"Ar/**Ar and U-Pb Geochronological Constraints on the
Thermal and Tectonic Evolution of the Connemara
Caledonides, Western Ireland
by
ANKE MARIA FRIEDRICH
M.S. University of Utah (1993)
B.S. University of Utah (1990)
'Vordiplom in Geologie' University of Karlsruhe, Germany (1988)
Submitted to the
Department of Earth, Atmospheric, and Planetary Sciences
in Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September, 1998
@ Massachusetts Institute of Technology, 1998. All rights reserved.
I
Certified by
Ce
"U
Oof Earth, Atmospheric, and Planetary Sciences
'Dpartment
e
bKip
V. Hodges
Thesis Supervisor
Accepted by
Ronald Prinn
Department Head
MASSACHUSETTS INSTJUTE
"Ar/ 9 Ar and U-Pb Geochronological Constraints on the
Thermal and Tectonic Evolution of the Connemara
Caledonides, Western Ireland
ANKE MARIA FRIEDRICH
Submitted to the Department of Earth, Atmospheric and Planetary Sciences at
the Massachusetts Institute of Technology on July 15, 1998 in partial
fulfillment of the requirements for the degree of Doctor of Philosophy in
Geology
ABSTRACT
The Connemara region of the Irish Caledonides is a classical example
of a regional-scale high-temperature metamorphic terrain. Its formation was
related to intrusion of a compressional continental magmatic arc, for which a
protracted thermal evolution was inferred based on a >75 Ma spread in U-Pb,
Rb-Sr, and K-Ar mineral dates. Such a history is inconsistent with field
observations which suggest a simple relationship between metamorphism
and syntectonic magmatism. This study was designed to explore the
significance of the large spread in apparent ages using higher resolution U-Pb
and 4"Ar/ 39 Ar geochronometers. The results indicate that arc magmatism,
sillimanite-grade metamorphism, anatexis, and late fluid infiltration spanned
only about 12 million years. Cooling following the metamorphic peak was
actually relatively rapid at 35*C/Ma until about 460 Ma, then 214*C/Ma until
450 Ma. Regional differences in 40Ar/ 3 9Ar cooling ages of >15 Ma are related
to spatial and temporal variations in magmatism, metamorphism, and
deformation, rather than differential unroofing of the orogen. 40Ar/ 39Ar dates
older than the onset of magmatism or younger than a regional Silurian
unconformity represent the combined effects of excess 4 0Ar contamination,
metasomatism, thermal resetting or alteration related to post-orogenic pluton
emplacement. This study shows that geochronologic data must be evaluated
in the context of careful field mapping, structural and petrologic analysis.
Geochronological data from Connemara suggest that arc magmatism
related to the Grampian orogeny in this region spanned a brief interval
between 475 and 462 Ma and was followed by rapid cooling. The oldest
recognized Grampian processes included high P/T metamorphism, followed
by intrusion of the Connemara Gabbros into Dalradian metasedimentary
rocks, regional-scale ductile deformation, and sillimanite-grade metamorphism between 474.5 and 470.1 Ma. Voluminous -467 Ma quartz diorites
only intruded in southern Connemara associated with more localized
deformation, anatexis and metasomatism between 468 and 462 Ma. Intrusion
of the 462.5 Ma Oughterard Granite marks the end of arc magmatism and
contractional deformation at Connemara. The compressional continental
magmatic arc at Connemara (the Grampian orogeny) was coeval with
continental arc magmatism in Scotland and Newfoundland, and postdates
ophiolite formation and obduction along strike in the AppalachianCaledonian orogen.
Thesis advisor: Dr. Kip V. Hodges
Title: Professor of Geology
ACKNOWLEDGMENTS
Financial Support for this dissertation was provided by the National
Science Foundation through a NSF grant awarded to Kip Hodges and Samuel
Bowring, a Geological Society of America Student Research Grant, and
several SRFC grants from the Department of Earth, Atmospheric and
Planetary Sciences. I would like to thank Drs. Cees van Staal, Greg Hirth, Sam
Bowring and Kip Hodges for serving on my thesis committee, for reading the
entire thesis and for very helpful comments and advice before, during and
after the thesis defense.
The research related to this dissertation would have been impossible
without the help, logistical support, field guidance, and friendship from
many Irish and British colleagues, foremost Bruce Yardley (Leeds University),
Paul Ryan (University College Galway), and Barry Long (Irish Geological
Survey). The ideas I expressed in this dissertation benefited from lively
discussions with Bruce Yardley, Paul Ryan, Bob Cliff, Geoff Tanner, Barry
Long, Martin Feely, Bernard Leake, Jack Soper, Cees van Staal and with the
MIT crowd including Meg Coleman, Nancy Harris, Audrey Huerta, Wiki
Royden, Meg Thompson, Clark Burchfiel, Drew Coleman, David Hawkins,
Chris Marone, Mark W. Martin, C.J. Northrup, Bill Olszewski, Jim Van
Orman, Mark Schmitz, Gunter Siddiqi and of course Sam Bowring and Kip
Hodges.
I would like to thank especially all of my friends in Galway,
Shanaheever, and Leenan for kindly hosting me and my field gearincluding several hundred kilograms of rocks, sledge hammers, and a bicycle
with no front wheel- and letting me in and out of their houses during any
time of the day -and night: Paul Ryan, Martin Feely, Roisin Moran, the
Hamiltons' from Lenaan and their Pub, and Frances, Thomas and Martin
from the Shannaheever Campground. Thanks to Gunter Siddiqi for helping
me explore Connemara during my first week in the field, for discovering the
secret of Inishbofin, and for sharing the wettest and foggiest day of this planet
on Cashel Hill, the Cashel Pub, the gabbros, and in a far-traveled R&V with
homemade German Apfelkuchen.
I would have never ended up at MIT if it hadn't been for a series of
larger or smaller coincidences involving Sonja Stotz, Lester Keller, Pat Miller,
and Thor Kallerud from the University of Utah Ski Team, John Bartley (Ph.D.
MIT 1981) my MS thesis advisor at the University of Utah, the Extensional
Faults that overprinted Contractional Faults in Utah, the field-based Tectonics
group at MIT that researched Extensional Faults in Contractional Orogens,
and a new advisor-Kip Hodges. Among the countless formal and informal
interactions with Kip, I enjoyed most the high level of scientific
communication, advice, editorial work and last but not least the camaraderie.
Thanks Kip!
The U-Pb lab at MIT, in short Sams' Lab, certainly is the scientific and
social center of the 11th floor. Foremost, I would like to thank Sam Bowring
for being a nonstop scientific inspiration by suggesting at least one Ph.D.-scale
research project per day ("aren't you done yet?"); for instant judgment of my
own ideas ("holy snipe"); for believing in the impossible ("those heat-lamps
work 24 hours a day ...Let's do it!"). Thanks a lot to Kathy Davidek ala Keefe
for teaching me the ropes of mineral processing and U-Pb clean lab
techniques, and for knowing where all the tools are hiding in the lab. Pat
Walsh deserves a special Dankeschdn for reducing MIT bureaucracy hurdles.
Bill Olszewski was a most thorough, patient and forgiving teacher of Argon
isotopic matters ("tape on the planchette-oh, well, you'll have to remove it. It
would be best to start over..."). Drew Coleman and Mark Martin are two fine
scientists who differ most in their musical choices for lab entertainment.
My fellow graduate students, the 1994 Wellesly field camp crowd
(especially Tracy Johnston), the Women of 2nd & 3rd west in McCormick
Hall, the 'housemasters' Kathy Hess and Charles Stewart, and the MIT Ski
Team contributed most to making my MIT experience a very pleasant and
special one. I will always remember the IAP field camp fires; BlackForest Ham
in Las Vegas, Kittery Shopping trips, Pizza dinners at the housemasters'; the
many hours of stimulating scientific and less scientific discussions at the CJ(Northrup)-and-DAVER-(Hawkins) Memorial Blackboard; watching the
sunrise from the 11th floor because you're still there; soccer on the astro turf
at 11:30 pm; Toscaninis' icecream. CJ Northrup, David Hawkins, Dawn
Summer, Mousumi Roy, Steve Karner, Gunter Siddiqi, Nancy Harris, Jim
VanOrman, Steve Parman, Erik Kirby, Mark Schmitz, Kirsten Nicolaysen,
and new and old fellow Kip-mates Dave Applegate, Martha House, Meg
Coleman, Audrey Huerta, Jose Hurtado, Arthur White, Julie Baldwin, Karen
Viskupic (notice a new pattern?), thanks for sharing these moments! I am
especially glad that Nancy 's and Audrey's final thesis push overlapped with
mine!
I would like to announce that the Anke-McCormickHall-BostonHilton Hotel and Restaurant service is now closed. Please check all your
belongings. A new hotel will soon open in another part of this hemisphere. I
thank Kim Olsen, the founding father of the Boston-Hilton chain and
Powerbar-Cafe, and all the customers over the years, Chris Carlson, Ebbe
Hartz, Meg Coleman, David Hawkins, Kirsten Nicolayson, Audrey Huerta,
Kalsoum Abassi, Reiner and Annette Haus and many more.
Thanks to Barb and Dave Noyse from Spruce Head Island, Maine, and
Julchen, Lisa, Annette and Reiner Haus, for adopting me into their families.
Finally I would like to thank my brother Heinz and my mother Uschi for
their unconditional support and love without which I could have never
stayed away from home for so long.
To the Memory of My Dad
Dr. med. Kurt Friedrich
(1906-1976)
TABLE OF CONTENTS
ABSTRA CT..........................................................................................3
ACKNOWLEDGMENTS............................................................................5
TABLE OF CONTENTS............................................................................8
CHAPTER 1:
INTRODUCTION..............................................................10
REFERENCES..........................................................................................................................15
CHAPTER 2: A SHORT-LIVED CONTINENTAL MAGMATIC ARC AT
CONNEMARA, WESTERN IRISH CALEDONIDES: IMPLICATIONS FOR THE AGE
OF THE GRAMPIAN OROGENY.............................................................18
2.1. A BSTRACT .....................................................................................................................
2.2. INTROD UCTION ................................................................................................................
2.3. REGIONAL SETTING ........................................................................................................
18
19
21
2.4. GEOLOGY OF THE CONNEMARA COMPLEX...........................................................................
2.5. U -PB RESULTS...............................................................................................................
2.6. THE GRAMPIAN OROGENY AT CONNEMARA..........................................................................
22
2.7. REGIONAL IM PLICATIONS.................................................................................................
ACKNOW LEDGMENTS............
...
................................................................
26
REFERENCES CITED ............................................................................
FIGURE C APTIONS...................................................................................................................
23
25
27
28
33
CHAPTER 3: GEOCHRONOLOGICAL CONSTRAINTS ON THE TECTONIC
EVOLUTION OF THE CONNEMARA CALEDONIDES, WESTERN IRELAND........41
3.1. A BSTRACT .....................................................................................................................
3.2. INTRODUCTION ................................................................................................................
3.3. TECTONIC SETTING .......................................................................
3.4 GEOLOGY OF THE CONNEMARA REGION ..............................................................................
3.5. PREVIOUS GEOCHRONOLOGY............................................................................................
3.6. NEW CONSTRAINTS ON THE TECTONIC AND MAGMATIC EVOLUTION OF CONNEMARA ..................
41
42
42
43
45
48
3.6.1 U-Pb Results: The Ages of Ductile Deformation in Southern Connemaraand Anatexis......... 48
3.6.2. U-Pb Results: Age of the Delaney Dome Formation.......................................................
50
3.6.3. * ArA9Ar Results: A Case Against ProtractedCooling...................................................
51
3.7. THE TIMING OF MAGMATISM, METAMORPHISM, AND DEFORMATION AT CONNEMARA .................
3.8. AGE AND SIGNIFICANCE OF THE MANNIN THRUST..................................................................
55
3.9. THE COOLING HISTORY OF CONNEMARA AFTER THE CESSATION OF ARC MAGMATISM..................
3.10. IMPLICATIONS FOR THE TECTONIC EVOLUTION OF CONNEMARA ..............................................
A CKNOW LEDGMENTS..............................................................................................................
REFERENCES C ITED ................................................................................................................
56
APPENDIX 3.2: SAMPLE PREPARATION AND ANALYTICAL METHODS................................................
FIGURE C APTIONS..................................................................................................................74
67
56
57
58
59
CHAPTER 4: CONSTRAINTS PROVIDED BY U-PB TITANITE GEOCHRONOLOGY
ON FLUID INFILTRATION IN THE CALEDONIDES OF CONNEMARA, WESTERN
IRELAN D ............................................................................................
87
4.1. A BSTRACT ......................................----
.....
-
. ......
-----------------------------------------------
87
.... 88
INTRODUCTION....................................................................
89
GEOLOGIC SETTING .....................................................................................
91
METAMORPHISM AND PREVIOUS GEOCHRONOLOGY ............................................................
93
METHODS.............----.----.---------------------------......................................................................
94
INTERPRETATION STRATEGY FOR U-PB TITANITE DATES........................................................
95
RESULTS ...................................------------........................................................................
4.2.
4.3.
4.4.
4.5.
4.6.
4.7.
4.7.1.
4.7.2.
4.7.3.
4.7.4
Staurolite Zone.........................................................................................................
Staurolite-SillimaniteTransition Zone ........................................................................
Sillimanite-Muscovite Zone........................................................................................
Migmatite Zone........................................................................................................100
4.8 DISCUSSION ....................................................................................................................
A CKNOW LEDGMENTS..............................................................................................................103
REFERENCES CITED.....................................................................................104
---FIGURE CAPTIONS...........................................................................................................--
95
96
97
101
108
CHAPTER 5: CONSTRAINTS ON THE DEFORMATION AND COOLING HISTORY
39
40
OF THE CALEDONIDES OF CONNEMARA, WESTERN IRELAND, FROM AR/ A R
THERMOCHRONOLOGY.......................................................................125
5.1. ABSTRACT ..................................---......---.....................................................................
5.2. INTRODUCTION.................................................................126
5.3. GEOLOGIC SETTING.................................................................127
5.3.1.
5.3.2.
5.3.3.
5.3.4.
125
Deformation History..................................................................................................128
Intrusive History......................................................................................................130
M etamorphic History................................................................................................130
Post-GrampianSedimentation, Igneous Activity, and Late-Stage FluidFlow.......................132
133
...... ............................................................
5.4. PREVIOUS THERMOCHRONOLOGY ..............
134
--......................................................................
-5.5. METHODS....................................39
137
5.6. *AR/ AR RESULTS AND INTERPRETATION ..............................................
5.6.1. NorthernmostGarnet-StauroliteZone...........................................................................137
5.6.2. Garnet-Stauroliteand Staurolite-Sillimanite TransitionZones...........................................
5.6.3. Sillimanite-K-feldsparZone........................................................................................139
5.6.4. Migmatitic Portionsof the Sillimanite-K-feldsparZone...........................141
.............................
5.7. INTERPRETATION OF *AR/3 9AR RESULTS .............
5.7.1. Significance of Category 1 Ages ..................................................................................
5.7.2. Significance of Category 2 Ages ............................................................................
......5.7.3 Significance of Category 3 Ages ....................................................................
138
144
145
147
147
5.8. TOWARD A COMPREHENSIVE MODEL OF THE THERMAL AND TECTONIC EVOLUTION OF CONNEMARA148
5.9. CONCLUSIONS.............-..........-----.-----..................................................................151
152
.............................................................
REFERENCES CITED .............
APPENDIX 5.1: SAMPLE DESCRIPTION........................................................158
Northern Connemara......................................................-.....................158
-.. . -------------...........................
Central Connemara...................................................-.....
.......................
Southern Connemara......................................................-
159
160
APPENDIX 5.2: ANALYTICAL METHODS......................................................161
APPENDIX 5.3: CONSTRUCTION OF SCHEMATIC CROSS SECTIONS..................................................163
191
--....................................................................
FIGURE CAPTIONS......................................-
CHAPTER 1: INTRODUCTION
Insight into the processes responsible for formation of the continental
lithosphere can be gained by understanding
the thermal evolution
of
continental crust. In stable continental regions heat transfer occurs mainly by
conduction, but advective processes, such as faulting, plutonism, and fluid
circulation, can result in high transient geothermal gradients in tectonically
active areas (e.g. DeYoreo et al. 1991, Huerta et al. 1996). Such gradients are
recognized today in metamorphic terrains through quantitative studies of
pressure-temperature paths (e.g. England and Thompson 1984, Hodges 1991).
Information about the timing of metamorphic events and the successive
cooling history is of first-order importance in determining the thermal
evolution of the crust and inferring rates of tectonic and surficial processes
(Thompson & England 1984, Royden & Hodges 1984). Knowledge of the
timing, intensity, and duration of events responsible for elevated transient
geothermal gradients, however, depends on our ability to extract meaningful
thermochronologic information from metamorphic terrains (e.g. Zeitler
1989).
4 0Ar/ 3 9
Ar
and U-Pb geochronology of metamorphic minerals provide
powerful tools to track the thermal evolution of orogenic belts (McDougall &
Harrison 1988, Heaman & Parrish 1991). Combined, these two methods can
be used to reconstruct large portions of the thermal history of an orogenic belt
between 760*C (the nominal U-Pb closure temperature for zircon) to roughly
170*C (4 Ar/ 3 9Ar in K-feldspar). As a result of recent improvements
in
analytical techniques, it is now possible to obtain U-Pb and 4"Ar/ 3 9Ar dates
with an analytical uncertainty of less than 0.3% and 1% for most samples,
respectively.
However, even the most precise dates are not necessarily geologically
meaningful. In some cases, excess "Ar or inherited acessory minerals can
thoroughly complicate the interpretation of geochronologic data.
Even if
such factors are not important, the exact significance of a date depends on
how a particular mineral-isotopic system responds to the changing thermal
structure in an orogen.
Open-system behavior in mineral-isotopic systems is usually regarded
as a thermally activated volume diffusion process (e.g. Everenden et al. 1960,
Hart 1964, Hansen & Gast 1967, cf. Lee 1993). Thus, dates should correspond to
the temperatures below which diffusive loss of the radiogenic daughter
becomes effectively insignificant (Dodson 1973).
Closure temperatures are
different for each mineral-isotopic system because the diffusivity of the
radiogenic daughter isotope depends on the crystal-chemistry of a mineral
(e.g. Giletti 1974a).
Determining these parameters is difficult because, in
natural samples, the diffusivity of the radiogenic daughter varies even within
a single mineral-isotopic system due to and differences in composition and
effective diffusion dimension (Giletti 1974b, Harrison et al. 1985, Mezger et al.
1991, Scaillet et al. 1992). Variations in each of these parameters can change
the closure temperature of a mineral by up to 100*C (Harrison et al. 1985,
Hames & Bowring 1994). Experimentally determined diffusion parameters
are only available for a limited range of compositions and diffusion
dimensions (e.g. Giletti 1974b, Harrison 1981, Harrison et al. 1985, Baldwin et
al. 1990, Foland 1993, Cherniak 1993); inappropriate application of these
parameters can lead to gross misinterpretation of geochronologic dates. Until
a complete experimental database becomes available, these factors are best
determined empirically in slowly cooled terranes, where the range in ages is
large enough to be resolvable by radiometric analysis.
The Connemara region of western Ireland was thought to be an ideal
location to understand better factors that control ages in a slowly cooled
metamorphic terrane. Slow cooling of Connemara had been assumed based
on reconnaissance geochronologic data suggesting a >100 million year history
of intrusive activity and subsequent cooling (e.g. Elias et al. 1988, Miller et al.
used
1991, Cliff et. al 1996). At Connemara, most of the commonly
chronometers can be sampled in a variety of lithologies and metamorphic
grade ranging from the upper greenschist to upper amphibolite facies. Each
metamorphic zone experienced a distinct thermal history that can be
evaluated as a function of composition and grain size.
The first part of my dissertation was designed to test the assumption
that Connemara is a slowly (<5*C/Ma) cooled terrane by determining the age
and duration of magmatism and high-temperature (>700*C) metamorphism
(Chapter 2). The results show that magmatism and metamorphism occurred
between 474 and 463 Ma. Cooling of the high-grade terrane from >600*C to <
200*C is restricted to between ~470 Ma, the timing of peak metamorphism,
and ~443 Ma, the age of the Silurian unconformity. Therefore, the thermal
pulse associated with the Grampian orogeny lasted roughly 25 Ma, a far
shorter interval than previously assumed (Chapter 3). The large spread in
mineral-isotopic
ages observed at Connemara cannot be explained by
protracted cooling following a single orogenic event.
The second part of my dissertation was aimed at further determining
the significance of the spread in published dates studies (e.g. Elias et al. 1988,
Miller et al. 1991) using
40Ar / 39 Ar
40Ar/ 39 Ar
thermochronology. The advantages of
thermochronology over conventional thermochronologic methods
(K-Ar, Rb/Sr) are that (1) measurements
of the parent and radiogenic
daughter are made during a single analysis, resulting in a higher analytical
precision and allowing in-situ intragranular analyses, and (2) that the
presence of any excess radiogenic daughter isotope (excess
40Ar)
can be
recognized by incremental heating of the sample by a resistance furnace or a
laser. My sampling focused on muscovite-, biotite-, and phlogopite-bearing
metamorphic rocks from the garnet, staurolite, and sillimanite metamorphic
zones, as well as from anatectic metasedimentary rocks and syn-and postorogenic magmatic rocks. To place limits on the permissible range of
4"Ar/ 39Ar
(and therefore K-Ar and Rb/Sr) cooling ages, I combined 4"Ar/
39Ar
thermochronological results with geological field observations, thin section
and electron microprobe analyses, high-precision U-Pb dating (Chapters 2,
3and 5), and results from paleomagnetic studies (Morris & Tanner 1977,
Robertson 1984; Chapter 5).
