0760
0.0526
0.0287
0.0275
0.0672
0.0275
0.0283
0.0286
0.0275
0.0521
0.0845
0.0719
0.0292
0.0571
0.0638
0.0642
0.0779
0.0468
0.0272
0.0282
0.0285
0.0810
0.0291
0.0301
0.0748
0.0684
0.0613
0.0435
0.0958
0.0286
0.0287
0.0811
0.0792
0.0288
206Pb*
238U
Isotopic ratios
±
(%)
3.6
4.0
6.6
7.8
2.8
9.6
8.3
7.9
4.4
4.9
9.3
2.7
7.3
5.9
8.3
12.3
3.1
5.0
14.4
4.2
10.2
5.0
3.4
4.7
7.9
1.8
3.0
4.4
10.7
4.8
4.1
5.3
28.5
6.9
6.1
2.3
1.3
4.1
1.4
0.9
4.0
2.6
2.7
1.2
0.9
2.3
0.9
6.5
0.7
6.9
12.0
1.0
1.4
12.6
1.1
1.7
3.7
2.6
0.8
0.7
0.7
1.1
1.9
9.9
0.8
2.0
0.7
5.2
1.0
0.8
±
(%)
0.65
0.32
0.61
0.18
0.31
0.42
0.31
0.34
0.27
0.19
0.25
0.35
0.88
0.12
0.83
0.97
0.31
0.28
0.87
0.27
0.16
0.74
0.76
0.17
0.09
0.40
0.37
0.43
0.93
0.17
0.50
0.13
0.18
0.14
0.13
error
corr.
488.8
472.3
330.7
182.1
175.1
419.5
174.7
180.0
181.5
175.0
327.3
522.8
447.6
185.7
358.1
398.6
401.3
483.3
294.7
172.9
179.2
181.1
502.0
184.8
191.1
465.2
426.2
383.8
274.2
589.9
182.0
182.2
502.4
491.5
182.8
206Pb*
238U
11.0
5.9
13.1
2.6
1.5
16.4
4.4
4.8
2.1
1.6
7.4
4.7
28.1
1.3
24.0
46.3
3.7
6.6
36.2
1.9
2.9
6.6
12.4
1.4
1.4
3.3
4.6
7.0
26.6
4.7
3.7
1.3
25.1
4.6
1.4
±
(Ma)
13.3
15.1
19.5
12.1
4.6
34.5
13.0
12.6
7.0
7.7
27.9
10.6
26.5
9.6
25.7
41.9
10.6
18.9
37.5
6.7
16.5
8.2
13.4
7.6
13.4
6.9
10.6
14.0
26.0
21.2
6.8
8.9
124.3
27.3
10.0
±
(Ma)
Apparent ages (Ma)
207Pb*
235U
463.2
469.4
347.0
169.7
175.2
444.6
171.3
173.0
174.3
169.7
353.8
509.7
445.9
178.3
370.0
415.1
417.2
474.1
300.9
173.7
176.0
179.7
501.6
176.6
186.6
470.4
430.0
384.5
279.3
575.7
179.8
183.3
565.1
501.2
179.6
338.0
455.3
457.8
-1.0
177.5
576.6
124.4
78.6
77.2
96.3
531.3
451.4
437.2
81.2
445.4
507.7
506.0
429.8
349.4
184.2
133.8
161.5
500.1
68.1
130.2
496.3
450.0
388.6
322.0
520.1
150.8
198.0
826.4
546.0
137.8
206Pb*
207Pb*
61.1
84.4
116.5
184.2
63.3
189.4
185.1
177.9
99.7
115.0
198.6
55.1
76.1
139.0
102.0
60.6
64.6
106.7
159.3
93.7
236.7
78.5
48.2
110.2
185.0
37.2
62.5
88.4
89.7
104.2
83.5
122.8
596.7
149.5
141.4
±
(Ma)
488.8
472.3
330.7
182.1
175.1
419.5
174.7
180.0
181.5
175.0
327.3
522.8
447.6
185.7
358.1
398.6
401.3
483.3
294.7
172.9
179.2
181.1
502.0
184.8
191.1
465.2
426.2
383.8
274.2
589.9
182.0
182.2
502.4
491.5
182.8
Best age
(Ma)
11.0
5.9
13.1
2.6
1.5
16.4
4.4
4.8
2.1
1.6
7.4
4.7
28.1
1.3
24.0
46.3
3.7
6.6
36.2
1.9
2.9
6.6
12.4
1.4
1.4
3.3
4.6
7.0
26.6
4.7
3.7
1.3
25.1
4.6
1.4
±
(Ma)
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
Orthogneiss JG061604-1
01
161
4536
02
162
3107
03
399
2277
04c
327
4212
04t
553
5646
05e
443
5803
06
311
2984
07
429
5706
08
830
10912
09
511
7751
10c
224
1297
10t
290
11561
11c
405
22387
11t
427
10364
12
326
3348
13c
629
8708
13t
325
15444
14
175
7634
15c
341
7384
15t
581
11864
16
251
5906
17
775
14382
18
365
26666
19
468
11114
20
210
4845
21c
645
26177
21t
461
12015
22
269
10435
23
337
11045
24
184
12167
25
571
14545
26c
282
10382
27
132
2240
28
319
5598
29
344
10588
315
Isotopic ratios
Apparent ages (Ma)
±
(Ma)
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Best age
(Ma)
12.9
17.7
9.0
35.3
8.1
5.6
18.8
36.9
7.2
6.4
35.3
11.8
11.4
11.0
20.6
30.6
7.5
6.7
11.6
13.8
35.3
7.4
22.2
17.1
5.9
±
(Ma)
457.1
518.1
468.1
508.5
200.2
498.7
444.8
414.9
493.6
446.2
480.3
211.8
483.8
468.5
489.3
518.2
488.0
494.8
525.9
484.6
447.9
476.2
384.3
411.4
187.6
11.5
4.7
5.7
19.3
5.8
11.8
18.3
24.6
206Pb*
207Pb*
37.9
189.5
40.6
41.4
73.1
27.1
154.8
46.9
105.3
53.6
45.4
68.0
226.5
68.7
161.4
99.2
51.7
20.6
41.7
44.0
59.6
47.4
84.2
45.8
34.8
778.9
491.3
507.2
755.8
425.5
794.5
855.5
475.5
±
(Ma)
570.0
547.8
513.3
538.7
985.3
536.7
556.2
396.5
518.9
614.2
384.8
240.5
743.5
544.6
667.2
561.0
520.2
512.9
508.2
1123.7
574.2
593.1
874.7
556.8
113.4
72.6
59.6
67.8
59.9
104.8
51.4
41.2
263.9
207Pb*
235U
13.0
38.3
10.4
30.1
13.2
6.8
31.0
31.9
19.8
11.0
29.4
12.4
45.4
15.2
35.6
31.4
11.1
6.7
12.1
17.0
32.4
10.7
26.8
16.9
5.8
859.3
540.9
532.7
887.0
698.2
874.2
872.7
878.8
±
(Ma)
476.3
523.7
475.8
514.0
276.5
505.6
463.3
412.1
498.1
474.6
464.1
214.2
531.8
481.6
521.9
526.2
493.7
498.1
522.6
613.6
469.1
496.8
462.9
434.1
182.3
21.2
11.4
13.3
21.7
19.3
16.5
17.6
58.8
206Pb*
238U
12.9
17.7
9.0
35.3
8.1
5.6
18.8
36.9
7.2
6.4
35.3
11.8
11.4
11.0
20.6
30.6
7.5
6.7
11.6
13.8
35.3
7.4
22.2
17.1
5.9
800.0
500.1
511.9
789.8
470.9
815.7
860.3
551.4
error
corr.
457.1
518.1
468.1
508.5
200.2
498.7
444.8
414.9
493.6
446.2
480.3
211.8
483.8
468.5
489.3
518.2
488.0
494.8
525.9
484.6
447.9
476.2
384.3
411.4
187.6
11.5
4.7
5.7
19.3
5.8
11.8
18.3
24.6
±
(%)
0.86
0.38
0.73
0.97
0.75
0.68
0.52
0.98
0.30
0.51
0.97
0.89
0.22
0.61
0.50
0.80
0.56
0.83
0.77
0.80
0.95
0.59
0.83
0.90
0.91
778.9
491.3
507.2
755.8
425.5
794.5
855.5
475.5
206Pb*
238U
2.9
3.6
2.0
7.2
4.1
1.2
4.4
9.2
1.5
1.5
7.6
5.7
2.4
2.4
4.4
6.2
1.6
1.4
2.3
3.0
8.2
1.6
6.0
4.3
3.2
0.41
0.34
0.35
0.68
0.27
0.54
0.75
0.39
±
(%)
0.0735
0.0837
0.0753
0.0821
0.0315
0.0804
0.0714
0.0665
0.0796
0.0717
0.0774
0.0334
0.0779
0.0754
0.0788
0.0837
0.0786
0.0798
0.0850
0.0781
0.0719
0.0767
0.0614
0.0659
0.0295
1.6
1.0
1.2
2.7
1.4
1.6
2.3
5.4
207Pb*
235U
3.4
9.4
2.7
7.5
5.4
1.7
8.3
9.4
5.0
2.9
7.9
6.4
11.0
4.0
8.7
7.7
2.8
1.7
3.0
3.7
8.6
2.7
7.2
4.8
3.5
0.1284
0.0792
0.0819
0.1244
0.0682
0.1312
0.1419
0.0766
U/Th
0.5985
0.6749
0.5978
0.6591
0.3130
0.6453
0.5783
0.5007
0.6333
0.5958
0.5795
0.2349
0.6883
0.6069
0.6719
0.6790
0.6261
0.6332
0.6731
0.8299
0.5872
0.6311
0.5776
0.5335
0.1967
3.8
2.9
3.3
4.0
5.1
2.9
3.0
13.8
206Pb
204Pb
0.9
1.0
2.2
1.2
19.0
1.1
1.8
2.2
1.1
1.3
1.3
6.1
1.2
1.2
0.7
1.1
1.2
1.3
1.3
1.8
1.7
1.3
1.5
1.0
17.1
1.1988
0.6365
0.6555
1.1768
0.5900
1.2330
1.3333
0.7213
U
(ppm)
Orthogneiss PK97-6-4-1A
01
471
14669
02
133
5153
04
175
7256
05
415
24215
06
1339
970
07
726
26669
08
399
9460
09
230
10450
10
132
6112
11
219
5375
12
216
6397
13
484
7556
14
227
4007
15
302
9874
16
393
6590
17
139
3940
18
528
26875
19
474
25490
21
433
13518
22
386
9674
23
411
10480
24
458
10239
25
688
1408
26
666
24597
27
949
13843
1.2
1.2
1.2
0.8
5.4
0.9
1.1
3.2
Spot
ID
Orthogneiss JG053104-2
010R
329
6789
01C
828
24428
01R
527
19951
02C
633
11402
02T
451
14854
03C
404
17986
03R
436
16151
04T
121
2858
316
562
637
364
644
177
150
626
539
548
360
394
436
337
315
443
807
322
395
377
451
461
213
291
1123
694
431
294
886
439
U
(ppm)
18117
3019
11360
14390
7018
4029
2023
7293
3456
10394
26682
16915
25423
37777
15826
12463
5574
1883
13146
7751
3495
5131
9018
34467
29586
23055
16364
31479
34094
206Pb
204Pb
Isotopic ratios
Apparent ages (Ma)
795.7
757.3
759.7
765.7
817.2
779.9
532.5
470.1
478.6
781.6
826.8
814.0
856.0
852.4
822.0
468.5
397.3
822.6
843.0
863.7
490.5
297.7
926.5
528.1
811.0
840.2
775.6
771.0
798.3
3.3
1.7
3.3
3.9
3.3
48.9
7.5
15.0
11.0
17.6
11.7
11.4
12.3
5.5
8.8
7.9
10.7
19.5
9.0
11.4
7.0
13.5
17.7
5.5
14.5
65.4
44.0
16.2
139.6
6.3
31.3
13.2
27.6
19.0
±
(Ma)
45.9
169.7
41.2
52.5
128.8
169.4
167.2
80.9
212.0
48.9
40.8
58.8
53.0
60.6
44.3
66.3
238.2
240.6
34.5
97.7
121.9
200.5
64.6
109.8
20.7
62.0
126.1
26.9
56.0
180.0
184.5
178.9
179.1
169.5
Best age
(Ma)
895.0
973.8
807.8
827.0
992.8
947.1
560.7
535.9
601.2
852.9
827.4
862.1
868.6
837.0
892.3
597.8
503.2
1111.8
858.5
960.0
820.2
833.9
1006.5
829.7
927.3
902.7
881.8
899.1
877.9
135.8
202.9
320.1
68.7
454.6
±
(Ma)
38.9
46.9
15.5
16.0
39.2
47.5
33.3
17.6
38.9
14.6
12.5
17.9
20.5
17.9
14.9
13.4
38.5
73.4
10.4
30.3
64.1
55.3
22.8
125.8
7.5
28.9
35.3
22.3
20.8
172.7
143.1
2.0
168.3
262.3
206Pb*
207Pb*
822.3
814.4
772.0
781.6
865.9
824.6
537.9
481.4
500.4
800.4
826.9
827.0
859.6
848.2
841.2
491.1
413.2
905.0
847.3
891.2
553.2
367.3
950.5
588.7
842.7
857.5
803.6
804.7
819.6
10.0
14.4
20.6
6.0
31.9
±
(Ma)
48.9
7.5
15.0
11.0
17.6
11.7
11.4
12.3
5.5
8.8
7.9
10.7
19.5
9.0
11.4
7.0
13.5
17.7
5.5
14.5
65.4
44.0
16.2
139.6
6.3
31.3
13.2
27.6
19.0
179.5
181.6
167.1
178.3
175.9
207Pb*
235U
795.7
757.3
759.7
765.7
817.2
779.9
532.5
470.1
478.6
781.6
826.8
814.0
856.0
852.4
822.0
468.5
397.3
822.6
843.0
863.7
490.5
297.7
926.5
528.1
811.0
840.2
775.6
771.0
798.3
3.3
1.7
3.3
3.9
3.3
±
(Ma)
0.95
0.12
0.73
0.52
0.34
0.19
0.28
0.59
0.12
0.45
0.46
0.44
0.69
0.36
0.57
0.45
0.31
0.19
0.39
0.35
0.92
0.84
0.51
0.98
0.64
0.80
0.28
0.95
0.68
180.0
184.5
178.9
179.1
169.5
206Pb*
238U
6.5
1.0
2.1
1.5
2.3
1.6
2.2
2.7
1.2
1.2
1.0
1.4
2.4
1.1
1.5
1.6
3.5
2.3
0.7
1.8
13.8
15.1
1.9
27.5
0.8
4.0
1.8
3.8
2.5
0.30
0.10
0.14
0.60
0.10
error
corr.