The results show that the distribution of 4"Ar/
39 Ar
cooling ages is very
different from that observed in previous geochronologic studies (e.g. Elias e t
al. 1988). The large spread in ages (>50 Ma) appears to be restricted to areas
that have experienced the effects of post-orogenic magmatism (the Galway
batholith) or brittle faulting (the Renvyle Bofin Slide). A much narrower age
range (< 10 Ma) occurs in subareas of the metamorphic zones in the Dalradian
fold and thrust belt. Most of this variation in ages is consistent with the
different closure temperature for different minerals. The largest difference in
cooling ages for a single mineral-isotopic system (c. 20Ma) occurs between the
lowest (garnet) and the highest metamorphic ('migmatite') zone of northern
and southern Connemara, respectively. On a scale of < 5 Ma, some of the
4 0Ar/ 39 Ar
cooling ages appear to vary at random for minerals with similar
compositions and grain sizes, indicating that the limit of resolution for
4 0Ar/ 39Ar
data may be c. 5 Ma (Chapter 4).
This dissertation consists of four chapters that have been written for
publication in international geologic journals. Chapter 2 presents new U-Pb
geochronologic constraints on the age of the oldest and youngest intrusions of
the continental magmatic arc at Connemara. The results show that the
continental magmatic arc at Connemara was much younger and shorter lived
that known previously, and help to resolve a long-standing controversy
about the age and significance of the Grampian orogeny. This paper has been
submitted to GEOLOGY with Samuel Bowring, Mark W. Martin, and Kip
Hodges as co-authors. Chapter 3 presents
intermediate
magmatism
and
coeval
additional
metamorphism
U-Pb
in
ages for
southern
Connemara, as well as 40Ar/ 39Ar data which shows that Connemara was not
a slowly cooled terrane. All high-precision
geochronologic
dates are
remarkably consistent with their relative ages as inferred from field
relationships.
This paper was presented as an abstract at a "Discussion
meeting on the Caledonides" in Glasgow, Scotland (September 9 & 10, 1997),
and has
been accepted for publication
in the JOURNAL
GEOLOGICAL SOCIETY OF LONDON; the co-authors
OF THE
are Kip Hodges,
Samuel Bowring, and Mark W. Martin. Chapter 5 contains the main portion
of the 4"Ar/
3 9Ar
data obtained during the course of this dissertation. These
data and their interpretation focused on the thermal history related to and
immediately following emplacement
of the Connemara
magmatic arc,
although the Connemara region preserved a long record of 4"Ar/ 3 9Ar data
that
are
not
related
to
cooling
following
the
amphibolite-facies
metamorphism recorded in the Dalradian country rocks. This chapter is coauthored by Kip Hodges and will be submitted for publication to TECTONICS.
Chapter 4 is part of an attempt to determine
metamorphism at each metamorphic grade.
the timing of peak
This chapter contains the
preliminary results of U-Pb titanite analyses from calcsilicate rocks from all
metamorphic grades. Some of the U-Pb titanite results for the staurolite and
sillimanite zone titanites are surprisingly young relative to the expected age
of peak metamorphism in northern Connemara, but are of the same age as
the 462 Ma U-Pb titanite dates for the highest metamorphic zones of
southern Connemara. We infer that these U-Pb titanite dates record the
timing of fluid infiltration which occurred post peak metamorphism at the
low metamorphic zones. This chapter represents research in progress, and
will be complemented
by U-Pb monazite data before submission
for
publication to the JOURNAL OF METAMORPHIC PETROLOGY, with coauthors Samuel Bowring and Kip Hodges.
References
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cooled biotites: Insights from the Southwestern U.S. Proterozoic Orogen.
Geochimica et Cosmochimica Acta, 59, 3205-3220.
Huerta, A. D., Royden, L. H. & Hodges, K. V. 1996. The Interdependence of
Deformational and Thermal Processes in Mountain Belts. Science, 273, 637639.
Kelley, S. P., Arnaud, N. 0., and Turner, S. P. 1994. High spatial resolution
40Ar
/ 39 Ar investigations using an ultra-violet laser probe extraction
technique. Geochimica et Cosmochimica Acta, 58, 3519-3525.
Leake, B. E., & Tanner P. W. G. 1994. The Geology of the Dalradian and
associated rocks of Connemara, western Ireland. Royal Irish Academy,
Dublin, 96 pp.
Lee, J.K.W. 1995, Multipath diffusion in geochronology: Contributions to
Mineralogy and Petrology, 120, p. 60-82.
McDougall, I., and Harrison, T. M., 1988, Geochronology
and
Thermochronology by the 44Ar/ 3 9Ar Method, New York: Oxford University
Press, 2 12pp.
Mezger, K., Rawnsley, C. M., Bohlen, S. R., & Hanson, G. N. 1991. U-Pb
garnet, titanite, monazite, and rutile ages: Implications for the duration of
high-grade metamorphism and cooling histories, Adirondack Mtns., New
York: Journal of Geology, 99, 415-428.
Miller, W. M., Fallick, A. E., Leake, B. E., Macintyre, R. M., & Jenkin, G. R. T.
1991. Fluid disturbed hornblende K-Ar ages from the Dalradian rocks of
Connemara, western Ireland. Journal of the Geological Society of London,
148, 985-992.
Morris, W. A. & Tanner, P. W. G. 1977. The use of paleomagnetic data to
delineate the history of the development of the Connemara Antiform.
Canadian Journal of Earth Sciences, 14, 2601-2613.
Royden, L. H., & Hodges, K. V. 1984. A technique for analyzing the thermal
and uplift histories of eroding orogenic belts: A Scandinavian example.
Journal of Geophysical Research, 89, 7091-7106.
Robertson, D. J. 1988. Paleomagnetism of the Connemara gabbros, western
Ireland. Geophysical Journal of the Royal Astronomical Society, 94, 51-64.
Scaillet, S., Feraud, G., Ballevre, M. & Amouric, M. 1992. Mg/Fe and
[(Mg,Fe)Si-A121 compositional control on argon behavior in high-pressure
white micas: A 4 Ar / 3 9Ar continuous laser-probe study from the Dora-Maira
nappe of the internal western Alps, Italy. Geochimica et Cosmochimica Acta,
56, 2851-2872.
Thompson, A. B. & England, P. C. 1984. Pressure-temperature-time paths of
regional metamorphism II: Their inference and interpretation using mineral
assemblages in metamorphic rocks. Journal of Petrology, 25, 929-954.
Zeitler, P. K. 1989. The geochronology of metamorphic processes. In: Daly, J.
S., Cliff, R. A., & Yardley, B. W. D. (eds), Evolution of metamorphic belts,133148.
CHAPTER 2: A SHORT-LIVED CONTINENTAL MAGMATIC ARC
CALEDONIDES:
IRISH
WESTERN
AT
CONNEMARA,
IMPLICATIONS FOR THE AGE OF THE GRAMPIAN OROGENY.
Anke M. Friedrich, Samuel A. Bowring, Mark W. Martin, and Kip V. Hodges
submitted to Geology, March, 1998.
2.1. Abstract
New U-Pb data from the Connemara region of Ireland indicate that
continental arc magmatism along the southern margin of Laurentia was
short-lived, between c. 475 and 463 Ma. Field mapping demonstrates that
intrusive activity at Connemara was synchronous with Grampian midcrustal deformation and upper amphibolite-facies metamorphism. U-Pb
zircon analyses indicate that the age of the two oldest intrusions, the
Currywongaun and Cashel-Lough Wheelaun gabbros, are 474.5 ± 1.0 Ma and
470.1 ± 1.4 Ma, respectively. U-Pb analyses of xenotime from the
postdeformational Oughterard granite constrain crystallization to 462.5 ± 1.2
Ma. The implied age of the Grampian orogeny at Connemara is substantially
younger than generally acknowledged, but consistent with other age
constraints for the 'evolution of the Laurentian-Iapetus plate boundary.
Development of the 475-463 Ma magmatic arc at Connemara postdates
Tremadoc-Early Arenig ophiolite obduction, and was broadly coeval with arc
magmatism along strike of the Appalachian-Caledonian orogen. This
implies that subduction beneath the Laurentian margin from the
Appalachians to Scotland did not develop until after ophiolite obduction.
The new data from Connemara imply that this subduction polarity reversal
occurred at or just prior to 475 Ma, and was much less diachronous along
strike than previously assumed. Major Laurentia-ward subduction did not
develop until after the Grampian-Taconian orogeny, and is not recorded in
the Grampian orogen, where plate convergence after c. 460 Ma was
accommodated by strike-slip faulting.
2.2. Introduction
The
Appalachian-Caledonian
orogen records destruction
of the
Laurentian passive margin during Iapetus ocean closure. The earliest
orogenic events include widespread ophiolite obduction and accretion of
oceanic terranes, which were followed by syn-tectonic arc magmatism in the
Laurentian margin (e.g. van Staal et al. 1998). In the British and Irish
Caledonides these events are attributed to the Grampian orogeny, which
correlates with the Taconian/Humberian orogeny of the northern
Appalachians (Figure 2.1A; e.g. Lambert and McKerrow 1976; Cawood et al.
1995; Karabinos et al. 1998, Swinden et al. 1997, Whalen et al. 1997). The
Grampian
orogeny
is
recorded
in
Laurentian
margin
rocks
(the
Neoproterozoic Dalradian Supergroup) by arc magmatism, contractional
deformation, and metamorphism, but also by turbidite sequences, ophiolites
and accreted island-arcs (Ryan and Dewey 1991). Correlating Grampian events
between these two tectonic settings is difficult, partly due to later
fragmentation of the Laurentian margin by transcurrent faulting, and
requires knowledge of the precise timing of these events (Figure 2.1B; e.g.
Hutton 1987). The timing of Humberian/Grampian ophiolite obduction and
terrane accretion in Newfoundland and western Ireland is well established
(e.g. Ryan and Dewey 1991, Cawood et al. 1995). A Late Cambrian to Lower
Ordovician (c. 515 to 475 Ma) oceanic island-arc, which formed above an
oceanward-dipping subduction zone, collided with the Laurentian margin in
late Arenig/early Llanvirn time (c. 475 to 465 Ma). A new subduction zone
developed beneath the Laurentian margin, the ophiolites and accreted
terranes during Middle Ordovician time (e.g. Dewey and Shackleton 1984;
Tucker and Robinson 1990; van Staal 1994; Cawood et al. 1995, Swinden et al.
1997, Whalen et al. 1997, van Staal et al. 1998). However, the duration of
Grampian events recorded within Dalradian Supergroup rocks ranges from
>490 to 460 Ma based on published radiometric ages of continental magmatic
arcs such as the one at Connemara. This implies that an active margin at
Connemara
was contemporaneous
with
passive
Laurentian
margin
sedimentation and ophiolite obduction in Newfoundland and the British
Isles. These timing relationships are inconsistent with field relationships
which clearly show that ophiolites are cut by magmatic arc rocks (e.g. in
Tyrone; Hutton et al. 1985), and pose a major problem for comprehensive
models of the Grampian orogen, which currently are explained in two
different ways. One model embraces significant along-strike variations in the
Laurentian margin, with ophiolite obduction and terrane accretion at a longlived and complicated Andean-type plate margin (e.g. Lambert and McKerrow
1976; Yardley et al. 1987; Dewey and Ryan 1990). The other model suggests that
the Dalradian block was exotic to the Laurentian margin, and did not accrete
until after the Grampian orogeny (e.g. Bluck and Dempster 1991). The latter
model is especially appealing because some exposures of the Dalradian
Supergroup, for example at Connemara, are located in an unusual tectonic
position with respect to the rest of the Laurentian margin (Figure 2.1B).
During the Grampian orogeny at Connemara, tholeiitic to calc-alkaline
magmas of the Connemara Gabbro and Gneiss Complex (Connemara
Complex) were emplaced into rocks of the Dalradian Supergroup (Fig. 1;
Yardley and Senior 1982; Leake 1989). Field relations indicate that igneous
activity began during and
outlasted
the main
phase
of
Grampian
deformation, locally referred to as D2 and D3; early mafic rocks exposed in the
north and west of the complex are deformed, but felsic intrusions farther east
are undeformed and cut all ductile structures (e.g. Leake 1989). The potential
of these relationships to provide important constraints on the timing and
duration of Caledonian orogenic processes was not fully realized in the past
because reliable age constraints were not available for most intrusive phases
at Connemara. The age of the youngest intrusions has been controversial
with age estimates ranging from 407 to >470 Ma (Kennan et al. 1987; Tanner e t
al. 1997). The oldest gabbroic intrusions are commonly regarded c. 490 Ma old
based on highly discordant U-Pb data for one pluton (Jagger et al.
1988).
Along with U-Pb zircon data indicative of intrusive activity as young as 465
Ma (Cliff et al. 1996), this finding is commonly used to infer a duration of arc
magmatism at Connemara of at least 25 million years.
Data presented in this study provide the first high-precision U-Pb ages
of the earliest and latest intrusive phases at Connemara, placing constraints
on the duration of arc magmatism and, hence, the Grampian orogeny. Our
results indicate that the Grampian orogeny at Connemara was essentially
coeval with the Humberian orogeny in Newfoundland, rendering complex
models of the Laurentian margin unnecessary.
2.3. Regional Setting
Northwestern Ireland exposes a spectacular cross section through the
Caledonian orogen from the Laurentian margin in the north to terranes
accreted to its margin farther south (Figure 2.1B). The former Laurentian
margin terminates at a major mid-Ordovician suture, the Fair Head-Clew
Bay Line. Tectonic units south of this collisional suture include remnants of
an intra-oceanic subduction zone and a juvenile island arc of TremadocArenig age, as well as a Tremadoc-Llanvirn forearc basin, the South Mayo
Trough. Sedimentary rocks of the South Mayo Trough record erosion of an
island arc, ophiolites, and a metamorphic terrane (e.g. Dewey and Ryan 1991).
To the south, a Silurian unconformity conceals most of the boundary
between the South
Mayo Trough
and the Connemara
terrane.
The
Connemara Complex and its Dalradian country rocks were thrust onto
Ordovician metavolcanic rocks during the Grampian orogeny (Figure 2.1B,
e.g. Tanner et al. 1989). Farther south, rocks of oceanic affinity that contain
Connemara-derived
clasts and ophiolites, the South Connemara Group,
probably formed as trench deposits over a Llanvirn-aged north-dipping
subduction zone (P. Ryan, personal communication). Most of this contact is
obscured by intrusion of the Devonian Galway batholith.
2.4. Geology of the Connemara Complex
The Connemara Complex consists of a series of mafic and ultramafic
intrusions, dominated by metagabbro throughout much of the region. In
southern Connemara, a younger series of calc-alkaline intrusions, which
range in composition from quartz diorite to granite are also included in the
suite (Figure 2.2; e.g. Leake 1989). The metagabbros contain metamorphic
amphibole which replaced primary pyroxene and plagioclase during postcrystallization hydration. In southern Connemara, the mafic bodies were
intruded by quartz diorites. The youngest intrusive unit of the igneous
complex is the undeformed Oughterard granite (Figure 2.2; e.g. Tanner et al.
1997).
The Connemara Complex intruded a variety of metasedimentary rocks
of the Dalradian Supergroup. These rocks record three deformation episodes
and two metamorphic events. The oldest is a Barrovian-type metamorphism
(M2; >6 kbars, c. 550*C, Yardley et al. 1987) related to crustal thickening. This
event is best preserved in northern Connemara where it is not overprinted by
the regional upper amphibolite-facies
metamorphism
(M3). This latter
metamorphic event (c. 5 kbars, 750*C, Yardley et al. 1987) culminated in the
anatexis of some Dalradian units in southern Connemara.
The earliest fabric (Si) recorded in Dalradian rocks occurs only as
inclusion trails within syn-D2 garnets and has unknown tectonic significance.
Early main phase deformation, locally referred to as D2, is represented mainly
by a penetrative schistosity and isoclinal folds at a variety of scales. The folds
were refolded by large-scale, F3 fold nappes and cut by minor brittle faults. A
penetrative schistosity, axial planar to F3 folds, developed synchronously
with M3 sillimanite-grade metamorphism (e.g. Tanner and Shackleton, 1979).
Emplacement of the Connemara gabbros occurred during contractional
ductile deformation. The gabbros of northern Connemara intruded prior to
the D3 deformation, possibly as early as the regional D2 deformation
(Wellings 1998), and provide a maximum age for D3 deformation.
In
southern Connemara the age relationships of the metagabbros with respect to
D2 structures are obscured by the intense D3/M3 overprint. Emplacement
during
early-D3
most
seems
likely,
because
the
gabbros
contain
metamorphosed xenoliths, were deformed by F3 folds and predated quartz
diorites (e.g. Tanner 1990) .
At a late stage of D3, the Connemara terrane was thrust southward
over metarhyolitic rocks of the Delaney Dome Formation along the Mannin
thrust (Leake et al. 1983). This thrust was folded by a F4 WNW-plunging
antiform (Figure 2.2; Leake, 1986). Another
macroscopic F4 fold, the
Connemara antiform, deformed the Dalradian country rocks to the north and
was intruded by the Oughterard granite. Thus the granite marks the end of
magmatism and deformation in Connemara.
2.5. U-Pb Results
In order to determine
the age and duration of magmatism
at
Connemara, we have conducted single-crystal U-Pb geochronologic studies of
the oldest mafic intrusions and the youngest granite. Zircons were air-abraded
for -40 hours until no crystal faces remained. All crystals were dissolved by
standard anion-based liquid chromatography after Krogh (1982); details of the
analytical protocol may be found in Bowring et al. (1993) and Hawkins and
Bowring (1997).
We analyzed ten zircon fractions of the plagioclase-hornblende gabbro
(sample AF47), which was collected from the same location as the gabbro
dated by Jagger et al. (1988; National Grid Reference L 843.440). The zircons are
typically clear, subhedral elongate crystals, roughly 200 Rm long. Eight of the
zircon fractions define a discordia with an upper intercept date of 470.1 ± 1.5
Ma (MSWD = 0.53) and a weighted mean
27Pb/20'Pb
date of 470.3 ± 0.5 Ma
(MSWD = 0.46; Table 1; Figure 2.3A). Two fractions were excluded from the
regression because their
207
Pb/
2 06
Pb dates of 494.8 Ma and 472.8 Ma indicate the
presence of an inherited component. We interpret the crystallization age of
this gabbro to be 470.1 ± 1.5 Ma. Jagger et al. (1988) chose to report their more
207 Pb/ 20'Pb
precise
date of 490 ± 1 Ma, which was based on the mean of six of
their more magnetic fractions, rather than their upper intercept date of 477 +
25 / - 6 Ma for all nine analyses, which overlaps within 2a with the 470 Ma
age of this study. We believe that their more discordant analyses resulted
from analyzing multigrain fractions of unabraded zircons.
Single zircon U-Pb analyses of a basic pegmatite (sample AF124) are the
first attempt to determine the radiometric age of the syn-D2 Currywongaun
intrusion. All four crystals were clear, prismatic -250 gm crystals. The
analyses cluster near concordia with weighted mean
2 07
Pb/
20
Pb/
23 8
U,
207
Pb/
235
U, and
Pb dates of 472.0 ± 0.3 Ma (MSWD = 1.18), 472.5 ± 0.3 Ma (MSWD =
20 6
0.25), and 474.5 ± 1.0 Ma (MSWD = 0.11), respectively (Table 2.1; Figure 2.3B).
We interpret the slight discordance between these three mean ages as a result
of limited Pb loss. In this case, the best estimate of the age of the
Currywongaun intrusion is the weighted mean
2 07
Pb/
Pb date of 474.5 ± 1.0
206
Ma.
Previous geochronologic investigations of the two-mica Oughterard
granite were based on the Rb-Sr method, but age interpretation has been
problematic because most of the major minerals are altered (e.g. Tanner et al.
1997). We analyzed zircon and xenotime from the Oughterard Granite
(sample AF97-0101; Table 2.1; Figure 2.3C). Zircon crystals from this sample
are either small clear, prismatic crystals or larger, fractured crystals with clear
cores and cloudy overgrowths. Single zircon analyses yield
20 7Pb/ 20 6Pb
dates of
580 Ma and 2199 Ma, respectively, indicating that these zircons are inherited
grains. The granite also contains abundant bipyramidal xenotimes. Five
single crystal analyses of clear yellow and some cloudy orange xenotimes
have a weighted mean
20 7Pb/ 2'Pb
date of 462.2 ± 0.5 Ma (MSWD = 0.93) and
define a discordia with an upper intercept date of 462.5 ± 1.2 Ma (MSWD
1.16), which is the best estimate of the crystallization age of this granite.
-
2.6. The Grampian Orogeny at Connemara
U-Pb geochronology of the Connemara gabbros and the Oughterard
granite shows that magmatism, mid-crustal deformation, and amphibolitefacies metamorphism occurred between 474.5 Ma and 462.5 Ma (Figure 2. 4).
This rapid succession of events at Connemara is consistent with field
relationships. In northern Connemara, mafic intrusions were intruded prior
to D2, but D3 deformation occurred while the contact aureole of these
intrusions still was hot (Wellings 1998). In southern Connemara, the hanging
wall of the late-D3 Mannin thrust was chilled during emplacement against
cold
footwall
rocks
as
they
experienced
prograde
greenschist-facies
metamorphism. This implies that the Connemara terrane was still relatively
hot (>500*C) in late-D3 time (e.g. Leake 1986). Similarly, the post-D4
Oughterard Granite intruded country rocks that had not yet cooled after highgrade metamorphism, based on the lack of a contact aureole around the
intrusion.
The Connemara Complex forms part of a contractional continental
magmatic arc. The early stages of contractional deformation resulted in
crustal thickening, recorded by M2 high P/T metamorphism (cf. Wellings
1998). A syn-D2 emplacement age of the 475 Ma northern Connemara
gabbros, as inferred by Wellings (1998), implies that arc magmatism above a
subduction
zone
occurred
during
crustal
loading.
In
Tyrone
and
Newfoundland crustal loading was related to ophiolite emplacement, but
followed by intrusion of arc magmatic rocks into the Laurentian margin and
the ophiolites. If ophiolite emplacement was the cause of crustal loading at
Connemara, and the Currywongaun gabbro was emplaced during the regional
D2/M2 event, the magmatic arc, i.e., a Laurentia-ward dipping subduction
zone existed simultaneous with ophiolite emplacement (at 475 Ma). In light
of existing regional tectonic models for the Grampian orogen these
relationships can be reconciled only if it is assumed that the subduction
polarity reversal was an extremely rapid event and began during ophiolite
obduction. Alternatively, existing tectonic models for the Connemara orogen
must be modified.