0.1314
0.1247
0.1251
0.1261
0.1351
0.1286
0.0861
0.0756
0.0771
0.1289
0.1368
0.1346
0.1420
0.1414
0.1360
0.0754
0.0636
0.1361
0.1397
0.1434
0.0791
0.0473
0.1546
0.0854
0.1341
0.1392
0.1279
0.1270
0.1318
1.9
0.9
1.9
2.2
2.0
±
(%)
6.9
8.4
2.9
2.9
6.7
8.4
8.0
4.6
9.8
2.6
2.2
3.2
3.5
3.1
2.6
3.4
11.3
12.2
1.8
5.1
15.0
17.9
3.7
28.0
1.3
5.0
6.4
4.0
3.7
0.0283
0.0290
0.0281
0.0282
0.0266
206Pb*
238U
1.2477
1.2302
1.1389
1.1592
1.3462
1.2527
0.6985
0.6066
0.6369
1.1995
1.2579
1.2579
1.3316
1.3056
1.2899
0.6220
0.5022
1.4383
1.3036
1.4053
0.7242
0.4358
1.5501
0.7856
1.2932
1.3269
1.2065
1.2089
1.2416
6.1
8.7
13.4
3.7
19.7
±
(%)
1.5
1.0
1.5
1.9
1.2
1.3
2.0
1.5
1.7
1.0
1.3
1.5
1.8
1.6
1.5
1.9
2.6
1.2
1.5
1.5
2.4
4.7
1.5
1.0
1.6
1.7
2.0
0.9
1.6
0.1934
0.1958
0.1789
0.1920
0.1891
207Pb*
235U
12.8
19.4
35.8
77.9
24.9
U/Th
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
05C
05R
06C
06R
07C
07R
08R
09C
09R
10C
11C
11R
12C
12R
13R
14C
14R
15C
15R
16C
16R
17R
18C
18R
19C
19R
20C
21C
21R
Orthogneiss JG063004-1
01C
1043
14734
01R
893
11938
03
365
5190
05
3714
9099
06T
387
4393
317
53
822
254
49
138
925
799
1345
1029
815
61
1391
1188
47
1227
1386
946
1281
763
38
185
564
28
U
(ppm)
267
6752
1075
469
1631
6520
5430
23033
10544
11477
1267
39990
8972
388
19568
34576
13632
30474
34610
1470
1185
25235
3387
206Pb
204Pb
Isotopic ratios
Apparent ages (Ma)
80.4
172.5
255.8
225.5
204.3
169.5
175.7
177.3
171.2
190.8
260.6
175.5
180.2
287.3
186.7
178.5
189.1
182.3
915.6
178.6
197.8
178.0
225.1
13.1
36.6
11.4
22.1
30.5
12.3
17.7
8.4
17.5
7.8
13.2
67.6
7.6
34.1
142.0
46.1
13.9
12.5
6.7
9.0
8.7
99.4
7.8
15.9
225.6
4.5
4.5
3.3
3.1
27.3
30.4
24.3
4.4
55.2
±
(Ma)
13.4
2.7
11.5
15.9
5.5
2.5
1.8
4.2
3.3
3.0
12.7
4.5
3.2
12.7
3.9
4.1
2.7
2.4
31.6
15.5
5.5
3.0
15.6
1652.6
1587.5
1131.0
783.1
925.8
183.5
1661.1
762.0
2385.7
838.1
515.1
207Pb*
235U
172.0
175.0
265.6
169.7
203.3
175.9
177.9
182.2
180.7
188.2
184.0
176.8
176.4
172.7
187.5
179.6
187.2
182.2
811.3
175.3
175.2
175.3
188.7
14.5
55.9
12.5
27.2
13.1
4.8
28.8
10.3
21.5
7.8
13.9
±
(Ma)
0.09
0.32
0.29
0.14
0.11
0.17
0.13
0.57
0.32
0.32
0.16
0.53
0.19
0.08
0.81
0.83
0.76
0.71
0.92
0.48
0.24
0.65
0.31
1599.8
1546.0
1112.7
762.8
913.4
173.6
1623.1
749.5
2247.9
828.7
467.3
206Pb*
238U
7.9
1.6
4.4
9.5
2.7
1.5
1.0
2.3
1.9
1.6
7.0
2.6
1.8
7.5
2.1
2.3
1.5
1.3
4.1
9.0
3.2
1.7
8.4
0.64
0.89
0.73
0.93
0.31
0.38
0.93
0.93
0.60
0.73
0.94
error
corr.
0.0270
0.0275
0.0421
0.0267
0.0320
0.0277
0.0280
0.0287
0.0284
0.0296
0.0290
0.0278
0.0277
0.0271
0.0295
0.0283
0.0295
0.0287
0.1341
0.0276
0.0275
0.0276
0.0297
1.0
4.1
1.2
3.8
1.5
2.8
2.0
1.5
1.1
1.0
3.1
±
(%)
87.3
4.8
15.1
69.8
24.9
8.9
7.8
4.1
5.7
5.0
43.1
4.8
9.6
88.7
2.6
2.7
1.9
1.9
4.5
18.5
13.5
2.7
27.3
0.2817
0.2710
0.1884
0.1256
0.1522
0.0273
0.2863
0.1233
0.4172
0.1372
0.0752
206Pb*
238U
0.0824
0.1852
0.2865
0.2486
0.2229
0.1817
0.1889
0.1907
0.1837
0.2067
0.2927
0.1887
0.1942
0.3270
0.2018
0.1922
0.2047
0.1967
1.4639
0.1923
0.2151
0.1916
0.2482
1.6
4.6
1.7
4.1
5.0
7.3
2.2
1.6
1.9
1.4
3.3
±
(%)
57.6
17.2
16.8
75.7
32.2
15.7
19.3
28.8
22.5
19.9
20.3
39.6
30.0
67.3
45.6
29.2
25.1
27.8
3.5
61.6
37.4
15.5
56.5
4.0917
3.7755
2.0461
1.1624
1.4887
0.1980
4.1344
1.1179
9.4811
1.2827
0.6608
207Pb*
235U
1.5
1.8
1.6
0.7
0.8
12.3
1.3
1.9
0.6
0.7
3.0
U/Th
±
(Ma)
13.4
2.7
11.5
15.9
5.5
2.5
1.8
4.2
3.3
3.0
12.7
4.5
3.2
12.7
3.9
4.1
2.7
2.4
31.6
15.5
5.5
3.0
15.6
Best age
(Ma)
172.0
175.0
265.6
169.7
203.3
175.9
177.9
182.2
180.7
188.2
184.0
176.8
176.4
172.7
187.5
179.6
187.2
182.2
811.3
175.3
175.2
175.3
188.7
22.8
38.3
22.6
27.2
13.1
4.8
15.0
10.3
25.7
7.8
13.9
±
(Ma)
-2220.0 1277.7
138.4 106.3
167.4 337.8
856.5 1662.5
216.5 580.0
81.4 208.3
145.2 180.8
111.7
80.6
42.1 129.1
222.2 109.2
1022.0 902.2
158.6
96.0
231.1 219.1
1368.5
8.4
175.8
36.3
164.2
35.5
212.6
29.0
183.7
30.5
1176.2
35.8
221.9 377.8
477.1 290.8
213.4
47.8
625.2 569.2
1720.5
1643.2
1166.3
762.8
913.4
173.6
1709.5
749.5
2505.6
828.7
467.3
1720.5
1643.2
1166.3
841.2
955.4
312.6
1709.5
798.8
2505.6
862.9
733.2
22.8
38.3
22.6
30.1
97.6
154.1
15.0
12.3
25.7
19.2
23.4
206Pb*
207Pb*
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
07
08
09
11
12
13
14
15
16
17R
18
19
22
2C
B1
B2
B3
B4
B5
B6
B6T
B7
B9
Paragneiss JG061504-4
01
597
82630
02
299
7215
03
151
25090
04
197
15895
05
154
5024
05t1
492
21967
06
98
15309
07
513
17530
08
217
61871
09
243
25649
10
376
20935
318
808
160
235
531
147
26
263
90
615
656
144
420
699
625
343
194
175
677
247
200
129
239
197
1051
750
330
106
301
440
477
525
241
512
800
812
259
U
(ppm)
86708
20559
23693
58166
22655
13410
25031
19734
7843
48492
17829
40564
84673
156689
21386
15064
20470
49738
29007
23338
40905
51917
28746
7405
7826
31997
165386
1162314
303994
286250
11365
38975
226956
30468
53797
107604
206Pb
204Pb
13.8
1.2
1.7
0.7
1.1
1.2
0.4
1.7
10.3
0.9
1.4
1.6
1.7
1.2
0.8
0.6
1.2
0.9
1.1
1.8
1.3
1.0
2.2
9.9
10.2
1.8
1.5
1.7
2.1
2.9
16.0
2.2
1.5
11.6
10.9
0.8
U/Th
0.2054
1.2216
1.2747
3.6742
1.2606
13.1298
1.5111
6.8180
0.1813
1.5137
5.0264
1.4400
3.4106
9.4482
1.0897
1.4612
1.3317
1.2181
1.6252
2.8027
9.6724
4.0783
3.2887
0.1888
0.1927
1.2327
1.3006
9.6153
9.1957
8.5739
0.2084
8.5649
4.3677
0.1952
0.2018
3.0608
207Pb*
235U
0.0284
0.1320
0.1413
0.2684
0.1353
0.5028
0.1550
0.3378
0.0281
0.1547
0.3112
0.1504
0.2483
0.4148
0.1194
0.1496
0.1440
0.1326
0.1642
0.2321
0.4277
0.2790
0.2347
0.0283
0.0274
0.1315
0.1335
0.4283
0.4043
0.3834
0.0273
0.3927
0.2977
0.0275
0.0283
0.2444
206Pb*
238U
Isotopic ratios
±
(%)
3.8
3.2
4.7
1.4
2.0
1.8
2.1
2.0
4.0
1.6
4.6
1.4
1.3
1.7
9.2
2.2
2.5
2.4
1.6
1.3
2.2
1.6
9.5
5.7
3.4
2.9
4.8
2.6
2.3
3.6
7.5
5.8
3.0
4.7
2.9
2.6
2.0
2.2
2.9
1.1
1.2
1.0
1.4
1.7
2.5
1.5
4.4
1.2
1.0
1.2
8.7
1.7
2.2
1.9
1.4
1.0
2.0
1.0
8.9
1.5
1.5
1.2
2.3
1.7
2.0
3.4
3.3
5.1
2.6
2.0
1.6
2.2
±
(%)
0.52
0.68
0.62
0.78
0.60
0.55
0.66
0.84
0.62
0.93
0.96
0.83
0.80
0.70
0.95
0.77
0.88
0.79
0.87
0.74
0.93
0.64
0.95
0.26
0.43
0.43
0.47
0.64
0.87
0.96
0.44
0.87
0.88
0.42
0.54
0.82
error
corr.