North-vergent folding and subsequent southward thrusting of the
Connemara terrane could not have lasted long because the major structural,
metamorphic and magmatic architecture of Connemara was assembled
between c. 475 and 463 Ma. During this time exhumation of the orogen began
and was followed
by strike-slip
faulting
that
prevented
prolonged
convergence. Removal of the Connemara terrane from the active margin
shortly after orogenesis but prior to Silurian time explains the lack of igneous
or metamorphic ages younger than c. 460 Ma.
2.7. Regional Implications
A younger age for the Grampian orogeny at Connemara helps to solve
a major conflict regarding the origin of the Dalradian terranes of Ireland and
Scotland. A c. 475 Ma onset of the Grampian orogeny postdates carbonate
platform sedimentation that is known to have characterized the southern
Laurentian margin until Arenig/Llanvirn time (Figure 2.4; e.g. Soper and
England 1995). This implies that the Dalradian rocks of Connemara could
have been part of the Laurentian passive margin until the onset of the
Grampian orogeny. An exotic origin of the Dalradian Supergroup rocks as
proposed by Bluck and Dempster (1991) is not required.
The Grampian magmatic
arc at Connemara
postdates ophiolite
obduction recorded along strike of the northern Appalachian-Caledonian
orogen and was coeval with arc syntectonic arc magmatism of the
Grampian/Humberian/Taconian orogeny (e.g. Tyrone, Hutton et al. 1985).
This orogenic event occurred during a brief interval, between c. 475 and 463
Ma, along the Laurentian margin from the northern Appalachians to
Scotland (Figure 2. 4; Figure 2.1B; cf. Rogers et al. 1994). In late Tremadoc to
Early Arenig time the Laurentia-Iapetus plate boundary was dominated by
obduction of supra-subduction zone ophiolites (e.g., van Staal et al. 1998).
Subduction polarity reversal, which resulted in Late Arenig to Llanvirn
contractional arc magmatism in the Laurentian margin, occurred at or just
prior to 475 Ma.
There is no evidence for subduction beneath the southeastern
Laurentian margin before Late Arenig time. Closure of the Iapetus Ocean
before c. 475 Ma must have occurred within Iapetus or along its southern
(Avalonian) margin. The earliest subduction beneath the Laurentian margin
is recorded by short-lived magmatic arcs like the one at Connemara.
However, major arc magmatism in the Laurentian margin did not occur
until c. 454 to 425 Ma (e.g. Tucker and Robinson, 1990; Cawood et al. 1995;
Karabinos et al. 1998), which is not documented in Ireland where intense
strike-slip faulting prevented prolonged convergence. The Grampian orogeny
at Connemara is consistent with arc magmatism along strike from at least
Newfoundland to Scotland (e.g., Rogers et al. 1994; Karabinos et al. 1998,
Swinden et al. 1997, Van Staal et al. 1998) and correlates with the
Taconian/Humberian orogeny of the northern Appalachians. It does not
support models
of a
continental
collision
between
Laurentia
and
Avalonia/Gondwana for the Ordovician period.
Acknowledgments
This study was supported by a NSF grant awarded to KVH and SAB and a
GSA student grant awarded to AMF. We thank Barry Long from the Irish
Geological Survey for collecting the Oughterard granite sample.
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Soper, N. J., & England, R. W. 1995. Vendian and Riphean rifting in N W
Scotland: Journal of the Geological Society, London, 152, 11-14.
Stacey, J. S., & Kramers, J. D., 1975, Approximation of terrestrial lead isotope
evolution by a two stage model. Earth and Planetary Science Letters, 26, 207221.
Steiger, R. H., & Jager, E. 1977.
Subcommission on geochronology:
convention on the use of decay constants in geo- and cosmochronology. Earth
and Planetary Science Letters, 60, 359-362.
Swinden, H. S., Jenner, G. A., Szybinski, Z. A. 1997. Magmatic and tectonic
evolution of the Cambrian-Ordovician Laurentian margin of Iapetus:
Geochemical and isotopic constraints from the Notre Dame Subzone.
Newfoundland. In: Shina, K., Whalen, J.B., Hogan J. (eds) Magmatism in the
Appalachian orogen. Geological Society of America, Memoir, 191, 337-365.
Tanner, P. W. G. 1990. Structural age of the Connemara gabbros. Journal of
the Geological Society, London, 147, 599-602.
& Shackleton, R. M. 1979. Structure and stratigraphy of the
Dalradian rocks of the Bennabeola area, Connemara, Eire. In: Harris, A. L.,
Holland, C. H., and Leake, B. E., (eds), Caledonides of the British Isles Reviewed: Geological Society, London, Special Publications, 8, 243-256.
Dempster, P. J., and Dickin, A. P. 1989. Time of docking of the
Connemara terrane with the Delany Dome Formation, western Ireland:
Journal of the Geological Society, London, v. 146, p. 389-392.
& Rogers, G. 1997. New constraints upon the structural
and isotopic age of the Oughterard Granite, and on the timing of events in the
Dalradian rocks of Connemara, western Ireland. Geological Journal, 32, 247263.
Tucker, R. D., & McKerrow, W. S. 1995. Early Paleozoic chronology: a review
in light of new U-Pb zircon ages from Newfoundland and Britain. Canadian
Journal of Earth Sciences, 32, 368-379.
Tucker, R. D., & Robinson, P. 1990. Age and setting of the Bronson Hill
magmatic arc: A reevaluation based on U-Pb zircon ages in southern New
England: Geological Society of America Bulletin, 102, 1404-1419.
Van Staal, C. R. 1994. The Brunswick subduction complex in the Canadian
Appalachians: record of the Late Ordovician to Late Silurian collision
between Laurentia and the Gander margin of Avalon. Tectonics, 13, 946-962.
Dewey, J. F., Mac Niocaill, C., McKerrow, W. S. 1998. The
Cambrian-Silurian tectonic evolution of the northern Appalachians and the
British Caledonides: history of a complex, west and southwest Pacific-type
segment of Iapetus. In: Blundell, D. J., Scott, A. C. (eds) Lyell: The past is the
key to the present. Geological Society, London, Special Publications, 143, 199242.
Wellings, S. A. 1998. Timing of deformation associated with the syn-tectonic
Dawros-Currywongaun-Doughruagh Complex, NW Connemara, western
Ireland. Journal of the Geological Society, London, 155, 25-37.
Whalen, J. B., Jenner, G. A., Longstaffe, F. J., Gariepy, C. 1997. Implications of
granitoid geochemical and isotopic (Nd, 0, Pb) data from the CambroOrdovician Notre Dame Arc for the evolution of the Central Mobile Belt,
Newfoundland Appalachians. In: Shina, K., Whalen, J.B., Hogan J. (eds)
Magmatism in the Appalachian orogen. Geological Society of America,
Memoir, 191, 367-395.
Yardley, B. W. D., Barber, J. P., and Gray, J. R. 1987. The metamorphism of the
Dalradian rocks of western Ireland and its relation to tectonic setting.
Philosophical transactions of the Royal Society, London, A, 321, 243-270.
Yardley, B. W. D., & Senior, A. 1982. Basic magmatism in Connemara,
Ireland: evidence for a volcanic arc? Journal of the Geological Society,
London, 139, 67-70.
TABLE 2.1. U-Pb ANALYSES OF ZIRCON AND XENOTIME FROM CONNEMARA
Fractions
(sg)
U
(ppm)
Pb
(ppm)
Age (Ma)*
Errors2a (%)
Concentration
Sample Weight
206 Pb' 208 Pbt 206 Pb6
204Pb 206Pb
238 U
207 Pb§
207 Pb
206 Pb 207 Pb 207 Pb
% err 235U
% err 206 Pb % err 238U
(.14)
corr
235U 206 Pb coef
Pb#
ED*
(pg)
Pb#
Lough Wheelaun gabbro [AF-47]
Z1a (4)
5.7
509.3
43.3
15356
0.223
0.07557
(.20)
0.05709
(.13)
469.6
473.9
494.8
0751
90
270
22a (4)
4.1
607.4
511
2078.0
0 235
0.07487 ( 17) 0 58297 (19)
0.05647
(.09)
4655
466.4
470.9
0.896
58
360
Z4a (4)
4.9
481.1
408
3513.1
0.244
0.07580 (14)
0.59005
(17)
005646
(.09)
471.0
4709
4704
0858
33
60.8
Z8a (4)
104
288.4
247
1892.0
0.238
0.075665 (20)
0.58891
(.26) 0.05646
(.17)
470.1
470.2
4704
0 763
77
329
Z1la (1)
1.0
2464
20.9
26893
0.248
0.07535 (19)
0.58618
(.22)
0.05642
(.12)
468.3
468.4
469.1
0.849
44
46.9
Zi (1)
9.3
2085
176
2681.6
0.249
0.07496 (.09)
0.58330
(.12)
0.05644
(.08)
466.0
466.6
469.6
0.747
3.5
46.5
Z2 (1)
5.5
294 5
24.8
2960.7
0.250
0.07489 (.08)
0.58282
(.12)
0.05644
(.08)
465.6
466.3
469.8
0.735
27
51 4
Z3 (1)
5.5
298.4
24.8
3926.9
0.235
007486 (.07)
0 58282 (.13)
0.05646
(.11)
465.4
466.3
470.7
0.549
2.0
67.5
Z4 (1)
4.8
283.1
23.7
3706.8
0.247
0.07472 (.08)
0.58156
(.11) 0.05645
(.08)
464.5
465.5
470.0
0737
18
643
Z5 (1)
68
307.1
259
4175.3
0.247
0.07507 (.07)
0.58497
(.10)
0 05652
(.07)
466.6
4677
472.8
0 754
24
72 5
0.59477
Basic pegmatite [AF-124]
Zi (1)
3.2
988.0
91 2
5783.6
0.354
0.07597 (.06)
0.59269
(.19)
0 05858
(.18)
472.0
4726
4752
0310
27
Z2 (1)
4.9
288.8
27 1
4986.2
0375
0.07602 (07)
0.59273
(.11) 0 05655
(.08)
4723
472 6
474 1
0.676
14
952
Z3 (1)
25
11644
105.6
8512.7
0.329 0.07596
(.10) 059237
(.15)
(.11)
4719
472.4
474.5
0.676
1.7
157.5
Z4 (1)
41
479.6
444
3021.9
0.358 0.07594
(.08) 0.59228
(.11) 0.05657
(07)
471 8
472.3
474.7
0.759
3.2
56.9
0.05656
1088
Oughterard granite [AF97-0101]
Z6 (1)
0.9
1093.2
825
26817
0.031
0.08055
(.20)
0.65903
(25)
005934
(15)
4994
514.0
5796
0.801
1.9
388
Z7 (1)
1.7
255.3
358
1445.5
0103
0.13038
(.20)
247569
(34)
0.13771
(27)
7900
12650
2198.7
0.619
25
24.3
Xi (1)
1.9
247986
1870.0
60561
0137
0.07277
(.12)
056461
(.14)
005627
(06)
4528
454.5
463.0
0891
357
986
X2 (1)
55
24910.1
1910.7
3751.1
0156
0.07240
(.14)
0.56127
(15)
0.05623
(.05)
450.6
452.4
461 4
0.937
1667
62 1
X4 (1)
0.1
1808315
133995
8388.7
0.099
007423
(.07)
057571
(.09)
0.05625
(.05)
4616
461.7
4623
0811
102
131.4
X5 (1)
17
27751.2
2109.2
7681.9
0.143
007313
(.13)
0.56724
(14)
0.05626
(.05)
4550
456.2
4625
0.934
283
1257
X8 (1)
01
224066 8
168315
6189.8
0126
007328 (12)
056826
(16)
005624
(10)
4559
456.9
461.9
0769
168
995
Note Allerrorsare reported as 2-c. Number at zircons ineach traction shownin parenthesesafter fraction name. Sampleweightsare estimatedusinga videomonitor with gndded
screen andare known to within 40% Z = Zircon.X = Xenotime CommonPb correctionswere calculatedusingthe model of Stacy and Kramers(1975)and the interpretedage of the sarple
* RadiogencPb.
t Measuredratio correctedfor spike and fractionation only Massfractionation correction 0.15%/amu* 004%/amu wasappled to singlecollectorDaly analysesand 0 12%/amu* 0 04%
for dynamic Faraday-Dalyanalyses
§ Correctedfor fractionation, spike, blank,and initial commonPb.
# total commonPb analysis. Totalprocedural blankfor Pb rangedfrom 0 65-3 7 pg and < 1.0pg for U. Blank isotopiccomposition."Pb/Pb = 19.10t 0 1,
"'PbP"Pb =15 71 * 0.1, 208Pb/204Pb = 38 65 ± 0.1
**Age calculations are based on the decayconstantsof Steigerand JAger(1977)
a
in
Figure Captions
Figure 2.1. Geologic setting of the Irish Caledonides. a: extent of the
Laurentian margin. S: Scotland, BI: British Isles, after Hibbard et al. (1995). b:
simplified tectonic map of the western Irish Caledonides, north of the Iapetus
suture, after Ryan and Dewey (1991). The Fair Head-Clew Bay Line (FH-CBL)
separates the Connemara terrane from other Dalradian Supergroup
exposures. Ordovician ophiolites occur near this suture in the South Mayo
and Tyrone. SMT: South Mayo Trough. DDF: Delaney Dome Formation, SCG:
South Connemara Group.
Figure 2.2. Simplified geologic map of Connemara. The sample localities are
shown as dark gray boxes. DDF: Delaney Dome Formation. The gabbros of
northern
Connemara
are the
Dawros
(Da) - Currywongaun
(Cw) -
Doughruagh (Do) Complex. The Cashel (Ca) - Lough Wheelaun (LW) Gabbro
occurs near the structural base of the intrusive complex.
Figure 2.3. U-Pb concordia diagrams. a: Cashel-Lough Wheelaun Gabbro
(sample AF47). b: Currywongaun Basic Pegmatite (sample AF 124). c:
Oughterard Granite (sample AF97-0101).
Figure 2.4. Age and duration of the Grampian Orogeny at Connemara. The
time scale is from (1) Tucker & McKerrow (1995), (2) Landing et al. (1997) and
(3) Davidek et al. (in press). Events in Newfoundland, some of which are also
documented in Scotland, are cited from: (4) e.g. Soper & England (1995); (5)
e.g. Cawood & Suhr (1992), Dewey et al. unpublished; (6) Karabinos et al.
(1998), Dewey et al. unpublished; (7) Tucker & Robinson (1990), and Cawood
et al. (1995); (8) Van Staal et al. (1998), Swinden et al. (1997); (9) Ryan & Dewey
(1991), Dewey & Ryan (1990).
b
Devonian - Carbon.
Silurian Rocks
Granite
Ordovician Rocks
Dalradian Rocks
Grenville
Basement
Major Faults
Connemara
L
50 km
Figure 2.1
and
Silurian,
rdovician Rocks,
undivided
1111U01V1EMVC~jCarboniferous,
474.5 1
on
a
Galway Batholith
D
Oughterard granite
Ca111111
Laugh Wheelaun
Mannin thrust
nan
470.1 1.4 Mae5lM
a
ughterard granite
r-7/-doregnis
W.
W42512M
Maflc Intrusions
Cas e
Delaney Dame
Formation
..Fold.
.
and Thrust Belt In
Daradban Supergroup
..
Galway Batholith
.Ma.ic
Figure 2.2
Terrane boundary
interred
(A)
474
0.0763006-Lough-Wheelaun
gabbro AF-47
Co
co 0.0758CMJ
470
6;za1l
0.0753 C466
0.0748-
Upper Intercept Age
470.1 Ma
0.581
(B)
0.0762
0.586
0.591
0.596
PbI235U
Basic pegmatite
0.0763
D
2 07
1.4 Ma
-
AF-124
473
Co
Co)
0.0761 -
472
0.0760-
C
C\i
0.0759 -
207 Pb/ 206 Pb Age
474.5 ±1.0 Ma
0.5907
0.5917
0.5927
P b/235 U
207
0.5937
0.5947
(C)
0.0745
3
Oughterard granite
AF97-01 01
46M,
0.0740
Co
c
0.0735
9
0.0730 -
0
0.0725
o
xx8
450
x
Intercept Age
0.0720Upper
0.0720462.5
0.555
Ma
0.560
207
0.565
0.570
Pb! 235 U
Figure 2.3
1.2 Ma
0.575
0.580
38
Northern
Appalachians
Time
[Ma]
South
Mayo
Connemara
Silurian
E
-440
unconformity
Ashgill
E
-450
strike-slip faulting
Caradoc
-
458 -
la
0
E
Oughterard Granite
[AF-9701 01]
Llanvirn
-
16
470
U-
0
Cashel-L. Wheelaun
Gabbro [AF47]
Currywongaun Basic
Pegmatite [AF124]
9
island arcLaurentia
collision
9
Arenig
(
-480 g
0 2
_j
|-0
Ci
.)
Tremadoc
3
j0
C
<483-
< 490 -
4490 15
-0
Cambrian
C
C
-
500
I
0
N
C
0
.o
CoC
CoC. Co)
w 0
Figure 2.4
CLs
CHAPTER 3: GEOCHRONOLOGICAL CONSTRAINTS ON THE
TECTONIC EVOLUTION OF THE CONNEMARA CALEDONIDES,
WESTERN IRELAND.
Anke M. Friedrich, Kip V. Hodges, Samuel A. Bowring & Mark W. Martin
accepted for publication in the Journal of the Geological Society, London; July,
1998.
3.1. Abstract
The Connemara region of the western Irish Caledonides records the
evolution of a short-lived Mid Ordovician (c. 475 to 463 Ma) continental
magmatic arc that intruded into Dalradian metasedimentary rocks during
regional ductile deformation. New geochronologic data of intermediate
intrusive phases at Connemara are consistent with the relative age succession
suggested by field relationships. Calc-alkaline plutons (quartz diorite) were
emplaced into the Dalradian sequence and into the older Connemara Gabbros
at about 467± 2 Ma, coeval with anatexis, manifested by 467± 2 Ma granitic
pegmatite and 468 ±2 Ma leucosome formation. A new U-Pb age of 475.9 ±2.2
Ma for a metarhyolitic rock of the Delaney Dome Formation, a unit restricted
to the footwall of the regionally important Mannin thrust, overlaps with UPb ages for the Connemara intrusive complex (475 to 463 Ma). The possibility
that the Delaney Dome Formation also represents part of the Connemara arc
casts doubt on the published interpretation of the Mannin thrust as a major
terrane boundary. Connemara was situated at an active plate margin until
about 463 Ma, after which sinistral strike-slip resulted in accretion of
Connemara to South Mayo, western Ireland. From a study of U-Pb and new
4 Ar/ 39Ar geochronological data we conclude that the cooling history of
Connemara following the Grampian orogeny was simple and brief, at a
minimum rate of 35*C/Ma between 468 and 460 Ma, and 14*C/Ma between
460 and 450 Ma.
3.2. Introduction
The Connemara region of the western Irish Caledonides is one of the
best-studied continental magmatic arcs in the northern hemisphere and a
classic regional-scale, high-temperature metamorphic belt (Figure 3.1; e.g.
Yardley et al. 1987; Leake 1989). Field relations indicate that magmatism,
metamorphism of the country rocks, and deformation are closely associated
in time and space. Previous geochronological studies of the Connemara
metamorphic complex resulted in a complex pattern of igneous
and
metamorphic ages, spanning many tens of millions of years, which has been
interpreted in terms of a complicated spatial and temporal tectonic and
thermal evolution (e.g. Elias et al. 1988; Cliff et al. 1996). However, recent UPb geochronological analyses of the oldest and youngest intrusive phases in
Connemara show that arc magmatism occurred over a period of less than 15
Ma (Friedrich et al. submitted, Chapter 2). In this paper we present additional
U-Pb
geochronological
metamorphism,
data
to
and syn-magmatic
refine
the
timing
deformation,
of
magmatism,
as well as
40Ar/ 3 'Ar
thermochronological data to assess the cooling history of the Connemara
terrane. The results confirm that the Grampian orogenesis at Connemara was
short-lived.
3.3. Tectonic Setting
Ophiolite obduction, subduction and transpression related to initial
closure of the Iapetus ocean led to destruction of the former Laurentian
margin of the British Isles and Ireland (Figure 3. 1). In western Ireland,
distinctive lithotectonic terranes record a mid-Ordovician event, the
Grampian orogeny, which ended with the transpressional accretion of exotic
fragments along the Laurentian margin in pre-Silurian time (e.g. Hutton
1987). Most importantly, these terranes escaped the main Silurian-Devonian
Caledonian
orogeny recorded
in other
portions
of the Caledonides,
permitting a unique view into pre-Silurian orogenic processes.
The Connemara terrane includes clastic continental rise and slope
deposits that are correlated with part of the Neoproterozoic
Dalradian
Supergroup in Scotland (Figure 3. 1; e.g. Harris et al. 1994). Most of these
rocks were deposited prior to opening of the Iapetus ocean, presently dated at
about 617 Ma (Kamo et al. 1989). Unmetamorphosed Silurian (Llandovery; <
443 Ma) sedimentary rocks unconformably
overlie Dalradian rocks in
Connemara, as well as Ordovician sedimentary and volcanic rocks of the
South Mayo Trough to the north (Figure 3. 1; e.g. Dewey & Ryan 1990). The
contact
between
mid-Ordovician
high-grade
metamorphic
rocks
of
Connemara and very-low grade rocks of the South Mayo Trough is inferred
to be a strike-slip terrane boundary along which Connemara was emplaced
from the west (e.g. Hutton 1987). The presence of Connemara adjacent to
South Mayo in Late Ordovician time is indicated by metamorphic and
igneous clasts of the Late Llanvirn to Caradoc (c. 460 to >443 Ma) Derryveeny
conglomerate on South Mayo rocks which are similar to rocks exposed in
Connemara (Figure 3.2; Graham et al. 1991).