180.8
799.3
851.9
1532.9
818.0
2625.7
929.1
1876.1
178.8
927.2
1746.5
903.4
1429.8
2236.9
727.4
898.6
867.3
802.6
980.1
1345.6
2295.3
1586.5
1359.0
179.7
174.3
796.6
807.8
2298.3
2188.7
2092.3
173.9
2135.2
1679.7
174.7
179.8
1409.8
206Pb*
238U
3.5
16.6
23.2
15.1
9.1
21.6
12.1
27.8
4.3
13.1
66.7
9.9
12.8
22.9
59.9
14.5
17.5
14.6
12.9
12.1
39.0
14.1
109.6
2.6
2.5
9.1
17.1
32.3
37.2
61.6
5.6
92.3
39.2
3.4
2.8
27.3
±
(Ma)
6.6
18.1
26.6
11.3
11.1
17.1
12.8
18.1
6.2
9.9
38.7
8.5
9.9
16.0
48.6
13.5
14.2
13.6
10.2
10.1
20.0
12.7
73.7
9.2
5.6
16.1
27.8
24.0
21.1
32.7
13.1
53.1
24.8
7.8
4.9
20.1
±
(Ma)
Apparent ages (Ma)
207Pb*
235U
189.7
810.5
834.5
1565.8
828.2
2689.0
934.9
2088.1
169.2
935.9
1823.8
905.7
1506.8
2382.5
748.4
914.5
859.6
808.9
980.0
1356.3
2404.1
1650.0
1478.4
175.6
178.9
815.6
846.0
2398.6
2357.7
2293.8
192.3
2292.8
1706.2
181.0
186.7
1422.9
301.9
841.5
788.4
1610.4
855.7
2737.0
948.5
2304.0
36.4
956.5
1913.2
911.3
1616.8
2509.6
811.6
953.1
839.8
826.3
979.8
1373.1
2497.5
1731.8
1654.1
121.2
239.7
867.6
947.4
2485.0
2507.3
2478.5
424.0
2436.5
1739.0
264.2
273.9
1442.7
206Pb*
207Pb*
73.8
49.5
77.0
16.6
32.4
24.8
32.1
19.3
74.6
11.9
24.3
16.5
14.2
21.0
60.3
29.3
24.3
31.1
16.3
17.4
13.5
21.8
56.8
130.2
71.4
53.8
87.9
33.7
19.0
16.9
149.9
48.5
25.9
97.9
55.8
28.6
±
(Ma)
180.8
799.3
851.9
1610.4
818.0
2737.0
929.1
2304.0
178.8
927.2
1913.2
903.4
1616.8
2509.6
727.4
898.6
867.3
802.6
980.1
1373.1
2497.5
1731.8
1654.1
179.7
174.3
796.6
807.8
2485.0
2507.3
2478.5
173.9
2436.5
1739.0
174.7
179.8
1442.7
Best age
(Ma)
3.5
16.6
23.2
16.6
9.1
24.8
12.1
19.3
4.3
13.1
24.3
9.9
14.2
21.0
59.9
14.5
17.5
14.6
12.9
17.4
13.5
21.8
56.8
2.6
2.5
9.1
17.1
33.7
19.0
16.9
5.6
48.5
25.9
3.4
2.8
28.6
±
(Ma)
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
10t1
11
11t1
12
13
14
15
16
16t1
17
18X
19
20
22
23
24
25
26
27
28
29
30
31c
31t1
31t2
32t1
33c
34c
34c2
34c3
34t1
34t2
35c
35t1
35t2
36c
319
354
266
91
238
811
299
651
514
191
186
854
197
565
U
(ppm)
1410.8
824.4
728.0
2525.2
178.3
736.5
172.2
21.6
45.1
5.0
20.6
3.2
33.9
9.8
25.9
42.2
3.6
52.8
5.4
Apparent ages (Ma)
924.2
1676.7
171.0
1029.2
175.6
16.1
9.5
65.1
39.9
26.2
16.9
18.5
36.3
20.4
13.0
15.3
7.3
23.3
9.4
12.8
25.3
17.9
25.3
8.9
35.9
35.7
Isotopic ratios
33.9
65.4
123.3
42.2
58.5
57.6
140.3
657.2
37.2
45.1
68.8
20.6
78.9
1877.2
502.8
779.9
1295.3
1233.3
1822.7
1889.0
1011.0
1441.3
956.3
937.0
777.7
934.6
910.8
2507.2
1881.4
1727.6
1625.1
958.6
1876.4
487.6
±
(Ma)
1410.8
877.7
886.2
2525.2
217.2
851.2
337.2
2118.2
1006.4
1676.7
369.6
1029.2
189.1
16.1
59.1
58.3
39.9
26.2
16.9
18.5
42.4
20.4
23.4
31.3
25.4
27.0
19.6
12.8
25.3
17.9
25.3
36.5
35.9
176.7
Best age
(Ma)
21.7
19.4
38.0
28.3
5.4
43.5
11.7
128.4
19.2
26.9
7.2
10.2
6.2
1877.2
370.1
874.8
1295.3
1233.3
1822.7
1889.0
981.6
1441.3
988.6
1013.1
851.3
1014.5
946.5
2507.2
1881.4
1727.6
1625.1
981.4
1876.4
453.9
±
(Ma)
1190.6
839.0
768.1
2468.9
181.0
765.5
183.9
428.2
948.8
1631.2
185.1
1001.3
176.5
16.0
12.5
51.7
22.2
19.7
56.1
13.8
28.0
15.9
11.6
14.5
8.7
18.5
8.9
13.3
21.8
17.8
15.5
12.8
20.6
42.2
206Pb*
207Pb*
25.1
9.8
25.9
34.9
3.6
52.8
5.4
10.1
21.6
31.6
5.0
11.3
3.2
1826.1
479.6
804.9
1324.8
1176.8
1555.0
1880.4
1001.7
1427.3
966.2
960.0
797.0
958.7
921.3
2403.1
1865.6
1702.8
1604.8
965.5
1776.2
481.7
±
(Ma)
1073.1
824.4
728.0
2401.0
178.3
736.5
172.2
183.8
924.2
1596.2
171.0
988.7
175.6
26.0
9.5
65.1
26.5
26.2
85.8
20.2
36.3
22.5
13.0
15.3
7.3
23.3
9.4
23.6
34.4
28.5
19.2
8.9
21.3
35.7
207Pb*
235U
0.82
0.37
0.53
0.57
0.63
0.94
0.46
0.15
0.81
0.68
0.70
0.77
0.48
1781.7
502.8
779.9
1343.1
1146.3
1365.6
1872.5
1011.0
1418.0
956.3
937.0
777.7
934.6
910.8
2282.1
1851.3
1682.7
1589.3
958.6
1692.1
487.6
±
(Ma)
2.5
1.3
3.8
1.7
2.0
7.6
3.2
5.6
2.5
2.2
3.0
1.2
1.8
0.88
0.60
0.95
0.73
0.88
0.99
0.77
0.88
0.86
0.79
0.75
0.63
0.89
0.76
0.85
0.84
0.89
0.71
0.49
0.58
0.69
206Pb*
238U
±
(%)
0.1811
0.1364
0.1196
0.4513
0.0280
0.1210
0.0271
0.0289
0.1542
0.2810
0.0269
0.1658
0.0276
1.7
2.0
8.9
2.2
2.5
7.0
1.2
3.9
1.8
1.5
1.7
1.0
2.7
1.1
1.2
2.1
1.9
1.4
1.0
1.4
7.6
error
corr.