3.4 Geology of the Connemara Region
The Connemara complex (the Connemara gabbro and orthogneiss
complex of Leake and Tanner 1994) is the largest magmatic arc exposed within
the British and Irish Caledonides. Its country rocks include a variety of
metapelitic,
calcsilicate, and
amphibolitic
lithologies.
Arc magmatism
triggered regional high-temperature, intermediate-pressure metamorphism,
culminating
with the anatexis of some Dalradian units in southern
Connemara (e.g. Leake & Tanner 1994). The intrusive complex consists of a
series of tholeiitic mafic intrusions, mainly gabbro, and a younger series of
calc-alkaline intrusions which range in composition from quartz diorite to
granite (Figure 3.2; e.g. Leake 1986, 1989). At its present erosion level, the
complex is interpreted as the lower portion of a mafic magma chamber,
disrupted by the intrusion of calc-alkaline plutons, and ductile deformation.
The final stage of arc magmatism is represented by undeformed granitic
intrusions, which are concentrated in the eastern portion of Connemara. The
largest of these is the Oughterard Granite pluton (Figure 3.2).
Ductile fabrics in the Dalradian country rocks record two main
deformational episodes (D2 &D3), both of which were associated with
metamorphism (e.g. Yardley 1976; Tanner & Shackleton 1979). The oldest
fabric (Si) is preserved only as inclusion trails in garnet and staurolitic
porphyroblasts. A penetrative schistosity (S2) and tight to isoclinal folds (F2)
developed synchronously with garnet + staurolite ± kyanite mineral
assemblages (M2) that are mainly preserved in northern Connemara. Across
much of Connemara, the S2 schistosity is folded by large-scale F3 folds and
reworked by a penetrative S3 schistosity developed in lithologies containing
sillimanite + biotite ± cordierite ± garnet assemblages (M3).
Connemara, in a zone of intense migmatization
In southern
of pelitic rocks, D3
deformation was accompanied by partial melting.
The gabbros of northern Connemara, represented by the DawrosCurrywongaun-Doughruagh complex (Figure 3.2.) are considered by Wellings
(1998) to have been emplaced during D2 deformation and deformed during
D3. Those in southern Connemara where probably intruded during the early
stages of D3 deformation (Tanner 1990), deformed by F3 folds, and intruded by
strongly foliated, syn-D3 quartz diorites. Subsequently, the Connemara
Complex was thrust southwards over rhyolitic rocks of the Delaney Dome
Formation
along the Mannin
greenschist-facies metamorphism
Thrust (Figure 3.2), which
of the footwall rocks.
resulted in
Thrusting was
associated with a penetrative shear foliation and a NNW-oriented mineral
lineation in both hanging wall and footwall lithologies (Tanner et al. 1989).
Movement on the Mannin Thrust outlasted regional deformation because
hanging wall units have been brecciated near the thrust plane (Leake 1989).
The Mannin Thrust was folded by a late WNW-plunging antiform, which is
similar to the SE-plunging F4 Connemara antiform that controls the presentday outcrop-pattern of Dalradian units (Figure 3.2; e.g. Leake 1986).
undeformed
The
Oughterard Granite and related granitic pods cut across
mesoscopic F3 folds and the S3 fabric (e.g. Tanner et al. 1997). At least some of
the smaller granitic pods west of the Oughterard Granite are pre-D4 in age
(Tanner et al. 1997). However, the main bodies of the Oughterard Granite
intrude the F4 Connemara antiform and cut the lithological contacts folded
around it. The intrusions
mark the end of magmatism
and major
deformation in the Connemara region (Figure 3.2).
3.5. Previous Geochronology
The earliest attempt to establish to date one of the intrusive bodies in
the Connemara Complex was that of Pidgeon (1969), who obtained discordant
U-Pb data for four multi-grain zircon fractions from a sample of the CashelLough Wheelaun Gabbro. By projecting a line through the data points of the
two coarsest fractions, Pidgeon estimated a 504 ± 10 Ma crystallization age
(recalculated based on the decay constants of Steiger and Juger (1977). A second
study of another sample from the same locality produced similar discordant
results (Jagger et al. 1988). Nine multi-grain zircon fractions loosely defined a
chord with an upper intercept of 477 +25/-6 Ma, but these authors regarded
the mean
27Pb/2Pb
age of these fractions (490 ± 1 Ma) as the crystallization
age of the gabbro. We recently conducted a third U-Pb study of a sample from
the Cashel-Lough Wheelaun Gabbro, this time using small seven single and
one multi grain zircon fraction that had been heavily abraded (Friedrich et al.
submitted, Chapter 2). The resulting data are much more concordant than
those obtained previously, and they define an upper intercept age of 470.1 ±
1.4 Ma, which we interpret as the crystallization age of this gabbro.
In
addition, we analyzed eight single zircons from a mafic pegmatite of the synD2 Currywongaun intrusion and interpret the age of this sample to be 474.5 ±
1.0 Ma. Based on these results, we regard the early, mafic phases of the
Connemara Complex to have crystallized between 475 and 470 Ma.
A variety of geochronological methods have been used to establish
ages for the younger phases of the Connemara Complex. Cliff et al. (1996)
determined a U-Pb zircon lower intercept age of 463
diorite intrusion, and a mean
207 Pb/ 20 6
Pb
age of 466
4 Ma from a quartz
6 Ma from a fine-
grained, unfoliated granite that cuts across migmatitic leucosomes. The age of
the undeformed Oughterard Granite has been especially controversial. Leggo
et al. (1966) obtained a Rb-Sr internal isochron age of 460 ± 7 Ma on
muscovite, which they regarded as a minimum age estimate for the granite.
In contrast, Kennan et al. (1987) determined a Rb-Sr whole rock errorchron
age of 407 ± 23 Ma for the same intrusion, concluding that it belonged to a
suite of Early Devonian 'Newer Caledonian Granites', like the Galway
Batholith. Leake (1988) marshalled field and geochemical arguments against
this interpretation and suggested that the 407 Ma date reflected isotopic
disturbance during retrogression.
Recently, Tanner et al. (1997) analyzed several samples of the
Oughterard granite, its satellite intrusions, and cross-cutting pegmatites and
obtained Rb-Sr dates ranging from 396 to 473 Ma, but regarded the oldest as
the best estimate of the minimum age of the granite. We have recently
obtained an age of 462.5 ± 1.2 Ma from the large mass of Oughterard granite
found in the east of the inlier (Figure 3.2) using the U-Pb xenotime technique
(Friedrich et al. submitted, Chapter 2). Additional U-Pb data published by
Cliff et al. (1996) suggest that magmatism
and metamorphism
in the
Connemara region continued until about 463 Ma. They interpreted a titanite
date of 466 ± 3 Ma and a monazite date of 463 ± 4 Ma for two pegmatites as
marking the minimum age of D3 deformation and M3 metamorphism in
eastern Connemara. Cliff et al. (1993, 1996) also obtained a c. 478 Ma U-Pb
titanite age for a metamorphic diopside rock from central Connemara,
attributing it to M3 metamorphism. In light of the then-accepted 490 Ma age
of Jagger et al. (1988) for the Cashel-Lough Wheelaun Gabbro, Cliff et al.
(1996) inferred a prolonged period of arc magmatism and metamorphism at
Connemara from 490 Ma until at least 463 Ma.
Age constraints for the late D3 Mannin Thrust are limited. Leake et al.
(1983) obtained a Rb-Sr errorchron age of c. 460 Ma for a geographically
separated set of mylonitic metarhyolitic samples of the footwall Delaney
Dome Formation. Tanner et al. (1989), analyzing contiguous sample of
mylonite from a single site, determined a Rb-Sr whole-rock isochron age of
443 ± 4 Ma, and Rb-Sr muscovite-whole-rock isochron ages of 448 ± 4 and 451
± 4 Ma from two samples. These authors interpreted the average of these
dates - 447 Ma - as the age of the main Mannin Thrust movement. Recently,
Cliff et al. (1996) obtained a Rb-Sr muscovite-feldspar isochron age of 456 ± 6
Ma for a syn-D4 muscovite-bearing quartz vein from central Connemara,
which suggests an older age for the Mannin Thrust.
Internally consistent Rb-Sr and K-Ar ages, as well as a few "Ar/"Ar
ages from Connemara, span a range of over 75 Ma. The oldest dates are c. 485
Ma K-Ar hornblende ages from the vicinity of the Connemara gabbros of
southern Connemara, whereas most K-Ar and Rb-Sr mica ages range from
455 to 440 Ma (Elias et al. 1988; Miller et al. 1991). Because the Connemara
gabbros were thought to be 490 Ma old, the K-Ar hornblende ages were
interpreted to indicate rapid cooling to the closure temperature for Ar
retention in amphibole (c. 500*C; Harrison 1981), followed by a 30 millionyear period of slow cooling, and a final phase of rapid cooling between 455
and 440 Ma (e.g. Elias et al. 1988; Cliff et al. 1996). These data also could be
interpreted as indicating a simple cooling history between 485 Ma and 440 Ma
at a relatively slow rate of c. 5*C/Ma. A few younger dates in the Connemara
region (c. 420 Ma) have been attributed to partial reheating in the vicinity of
the c. 400 Ma Galway Batholith (e.g. Leggo et al. 1966; Elias et al. 1988).
3.6.
New Constraints on the Tectonic and Magmatic
Evolution of Connemara
Although the U-Pb data published in a companion paper (Friedrich et
al. submitted, Chapter 2) demonstrate that Connemara arc magmatism was
restricted to only a few million years, several important questions remain
regarding the thermal evolution and deformational history of the region
during Grampian orogenesis. For example, is it possible to constrain better
the absolute ages and durations of specific deformational events? Does the
Mannin Thrust represent a fundamental suture between exotic terranes and,
if so, when did docking occur? What was the cooling history of the region
subsequent to magmatic activity? We have addressed these questions with
U-Pb and "Ar/ 3 'Ar geochronological studies of selected samples from
southern Connemara.
3.6.1 U-Pb Results: The Ages of Ductile Deformation in Southern
Connemara and Anatexis
In
order to bracket better the timing
of syn-tectonic
pluton
emplacement in the Connemara Complex, we analyzed zircons from two
samples collected in the migmatite zone of southern Connemara (Figure 3.2):
a hornblende-bearing quartz diorite gneiss (AF43; National Grid Reference L
814.410) and a concordant granite pegmatite (AF44; L 814.410 ).
Sample
AF43
contains
two
morphologically
distinct
zircon
populations: clear prismatic crystals up to 400 gm long, and inclusion-rich,
multi-faceted crystals (< 200 gm). The six single zircon analyses from both
populations are concordant with weighted mean
2 07
Pb/
206
Pb,
20 6
Pb/
238
U, and
207 Pb/U
dates of 467.9 ± 1.2 Ma, 466.2 ± 0.5 Ma, and 466.5 ± 0.6 Ma,
respectively (Table 3.1, Figure 3.3A).
We interpret the crystallization age of
the quartz diorite intrusion to be 467 ±2 Ma. Sample AF44 also contains
multi-faceted and prismatic zircons (<200 gm). Four analyses are concordant
with weighted mean
207Pb/ 2"Pb,
2
"Pb/238U, and
2 07
Pb/ 2 5U dates of 467.4 ± 2.1
Ma, 466.6 ± 1.8 Ma, and 466.8 ± 1.6 Ma, respectively (Table 3.1, Figure 3.3B).
One of the zircon analyses (AF44-Z9) is -2% discordant, with a 2 07Pb/
235
U date
of 477.5 ± 0.6 Ma, which we attribute to the presence of a minor inherited
component. We interpret that sample AF44 crystallized at 467 ± 2 Ma.
Together, the data from these two samples indicate that main ductile (D3)
deformation occurred lasted until c. 467 Ma in southern Connemara.
This interpretation is consistent with some of the U-Pb data presented
by Cliff et al. (1996) for syndeformational
intrusive
activity, but is
inconsistent, for example, with their U-Pb titanite ages from central
Connemara, which suggest that M3 occurred much earlier (-478 Ma) in this
area. To explore this problem further, we analyzed monazite and zircon from
a metapelitic migmatite sample (AF45; National grid Reference L 814.410;
Figure 3.2) collected near samples AF43 and AF44 in southern Connemara.
Zircons in sample AF45 are either prismatic or multi-faceted crystals (< 350
gm). Zircons from both populations are discordant with
2 7Pb/ 2
6Pb dates
ranging from 817 to 2636 Ma suggesting that these zircons are detrital (Table
3.1). Monazite in the melanosome occurs both as inclusions in biotite and
garnet and at grain boundaries between muscovite and quartz. In the
leucosome, they are found as inclusions in quartz and albite. Most crystals are
subhedral, clear, and yellow, with a diameter up to 180 gm (cf. Appendix 3.1).
One monazite crystal from AF45 is reversely discordant (Table 3.1, Figure
3.3C). Incomplete dissolution of monazite or an excess component of 2"Pb
may be responsible for this reverse discordance, resulting in an unreliable
/ 23 8U date (e.g. Scharer 1984). We prefer the 207 Pb/ 235U date of 465.8 ± 4.0
206 p
Ma as the best estimate of the crystallization age of this monazite. The other
monazite of this sample is slightly older, but nearly concordant with a
2 07Pb/ 2..
U date of 467.0 ± 0.4 Ma (Table 3.1, Figure 3.3C). Our best estimate for
the time of monazite crystallization in AF45 is 466
+2
Ma, which we regard as
a robust estimate for the age of M3 metamorphism in central Connemara.
We are attempting other ways to evaluate the Cliff et al. (1996) suggestion of a
478 Ma event: in particular, we are dating titanites from several localities of
central Connemara. Although this study is incomplete, the results thus far do
not provide evidence for titanite crystallization older than 466 Ma.
3.6.2. U-Pb Results: Age of the Delaney Dome Formation
The tectonic significance of the Mannin Thrust is not well understood
because neither the age nor the palaeotectonic affinity of its footwall rocks, the
Delaney Dome Formation, is known with relation to the Connemara
Complex. If the Delaney Dome volcanic units were products of the
Connemara volcanic arc, the Mannin Thrust would simply be an intra-arc
structure. On the other hand, the Delaney Dome Formation and Connemara
Complex may represent distinctive and widely separated tectonostratigraphic
terranes sutured along the Mannin Thrust. Although the final resolution of
such questions requires more geochemical study (cf. Leake & Singh 1986), new
U-Pb data show that Delaney Dome volcanism and Connemara Complex
plutonism occurred at the same time and therefore both could have formed
synchronously within the same arc.
Zircons from a mylonitic metarhyolite sample of the Delaney Dome
Formation (AF94-DD2; National Grid Reference L 647.479; Figure 3.2) belong
to one of three morphological populations: clear, short (c. 100 gm) crystals;
cloudy, large (c. 200 gm) crystals; and clear prismatic crystals up to 160 g m
long. Zircons from the former two populations yielded discordant analyses
with
2 07 Pb/ 2
06Pb
dates ranging from 992 to 1268 Ma (Table 3.1). We interpret
these zircons to have been incorporated from a pre-eruptive source. Five
zircons of the prismatic population (Z5, and Z9 - Z12; Table 3.1; Figure 3.3D)
are discordant and yield a weighted mean
2 0zPb/ 20Pb
date of 475.9 ± 2.2 Ma. A
linear regression through these five analyses yields an upper intercept age of
474.6 ± 5.5 Ma, which we interpret as the approximate age of the Delaney
Dome Formation.
3.6.3. 'Ar/'Ar Results: A Case Against Protracted Cooling
Previous K-Ar and Rb-Sr geochronological studies suggested that the
Connemara terrane had a protracted cooling history, extending as far back as
Late Cambrian/Early Ordovician time (e.g. Elias et al. 1988; Miller et al. 1991).
This conclusion is inconsistent with Middle Ordovician U-Pb ages for arc
magmatism and peak metamorphism in southern Connemara, which imply
that significant cooling did not occur until after 465 Ma. Our
were aimed at testing
the protracted
cooling
40Ar/ 3 9Ar
hypothesis
studies
through
a
combination of resistance furnace and laser mapping experiments on micas
from the Connemara region.
We analyzed biotite from a biotite schist of the Lakes Marble Formation
of the Dalradian Supergroup (AF24; National Grid Reference L 815.461),
muscovite from a concordant pegmatite (AF23; L 815.461) intruding the same
formation, and muscovite from a second concordant pegmatite intruding an
amphibolite of the Streamstown Formation of the Dalradian Supergroup
(AF31; L 821.468). All three samples were collected in the northern portion of
the migmatite zone, an area presumably unaffected by intrusion of Late
Silurian-Devonian granites like the Galway Batholith (cf. Jenkin et al. 1992;
Figure 3.2). Examination of the micas with a binocular microscope and a
scanning electron microscope revealed no obvious inclusions. The major
element composition of each mineral was determined using the JEOL 733
electron microprobe at MIT (Table 3.2). Detailed compositional profiles
revealed no discernible major element zoning.
We initially performed incremental heating experiments on all three
samples, by using the resistance furnace to extract gas from the AF31
muscovite and the defocused laser on the AF24 biotite and the AF23
muscovite (Table 3.3). These experiments resulted in essentially flat release
spectra, displaying statistically definable plateaus (Figure 3.4). Assuming an
initial
40Ar/
36
Ar ratio of 295.5, the plateaus correspond to ages of 460 ± 2 Ma
(AF31 muscovite), 454 ± 2 Ma (AF23 muscovite), and 450 ± 5 Ma (AF24
biotite). All of the micas released Ar with very high
40Ar / 3 6Ar
ratios
throughout the experiments, such that isotope correlation diagrams were of
little use in testing the results for excess
40Ar
contamination. Although we
cannot preclude the existence of such contamination, the highly radiogenic
nature of the Ar in these samples would require extreme amounts of excess
4 0Ar
contamination to significantly effect the calculated ages for each heating
increment. We interpret the plateau ages of these micas the time at which the
samples cooled through the closure temperature range for Ar diffusion in
muscovite and biotite.
Extracting cooling history information from these data depends on the
assumptions used for the diffusion geometry and the critical diffusion
dimension (aeff) for 40Ar in biotite and muscovite. The AF 24 biotite is similar
in major element composition to the Cooma biotite studied experimentally
by Harrison et al. (1985) and Grove & Harrison (1996), whose data were
combined to obtain diffusion parameters of E = 47.1 kcal/mol and D. = 0.075
cm 2/s for a cylindrical diffusion geometry. Based on their experimental
results, Harrison et al. (1985) suggested a value of aff = 150 gm for biotites of
intermediate Fe/(Fe+Mg) ratio, similar to those in sample AF24. Based on
this value, the combined Harrison et al. (1985) and Grove & Harrison (1996)
diffusion results for Cooma biotite, and a nominal cooling rate of 5*C/Ma, we
used the equations of Dodson (1973) to estimate a bulk closure temperature of
302*C. Using the experimental results of Robbins (1972) for muscovite (as
reanalyzed by Hames & Bowring, 1994: E = 43 kcal/mol and D. = 0.00039
cm 2/s) and assuming again a cylindrical geometry and aff = 150 pm (cf.
McDougall & Harrison, 1988), we calculate a bulk closure temperature of
326*C, only slightly higher than the biotite value. Assuming this approach is
correct, the Connemara muscovite and biotite data presented here are
consistent with an average cooling rate of only about 2-3*C/Ma over the 460450 Ma interval.
However, several lines of evidence argue against this very slow cooling
interpretation of the 4 0Ar/ 39Ar data from Connemara. The most significant is
that the AF31 muscovite used for incremental heating, a single {001} cleavage
fragment with a grain diameter of
-
6.4 mm, was much larger than the AF23
muscovite used for incremental heating (an 0.8 mm {001} cleavage fragment)
and yielded a significantly older plateau age. This suggests that af for these
micas may correspond to the physical grain size and not to a much smaller
subgrain dimension. Indeed, numerous
4 Ar/ 3 9Ar
laser mapping studies of
biotite and muscovite show diffusion gradients on a scale much greater than
150 gm (Phillips & Onstott, 1988; Onstott et al. 1991; Phillips 1991; Hames &
Bowring 1994; Hodges et al. 1994; Hodges & Bowring 1995).
If we use the
Hames & Bowring (1994) re-analysis of Robbins' (1972) data, and assume that
a,
corresponds to one-half the physical grain size for each muscovite, the
bulk closure temperature of the AF31 muscovite is 463*C, that for the AF23
muscovite is 377*C, and the average cooling rate between the two closure ages
becomes ~14'C/Ma. A similar calculation for AF24 biotite, using one-half the
physical grain size (0.4 mm) as a, , results in a bulk closure temperature of
345*C for a cooling rate of 14'C/Ma. However, simple linear cooling at
14*C/Ma from AF31 muscovite closure to AF24 biotite closure would predict
a slightly older age for the biotite than the one actually measured. This may
suggest a decrease in cooling rate between 455 and 450 Ma to something like
10*C/Ma, but the combined uncertainties in the
4
Ar/ 39Ar data and in
available diffusion data preclude more detailed calculations.
Another method for evaluating various cooling models is to search for
age gradients in the Connemara micas. Dodson (1986) suggested that slowly
cooled mineral
grains should
show discernible
age gradients if ae
corresponded to the physical grain radius, and he derived mathematical
algorithms to describe closure temperature as a function of radial distance in
mineral grains. Using the Hames & Bowring (1994) diffusion values, a grain
with the dimension of the incrementally heated AF31 muscovite should
exhibit a core-to-rim age gradient of 33 Ma for an average cooling rate of
5*C/Ma and up to 149 Ma for an average cooling rate as slow as 1*C/Ma.
Since such gradients can be resolved easily with the laser
44Ar/ 3 9Ar
microprobe, we performed a laser mapping experiment on a second {001}
cleavage fragment from the same AF31 muscovite crystal that had provided
the incrementally heated fragment (Table 3.4). The results are illustrated in
Fig. 5 as a plot of apparent age vs. distance from the grain center, calculated as
a fraction of the grain radius.
All 27 fusion spots yield a 39Ar-weighted mean age of 459.1 ± 1.6 Ma,
indistinguishable from the plateau age for the other AF31 muscovite grain.
Although there is some scatter in Fig. 3.5 beyond that which might be
expected from analytical imprecision alone - possibly due to the existence of
fast diffusion pathways within the crystal (cf. Hodges & Bowring, 1995) - there
is no indication of a systematic decrease in apparent
4 Ar /
39
Ar age with radial
distance. If there is an age gradient in this sample related to cooling, it must be
of a magnitude too small to be resolved with the analytical capabilities of the
microprobe. Given that the standard deviation of all mapped ages in the AF31
crystal is 6.5 Ma, a cooling rate of at least 14*C/Ma would yield no analytically
discernible gradient. We conclude that the region of Connemara from which
our 4 Ar/
39Ar
samples were collected experienced relatively rapid cooling (>
14*C/Ma) over the 450-460 Ma period, consistent with the brevity of
Connemara arc magmatism.
The Timing of Magmatism, Metamorphism, and
Deformation at Connemara
3.7.
The new data presented here, as well as those in Friedrich et al.
(submitted), are consistent with the relative sequence of events established
through field observations. For example, the U-Pb ages of sequentially
emplaced gabbro, quartz diorite, and granite intrusions are 470, 467, and 463
Ma (Figure 3.6A). The total duration of arc magmatism at Connemara was no
more than about 12 million years, much shorter than previously thought.
The timing of peak metamorphism in the migmatite zone (>700*C:
Barber and Yardley 1985; Treloar 1985) is well constrained at 468 to 466 Ma by
our results from south-central Connemara and those of Cliff et al. (1996) for
eastern Connemara (Figure 3.6C). High temperature conditions (>500*C)
lasted at least until 463 Ma based on U-Pb titanite dates from Cliff et al. (1996)
and Friedrich (unpublished data, Chapter 4). The age of the M3 metamorphic
peak falls within the age range for Connemara magmatism and closely
matches the U-Pb age of intermediate-composition plutons, suggesting a
direct causal relationship. This is supported by the observation that the
migmatitic leucosomes are most abundant near quartz-diorite intrusions.
Such a relationship obviates the need to postulate other heat sources for
metamorphism (cf. Cliff et al. 1996). Since both the quartz-diorite intrusions
and anatexis were coeval with the most intense ductile deformation (still
D3?), this deformational event (D3') can be confidently assigned a 468 - 466
Ma age (Figure 3.6B).
U-Pb ages of the intrusive rocks also help to constrain the ages of other
deformational events (Figure 3.6B). The 474.5 Ma Currywongaun gabbro was
emplaced during D2 deformation, whereas the 470.1 Ma Cashel gabbro in
southern Connemara intruded during early D3 deformation. An age
difference of only a few million years between D2 deformation and the onset
of D3 deformation is consistent with field observations
and thermal
modeling results from the aureole in northern Connemara (Wellings 1998).
3.8. Age and Significance of the Mannin Thrust
The Mannin Thrust post-dates the crystallization of intermediatecomposition meta-intrusive rocks in its hanging wall and metarhyolites of
the Delaney Dome Formation in its footwall. The 467 Ma quartz diorite age
reported in this paper suggests a maximum age for the thrust (Figure 3.6 A,
B). A minimum age derives from the observation that a late fold deforms the
Mannin Thrust. The F4 Connemara antiform is intruded by the 462.5 Ma
Oughterard Granite (Figure 3.2). Assuming that F4 folding occurred at the
same time across Connemara, the major period of movement along the
Mannin Thrust is bracketed between 466.5 and 462.5 Ma (Figure 3.6 B). The
447 Ma age for the Mannin Thrust proposed by Tanner et al. (1989) is
inconsistent with our U-Pb age for the Oughterard Granite, unless one is
prepared to speculate that D4 deformation is diachronous across Connemara
or at least may have lasted 15 Ma. Either of these possibilities seems unlikely
in light of overwhelming evidence for a brief Grampian orogenic event.
Rather, we suspect that the age determined by Tanner et al. (1989) reflects a
late brittle fault reactivation, related fluid transport, or late-stage regional
cooling below the Rb-Sr closure temperature for muscovite.
3.9. The Cooling History of Connemara after the Cessation
of Arc Magmatism
U-Pb ages for magmatism and metamorphism at Connemara provide
an upper age limit for the period of post-magmatic, late-metamorphic cooling
that should be recorded by "Ar/
3
Ar, K-Ar and Rb-Sr cooling ages. Regional-
scale upper amphibolite-facies metamorphism
exceeded temperatures of
700*C in southern Connemara (e.g. Yardley et al. 1987). Such temperatures
would be expected to reset all mineral-isotopic systems involving ''K or 87Rb
decay. A minimum age for cooling of rocks at Connemara is given by the
proximity of the presently exposed rocks to the Silurian unconformity (c. 443
Ma; Figure 3.2). Therefore, cooling of southern Connemara from >700*C to
near surface temperatures is restricted to between c. 468 and c. 443 Ma. Any
'"Ar/
39
Ar or Rb-Sr dates that are older than
468 Ma do not reflect cooling of
-
southern Connemara after magmatism and high-grade metamorphism, and
must reflect isotopic disequilibrium or excess
4
Ar contamination. Similarly,
dates younger than c. 443 Ma must reflect open-system behavior unrelated to
simple cooling after metamorphism.
4'Ar/
9Ar
ages of micas from southern Connemara indicate that this
portion of Connemara cooled through the 460 to 350'C temperature range
between 460 and 450 Ma, probably at a rate of at least 14*C/Ma (Figure 3.6 C).
Combined with the 468 Ma of peak metamorphism, these data imply rapid
cooling at 35 *C/Ma between 468 and 460 Ma. We have found no evidence to
support the widely held conviction that the cooling history of Connemara
was complex and protracted (e.g. Elias et al. 1988). Instead, Connemara appears
to have had a relatively simple cooling history subsequent to a brief period of
arc magmatism and Grampian orogenesis.
3.10.
Implications
for the
Tectonic
Evolution
of
Connemara
At present, we have no evidence for magmatic or metamorphic
activity that may have affected the Dalradian Supergroup at Connnemara
prior to 474 Ma. Older zircon growth events are evidenced by
20 7Pb/ 2 06Pb
dates
for Connemara samples ranging from 2636 Ma to less than 600 Ma (Table 3.1).
However, these zircons are either detrital (in metasedimentary units) or
inherited (in igneous units). The youngest of the inherited ages is a 580 Ma,
obtained for a zircon from the Oughterard granite. This age postdates the
currently accepted minimum age for deposition of the Dalradian Supergroup,
implying that this zircon was derived from a different, younger (but
unidentified) source.
Late stages of continental
arc magmatism
at Connemara
were
accompanied by orogen-perpendicular shortening along the Mannin Thrust
and the Connemara Antiform. Given the presence of igneous rocks of similar
age in both hanging wall and footwall, as well as the short time span available
for thrust movement, the fault is not likely to represent a fundamental
terrane boundary. More likely, the Delaney Dome Formation formed as a
surface equivalent of the Connemara Complex and the Mannin Thrust is an
intra-arc structure.
Continental arc magmatism at Connemara and associated regional
metamorphism completely postdate Lough Nafooey arc magmatism
(Tremadoc-early Arenig) at South Mayo (e.g., Dewey & Ryan 1990).
Emplacement of Connemara adjacent to the South Mayo terrane probably
occurred after high-temperature
deformation at Connemara because the
metamorphic grade at South Mayo is very low. Since the Mannin Thrust
developed during compressional deformation and arc magmatism, it seems
certain that this structure also predates terrane accretion. Based on our
geochronological results, emplacement of the Connemara terrane adjacent to
South Mayo occurred after c. 462.5 Ma, but prior to deposition of the Late
Ordovician Derryveeny conglomerate.
Acknowledgments
This research was funded by a U.S. National Science Foundation grant
to K.V. Hodges and S.A. Bowring, and a GSA student grant to A. M. Friedrich.
A. M. F. is especially grateful to B.W.D.Yardley, P.D. Ryan, M. Feely, C.B.
Long,
P.W.G. Tanner, and B.E. Leake for introduction to the field area and logistical
support.
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orogen. Geochimica et Cosmochimica Acta, 59, 3205-3220.
Hames, W. E. & Bowring, S. A. 1994. 4 Ar/ 9Ar age gradients in mica
from a high-temperature-low pressure metamorphic terrain: Evidence for
very slow cooling and implications for the interpretation of age spectra.
Geology, 22, 55-58.
Hutton, D. H. W. 1987. Strike-slip terranes and a model for the evolution of
the British and Irish Caledonides. Geological Magazine, 124, 405-425.
Jagger, M. D., Max, M. D., Aftalion, M. & Leake, B. E. 1988. U-Pb zircon ages of
basic rocks and gneisses intruded into the Dalradian rocks of Cashel,
Connemara, western Ireland. Journal of the Geological Society, London, 145,
645-648.
Jenkin, G. R. T., Fallik, A. E. & Leake, B. E. 1992. A stable isotope study of
retrograde alteration in SW Connemara, Ireland. Geological Journal, 110, 269288.
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A precise U-Pb zircon and baddeleyite age for the Longe Range dikes,
southeast Labrador. Geology, 17, 602-605.
Kennan, P. S., Feely, M. & Mohr, P. 1987. The age of the Oughterard Granite,
Connemara, Ireland. Geological Journal, 22, 273-280.
Krogh, T. E. 1973. A low-contamination method for hydrothermal
decomposition of zircon and extraction of U and Pb for isotopic age
determination. Geochimica et Cosmochemica Acta, 37, 485-494.
1982. Improved accuracy of U-Pb zircon ages by creation of more
concordant systems using the air abrasion technique. Geochimica et
Cosmochimica Acta, 46, 637-649.
Leake, B. E. 1986. The geology of SW Connemara, Ireland: A fold and thrust
Dalradian metagabbroic-gneiss complex. Journal of the Geological Society,
London, 143, 221-236.
1988. The age of the Oughterard Granite, Connemara, Ireland:
Comments. Geological Journal,23, 271-272.
1989. The metagabbros, orthogneisses and paragneisses of the
Connemara Complex, western Ireland. Journal of the Geological Society,
London, 146, 575-596.
& Singh, D. 1986. The Delaney Dome Formation, W. Ireland, and
the geochemical distinction between ortho- and para-quartzofeldspathic rocks.
Mineralogical Magazine, 50, 205-215.
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1:63.360 geological map and cross sections. Royal Academy, Dublin.
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southward thrusting of the Dalradian rocks of Connemara, western Ireland.
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,
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Transactions of the Royal Society, London, A, 321, 243-270.
Appendix 3.1
Backscattered Electron Images of Monazite AF45
e.tri.r++
-4..wpp.
.yer.s..
--. ..
a
,
* W
"-rix.---v
--..
no.,.se-A
.* -r.-+w-,cgggy,
Appendix 3.2: Sample Preparation and Analytical Methods
For U-Pb analysis, zircons and monazites were separated by standard
techniques. The zircons were air-abraded for about 40 hours until all crystal
faces were removed (Krogh 1982). Pb and U from zircons and monazites were
isolated by HCl-based anion exchange chromatography modified after Krogh
(1973). Further details of analytical protocols may be found in Bowring et al.
(1993) and Hawkins et al. (1997).
For 4 0Ar/ 39Ar analysis, we separated biotite and muscovite by handpicking large (0.8 to 6.4 mm diameter), subhedral to euhedral single crystals
from uncrushed or coarsely crushed hand samples. Gas extraction was
accomplished in three ways: furnace incremental heating, laser incremental
heating, and laser spot-fusion mapping.
The
first method
involved
progressive heating of a thick (c. 0.4 mm) muscovite crystal (sample AF31) in
a double-vacuum resistance furnace at temperatures maintained to within
about 5*C by a thermocouple for a period of 10 minutes per increment. Laser
incremental heating was performed by training a defocused, Coherent Ar-ion
laser on single crystals (samples AF23 and AF24) for two minutes at
successively higher power levels (as controlled by varying the plasma tube
current). Spot-fusion mapping was done on the (001) cleavage surface of one
large (6.4 mm) muscovite crystal (sample AF31). For this procedure, the laser
beam was focused to a spot size of c. 50 gm, and gas was extracted by firing
five, 200 ms bursts of the laser at an approximate power of 15 W. The result
was a melt pit varying from 70-300 gm across, depending on the varying
responses of different parts of the grain to the laser energy. Additional details
of 4 Ar/ 39Ar geochronological procedures at MIT may be found in Hodges &
Bowring (1995) and Hodges et al. (1994).
Table 3.1. U-Pb analyses of zircon and monazite from southern Connemara
Concentration
Errors 2-a (%)
Sample
Weight
U
Pb
Fraction
(pg)
(ppm)
(ppm)
204 Pb
206 Pb* 208 Pbt 206 Pb
207 PUV
Age (Ma)**
207 PbV
206 Pb 207 Pb 207 Pb
corr.
Pb§
Eb*
206 Pb
coef.
(pg)
Pb§
206 Pb
238 U
% err
235 U
% err
206 Pb
% err
238 U 235 U
Quartz Diorite Gneiss (AF43)
Z1(1)
12.8
353.3
30.0
834.4
0.178
0.07532
(.17)
0.58618
(.20)
0.05644
(.10)
468.1
468.4
469.8
0.873
26.3
13.8
Z2(1)
12.7
344.1
27.2
1422.0
0.129
0.07499
(.18)
0.58295
(.20)
0.05638
(.09)
466.2
466.4
467.2
0.901
14.7
22.7
Z3(1)
8.7
499.6
35.6
2549.9
0.034
0.07500
(.17)
0.58342
(.19)
0.05642
(.09)
466.2
466.7
468.9
0.884
8.2
37.4
Z4(1)
6.4
227.8
24.5
208.4
0.259
0.07504
(.45)
0.58396
(.53)
0.05644
(.26)
466.5
467.0
469.7
0.868
36.1
3.4
Z5(1)
7.3
259.8
23.5
471.4
0.215
0.07490
(.47)
0.58234
(.52)
0.05639
(.21)
465.6
466.0
467.8
0.919
19.8
7.9
Z6(3)
4.2
278.6
25.1
566.4
0.234
0.07577
(.58)
0.59121
(.63)
0.05659
(.23)
470.8
471.6
475.6
0.933
10.2
9.6
Z7(1)
2.6
324.2
27.3
1064.9
0.239
0.07518
(.83)
0.58411
(.85)
0.05635
(.17)
467.3
467.1
466.1
0.980
3.9
18.2
Granite Pegmatite (AF44)
Z1(1)
7.3
355.0
32.7
265.4
0.100
0.07496
(.26)
0.58338
(.50)
0.05645
(.40)
465.9
466.6
470.0
0.599
49.2
3.9
Z2(1)
8.3
426.6
35.4
489.9
0.087
0.07525
(.21)
0.58482
(.29)
0.05636
(.19)
467.7
467.6
466.7
0.752
35.4
7.4
Z5(1)
9.5
187.2
16.5
840.1
0.245
0.07536
(.39)
0.58586
(.42)
0.05638
(.15)
468.4
468.2
467.5
0.937
10.3
14.6
Z9(1)
4.4
329.1
26.9
1538.2
0.195
0.07509
(.48)
0.58640
(.50)
0.05664
(.13)
466.7
468.6
477.5
0.968
4.6
25.6
Z1 2(1)
3.6
614.5
48.2
5151.2
0.164
0.07475
(.10)
0.58095
(.26)
0.05637
(.24)
464.7
465.1
467.0
0.380
2.1
83.7
Zi (1)
276.5
139.2
5412.8
0.344
0.38250
(.14)
9.39740
(.15)
0.17819
(.05)
2087.9 2377.6
2636.1
0.944
6.6
116.4
Z2(1)
207.5
24.8
1306.5
0.078
0.12133
(.43)
1.15049
(.92)
0.06877
(.76)
738.2
891.8
0.563
2.3
19.7
Z3(1)
233.2
47.9
696.9
0.099
0.18975
(.27)
2.07218
(.37)
0.07921
(.24)
1120.0 1139.7
1177.3
0.759
18.6
11.8
Z4(1)
233.1
47.5
269.2
0.114
0.16622
(.37)
1.74327
(.42)
0.07606
(.17)
991.2
1024.7
1096.8
0.915
31.8
4.5
Z5(1)
87.2
9.5
368.1
0.037
0.10237
(.83)
0.93624
(.92)
0.06633
(.32)
628.3
670.9
816.9
0.939
10.7
5.6
Metapelite (AF45)
777.5
Table 3.1. continued
Errors 2-a (%)
Concentration
Sample
Weight
U
Fraction
.(g)
(ppm)
Pb
(ppm)
206 Pb* 208 Pbt 206 Pby
204 Pb
207 Pby
Age (Ma)**
207 PbV
2
b207 Pb 207 Pb
corr.
Pb§
Pb*
Pb§
206 Pb
238 U
% err
235 U
% err
206 Pb
% err
238 U
235 U
206 Pb
coef.
(pg)
Z6(1)
7.1
161.6
44.2
1316.5
0.340
0.21675
(.22)
2.70029
(.24)
0.09036
(.09)
1264.7 1328.5
1433.0
0.933
11.3
27.1
Z7(1)
2.2
669.2
153.6
2344.5
0.097
0.21329
(.18)
4.02098
(.19)
0.13673
(.05)
1246.3 1638.4
2186.2
0.958
8.1
41.1
M1(1)
3.6
7284.5
1574.0
23475.8
2.269
0.07510
(.08)
0.58402
(.09)
0.05640
(.05)
466.8
467.0
468.1
0.833
5.3
1058.9
M2(1)
4.3
9137.5
2290.4
20702.4
2.802
0.07505
(.07)
0.58210
(.08)
0.05625
(.05)
466.5
465.8
462.3
0.829
9.0
1092.0
M4(1)
10.0
312.3
144.5
3867.3
6.093
0.07458
(.13)
0.57664
(.17)
0.05608
(.10)
463.7
462.3
455.5
0.793
3.8
376.8
M12(1)
1.9
1368.8
835.9
8661.4
8.470
0.07393
(.32)
0.57457
(.36)
0.05636
(.16)
459.8
461.0
466.7
0.901
14.1
1132.5
M13(1)
7.0
2272.5
909.0
4892.7
5.392
0.07149
(.20)
0.55488
(.26)
0.05629
(.16)
445.1
448.2
463.8
0.798
14.7
432.3
Metarhyolite (AF94-DD2)
Z4(1)
2.9
115.5
19.9
2263.6
0.119
0.16785
(.14)
1.70760
(.21)
0.07378
(.15)
1000.3 1011.4
1035.5
0.698
1.6
35.8
Z5(1)
0.8
436.8
33.9
754.1
0.154
0.07461
(.30)
0.58163
(.60)
0.05654
(.49)
463.9
465.5
473.6
0.578
2.3
11.9
Z7(1)
0.9
679.9
80.3
3562.6
0.073
0.11843
(.16)
1.35468
(.17)
0.08296
(.07)
721.5
869.6
1268.3
0.923
1.3
54.7
Z8(1)
1.2
354.2
48.4
2934.4
0.032
0.14394
(.10)
1.43300
(.14)
0.07221
(.08)
866.9
902.8
991.8
0.782
1.4
43.0
Z9(1)
0.9
627.1
36.1
2630.0
0.113
0.05719
(.22)
0.44633
(.30)
0.05660
(.19)
358.5
374.7
476.1
0.779
0.8
40.8
Z10(1)
1.0
499.4
23.5
2104.4
0.199
0.04343
(.26)
0.33903
(.32)
0.05661
(.18)
274.1
296.4
476.6
0.835
0.7
35.0
Z11(1)
1.2
468.1
29.2
2397.3
0.159
0.05968
(.15)
0.46561
(.30)
0.05659
(.25)
373.7
388.1
475.4
0.541
0.9
38.6
Z12(1)
1.1
284.0
16.1
979.7
0.156
0.05446
(.32)
0.42488
(.43)
0.05658
(.28)
341.9
359.5
475.2
0.757
1.1
15.6
*Radiogenic Pb. t Measured ratio corrected for spike and fractionation only. Mass fractionation correction of 0.15%/amu ± 0.04%/amu was applied to single collector
Daly analyses and 0.1 2%/amu ± 0.04% for dynamic Faraday-Daly analyses. V Corrected for fractionation, spike, blank, and initial common Pb.
§ total common Pb in
analysis. Total procedural blank for Pb ranged from 0.65-3.7pg and < 1.0pg for U. Blank isotopic composition: "Pb/"Pb = 19.10 ± 0.1, 207Pb/204Pb =15.71 ± 0.1,
"Pb/Pb = 38.65 ± 0.1. All errors are reported as 2-;. Number of zircons in each fraction shown in parentheses after fraction name. Z= Zircon, M= Monazite.
Sample weights are estimated using a video monitor with gridded screen and are known to within 40%. Common Pb corrections were calculated using
the model of Stacy and Kramers (1975) and the interpreted age of the sample. **Age calculations are based on the decay constants of Steiger and JAger (1977).
Table 3.2. Microprobe analyses of biotite and muscovite.
AF24
AF23
AF31*
§
TiO 2
A12 03
Cr 203
FeOT
MnO
MgO
CaO
Na2O
K2 0
NiO
35.05
3.28
18.60
nd
21.65
0.32
7.13
0.02
0.28
8.43
0.02
45.11
0.45
32.27
nd
2.57
0.06
0.76
nd
0.59
9.68
nd
45.30
0.29
32.29
nd
3.27
0.03
0.87
0.00
0.69
9.55
nd
(.57)
(.11)
(.85)
nd
(.14)
(.02)
(.07)
(.00)
(.04)
(.09)
nd
TOTAL
94.79
91.48
92.30
(1.35)
SiO2
Xann
0.5
(Fe + Mg)/ Als
0.124
0.156
Sii, / (Si,, + AI,,)
0.785
0.78
Na / (Na + K)
0.084
0.09
* Average of 10 stations across the 6.4 mm muscovite
§ Standard deviation of 10 analyses
Table 3.3.
40
Increment
Ar 39 Ar incremental heating data for biotite and muscovite from Connemara
"Ar/"Ar
Atomic ratios
("Ar/"Ar).,,.