207Pb*
235U
3.1
3.4
7.0
3.1
3.3
8.1
7.0
36.6
3.1
3.3
4.2
1.6
3.9
0.3184
0.0811
0.1286
0.2317
0.1946
0.2360
0.3371
0.1698
0.2460
0.1599
0.1564
0.1282
0.1560
0.1517
0.4248
0.3327
0.2983
0.2796
0.1603
0.3002
0.0786
±
(%)
2.2304
1.2847
1.1306
10.3756
0.1952
1.1253
0.1986
0.5245
1.5459
3.9855
0.2000
1.6810
0.1899
1.9
3.3
9.3
3.0
2.8
7.0
1.6
4.4
2.1
1.9
2.3
1.6
3.0
1.5
1.4
2.6
2.2
1.9
2.1
2.5
11.0
206Pb*
238U
5.3
1.1
1.7
1.0
11.9
1.5
11.7
9.9
1.4
1.7
14.8
0.5
14.5
5.0405
0.6038
1.2093
2.6868
2.1866
3.6249
5.3718
1.6820
3.0785
1.5897
1.5740
1.1923
1.5707
1.4777
9.6617
5.2795
4.3494
3.8571
1.5881
4.7504
0.6071
U/Th
131109
183644
11713
223276
34025
67215
26161
1529
76329
36742
22889
59035
35817
1.4
0.9
0.9
0.6
0.9
0.7
4.3
1.4
1.1
1.5
1.3
2.5
3.1
1.2
0.6
2.8
0.6
1.0
1.9
1.5
0.6
206Pb
204Pb
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
36t1
37c
37t1
38c
38t1
39c
39t1
40t1
41c
42c
42t1
43c
43t1
75354
9896
29765
17349
33360
47094
95740
39668
91598
60537
42242
79431
52436
39939
68933
32628
19238
40637
59226
69131
6644
Quartzite JG062505-3
01
315
02
170
03
304
04
97
05
638
06
404
07
395
08
387
09
638
10
443
11
427
12
1022
13
505
14
533
15
333
16
190
17
114
18
294
19
763
20
319
21
143
320
294
94
115
245
564
188
554
265
203
623
187
176
368
91
417
195
585
66
824
141
239
548
91
356
318
452
96
108
132
132
507
147
314
791
112
113
U
(ppm)
30957
10404
18167
39713
49204
35425
48118
29280
30786
36676
17498
37339
21512
19814
29248
11686
53088
12592
57138
27436
17654
45096
12627
40327
68633
42384
4533
5186
15904
35576
85686
8122
62509
89036
4922
10127
206Pb
204Pb
0.9
0.4
1.0
3.5
1.5
0.8
0.9
2.0
1.6
3.1
0.9
0.8
1.6
1.2
4.4
2.6
3.4
1.4
2.9
1.0
0.8
6.9
0.5
1.2
2.1
0.9
1.2
0.6
0.9
1.1
1.9
0.7
2.7
3.4
0.8
1.1
U/Th
2.1836
1.4748
2.4919
5.0880
1.5388
5.0049
1.6123
2.5413
2.0077
1.6078
1.2500
10.1411
0.6508
10.2244
1.5608
9.5841
1.6734
10.7275
1.5002
6.3252
2.0089
1.6127
2.3823
7.9193
10.4152
1.6161
0.5713
0.5820
2.1818
9.5829
11.1573
0.6206
3.1047
2.1448
0.5613
1.5774
207Pb*
235U
0.1972
0.1542
0.2155
0.3178
0.1557
0.3079
0.1614
0.2049
0.1859
0.1633
0.1371
0.4496
0.0825
0.4517
0.1562
0.4300
0.1659
0.4270
0.1509
0.3669
0.1860
0.1615
0.2067
0.3722
0.4538
0.1622
0.0776
0.0797
0.2011
0.4387
0.4521
0.0796
0.2468
0.1939
0.0755
0.1597
206Pb*
238U
Isotopic ratios
±
(%)
2.6
3.6
1.8
2.5
3.1
6.1
1.9
5.6
1.8
3.7
1.7
1.4
3.7
1.7
1.2
4.3
2.2
3.6
3.2
2.1
8.1
1.9
1.4
3.7
1.5
1.7
3.9
3.5
2.0
1.4
2.2
3.2
1.4
1.9
5.6
1.7
1.0
2.4
1.4
1.8
2.7
3.5
1.5
5.4
1.2
2.7
1.0
1.0
3.6
1.3
1.0
3.2
1.9
3.5
2.3
1.8
6.4
1.6
1.0
3.4
1.0
1.0
1.7
1.8
1.8
1.3
2.1
1.6
1.0
1.6
4.4
1.1
±
(%)
0.38
0.67
0.75
0.73
0.87
0.58
0.81
0.96
0.70
0.75
0.60
0.71
0.97
0.78
0.82
0.74
0.86
0.96
0.73
0.84
0.79
0.82
0.70
0.90
0.68
0.59
0.44
0.52
0.91
0.90
0.95
0.49
0.71
0.85
0.78
0.64
error
corr.