"Ar/"Ar
( Ar/"Ar)..
"ArK moles
3.47E-04
3.14E-04
1.65E-04
2.01E-05
3.12E-05
2.57E-05
2.05E-05
3.15E-05
1.77E-05
9.27E-05
6.62E-05
5.88E-05
3.70E-03
3.71E-02
3.68E-02
3.89E-02
3.74E-02
3.55E-02
3.62E-02
3.60E-02
3.57E-02
3.65E-02
3.59E-02
3.55E-02
3.85E-02
3.48E-02
4.01E-02
7.69E-02
1.26E-03
9.30E-04
6.41E-04
3.80E-04
1.58E-04
3.12E-04
3.07E-04
1.35E-04
3.38E-04
4.41E-04
6.71E-04
2.18E-04
4.48E-03
3.85E-02
6.17E-16
7.OOE-16
1.43E-15
6.46E-15
9.24E-15
1.08E-14
8.37E-15
8.33E-15
8.29E-15
2.16E-15
1.66E-15
3.60E-15
7.42E-17
1.03E-17
3.97E-04
3.49E-04
2.60E-04
1.34E-04
9.78E-05
8.96E-05
1.08E-04
1.25E-04
1.42E-04
3.98E-04
1.23E-04
4.81E-04
5.14E-02
4.15E-02
3.91E-02
3.87E-02
3.62E-02
3.74E-02
3.58E-02
3.56E-02
3.54E-02
3.72E-02
3.56E-02
3.45E-02
8.51E-04
8.20E-04
8.32E-04
7.01 E-04
4.94E-04
1.75E-04
3.61 E-04
5.65E-04
6.25E-04
5.86E-04
5.52E-04
3.32E-03
2.86E-15
2.55E-15
3.33E-15
6.19E-15
8.26E-15
8.98E-15
7.08E-15
6.11E-15
5.36E-15
2.00E-15
1.68E-15
4.33E-16
Cum.% "Ar
40
Ar*%
Age (Ma)
Age Error (w J)
Age Error (w/o J)
1.0
2.1
4.4
14.9
29.9
47.4
60.9
74.4
87.8
91.4
94.0
99.9
100.0
100.0
Total Gas Age t t:
"Ar Wtd. Mean Age:
83.7
99.6
99.7
99.2
99.0
99.7
99.7
99.4
99.7
99.3
98.5
99.6
99.1
91.2
381.6
424.4
439.3
458.5
449.6
454.4
458.6
448.0
455.6
458.0
423.5
467.7
410.5
209.3
452.6
452.9
44.8
37.9
23.2
12.9
12.4
12.7
12.7
12.4
12.6
17.1
14.9
14.3
406.2
2367.3
3.8
2.0
(43.6)
(36.3)
(20.2)
(5.0)
(4.1)
(4.6)
(4.3)
(4.0)
(4.3)
(12.2)
(10.0)
(7.6)
(406.0)
(2,367.3)
5.2
9.9
15.9
27.2
42.3
58.7
71.6
82.7
92.5
96.2
99.2
100.0
Total Gas Age tt:
"Ar Wtd. Mean Age:
67.8
91.7
96.7
98.7
99.7
99.1
99.0
98.8
99.0
98.0
99.7
99.5
231.0
371.8
411.0
422.9
453.1
437.1
453.6
455.2
458.6
434.9
459.8
471.0
430.4
429.7
37.5
38.3
30.1
16.5
12.8
10.5
13.6
16.4
18.6
46.7
16.1
71.2
30.1
5.6
(37.5)
(38.3)
(30.1)
(16.5)
(12.8)
(10.5)
(13.5)
(16.3)
(18.6)
(46.7)
(16.0)
(71.2)
Sample AF23m @@
Tube current [A]
11.5
12
12.5
13
13.5
14
14.5
15
16
17
18
20
21
22
5.43E-04
2.19E-06
1.09E-06
1.86E-05
2.61E-05
1.15E-07
1.37E-07
1.20E-05
1.34E-07
1.61E-05
4.19E-05
5.85E-06
2.11E-05
2.81E-04
Sample AF24b ##
Tube current [A)
10.5
11
11.5
12
12.5
13
13.5
14
15
16
18
20
1.08E-03
2.72E-04
1.02E-04
3.48E-05
2.08E-08
2.22E-05
2.58E-05
3.26E-05
2.54E-05
5.99E-05
1.29E-06
6.59E-06
Table 3.3. continued
Increment
"Ar/*Ar
Atomic ratios
(3Ar/*Ar).,.
"Ar/Ar
("Ar/ 4 Ar).,.
39
ArK moles
t
Cum.% "Ar
5
4
Ar*%
#
Age (Ma)
Age Error (w J)
Age Error (w/o J)
*
NAF31mIl.f
Temperature [K]
950
1000
1050
1100
1150
1200
1250
1300
1350
1600
1900
2.78E-03
1.85E-03
3.08E-04
1.97E-05
4.35E-06
3.75E-06
2.03E-05
2.24E-05
1.64E-05
1.27E-05
1.08E-06
3.50E-05
3.33E-05
3.08E-05
2.15E-06
1.17E-06
4.79E-06
2.72E-06
1.36E-05
7.52E-06
5.13E-06
4.63E-05
6.14E-03
1.45E-02
3.19E-02
3.54E-02
3.55E-02
3.55E-02
3.52E-02
3.51E-02
3.54E-02
3.54E-02
4.11E-02
5.54E-05
3.81E-05
2.59E-05
1.29E-05
2.67E-05
2.18E-05
2.90E-05
6.73E-05
2.92E-05
3.27E-05
1.30E-04
2.10E-14
4.59E-14
2.14E-13
1.47E-12
1.39E-12
8.33E-13
3.38E-13
2.17E-13
3.88E-13
3.56E-13
5.20E-14
0.4
17.8
472.5
24.8
(24.4)
1.3
45.2
502.1
10.5
(9.6)
5.3
90.7
465.3
5.8
(4.1)
32.9
99.2
459.3
4.0
(.3)
59.0
99.6
459.6
4.0
(.3)
74.6
99.6
459.9
4.1
(.6)
81.0
99.1
460.7
4.0
(.5)
85.1
99.1
462.4
4.4
(1.8)
92.3
99.3
459.4
4.1
(1.0)
99.0
99.4
460.0
4.1
(.7)
100.0
99.7
403.5
6.2
(5.1)
Total Gas Age tt:
459.9
4.3
"Ar Wtd. Mean Age:
459.9
1.3
All uncertainties indicate 2 a errors In Individual measurements: t number of moles of K-derived "Ar ("ArK) released during each heating step. § cumulative percentage
of "ArK after each heating increment. # percentage of radiogenic *Ar ("oAr*) In the total "Ar for each analysis. ** Uncertainties, quoted at 2 a, include propagated error
in the Irradition parameter J. Uncertainties in parantheses indicate the contribution of analytical error to the overall uncertainty. tt Total gas ages calculated by summing
4
Ar* and "ArK for each increment. Assigned uncertainties (2 a) include the propagated errors In J and Isotopic measurements. Age calculations are based on the decay
constants of Stelger and Jager (1977). Typical system blanks are M/e 40Ar and "Ar (in moles) 1.3 x 10 -17and 1.0 x 10 1' for the furnace and 5.6 x 10 -1'and 4 x 10 -18
for the laser. §§ Heating was accomplished with a laser. J value = 0.01035 ± 0.000031. ## Heating was accomplished with a laser. J value = 0.010325 ± 0.000015.
***Heating was accomplished with a resitance furnace. J value = 0.01032 ± 0.000051.
Table 3.4. 4 Ar
Fusion spot
39
"Ar/ 4"Ar
Ar laser mapping data of muscovite from Connemara
Atomic ratios
("Ar/"Ar)error
"Ar/"0Ar
**t
Sample AF31m
("Ar/"Ar)error
"ArK moles
4
Ar*%
9
Age (Ma)
Age Error (w J) Age Error (w/o J)
##
**
(6.7)
7.8
460.8
91.9
1.13E-12
4.02E-04
3.26E-02
3.43E-05
2.68E-04
1
(12.2)
13.1
452.9
90.2
3.84E-15
3.05E-04
3.27E-02
8.87E-05
3.23E-04
2
(6.7)
8.2
454.5
89.4
9.48E-15
3.68E-04
3.22E-02
3.67E-05
3.53E-04
3
(36.5)
36.9
479.7
98.1
1.26E-15
8.99E-04
3.33E-02
2.74E-04
5.50E-05
4
(21.4)
21.9
464.8
82.1
2.99E-15
1.07E-03
2.89E-02
1.03E-04
6.01E-04
5
(10.3)
11.3
450.4
95.4
5
5.43E-1
5.33E-04
3.48E-02
6.72E-05
1.48E-04
6
(18.7)
19.3
460.7
86.4
1.82E-15
1.18E-03
3.07E-02
7.37E-05
4.54E-04
7
(9.2)
10.3
458.4
83.5
3.03E-15
5.OOE-04
2.98E-02
4.31E-05
5.53E-04
8
(19.7)
20.2
445.2
60.9
1.43E-15
6.90E-04
2.25E-02
8.11E-05
1.32E-03
9
(4.4)
6.5
453.8
83.0
1.28E-1 4
3.11 E-04
3.OOE-02
1.11 E-05
5.68E-04
10
(5.2)
7.0
461.9
98.9
8.44E-15
3.96E-04
3.50E-02
1.94E-05
2.79E-05
11
(12.0)
12.9
457.7
97.7
1.73E-15
5.78E-04
3.50E-02
8.20E-05
6.89E-05
12
(10.1)
11.2
469.1
99.3
6.28E-15
8.13E-04
3.46E-02
2.18E-05
1.39E-05
13
(4.2)
6.3
450.6
98.8
6.09E-15
2.70E-04
3.60E-02
2.47E-05
3.08E-05
14
(4.1)
6.2
457.3
99.2
8.39E-15
3.1OE-04
3.56E-02
1.68E-05
1.82E-05
15
(4.7)
6.7
460.0
97.8
1.43E-14
3.88E-04
3.48E-02
1.OOE-05
6.43E-05
16
(5.9)
7.6
462.2
99.1
8.47E-15
4.79E-04
3.51E-02
1.64E-05
2.30E-05
17
(4.9)
6.8
459.4
99.7
9.07E-15
4.01E-04
3.56E-02
1.57E-05
9.81E-08
18
(5.7)
7.4
459.5
99.4
4.57E-15
3.66E-04
3.54E-02
3.19E-05
1.21 E-05
19
(4.4)
6.5
458.8
99.1
8.68E-15
3.44E-04
3.54E-02
1.70E-05
2.30E-05
20
(9.6)
10.7
458.7
98.6
1.97E-15
3.OOE-04
3.52E-02
7.39E-05
3.88E-05
21
(7.2)
8.6
457.3
98.8
5
6.09E-1
5.77E-04
3.54E-02
2.36E-05
3.20E-05
22
(4.2)
6.3
461.2
99.1
1.11E-14
3.37E-04
3.52E-02
1.29E-05
2.34E-05
23
(4.9)
6.8
458.2
94.3
9.31 E-1 5
3.53E-04
3.37E-02
1.86E-05
1.86E-04
24
(5.5)
7.2
458.7
93.8
9.45E-15
4.21E-04
3.35E-02
1.59E-05
2.03E-04
25
(4.1)
6.3
467.6
99.2
7.11E-15
2.63E-04
3.47E-02
2.19E-05
1.78E-05
26
(7.8)
9.1
462.4
98.0
6.18E-15
E-04
6.21
3.47E-02
2.20E-05
5.83E-05
27
* all uncertainties indicate 2 a errors in individual measurements. t number of moles of K-derived "Ar ("ArK) released during each heating step.
§ percentage of radiogenic "Ar ("Ar*) in the total "Ar for each analysis. ## Uncertainties, quoted at 2 a, include propagated error in the
irradition parameter J. Uncertainties in parantheses indicate the contribution of analytical error to the overall uncertainty. Assigned uncertainties
(2 a) include the propagated errors In J and Isotopic measurements. Age calculations are based on the decay constants of Steiger
and JAger (1977). Typical system blanks are M/e "Ar and "Ar (in moles) 1.3 x 10 -"1and 1.0 x 109 for the furnace and 5.6 x 10-16 and
4 x 10 -'8 for the laser, respectively. ** Heating was accomplished with a laser. J value = 0.010314 ± 0.000015 [2 a].
Figure Captions
Figure 3.1. Location map of Connemara in the western Irish Caledonides.
The former Laurentian margin includes basement and rocks of the Dalradian
Supergroup. IS-Iapetus Suture.
Figure 3.2. Simplified geological map of Connemara. This map shows the
sample locations, the Silurian rocks, and the late Ordovician Derryveeny
conglomerate. C-LWG - Cashel-Lough Wheelaun Gabbro; DCD-C - Dawros-
Currywongaun-Doughruagh Complex of northern
Delaney Dome Formation; OG - Oughterard Granite.
Connemara;
DDF -
Figure 3.3 U-Pb concordia diagrams for: (a) a quartz diorite gneiss (sample
AF43), (b) a metapelite (sample AF45), (c) a concordant granite-pegmatite
(sample AF44), and (d) a metarhyolite from the Delaney Dome Formation
(sample AF94-DD2).
Figure 3.4. Argon release spectra for muscovite and biotite constructed using
the data in Table 3. (a) biotite AF 24, (b) muscovite AF23, and (c) muscovite
AF31.
Figure 3.5. 4 Ar/ 3 'Ar age distribution as a function of position within a 6.4
mm muscovite crystal (sample AF31). The error bars are plotted as the 2 a
analytical errors. The x axis is expressed as r/a, where r is the crystal radius (3.2
mm) and a is the distance between the crystal core and centre of the hole
produced by the laser.
Figure 3.6. Timing of magmatism, deformation, metamorphism and cooling
in Connemara. (a) shows the ages of intrusive rocks from this paper and
Friedrich et al. (submitted). (b) shows the timing constraints on deformational
events D2 to D4, including the Mannin Thrust, as well as the deposition of
the Derryveeny conglomerate and the Silurian rocks. (c) Temperature-time
plot for southern Connemara. Peak metamorphic temperatures are taken
from Treloar (1985) and Barber & Yardley (1985). Boxes show the 2a
uncertainties in the age data and approximate uncertainties in temperature.
N
&co
d
IS
4Great
ti
8W
N Mayo
S Mayo
54 N
G
Connemara
Fig. 2
. .......
50 km
Dalradian
Devonian - Carbon.
Silurian Rocks
LLLJSupergroup
Pre-Dalradian rocks
Grant
Granite
/Basement
Ordovician Rocks
Figure 3.1
Major Faults
Carboniferous rocks
Silurian rocks
e
o
., ..
Derryveeny Conglomerate
Galway Batholith
Connemara Gabbro and Gneiss Complex
Oughterard Granite
NX
,
Quartz-diorite gneiss
Mafic intrusions
Delaney Dome
Formation
Fold and Thrust Belt in
Dalradian Supergroup
w
Figure 3.2
Fold axis(F4)
78
0.0765
0.075
D 0.0760
CZ) 0.074
00
Co
Co)
C\ 0.0755
0
0.072
o
0.0750
CD
0.071
0.0745
0.547
207 Pb/
0.074
0.0757
0.567
0.557
23 5 U
207
0.587
0.577
Pb/ 235 U
Delaney Dome FM
(AF94-DD2)
450
Z5
CO0.064
Cf)
\ 0.0753
Co
o
N~
0.054
Co
0
0.0749
I
N~
Upper Intercept Age
474.6 Ma ±5.5 Ma
0.044
0.0745
0.5782
0.5822
207
0.5862
0.5902
pb/ 235 U
Figure 3.3
Z10
0.329 0.379
0.429 0.479
207
0.529 0.579
Pb/ 235 U
80
(A)
600
AF-24 biotite
510
2420
C 330
Plateau Age
450 i5 Ma
240
20
40
80
60
100
39Ar %
(B)
600
AF-23 muscovite
510
2 420
a)
330
Plateau Age
454 ± 2 Ma
240
20
40
60
80
1U
39Ar %
(C)
600
AF-31 muscovite
510
420
4:
c-
330
Plateau Age
460.1 ±1.5 Ma
240
20
40
60
39Ar %
Figure 3.4
80
100
82
500
ucvt
- F3
AF-31 muscovite
radius = 3.2 cm
490
480 -
-*
470 -
460450-
440 -
430.
0
core
0.1
0.2
0.3
0.4
Figure 3.5
0.6
0.7
0.8
0.9
1
rim
84
Time [Ma]
I , , ,
460
470
480
I ,
I , , , I I
I
I,
450
,
, ,
, ,
II,,
440
I, , , I
, , ,
il
Oughterard Granite (AF97-01 01)
A
Granite Pegmatite (AF-44)
Quartz Diorite (AF-43)
Cashel-Lough Wheelaun
Gabbro(AF-47)
Currywongaun Basic Pegmatite
(AF-1 24)
Delaney Dome
Fm (AF94-DD2)
B
+D2+
7 --
D3 -- ?
Mannin
Thrust
D4
Time [Ma]
Figure 3.6
85
Derryveeny
-Conglomerate ?
Deposition
.of Silurian
Sediments
-
..-.
..
,....
4.-..:-.r.v..w.-..-s
:..-...--....
..
A..
..
.
s....
g.
.
-
--.-.
:-.....
s
-A
senva
>,wi,-a-.--.id.i+1--1ri---rn--11h
-.i--T-47
..-.--.
-s-as
CHAPTER 4: CONSTRAINTS PROVIDED BY U-PB TITANITE
GEOCHRONOLOGY
ON FLUID INFILTRATION
IN THE
CALEDONIDES OF CONNEMARA, WESTERN IRELAND
Anke M. Friedrich, Samuel A. Bowring, and Kip V. Hodges
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts
Institute of Technology, Cambridge, MA 02139. E-mail: amfriedr@mit.edu.
Manuscript in preparation.
4.1. Abstract
Continental arc magmatism in the Dalradian rocks of the Connemara
Caledonides is closely associated in time and space with regional-scale
amphibolite-facies metamorphism. This relationship permits two ways of
determining the timing of peak metamorphism. U-Pb analyses of mafic
plutonic rocks in a previous study implied that the associated sillimanitegrade peak metamorphism occurred at c. 470 ± 1 Ma. Metasomatic diopside
rocks of the upper sillimanite zone that formed by fluid infiltration were
thought to also constrain the timing of peak metamorphism. In this study we
analyzed single titanite crystals of these diopside rocks along with other
calcsilicate rocks from lower and higher metamorphic grades by the U-Pb
TIMS method to determine the timing of metamorphism. The results
indicate that titanite crystallization in calcsilicate rocks from all metamorphic
grades occurred at c. 462 ± 1 Ma. This age postdates the presumed
metamorphic peak by up to 8 million years, but overlaps with the age of a
post-metamorphic granite pluton, which, combined with petrographic
evidence indicates that fluid infiltration occurred late during or after peak
metamorphism.
The source of fluids infiltrating the Dalradian
metasedimentary rocks therefore may be related to the emplacement of felsic
late-magmatic phases rather than from within Dalradian schists. Late- to
post-metamorphic fluid infiltration affected all metamorphic grades, a
volume much larger than the thermal aureole associated with intrusion of
the felsic magmas. U-Pb dating of calcsilicate rocks in the regional-scale
contact-aureole may not be a reliable indicator of the timing of peak
metamorphism, but instead is a precise indicator of the timing of fluid
infiltration.
4.2. Introduction
The metamorphic history of the Caledonides of Connemara, western
Ireland, can be divided into two periods: one characterized by greenschistamphibolite facies conditions at relatively high pressures, and another
younger period distinguished by lower pressures and substantially higher
temperatures. These have been referred to as M2 and M3, respectively, by
Yardley (1976) and Tanner & Shackleton (1979). The regional extent of M2 is
not well understood, largely because M3 was so intense throughout central
and southern Connemara that it obliterated all but a few vestiges of M2
assemblages. On the other hand, the pressure-temperature conditions of M3
have been studied in detail (e.g. Yardley 1976, Yardley et al. 1980, Barber &
Yardley 1985, Treloar 1985, Fergusen & Al-Ameen 1986, Yardley et al. 1987).
Peak M3 temperatures, as recorded by metamorphic mineral assemblages,
increase progressively from central Connemara southward toward
voluminous outcrops of intrusive rocks of the Connemara igneous complex,
the remnants of a continental magmatic arc. Consequently, most geologists
working in the area regard M3 as something like a regionally developed
contact metamorphic aureole developed around the arc. This relationship
suggests two possible approaches to evaluating the age and duration of M3:
dating various intrusive phases of the Connemara complex, or directly dating
minerals that crystallized during the M3 period.
A combination of the two approaches was used by Cliff et al. (1996) to
infer that M3 was a protracted event. Citing limited U-Pb geochronologic data
suggesting that early intrusive phases of the Connemara arc may be as old as
c. 490 Ma (Jagger et al. 1988), as well as U-Pb data for metamorphic minerals
with high closure temperatures (titanite and monazite) implying
crystallization ages between 478 and 463 Ma, Cliff and co-workers concluded
that M3 may have lasted for nearly 30 million years. However, more
systematic, higher-precision U-Pb geochronology of the intrusive complex
demonstrates that the entire arc developed between 474 and 463 Ma (Friedrich
et al. in review, Chapter 2). It is now clear that the Jagger et al. (1988) 490 Ma
date reflects an incorrect interpretation of highly discordant data, but an
inconsistency remains between the new geochronologic data for the
Connemara complex and some of the data previously published by Cliff et al.
(1996): do dates older than 470 Ma imply crystallization during M2, such that
they provide age constraints on that earlier event, or did M3 begin before
magmatic activity in the Connemara arc? Here we address this problem
through U-Pb
dating of titanite from
rocks spanning the range
of
metamorphic grades encountered at Connemara, including some samples in
which M2 assemblages predominate and some in which M3 assemblages are
more prevalent.
4.3. Geologic Setting
The Connemara terrain consists of Dalradian metasedimentary rocks
that correlate with other sedimentary deposits along the former Laurentian
margin of the northern Appalachians and the British Caledonides (Figure
4.1).