1160.1
924.4
1257.9
1778.7
933.0
1730.6
964.7
1201.7
1099.2
975.3
828.4
2393.4
511.1
2402.7
935.6
2305.7
989.3
2292.3
906.3
2014.6
1099.8
965.1
1211.4
2039.8
2412.3
969.2
481.6
494.0
1181.3
2344.9
2404.4
493.9
1422.1
1142.7
469.3
955.2
206Pb*
238U
10.6
21.0
15.5
28.5
23.4
53.6
13.6
58.9
12.4
24.8
8.0
20.0
17.9
26.2
8.7
62.5
17.6
67.0
19.6
31.3
64.4
14.1
11.0
58.7
20.1
9.0
8.0
8.7
19.7
25.0
41.6
7.4
12.8
16.8
19.7
9.8
±
(Ma)
18.4
21.8
13.1
21.3
19.0
51.9
11.7
40.6
11.9
22.9
9.7
13.1
15.0
15.6
7.6
40.0
14.2
33.7
19.3
18.8
54.8
12.0
10.2
33.6
13.7
10.6
14.5
13.2
13.9
13.0
20.3
12.4
10.9
13.0
20.5
10.7
±
(Ma)
Apparent ages (Ma)
207Pb*
235U
1175.8
920.1
1269.7
1834.1
946.0
1820.2
975.0
1284.0
1118.1
973.3
823.4
2447.7
509.0
2455.3
954.8
2395.6
998.5
2499.8
930.5
2021.9
1118.5
975.1
1237.3
2221.9
2472.4
976.5
458.8
465.7
1175.3
2395.5
2536.4
490.2
1433.9
1163.4
452.4
961.3
1204.8
909.8
1289.8
1897.5
976.4
1924.3
998.3
1424.3
1155.2
968.6
809.8
2493.2
499.5
2499.1
999.2
2473.0
1018.7
2673.0
988.3
2029.5
1155.1
997.7
1282.7
2394.2
2522.1
992.8
346.4
328.3
1164.1
2438.8
2643.6
473.4
1451.4
1202.1
367.5
975.5
206Pb*
207Pb*
48.3
54.9
23.4
30.8
31.0
89.7
22.4
28.2
24.9
49.2
28.9
16.9
20.1
17.9
14.3
49.3
22.9
17.2
44.2
20.6
98.5
22.2
19.8
27.6
18.3
27.9
80.0
68.3
16.2
10.5
10.8
61.4
19.1
19.3
79.3
26.9
±
(Ma)
1204.8
924.4
1289.8
1897.5
933.0
1924.3
964.7
1424.3
1155.2
975.3
828.4
2493.2
511.1
2499.1
935.6
2473.0
1018.7
2673.0
906.3
2029.5
1155.1
965.1
1282.7
2394.2
2522.1
969.2
481.6
494.0
1164.1
2438.8
2643.6
493.9
1451.4
1202.1
469.3
955.2
Best age
(Ma)
48.3
21.0
23.4
30.8
23.4
89.7
13.6
28.2
24.9
24.8
8.0
16.9
17.9
17.9
8.7
49.3
22.9
17.2
19.6
20.6
98.5
14.1
19.8
27.6
18.3
9.0
8.0
8.7
16.2
10.5
10.8
7.4
19.1
19.3
19.7
9.8
±
(Ma)
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
321
328
541
355
79
204
640
104
474
208
1222
331
391
332
451
92
261
592
275
270
710
167
261
238
526
915
335
810
202
333
221
289
489
181
549
117
473
U
(ppm)
34518
79411
27255
4142
16953
56612
5820
51562
18141
95533
78649
30351
60699
64448
12629
25804
85892
47440
19526
99923
42061
19436
24217
85491
41226
37891
18051
25817
39092
53420
54411
43926
46003
108409
4868
113880
206Pb
204Pb
2.2
3.0
1.5
0.7
0.6
0.9
0.9
4.4
0.7
9.7
0.8
1.5
0.6
1.7
0.8
1.1
1.6
1.7
0.6
1.4
0.7
1.8
0.8
1.4
1.3
1.3
2.6
1.2
0.9
0.9
1.5
3.5
1.4
2.8
1.2
1.1
U/Th
1.4885
4.6361
0.6574
2.5374
1.8812
3.1831
0.6042
1.6332
1.5001
1.5844
10.6322
1.2396
10.0840
4.4427
1.5955
1.4771
3.2981
4.9770
1.2974
3.8045
10.4948
2.0385
1.6095
4.8211
0.6376
3.5948
1.5898
2.0837
1.4016
5.3246
3.8360
1.5150
5.3185
4.7505
0.6110
10.0392
207Pb*
235U
0.1474
0.3078
0.0835
0.2143
0.1741
0.2257
0.0793
0.1632
0.1473
0.1590
0.4589
0.1344
0.4472
0.2971
0.1610
0.1499
0.2526
0.3113
0.1368
0.2779
0.4613
0.1860
0.1610
0.3076
0.0803
0.2627
0.1580
0.1905
0.1455
0.3344
0.2825
0.1542
0.3365
0.3086
0.0798
0.4431
206Pb*
238U
Isotopic ratios
±
(%)
2.9
1.9
6.9
2.8
3.7
4.1
5.4
1.5
6.1
2.3
2.0
1.9
2.3
2.0
3.9
2.4
1.6
3.2
1.8
1.4
2.3
3.0
2.8
2.5
5.9
3.0
3.5
1.8
2.1
1.9
1.6
1.2
1.5
2.0
4.7
1.5
2.2
1.3
5.4
1.5
2.9
3.5
3.6
1.0
5.3
2.1
1.0
1.0
1.9
1.5
3.3
1.5
1.1
3.1
1.0
1.0
2.2
2.5
1.8
2.1
4.7
2.5
2.7
1.5
1.3
1.0
1.0
1.0
1.0
1.6
2.8
1.2
±
(%)
0.76
0.68
0.78
0.53
0.79
0.87
0.67
0.68
0.87
0.90
0.51
0.52
0.79
0.75
0.84
0.63
0.69
0.95
0.56
0.74
0.95
0.85
0.66
0.85
0.80
0.83
0.75
0.84
0.61
0.54
0.62
0.83
0.65
0.78
0.60
0.80
error
corr.
886.6
1729.9
516.7
1251.8
1034.9
1311.7
492.2
974.4
885.9
951.3
2434.5
812.8
2382.8
1677.1
962.1
900.4
1451.8
1747.3
826.6
1580.9
2445.3
1099.4
962.6
1729.0
497.9
1503.8
945.6
1124.0
875.5
1859.5
1603.9
924.2
1869.8
1733.8
494.9
2364.3
206Pb*
238U
18.1
19.4
26.8
16.8
27.9
42.0
17.2
9.0
44.2
18.4
20.3
7.6
37.0
21.7
29.4
12.6
14.7
47.1
7.9
14.0
44.1
25.5
16.4
32.3
22.7
33.4
23.4
15.5
10.5
16.2
14.5
8.6
16.2
24.2
13.3
24.1
±
(Ma)
17.4
15.7
28.0
20.2
24.5
31.5
20.6
9.3
37.3
14.4
18.2
10.8
21.7
16.2
24.4
14.4
12.8
27.3
10.3
10.9
21.1
20.1
17.4
21.2
23.5
23.7
22.0
12.4
12.6
15.9
13.3
7.4
13.2
17.1
17.9
14.0
±
(Ma)
Apparent ages (Ma)
207Pb*
235U
925.7
1755.8
513.0
1282.8
1074.5
1453.1
479.9
983.1
930.4
964.1
2491.5
818.7
2442.5
1720.3
968.5
921.0
1480.6
1815.4
844.6
1593.7
2479.5
1128.5
973.9
1788.6
500.8
1548.4
966.2
1143.5
889.6
1872.8
1600.3
936.5
1871.8
1776.2
484.2
2438.4
1019.9
1786.8
496.4
1335.1
1155.8
1666.4
421.2
1002.4
1037.6
993.4
2538.3
834.6
2492.6
1773.4
982.9
970.8
1522.1
1894.6
892.2
1610.7
2507.6
1184.7
999.5
1858.8
514.2
1609.7
1013.4
1180.5
924.8
1887.6
1595.6
965.3
1874.1
1826.4
433.9
2500.8
206Pb*
207Pb*
37.5
25.2
96.3
45.3
44.8
37.6
89.1
22.2
60.3
20.5
28.3
34.2
24.3
23.4
43.2
37.7
22.4
17.6
30.8
17.1
12.0
30.4
42.1
24.2
78.9
30.8
47.0
19.5
34.6
28.3
24.1
14.0
21.3
23.0
83.0
15.3
±
(Ma)
886.6
1786.8
516.7
1335.1
1155.8
1666.4
492.2
974.4
885.9
951.3
2538.3
812.8
2492.6
1773.4
962.1
900.4
1522.1
1894.6
826.6
1610.7
2507.6
1184.7
962.6
1858.8
497.9
1609.7
945.6
1180.5
875.5
1887.6
1595.6
924.2
1874.1
1826.4
494.9
2500.8
Best age
(Ma)
18.1
25.2
26.8
45.3
44.8
37.6
17.2
9.0
44.2
18.4
28.3
7.6
24.3
23.4
29.4
12.6
22.4
17.6
7.9
17.1
12.0
30.4
16.4
24.2
22.7
30.8
23.4
19.5
10.5
28.3
24.1
8.6
21.3
23.0
13.3
15.3
±
(Ma)
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
322
42
305
63
193
403
844
U
(ppm)
1997
24803
7291
26288
64509
46504
206Pb
204Pb
Isotopic ratios
Apparent ages (Ma)
333.9
995.8
1102.6
1740.0
1739.6
997.8
182.1
87.3
14.3
30.5
20.0
26.6
11.6
24.7
30.7
51.3
47.5
28.5
41.2
37.3
157.3
15.2
14.8
147.7
35.4
157.4
121.1
70.4
13.2
30.1
137.0
12.6
18.9
104.7
176.6
30.0
23.6
80.4
15.6
25.4
±
(Ma)
30.0
12.9
13.5
48.2
10.9
24.1
1306.5
1203.4
1006.4
1151.5
1362.5
1390.9
2666.0
781.4
878.0
734.2
2293.1
962.7
951.1
1042.6
901.7
1006.0
786.3
903.6
2504.0
388.7
2654.0
723.0
1539.4
907.7
1164.6
1911.3
2488.6
1128.4
206Pb*
207Pb*
450.2
971.7
1113.6
1704.2
1707.7
980.6
66.4
31.0
8.2
13.0
13.3
20.5
9.6
7.3
9.6
24.3
74.8
11.3
12.9
14.5
46.7
7.7
6.3
43.5
24.5
28.4
69.6
18.3
20.5
18.2
47.4
10.0
15.5
37.2
±
(Ma)
12.5
12.8
16.6
56.7
14.8
32.7
1082.3
1138.5
943.9
1133.7
1310.6
1354.3
2543.0
738.9
847.6
700.4
1972.8
968.1
919.9
1078.3
891.8
984.2
772.9
917.9
2489.1
488.4
2611.9
738.2
1453.8
836.5
1089.1
1871.0
2476.2
1149.2
207Pb*
235U
473.2
961.0
1119.2
1675.3
1681.9
973.0
31.0
9.3
9.7
11.7
17.2
28.5
15.2
5.1
6.0
27.2
120.4
10.5
5.9
11.6
16.2
8.7
6.7
6.1
32.7
10.5
28.1
7.4
32.1
21.5
15.7
15.0
25.4
13.3
±
(Ma)
0.33
0.70
0.81
0.66
0.76
0.95
974.4
1104.8
917.3
1124.5
1279.1
1331.2
2391.8
725.0
836.0
689.9
1682.1
970.4
907.0
1096.0
887.8
974.4
768.3
923.9
2470.9
509.9
2558.0
743.2
1395.8
809.9
1051.7
1835.0
2461.0
1160.2
206Pb*
238U
2.7
1.4
1.6
3.8
1.0
3.6
0.34
0.20
0.85
0.59
0.82
0.86
0.74
0.54
0.46
0.86
0.95
0.64
0.33
0.53
0.25
0.79
0.80
0.10
0.60
0.29
0.18
0.30
0.96
0.89
0.23
0.80
0.74
0.23
error
corr.
0.0762
0.1608
0.1896
0.2968
0.2981
0.1629
3.4
0.9
1.1
1.1
1.5
2.4
0.8
0.8
0.8
4.2
8.1
1.2
0.7
1.2
1.9
1.0
0.9
0.7
1.6
2.1
1.3
1.1
2.6
2.8
1.6
0.9
1.2
1.3
±
(%)
8.2
2.1
2.0
5.8
1.3
3.8
0.1632
0.1869
0.1529
0.1906
0.2195
0.2294
0.4492
0.1190
0.1385
0.1130
0.2981
0.1625
0.1511
0.1853
0.1476
0.1632
0.1266
0.1541
0.4671
0.0823
0.4871
0.1222
0.2418
0.1339
0.1772
0.3293
0.4649
0.1972
206Pb*
238U
0.5579
1.6038
1.9943
4.3570
4.3755
1.6268
10.0
4.5
1.3
1.9
1.8
2.7
1.0
1.4
1.7
4.8
8.6
1.8
2.1
2.2
7.9
1.2
1.2
7.2
2.6
7.3
7.4
3.5
2.7
3.2
7.1
1.2
1.7
5.4
±
(%)
0.6
0.6
0.5
1.1
0.6
1.2
1.9035
2.0687
1.5334
2.0543
2.6356
2.7953
11.2376
1.0704
1.3042
0.9933
5.9792
1.5946
1.4744
1.8920
1.4067
1.6361
1.1409
1.4695
10.6044
0.6176
12.0969
1.0689
3.1859
1.2792
1.9229
5.3135
10.4575
2.1012
207Pb*
235U
1.7
0.7
1.5
1.3
0.8
1.6
1.1
2.1
6.7
1.5
1.5
1.1
1.9
0.7
1.7
0.7
10.2
0.6
0.2
0.9
1.5
0.7
1.7
2.0
0.9
2.1
0.9
1.5
U/Th
887.8
974.4
768.3
923.9
2504.0
509.9
2654.0
743.2
1539.4
809.9
1203.4
917.3
1151.5
1362.5
1390.9
2666.0
725.0
836.0
689.9
2293.1
970.4
907.0
473.2
961.0
1102.6
1740.0
1739.6
973.0
12.6
18.9
104.7
16.2
8.7
6.7
6.1
35.4
10.5
121.1
7.4
13.2
21.5
87.3
9.7
30.5
20.0
26.6
11.6
5.1
6.0
27.2
47.5
10.5
5.9
12.5
12.8
23.6
80.4
15.6
32.7
±
(Ma)
1911.3
2488.6
1128.4
Best age
(Ma)
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
94
95
96
97
98
100
Quartzite AP061304-A
01
101
803
02
88
6028
03
710
91258
04
257
10683
05
241
19678
06
128
30502
07
176
91288
08
529
97913
09
496
80837
12
371
6899
13
618
13182
14
491
67081
15
395
41951
18
124
35454
19
172
2941
20
490
50446
21
709
83091
22
69
9117
23
52
17323
24
199
5121
25
14
1563
26
170
17028
27
739
153048
28
261
24269
29
51
12448
30
190
62200
31
66
51945
32
77
19951
323
271
127
70
261
24
281
357
128
238
139
540
181
54
677
152
46
144
734
162
389
299
244
73
137
227
178
128
53
210
56
39
184
596
164
47
143
U
(ppm)
67360
12354
1680
149233
3972
7157
12024
7693
4647
75826
144222
49994
7080
3571
13695
5700
36024
74445
49238
101138
20097
11015
4723
15083
13284
70936
2661
17055
52534
3539
900
91594
81718
32997
24662
14862
206Pb
204Pb
1.1
0.5
0.8
1.0
2.2
1.2
11.0
0.3
1.1
1.3
0.8
0.7
1.2
2.6
1.3
0.4
0.8
2.7
1.6
1.2
1.3
1.2
0.7
0.5
1.5
1.6
1.4
0.9
1.1
0.7
0.6
2.4
8.4
2.4
0.4
1.6
U/Th
207Pb*
235U
2.3
4.7
19.7
2.2
9.5
6.8
4.5
5.4
4.4
1.8
1.0
3.6
5.1
6.4
4.3
9.1
2.1
1.4
1.0
1.0
1.4
2.9
8.3
4.8
1.8
1.2
10.8
2.4
2.1
12.2
15.4
2.3
2.4
7.4
2.3
3.1
±
(%)
0.1332
0.1935
0.0895
0.4109
0.1801
0.1509
0.1525
0.0830
0.1097
0.5001
0.4889
0.1609
0.1819
0.0961
0.1365
0.1671
0.2260
0.1974
0.6092
0.4296
0.1892
0.1389
0.0721
0.1292
0.2682
0.4499
0.0901
0.5251
0.1901
0.0884
0.2753
0.4142
0.0822
0.2983
0.4787
0.2373
206Pb*
238U
Isotopic ratios
1.2077
2.0367
0.6825
9.0885
1.9894
1.5559
1.5496
0.6657
0.9237
13.0214
13.2450
1.5870
2.0321
0.8357
1.2196
1.5027
2.6252
2.1933
24.2362
9.7343
2.0711
1.2856
0.5897
1.2424
3.6961
10.5128
0.7038
14.4033
2.1486
0.7525
3.3784
9.6454
0.6626
5.9001
11.3694
3.2680
1.4
2.2
1.7
2.1
1.9
1.3
3.6
1.9
1.5
1.7
0.7
0.7
0.7
1.2
0.9
1.1
0.9
1.3
0.7
0.7
0.7
1.0
1.7
2.4
0.7
1.0
0.9
1.0
0.7
1.3
2.6
2.2
0.7
6.5
1.3
1.4
±
(%)
0.58
0.46
0.09
0.95
0.20
0.20
0.79
0.36
0.34
0.92
0.71
0.19
0.14
0.19
0.20
0.12
0.43
0.87
0.71
0.71
0.49
0.34
0.21
0.50
0.38
0.81
0.09
0.43
0.34
0.11
0.17
0.95
0.29
0.88
0.54
0.43
error
corr.