For several thousand kilometers along strike, these rocks record
orogenic events related to closure of the Iapetus ocean, including island-arc
continent collision, ophiolite obduction, and rare continental arc magmatism
(e.g. van Staal et al. submitted).
Late-orogenic strike-slip resulted in
fragmentation of this collisional orogen (the Taconian/Grampian orogen),
especially in the British and Irish Caledonides.
As a consequence, the
Connemara terrane lies to the south of the rest of the former Laurentian
margin and a Cambro-Ordovician island arc (South Mayo) and a forearc
terrane (Figure 4.1). The actual terrane
boundary is mostly concealed by
lower Silurian rocks that lie unconformably on both Connemara and South
Mayo rocks, but the terranes can easily be distinguished because the island arc
terrane experienced metamorphism only up to the lower greenschist facies,
whereas the Connemara terrane reached a high metamorphic grade. Rocks of
the Dalradian Supergroup at Connemara share an early metamorphic history
(M2) with other Dalradian exposures to the north of a main Caledonian
suture in this region - the Fair Head-Clew Bay line - but they are the only
Dalradian rocks in Ireland to have been intruded by a continental magmatic
arc and to have experienced a high-temperature (M3) metamorphic event
(Figure 4.1, e.g. Yardley et al. 1987). The Connemara arc complex includes
large volumes of mafic to felsic intrusive rocks that dominate exposures in
the southern part of the terrane and scattered mafic/ultramafic intrusions in
northern Connemara (e.g. Leake 1986, 1989, Leake and Tanner 1994). These
bodies intruded during regional ductile deformation that affected both, the
magmatic arc and the metasedimentary country rocks. Deformation in the
country rocks included development of a schistosity that is now preserved
only as inclusion trails in garnets (Si), formation
recumbent
fold
(F2) and associated penetrative
of a regional-scale
schistosity (S2),
and
development of open to tight folds (F3) at a variety of scales and a penetrative
schistosity at higher metamorphic grades (S3; e.g. Tanner & Shackleton 1979).
The youngest arc-related deformation is represented by a broad range-scale,
upright antiform (Connemara antiform, F4), and a shear zone whose final
and brittle expression is the Mannin thrust, along which all of Connemara
was transported southward over greenschist-facies metarhyolite rocks (Leake
et al. 1983, Tanner et al. 1989).
The outcrop pattern of the Connemara Dalradian rocks is controlled by
the shallowly east-plunging Connemara antiform (Figure 4.1; Leake & Tanner
1994). The Silurian unconformity strikes approximately parallel to the
youngest Dalradian units folded around this antiform,
implying that
eastward tilting occurred after Silurian deposition. Therefore, a geologic map
of Connemara is also a cross-section through the pre-unconformity structural
architecture of Connemara, with a structural level decreasing from west to
east.
4.4. Metamorphism and Previous Geochronology
The metamorphic grade increases southward across Connemara over a
distance of >30 km. It ranges from staurolite to upper sillimanite grade,
including a partial melting zone near the intrusive complex (Figure 4.2). The
location of the E-W striking sillimanite-in isograd is inferred to mark the
transition between the area only weakly affected by M3 to the north and the
area strongly affected by M3 to the south (Tanner & Shackleton 1979). The
diagnostic (M2) pelitic subassemblages in the garnet/staurolite zone of
northern Connemara are staurolite ± garnet + muscovite ± Al-silicate ±
chlorite, coexisting with quartz + plagioclase + biotite (Boyle & Dawes 1991).
Samples saturated in A 2SiO,5 contain either kyanite or andalusite. Previous
researchers have identified only limited M3 effects in these rocks, including
the development of "retrograde" muscovite + chlorite subassemblages in
metapelitic rocks and serpentinization of calc-silicate rocks (e.g. Cruse & Leake
1968, Boyle & Dawes 1991).
Farther south, the M3 sillimanite-in isograd marks the first appearance
of prismatic or fibrolitic sillimanite in the presence of staurolite ± garnet +
quartz + plagioclase + biotite + ± muscovite. (Yardley 1976). Staurolite ceases
to be part of the stable assemblage south of the staurolite-out isograd. The
typical pelitic mineral assemblage in the sillimanite-muscovite zone is garnet
+ sillimanite + muscovite + biotite + plagioclase + quartz + plagioclase.
Toward the south of this zone,
sillimanite
muscovite is replaced by
and an upper sillimanite
corresponds
to
trondhjemitic
zone
of
widespread
K-feldspar +
zone can be defined. It broadly
development
of
granitic
and
leucosomes, presumably anatectic melts, that define the
"migmatite zone" of Barber & Yardley (1985). The typical pelitic mineral
assemblage in this zone is sillimanite + biotite + cordierite + K-feldspar +
plagioclase + quartz + silicate melt.
Metamorphic minerals in calcsilicate rocks at Connemara are
not powerful
indicators of metamorphic grade. Most contain calcite +
dolomite+ quartz + phlogopite
tremolite) or diopside.
+ titanite ± clinoamphibole
(typically
In the sillimanite-muscovite zone, a metasomatic
rock consisting predominantly of diopside occurs at a particular horizon of
the Connemara marble formation (Yardley et al. 1991a, b, Tanner 1991). The
formation of this diopside rock and other calcsilicate rocks is an important
topic of this paper and will be discussed in more detail in below.
Peak metamorphic conditions in northern Connemara reached c. 550
50 *C and between 6 to 8 kb during the M2 metamorphism (Boyle & Dawes
1991). Thermodynamic modeling of zoned garnets indicates that prograde
garnet growth occurred from ~2 to > 8 kb (Boyle & Dawes 1991). In southern
Connemara, peak M3 temperatures reached ~ 750"C at
-
5.5 kb during
formation of migmatitic restite, and declined to < 3 kb during crystallization
of the leucosome
(Barber & Yardley 1985, Treloar
1985).
Peak M3
temperatures near the sillimanite-in isograd were estimated at c. 550 to 600 0C
(Yardley et al. 1987). Temperatures during peak-metamorphic metasomatism
in the sillimanite-muscovite zone were estimated by Yardley et al. (1991) to
have been c. 620-650 *C.
Cliff et al. (1993, 1996) attempted to constrain the age of M3 sillimanitegrade metamorphism through U-Pb geochronology of samples from central
and eastern Connemara. They obtained U-Pb isotopic data for titanites
separated from three samples of the diopside rock within the Connemara
Marble Formation, collected over a distance of 12 km along strike in central
Connemara. One concordant fraction and the upper intercept of a discordia
array defined by three fractions led the authors to infer a c. 478 Ma age for M3
in this area.
However,
other samples, from
eastern and northern
Connemara, yielded significantly younger U-Pb dates. One concordant
titanite analysis for separates from a calcsilicate rock collected in the east
indicated an age of 466 ± 3 Ma (2y), and a presumably synmetamorphic
cordierite-bearing pegmatite yielded U-Pb monazite dates of c. 463 Ma. One
discordant titanite fraction separated from a calcsilicate rock of the staurolite
zone of northern Connemara yielded a
20 7Pb/
6Pb
20
date of
469 ± 15 Ma.
Although this sample was collected from an area where M3 effects were
generally regarded as minimal, U-Pb data for it were combined with the other
results to infer a complex spatio-temporal evolution of M3 at Connemara,
such that the timing of peak conditions varied by roughly 10-15 million years
from west to east (Cliff et al. 1996). In this model, the most likely explanation
for such diachroneity would be large transients in the thermal structure of the
region during M3 as a consequence of spatially variable magmatic activity in
the Connemara arc. However, recently obtained U-Pb dates for zircons from
the syn-D3 Cashel-Lough Wheelaun gabbro, the structurally oldest intrusion
of the southern Connemara complex, show that magmatic activity did not
begin until c. 470 Ma (Friedrich et al. in review, Chapter 2). This implies that
the 478 Ma titanite date of Cliff and coworkers represents M2 mineral growth,
or that M3 began before development of the Connemara arc. We pursued this
issue further by analyzing titanites from across the Connemara terrane with
the U-Pb method.
4.5. Methods
Samples were prepared following standard procedures, including
preliminary separation of titanite-rich aggregates using magnetic and heavy
liquid techniques. Euhedral titanite crystals, optically clear and inclusion-free,
were hand-picked from each aggregate. Each was photographed, measured,
and washed in acetone and diluted HNO 3 before dissolution (e.g. Figure 4.3).
Each crystal was dissolved in a teflon capsules containing 50% HF and a
mixed spike composed of
20
Pb- 233U-235U. Lead and Uranium were separated
using HCl and HBr -based ion chromatography modified after Krogh (1973).
All samples were analyzed on a VG Sector 54 mass spectrometer at the
Massachusetts Institute of Technology. A Stacey & Kramers (1975) model was
used to correct for common Pb in each sample. Most analyzed titanites were
highly radiogenic (2 16 Pb/ 204Pb >500), such that the calculated age is essentially
insensitive to the common Pb correction. Further details of the analytical
protocol may be found in Hawkins & Bowring (1997) & Bowring et al. (1993).
4.6. Interpretation Strategy For U-Pb Titanite Dates
The U-Pb dates obtained in this study could reflect the timing of
titanite crystallization or, alternatively, the timing of cooling of a sample
through the U-Pb closure temperature
interpretations
metamorphic
we choose
for titanite. Which
depends on the relationships
conditions, the nominal
of these
among peak
closure temperature for titanite
crystals of a given size, and the textural characteristics of titanite in any given
sample. We assume that Pb loss from titanite is governed by grain-scale
volume diffusion and use the Pb diffusion data of Cherniak (1993) to calculate
closure temperatures as a function of grain size and cooling rate (Dodson,
1973). The brevity of magmatic activity at Connemara suggests that post-M3
cooling was probably rapid, perhaps on the order of 50-100*C/Ma. The grain
sizes for crystals analyzed in this study range from 100 to 500 Rm; for a cooling
rate of 100*C/Ma, this implies closure temperatures between about 650 and
720 *C. This range is higher than estimated peak M2 and M3 metamorphic
temperatures in the staurolite and sillimanite-muscovite zones at
Connemara, but is less than the estimated peak M3 temperatures in the
migmatite zone (Yardley et al. 1987, Barber & Yardley 1985; Boyle & Dawes,
1991). In cases when the estimated closure temperature of a particular titanite
is greater than the estimated peak metamorphic temperature at the sampling
locality, we interpret the U-Pb titanite date as reflecting crystallization.
Otherwise, we infer that the date reflects cooling through the closure
temperature subsequent to crystallization.
In previous geochronologic studies of titanite from central Connemara,
it was assumed that the crystallization age of titanite from any locality was a
robust estimator of the age of peak M3 metamorphism at that locality (e.g.
Cliff et al.
1996). This would be true only if the titanite grew at peak
temperatures as part of the metamorphic assemblage. In most samples we
studied, the titanite crystals were euhedral and concentrated along grain
boundaries; while this is consistent with their being part of the metamorphic
assemblage, it is also consistent with crystallization after the metamorphic
peak or during metasomatic events unrelated to regional M2 or M3
metamorphism. Thus, we refer in this paper to "titanite crystallization
events" which may or may not be of the same age as "peak metamorphic
events". In the discussion section, we attempt to distinguish between these
possibilities through
comparison
of the
titanite
results
with
other
geochronologic constraints.
4.7. Results
The results of U-Pb titanite analyses from each metamorphic zone are
shown in Figure 4.4 and Table 4.1. The samples are described from north to
south, from low to high metamorphic grade.
4.7.1.
Staurolite Zone
Our attempts to determine
the age of titanite crystallization in
staurolite zone rocks were limited to the analysis of one impure marble
sample (AF40) collected from the Lakes Marble Formation (Figure 4.2,
National Grid Reference L 662 583). This rock consists mainly of calcite with
subordinate quartz and phlogopite. A weak foliation is defined well where
phlogopite and its alteration products are present. Small crystals of rutile and
muscovite occur along some grain boundaries. Quartz, pyrite and apatite are
abundant as inclusions
in calcite. Titanite occurs as euhedral crystals,
principally along calcite grain boundaries (Figure 4.5a), but not within calcite,
suggesting either: 1) crystallization as part of the prograde metamorphic
assemblage; or 2) late crystallization from M3 or post-M3 metasomatic fluids.
One titanite crystal (S3) yielded U-Pb data that plots concordantly with a date
of 462 ± 2 Ma (Table 4.1; Figure 4.4), whereas another analysis plots
concordantly with a date of 469 ±5 Ma. We interpret these date as indicative
of the crystallization ages of some of the titanites in this rock. Two other
crystals (S1 and S4) are discordant, with
2
zPb/
20Pb
dates of 499.8 ± 4.6 and
499.7 15.0 Ma, respectively Ma, suggesting that the sample also contains
titanite or other inclusions of Cambrian or Proterozoic age (Figure 4.4).
Analysis of abraded titanite crystals from AF40 is in progress, with the goal of
better defining age (or ages) of the inherited component.
4.7.2. Staurolite-Sillimanite Transition Zone
Sample AF37 represents a calcsilicate layer within the Lakes Marble
Formation, collected between the "sillimanite-in"
and "staurolite-out"
isograds in central Connemara (Figure 4.2, National Grid Reference L 842 540).
At this grade, metamorphic assemblages include minerals that grew during
both M2 and M3, though distinguishing between the two metamorphic
groups is not easy in the metacarbonate rocks of Connemara. The AF37
sample mainly consists of calcite with subordinate quartz, pyrite, phlogopite,
and titanite. Phlogopite is partly altered, resulting in formation of retrograde
clinochlor.
The predominant
S2 foliation
in this
rock is defined
crystallographically by calcite crystals. A second, weaker foliation (S3?) occurs
at a low angle to the first, defined by the alignment of phlogopite, clinochlor,
pyrite and titanite crystals (Figure 4.5b). Late veins cut across the calcite
crystals, but are poorly defined around phlogopite, clinochlor, pyrite and
titanite crystals, indicating a possible relationship between the veins the
phlogopite alteration. Titanite crystals are abundant at grain boundaries
between phlogopite, pyrite and clinochlor but has not been found within
calcite crystals (Figure 4.5c), suggesting that it may have grown during the M3
overprint or during a post-M3 metasomatic event.
Three titanite crystals
were analyzed from this sample. One analysis (S1) yielded a reversely
discordant ellipse on the concordia diagram, with 206Pb/ 238U, 207 Pb/ 235U, and
207
pb / 206 Pb dates of 461.8 ± 17.4 Ma, 458.8 ± 17.6 Ma, and 442. 12.7 Ma,
respectively (Figure 4.4, Table 4.1). Although there are several possible
explanations for reverse discordancy (e.g. Mattinson 1973; Hawkins &
Bowring 1997), the most likely in this instance is a slight error in the common
Pb correction. Since the least sensitive U-Pb isotopic subsystem to such
corrections is
20
Pb/ 2 38U, we regard 461 ± 17 Ma as our best estimate of the
crystallization age of this titanite. Two other analyses are less discordant with
206Pb/ 238U, 207 Pb/235U,
and 207 Pb/
20 6
Pb dates of 462.0 ± 7.6 Ma, 464.9 ± 8.1 Ma, 480
±12 Ma and 468 ±19 Ma, 469 ± 21 Ma, 474 ±31 Ma, for titanite analyses S3 and
S6, respectively (Figure 4.4, Table 4.1). These preliminary data that indicate
titanite crystallization in this rock may have occurred at c. 469 and 462 Ma, but
uncertainties of these analyses are large.
4.7.3. Sillimanite-Muscovite Zone
While examining calcsilicate samples from the sillimanite-muscovite
zone for possible U-Pb geochronology, we were able to distinguish two
groups of titanite-bearing rocks with different metamorphic/metasomatic
histories.
The first group includes samples containing pristine or only
slightly altered
M3
metamorphic
assemblages.
The
second
includes
retrogressed samples that show abundant evidence for metasomatic alteration
subsequent to M3. We analyzed titanites from both kinds of rocks in order to
evaluate the effects of retrogression on U-Pb systematics.
4.7.3.1.Non- to Moderately- Retrogressed Samples. In order to compare our
results directly to those of Cliff et al. (1993, 1996), we concentrated our efforts
in the sillimanite-muscovite zone on samples of the metasomatic diopside
rocks described in detail by Yardley et al. (1991a) and Cliff et al. (1993).
According to their interpretation, this rock type formed by infiltration of SiO 2 rich fluids that were derived from the overlying metapelite rocks during peak
M3 metamorphism. It occurs in a well-defined tectonostratigraphic position
between the Barnanoraun Formation metapelites and underlying calcsilicate
rocks of the Connemara Marble Formation. Sample AF6855 was collected by
Bruce Yardley at Creggs Quarry (Figure 4.2, National Grid Reference L 716 534)
and was part of the same rock analyzed by Cliff et al. (1996) as sample number
6855. Two other samples (AF33 and 7521), were collected from the same
horizon as exposed at Glencoghan, approximately 12 km east of Creggs Quarry
(Figure 4.2, L 812 500 and 808 498, respectively). Some of the diopside rocks are
altered to varying degrees, visible in thin section by a dense fracture network
and alteration of diopside. Especially in sample AF7521, titanite occurs at
diopside grain boundaries, near fractures and within alteration products
(Figure 4.5 e-g).
We analyzed four single titanite crystals from AF6855, ranging in grain
size from 530 to 250 gm. All are slightly discordant but have similar 20 6Pb/ 2 38 U
and 2 07 Pb/ 23 5U dates of c. 462 Ma (Figure 4.4, Table 4.1). There is no simple
correlation between U-Pb age and grain size, which is consistent with
crystallization at temperatures lower that the U-Pb closure temperatures for
titanite at any of the grain sizes represented by the AF6855 crystals. We regard
the weighted mean 20"Pb/2 8U date of 461.9 ± 1.7 Ma (MSWD = 11.2) for the
three most concordant fractions as the best estimate of the crystallization age
of these titanites.
We analyzed three titanite crystals from AF33. Two are concordant
with relatively low analytical uncertainties and 20 Pb/ 2 38U dates of 461.9 ± 0.6
Ma and 461.4 ± 0.6 Ma, respectively. The others are slightly discordant but
with sufficiently high analytical uncertainty that their error ellipses overlap
concordia. The weighted mean 2 06Pb/1 8U date of 462.1 ± 1.0 Ma (MSWD =
3.07) for all four fractions is taken to be the crystallization age of titanite in
AF33.
Sample 7521 was collected less than 1 kilometer northwest of the AF33
locality. Of the titanites separated from it, we selected four for U-Pb analysis.
Their grain sizes ranged from 150 to 350 gm. One analysis is reversely
discordant with
206
Pb/ 23 8U and
207
Pb/23U dates of 463.6 ± 1.0 and 462.0 ± 2.5 Ma.
Two analysis are normally discordant with
2 07
Pb/
Pb dates of 463 ±19 and
206
462.6 ± 4.5 Ma (Figure 4.4, Table 4.1). One of the analyses is concordant with a
206
Pb/ 238U date of 462.0 ± 1.0 Ma. We interpret this date as the best estimate of
the titanite crystallization in this sample.
The
-
16 million year inconsistency between our results for diopside
rock samples and the results published by Cliff et al. (1993, 1996) for titanites
from the same unit bear further scrutiny. We explored the possibility that an
inappropriate choice of common Pb correction could result in an error in our
estimated ages. However, our AF6855 titanites, which are from the same
sample as some of the Cliff et al. (1993, 1996) titanite fractions, were highly
radiogenic (2 6Pb/ 2 'Pb >500), such that their plotting coordinates on the
concordia diagram are relatively insensitive to the magnitude of the common
Pb correction (Figure 6). Applying significantly
different common
Pb
corrections to the Cliff et al. (1993, 1996) analyses would be no more effective
at reconciling the data, so another explanation is required.
In the original Cliff et al. (1993) study, titanite was separated from three
diopside rock samples from west-central Connemara, including 6855. Four
multigrain fractions were analyzed. All were discordant, two reversely and
two normally; of these, two fractions from the 6855 sample, both reversely
discordant, were oldest. The authors fitted a chord through all four analyses,
obtaining an upper intercept date of c. 478 Ma. Cliff et al. (1996) reported a
correction of the spike composition for the 6855 analyses, which had the effect
of shifting the ellipses for the two reversely
discordant fractions to
concordancy at 478 Ma. However, Cliff (personal communication, July, 1998)
still allows for the possibility of further analytical problems with the 6855
data. If these data are disregarded, interpretation of the other two fractions
analyzed by Cliff et al. (1993) is not straightforward. One fraction is slightly
discordant with a c. 461 Ma
2 06
Pb/ 23 sU date, similar to those we obtained for
titanites from our samples of the diopside rock. The other fraction dated by
Cliff et al. (1993) was more discordant, potentially implying a mixture of c.
461-463 Ma titanite with an older inherited component (perhaps similar to
that identified in the staurolite zone sample AF40, described above).
4.7.3.2. Retrogressed Sample.
Sample AF71 is from a calcsilicate horizon
within the Lakes Marble Formation at Maumeen (Figure 4.2, National Grid
Reference L 914 508). Primary metamorphic minerals in this rock include
chainsilicates altered beyond recognition, quartz, K-feldspar, and titanite, all
of which are extensively altered. Small (1 mm) fractures pervade the sample
and have been filled with a mineral aggregate of uncertain composition. In
thin section, these fractures are particularly well developed in euhedral
titanite crystals. One titanite crystal yielded a concordant analysis with
206
Pb
238
U,
Pb/
207
235
U, and
207 Pb/ 206Pb
dates of 461.4 ± 9.8, 461.8 ± 9.2, and 464
± 11 Ma. Another titanite crystal from this sample yielded a concordant U-Pb
date of c. 403.7 ± 8.1 Ma (20 6Pb/ 238 U; Figure 4.4, Table 4.1). Although this
analysis has a large uncertainty, it is clearly much younger than other titanites
analyzed from the sillimanite-muscovite zone. The approximate age is
similar to that of the large Galway batholith, exposed ~10 km to the south
(Figure 4.2). One viable interpretation of the AF71 data is that highly
channelized metasomatic fluids associated with Galway batholith intrusion
caused extensive Pb loss from the AF71 titanite at c. 403 Ma. Preliminary UPb analyses of titanite in this sample indicates that titanites may have
crystallized at c. 462 and at 403 Ma.