806.1
1140.5
552.5
2219.0
1067.5
905.9
914.8
513.8
671.1
2614.1
2565.9
962.0
1077.3
591.4
825.0
996.2
1313.6
1161.1
3066.8
2303.9
1117.3
838.4
448.9
783.2
1531.6
2394.9
555.9
2721.0
1121.8
546.2
1567.8
2234.3
509.5
1682.9
2521.4
1372.7
206Pb*
238U
10.3
22.6
9.0
40.0
19.1
11.4
30.5
9.4
9.5
36.5
14.8
6.3
7.0
6.7
6.6
9.8
10.7
13.4
17.1
13.6
7.2
7.7
7.4
17.8
9.6
19.2
4.9
22.7
7.5
6.9
35.9
41.2
3.4
96.6
26.5
16.7
±
(Ma)
12.9
32.2
81.1
20.5
64.1
42.3
27.9
21.8
21.6
17.3
9.3
22.4
34.6
29.4
24.2
55.4
15.5
10.1
9.7
9.1
9.7
16.3
31.2
26.9
14.6
11.0
45.4
22.4
14.8
53.1
121.5
21.1
9.6
64.2
21.9
24.4
±
(Ma)
Apparent ages (Ma)
207Pb*
235U
804.1
1127.8
528.3
2347.0
1111.9
952.8
950.3
518.1
664.3
2681.2
2697.3
965.1
1126.3
616.8
809.6
931.5
1307.7
1178.9
3277.9
2410.0
1139.3
839.3
470.7
820.0
1570.5
2481.0
541.1
2776.6
1164.6
569.7
1499.4
2401.5
516.2
1961.2
2553.9
1473.5
798.7
1103.6
425.1
2460.1
1199.8
1062.9
1033.5
537.2
641.4
2732.2
2797.2
972.3
1222.2
711.1
767.6
781.3
1298.0
1211.7
3409.7
2500.8
1181.4
841.7
578.6
921.0
1623.2
2552.3
479.0
2817.4
1245.0
664.6
1404.0
2546.5
546.2
2269.3
2579.8
1621.7
206Pb*
207Pb*
39.5
84.0
440.6
11.8
183.2
135.1
56.2
109.7
89.8
11.5
11.5
71.9
99.0
132.8
89.7
189.8
37.0
13.8
10.9
11.8
24.5
55.9
176.2
85.0
31.5
11.8
238.9
34.7
39.4
259.9
293.1
11.8
49.7
59.9
32.8
52.6
±
(Ma)
905.9
914.8
513.8
671.1
2732.2
2797.2
962.0
1222.2
591.4
825.0
996.2
1298.0
1211.7
3409.7
2500.8
1181.4
838.4
448.9
783.2
1623.2
2552.3
555.9
2817.4
1245.0
546.2
806.1
1103.6
552.5
2460.1
11.8
3.4
59.9
32.8
52.6
11.4
30.5
9.4
9.5
11.5
11.5
6.3
99.0
6.7
6.6
9.8
37.0
13.8
10.9
11.8
24.5
7.7
7.4
17.8
31.5
11.8
4.9
34.7
39.4
6.9
10.3
84.0
9.0
11.8
±
(Ma)
2546.5
509.5
2269.3
2579.8
1621.7
Best age
(Ma)
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
Spot
ID
33
34
35
36
37
38
40
41
42
43
44
46
47
48
49
50
51
52
55
56
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
324
Spot
ID
320
56
315
68
153
27
125
135
68
38
69
258
177
121
72
249
89
127
546
142
94
366
230
115
280
434
147
93
U
(ppm)
13261
13307
9717
5729
61747
4046
25412
23638
3077
14639
11630
34662
14551
37721
2427
45270
10076
9992
2996
22106
19470
80206
6272
6349
4208
1642
11967
1113
206Pb
204Pb
U/Th
0.7793
1.3384
2.2886
13.0184
14.5884
0.8420
1.6954
1.8421
0.7882
10.7864
4.0276
1.5388
1.9771
1.5506
3.0704
1.6927
2.0756
1.5725
1.5037
1.2759
2.1789
1.5748
1.6662
0.7830
0.6051
1.6366
0.8148
0.6346
207Pb*
235U
4.2
8.3
4.4
2.8
3.6
36.2
2.9
2.8
16.1
2.6
3.6
5.9
2.8
4.0
6.1
2.0
5.0
4.6
5.3
5.8
2.8
2.0
3.4
4.6
9.3
11.5
5.7
20.9
±
(%)
0.0890
0.1418
0.1872
0.4910
0.4879
0.0830
0.1617
0.1720
0.0862
0.4565
0.2815
0.1504
0.1766
0.1482
0.2153
0.1600
0.1880
0.1538
0.1354
0.1302
0.1889
0.1567
0.1650
0.0897
0.0716
0.1682
0.0957
0.0829
206Pb*
238U
1.0
1.7
3.1
2.3
2.5
2.9
1.2
2.2
2.1
0.9
1.1
5.3
0.9
0.8
2.8
1.3
0.8
1.7
4.2
4.2
0.9
1.6
0.7
0.8
6.7
1.3
0.8
1.7
±
(%)
0.23
0.20
0.71
0.79
0.68
0.08
0.40
0.76
0.13
0.33
0.30
0.89
0.33
0.21
0.46
0.67
0.17
0.37
0.78
0.74
0.32
0.82
0.22
0.18
0.72
0.11
0.14
0.08
error
corr.
549.3
854.7
1106.3
2575.0
2561.7
514.1
966.2
1023.2
533.3
2423.9
1598.8
903.2
1048.5
891.0
1256.9
956.6
1110.6
922.0
818.5
788.9
1115.6
938.5
984.4
554.0
446.0
1002.2
589.1
513.7
206Pb*
238U
5.1
13.2
31.4
47.8
51.8
14.2
10.5
20.4
10.8
17.4
15.2
44.2
8.9
6.8
32.3
11.9
8.7
14.7
32.2
31.5
9.3
14.2
6.8
4.5
29.0
11.9
4.4
8.6
±
(Ma)
585.1
862.5
1208.8
2681.0
2788.8
620.2
1006.8
1060.6
590.2
2504.9
1639.8
946.0
1107.7
950.7
1425.3
1005.8
1140.8
959.4
931.9
835.0
1174.3
960.3
995.7
587.2
480.5
984.4
605.2
499.0
207Pb*
235U
18.7
48.4
30.9
26.8
34.2
169.6
18.6
18.6
72.3
24.0
29.6
36.4
19.0
24.5
46.8
12.7
34.0
28.4
32.6
32.8
19.8
12.4
21.8
20.5
35.6
72.4
26.1
82.7
±
(Ma)
726.4
882.7
1396.8
2761.9
2957.5
1029.2
1096.2
1138.4
815.2
2571.2
1692.7
1047.0
1226.0
1091.6
1686.7
1114.5
1198.7
1046.3
1210.6
960.0
1284.1
1010.5
1020.7
717.6
648.8
944.9
665.8
432.1
206Pb*
207Pb*
87.1
169.0
59.2
28.5
42.6
754.2
53.3
36.3
336.2
40.7
64.0
55.0
52.3
77.9
100.0
29.5
96.4
85.6
65.4
79.7
52.4
23.3
68.0
96.0
138.3
234.1
121.5
469.5
±
(Ma)
788.9
1284.1
938.5
1020.7
554.0
446.0
1002.2
589.1
513.7
1096.2
1138.4
533.3
2571.2
1692.7
903.2
1226.0
891.0
1686.7
1114.5
1198.7
922.0
549.3
854.7
1396.8
2761.9
2957.5
Best
age
(Ma)
31.5
52.4
14.2
68.0
4.5
29.0
11.9
4.4
8.6
53.3
36.3
10.8
40.7
64.0
44.2
52.3
6.8
100.0
29.5
96.4
14.7
5.1
13.2
59.2
28.5
42.6
±
(Ma)
Apparent ages (Ma)
0.5
3.2
2.3
1.5
2.1
0.4
1.0
1.0
0.4
0.8
1.0
3.7
0.5
0.7
0.9
1.7
2.2
1.5
3.2
1.0
0.5
2.1
4.6
1.0
8.1
1.4
0.8
1.1
Isotopic ratios
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass
Spectrometry
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
94
95
96
98
99
100
101
102
103