4.7.4 Migmatite Zone
We studied one sample from the migmatite zone, a calcsilicate rock
(AF26) collected within the Lakes Marble Formation (Figure 4.2, National
Grid Reference L 814 465).
It contains calcite + quartz and subordinate
tremolite and titanite. Three individual titanite crystals (130-200 pm) yielded
concordant and mutually consistent U-Pb results (Figure 4.4, Table 4.1): the
weighted mean
Pb/ 2 3 'U and
20 6
207
Pb/
U dates are 462.8 ± 0.5 (MSWD = 0.49)
235
and 462.8 ± 0.7 Ma (MSWD = 0.09), respectively. Given petrologic constraints
on the peak M3 temperatures in the part of Connemara where AF26 was
100
collected, we might expect these titanite dates to reflect cooling through range
of U-Pb closure temperatures appropriate for 130-200 gm grain sizes (660680*C at a cooling rate of 100*C/Ma). However, the 462.8 Ma age for AF26
titanites is only slightly younger than our best available U-Pb constraint on
the timing of M3 peak metamorphism in the migmatite zone: a 468.4 ± 1.5 Ma
upper intercept date for monazites from a metapelitic sample collected about
8 km SSW of the AF26 locality (Friedrich et al. in press, Chapter 2). This
similarity suggests one of several possible thermal histories: 1) rapid cooling
of the migmatite zone after M3 metamorphism at c. 468 Ma; 2) a somewhat
earlier metamorphic peak at the AF26 locality, followed by cooling at a more
moderate rate through the titanite closure temperature interval at 463 Ma; or
3) extremely rapid cooling of the migmatite zone after 468 Ma metamorphism
followed by post-M3 crystallization of the AF26 titanites below their closure
temperature at c. 463 Ma. The latter interpretation is consistent with the rapid
cooling rate (c. 50*C/Ma) inferred previously (Friedrich et al. in press; Chapter
3).
4.8 Discussion
The most significant result of our U-Pb research on Connemara
titanites is that all samples, with the exception of the highly altered AF71
rock, yield dates of 461-463 Ma. This is true regardless of sampling locality,
metamorphic grade, relative intensity of M2 or M3 metamorphic effects, or
grain size. In central Connemara, the dominant mineral assemblages in
metamorphic
rocks
crystallized
during
M3
and
peak
metamorphic
temperatures were lower than the nominal U-Pb closure temperature for
titanite. Following the interpretive protocols of Cliff et al. (1996), we might
conclude from our titanite data that the age of M3 is 461-463 Ma. Given direct
evidence for a 468.4 Ma age for M3 metamorphism in southern Connemara
(Friedrich et al. in review, Chapter 2), this line of reasoning would also
101
suggest a north to south increase in the age of M3 peak metamorphism.
However, this interpretation would require that the titanites from southern
Connemara yield cooling ages that are, fortuitously, precisely the same as
crystallization ages for titanites from central and northern Connemara. The
northern Connemara data pose an additional problem; why did titanites of
M3 age crystallize in rocks that otherwise contain no petrographic evidence of
an M3 overprint on M2 prograde assemblages?
A more likely scenario is that none of the analyzed titanites from
Connemara crystallized at peak M3 conditions but instead crystallized late in
the metamorphic history of the region. Several lines of evidence favor this
interpretation. First, the Oughterard granite, which is the youngest phase of
the Connemara arc complex and widely is regarded as younger than the M3
event based on field relationships (cf. Tanner et al. 1997) has a crystallization
age of 462.5 Ma (Friedrich et al. in review; Chapter 2). Second, a syn-D3 quartz
diorite intrusion in southern Connemara yielded a U-Pb zircon
crystallization age of 466.5 ± 0.6 Ma (Friedrich et al. in press; Chapter 3),
similar to the 468.4 ± Ma M3 monazite age from a nearby metapelitic gneiss
(Friedrich et al. in press; Chapter 3). Third, between the sillimanite-in and
staurolite-out isograds, micas and amphiboles that crystallized as part of the
M3 assemblages yielded 470-475 Ma 4"Ar/WAr cooling dates (Chapter 5). This
provides an absolute minimum constraint on the age of M3 in central
Connemara. Since the 40Ar/ 39Ar closure temperatures for the dated minerals
are substantially lower than U-Pb closure temperatures of the titanites
analyzed for this paper (McDougall & Harrison 1988), it seems inescapable
that the 461-463 Ma titanites from central Connemara crystallized roughly
eight million years after the peak of M3 metamorphism.
The uniformity of nearly all concordant and near-concordant titanite
dates from Connemara suggests that this mineral crystallized throughout the
region during a metasomatic event. The occurrence of 461-463 Ma titanites in
far-northern and far-southern Connemara suggests that fluid infiltration
occurred rapidly, within no more that two or three million years, over
102
distances of >30 kilometers across regional strike. This scenario is similar to
that proposed by Yardley et al. (1991a, b), Cliff et al. (1993), and Cliff et al.
(1996), but differs in one important aspect: we regard the metasomatic event
as substantially younger than peak M3 metamorphism. This argues against
the suggestion of Yardley et al. (1991a) that the source of the metasomatic
fluids was prograde metamorphism of overlying metapelitic units (see also
the arguments of Tanner (1991)). A more likely source may be the magmatic
fluids evolved from the Connemara intrusive complex to the south during
the waning stages of that magmatic event. The c. 403 Ma titanite dates from
AF71 may indicate a similar metasomatic event related to intrusion of the
Galway batholith.
Finally, our results suggest caution in assuming that minerals
crystallizing during fluid infiltration events provide accurate age constraints
for metamorphic events (cf. Cliff et al. 1993). Pulses of high-temperature fluid
flow may occur episodically throughout the thermal evolution of regional
metamorphic terranes, leaving behind diachronous metasomatic mineral
assemblages that would yield a variety of apparent ages. In some cases - and
apparently the Grampian orogeny at Connemara was one - the most
significant fluid infiltration event may not coincide with the metamorphic
peak. The interpretation of metasomatic mineral dates as peak metamorphic
ages requires independent evidence that metasomatism and metamorphism
were coeval. In any case, our data and those of others such as Cliff et al. (1993,
1996) show that U-Pb geochronology of metasomatic minerals is a powerful
way to explore the ages and regional significance of fluid infiltration events.
Acknowledgments
This study was supported by a NSF grant awarded to Kip Hodges and Samuel Bowring.
AMF especially thanks Bruce Yardley and Bob Cliff for providing some critical samples.
103
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104
Krogh, T. E. 1973. A low-contamination method for hydrothermal
decomposition of zircon and extraction of U and Pb for isotopic age
determination. Geochimica et Cosmochemica Acta, 37, 485-494.
Leake, B. E. 1986. The geology of SW Connemara, Ireland: A fold and thrust
Dalradian metagabbroic-gneiss complex. Journal of the Geological Society,
London, 143, 221-236.
1989. The metagabbros, orthogneisses and paragneisses of the
Connemara Complex, western Ireland. Journal of the Geological Society,
London, 146, 575-596.
& Tanner, P. W. G. 1994. The geology of the Dalradian and
associated rocks of Connemara, Western Ireland: a report to accompany the
1:63.360 geological map and cross sections. Royal Academy, Dublin.
Singh, D. & Halliday, A. N. 1983. Major southward
thrusting of the Dalradian rocks of Connemara, western Ireland. Nature, 305,
210-213.
McDougall, I., & Harrison, T.M., 1988, Geochronology and Thermochronology
by the 4 Ar/ 39Ar Method: New York, Oxford University Press, 212 p.
Mattinson, J.M., 1973, Anomalous isotopic compositon of lead in young
zircons: Carnegie Institution of Washington Yearbook, 72, p. 613-616.
Stacey, J. S. & Kramers, J. D. 1975. Approximation of terrestrial lead isotope
evolution by a two stage model. Earth and Planetary Science Letters, 26, 207221.
Steiger, R. H. & Jhger, E. 1977. Subcommission on geochronology: convention
on the use of decay constants in geo- and cosmochronology. Earth and
Planetary Science Letters,, 60, 359-362.
Tanner, P. W. G. 1991. Metamorphic fluid flow. Discussion of the Yardley et
al. , 1991a. Nature, 352, 483-484.
& Shackleton, R. M. 1979. Structure and stratigraphy of the
Dalradian rocks of the Bennabeola area, Connemara, Eire. In: Harris, A. L.,
Holland, c. H., & Leake, B. E. (eds) The Caledonides of the British Isles Reviewed. Geological Society, London, Special Publications, 8, 243-256.
Dempster, P. J. & Dickin, A. P. 1989. Time of docking of the
Connemara terrane with the Delany Dome Formation, western Ireland.
Journal of the Geological Society, London, 146, 389-392.
& Rogers, G. 1997. New constraints upon the structural
and isotopic age of the Oughterard Granite, and on the timing of events in the
105
Dalradian rocks of Connemara, western Ireland. Geological Journal, 32, 247263.
Treloar, P. J. 1985. Metamorphic conditions in central Connemara, Ireland.
Journal of the Geological Society, London, 142, 77-86.
Yardley, B. W. D. 1976. Deformation and metamorphism of Dalradian rocks
and the evolution of the Connemara cordillera. Journal of the Geological
Society, London, 132, 521-542.
Barber, J. P. & Gray, J. R. 1987. The metamorphism of the Dalradian
rocks of western Ireland and its relation to tectonic setting. Philosophical
transactions of the Royal Society, London, A, 321, 243-270.
Bottrell, S. H., & Cliff, R. A. 1991a. Evidence for a regional-scale
fluid loss event during mid-crustal metamorphism. Nature (London), 349,
151-154.
& Cliff, R. A. 1991b. Metamorphic fluid flow. Reply to
discussion by Tanner, P. W. G. 1991. Nature, 352, 484.
Leake, B. E. & Farrow, C. M. 1980. The metamorphism of Fe-rich
pelites from Connemara, Ireland. Journal of Petrology, 21, 365-399.
106
F- --
"
TABLE 4.1. U-Pb RESULTS OF SPHENE FROM CONNEMARA
Sample
Weight
U
Pb
Fractions
(ug)
(ppm)
(ppm)
Age (Ma)**
Errors 2c (%)
Concentration
206 Pb* 208 Pbt 206 Pb4
204 Pb 206 Pb 238 U % err
207 Pb6
207 b
corr.
Pb#
PX
238 U
235 U 206 Pb coef
(pg)
Pb#
206 Pb 207 Pb 207 Pb
235 U % err 206 Pb % err
Calcite marble [AF-40]
S1
4.0
680.4
56.4
572.5
0.146
0.07331
0.57836
0.05722
456.1
463.4
499.8
0.706
s3
20
223.9
19.9
265.7
0.161
0.07436
0.57741
0.05632
462.3
462.8
465.0
0.767
s4
4.0
121.3
11.5
206.1
0.132
0.07479
0.58994
0.05721
464.9
470.8
499.7
0.704
s6
3.8
1412
11.5
315.2
0.125
0.07164
0.55691
0.05638
446.0
449.5
467.5
0.766
s9
3.8
76.2
10.3
84.6
0117
0.07560
0.58815
0.05643
469.8
469.7
469.1
0.783
Calcstlicate rock [AF-37)
460.8
457.8
442.9
0.963
(.52) 462.0
464.9
479.4
0.852
0.267
0.07535 (2.05) 0.58747 (2.52) 0.05655 (1.37) 468.3
469.3
474.0
0.839
0.06463 (1.00) 0.48807 (1.71) 0.05477
S1
0.7
255.9
24.2
172.4
0250 0.07410 (1.89) 0.56971 (1.99) 0.05576 (.54)
s3
3.0
138.3
12.3
298.0
0.204 0.07429 (.82) 0.58068
s6
1.8
11.9
63.8
73.0
(.99) 0.05669
Calcsilicate rock [AF-71]
s6
64
49.7
61
73.7
0.042
403.7
403.6
403.0
0.646
s7
7.3
45.2
8.2
66.5
0.039 0.08616 (.69) 0.65478 (1.19) 0.05512
532.8
511.4 416.9
0.637
S8
12.0
19.3
1.9
121.7
0.031 0.07418 (1.45) 0.55479 (219) 0.05424
461.3
448.1
s10
120
27.4
28
130.7
0.059
0.07420 (.97) 0.57589 (1.11) 0.05629
461.4
461.8 463.7
0.894
459.0
460.1
465.6
0.947
1351
461.9 462.3
464.6
0.871
85.2
381.2 0.711
Diopside rock [AF-68551
s1
1.0
430.8
36.2
1489.3
0.218
0.07380 (.20) 0.57321 (.21) 0.05633 (07)
s4
40
650.9
51.3
1438.3
0.129
0.07427 (.12) 0.57665
s5
174.0
359.1
30.3
2339.3
0.243
0.07417 (.10) 0.57566 (12) 0.05629 (.07)
461.3 461.7 4637
s6
18.4
421.9
33.3
1350.8
0129
0.07440 (.08) 0.57753 (.12) 0.05630 (.08)
462.6
462.9
(14) 0.05631 (.07)
0.813
12.6
464.2
0.695
27.2
Diopside rock [AF-331
S3
55.6
28.8
2.7
436.5
0.276 0.07437 (.30) 0.57794
(.47) 0.05636
(.34) 462.5
463.1
466.5
0.690
S4
111.0
128.4
14.8
526.3
0.610 0.07428 (.08) 0.57652
(21) 0.05629
(.19) 461.9 462.2
4638
0.478
S5
98.0
310.1
26.1
450.7
0.095 0.07420 (.09) 0.57522
(.13) 0.05623
(.09) 461.4 461.4
461.4 0.712
S8
2.5
679.3
72.4
430.0
0.461 0.07445 (.24)
0 57944
(.43) 0.05645
(.35) 462 9
464.1
470.1
0.589
17.9
129.2
326.0
19.3
Diopside rock [AF-7521]
S1
10.0
379.2
575.3
0.166 0.07429 (.15) 0.57620
(.43) 0.05625
462.0
4620
462.3
0.487
S5
4.4
292.0
1514
0.121
0.07345 (.34) 0.56993
(.97) 0.05628
456.9
458.0
463.3
0.483
s6
3.9
1382.3
437.6
0.081
0.07456 (.19) 0.57624
(.54) 0.05605
463.6
462.0
454.4
0.449
s8
27.8
413.1
1068.2 0.197 0.07316 (.10) 0.56752 (.23) 0.05626
455.2
456.4
462.6
0.499
Calcstlicate rock [AF-26]
S1
10.0
130.6
667.0
0.234
0.07445 (.28) 0.57747
0.05625 (.19) 462.9
462.8
462.4
0.833
S3
8.0
347.8
1132.5
0.239
0.07445 (.14) 0.57755
0.05627 (.11) 462.9
462.9
462.9
0.801
462.5
463.7 0.762
S4
1.7
897.7
1362.8 0.160 0.07434 (.25) 0.57697
0.05629 (.22) 462.3
* Radiogenic Pb
t Measured ratio corrected for spike and fractionation only. Massfractionation correction of0.15%/amu t 0.04%/amu was appfied to single collector Daly analyses and
0.12%/amu ± 0.04% for dynamic Faraday-Daly analyses
§Corrected for fractionation, spike, blank, and initial common Pb
# total common Pb in analysis. Total procedural blank for Pb ranged from 0.65-3.7 pgand < 1.0pqfor U. Blank isotopic composition- PbP-Pb = 19.10 ± 0.1,
"'Pb/-Pb =15.71± 0.1, "PbP"Pb = 38.65 t 0.1.
**Age calculations are based on the decay constants of Steiger and Jaqer (1977)
107
Figure Captions
Figure 4.1. Simplified tectonic map of the British and Irish Caledonides. This
Figure shows the unusual tectonic position of the Connemara Dalradian
rocks relative to the rest of the Dalradian exposures.
Figure 4.2. Simplified geologic map of Connemara with sample localities and
metamorphic isograds. OG- Oughterard Granite (462.5 Ma; Chapter 2). The
metamorphic grade of the Dalradian rocks is indicated by metapelitic
assemblages. Staurolite and sillimanite coexist between the sillimanite-in
isograd (Sill-in) and the staurolite-out isograd (Staur-out). The migmatite
zone is characterized by anatectic melting of metapelitic rocks with formation
of leucosome (melt) and paleosome (restite), and in general by a close spatial
relationship between intrusive rocks of the Connemara complex and the
Dalradian metasedimentary rocks.
Figure 4.3. Single titanite crystals of sample AF37 and AF40 dissolved for UPb analysis.
Figure 4.4. U-Pb concordia diagram for titanite analyses from Connemara
calcsilicate rocks.
Figure 4.5. Photo micrographs showing the selected titanite crystals within
calcsilicate rocks. (A) Titanite at calcite grain boundaries of sample AF40. (B)
Primary foliation (S2?) is defined by calcite crystals, whereas
phlogopite+clinochlor+pyrite+ titanite appear to form a second foliation (S3?)
at c. 30 degree angle. (C) Titanite in sample AF37 occurs associated with
phlogopite+clinochlor+pyrite. (D) Sample AF71 is very altered. Titanite
occurs within alteration products and within more pristine calcsilicate
minerals. (E-G) Titanite in the diopside rock AF7521 occurs at diopside grain
boundaries and within unidentified alteration products. Alteration is
inhomogeneous, from little alteration (E) to intense fracturing and
retrogression (G). (H) Titanite in sample AF26 occurs within and at calcite
grain boundaries. This rocks does not show any signs of retrogression.
Figure 4.6. Schematic U-Pb concordia diagram of the titanite analysis with
the lowest radiogenic 206Pb/"Pb ratio for various initial Pb compositions (at
4.5, 1, 0.8, 0.55, 0.5, 0.46, .4 Ma) to show the insensitivity of common Lead
correction on the sample age.
108
N
<Great Brita
8W
50 km
Devonian - Carbon.
Dalradian
Silurian Rocks
Supergroup
GraniteGraniteBasement
Pre-Dalradian rocks/
Ordovician Rocks
Major Faults
Figure 4.1
109
110
-
1 100 15'
South Ma)
AF45
A
-- a
+ + +
~.....
.
++
+;
.
G l . a . .ah
lt
.. . .
/
O
.
Carboniferous rocks
-
Connemara Complex
rocks
Silurian
Sia
rFold
Oughterard Granite
Derryveeny
Conglomerate
Quartz-diorite gneiss
intrusions
Galway Batholith
Figure 4.2
.
Delaney Dome
Formation
and Thrust Belt in
Dalradian Supergroup
Fold axis(F4)
.
- - -.
metamorphic
Isograds
sample
SAF71-AF
localities
112
300 gm
Figure 4.3
113
114
460
0.0746
0.0736
450
00726
s6
0.0716
0.0706
0.5424
00,5
0.5624
0.5524
Pbg 235 U
AF-3 7 Calcsisilicate
0.0765.
c
1
0.5724
207
470
0.0755
CM
46
0.0745
0.0735
450 si
s3
0.0725
0.0715
0.5500
0.5600
0.5700
0.5800
207 Pb 235 U
*0" AF-71 calcsilicate
(11
00744
00724
440
CO
Cl)
0.0704
420
0-O
0*06
400
00644
0.0624
04683
0.4883
05083
0.5283
207
0.5483
Pb/ 235 1
Figure 4.4
115
0.5683
0.5883
116
Diopside Rocks
0.0749
0.0747
0.0745
0.0743
o
CO)
'--
0.0741
0
0 0739
0.0737
00735
00733
..
05669
05689
0.5709
05729
0.5749
0.5769
0.5789
05809
207 Pbf235 U
0.0746
0.0744
o
0.0742
0.0740
00738
0.5
207 Pb 235 U
0.0750
0.0748
Co
CO,
0.0746
0.0744
CO
0
00738
0.5715
0.5735
0.5755
0.5775
207 Pb/ 35 U
Figure 4.4 continued
117
0.5795
0.5815
0.5835
118
D
0.0746
Sl
462
0.0744
S4
0.0742
460
0.0740
ca. 463 Ma
.
0.0738.
0.5715
0.5735
0.5755
. .
.
. .
. ..
0.5795
0.5775
207 Pb/ 235 U
Figure 4.4 continued
119
. .
0.5815
.
. .
.
0.5835
.
120
.
. .
.,.--
_--,
.,
-..
-
ts.-.....-
3.
w-+-s'er-.-145.<-<
Be
Figure 4.5
121
-.
122
Effect of the Common Lead correction (S&K'75) on the Age of the Sample (with the lowest
206Pb/204Pb ratio of all dated sphenes
0.0773
AF7521 S5
0.0768
480
measured
206
0.0763
Pb/ 204 Pb =
151.4
470
0.0758
0
0.0753
grey shade e p
0.0748
original ellipse
(see data table)
0.0743
2
1.5
0.0738
0.0733
0.0728
Estimated Age of common
0.55
0.46 0.5
450
0.05 0.05
0.1
Lead for correction [in Ga]
correction used in age calculation [Ga]
0.0723
0.5577
0.5627
0.5677
0.5727
0.5777
207
0.5827
pb/ 235U
Figure 4.6
0.5877
0.5927
0.5977
0.6027
124
CHAPTER 5: CONSTRAINTS ON THE DEFORMATION AND
COOLING HISTORY OF THE CALEDONIDES OF CONNEMARA,
WESTERN IRELAND, FROM "AR/3 9 AR THERMOCHRONOLOGY
Anke M. Friedrich and Kip V. Hodges
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts
Institute of Technology, Cambrige, MA 02139. E-mail: amfriedr@mit.edu.
Manuscript in preparation.
5.1. Abstract
The thermal evolution of a regional-scale (>25 km) amphibolite-facies
contact aureole of a syn-deformational continental magmatic arc has been
determined using detailed 44Ar/ 39Ar thermochronology of micas. An
important aspect of this analysis is the distinction of cooling ages from those
4 0Ar/ 3 9Ar dates not related to cooling after high-temperature metamorphism
by considering evidence from the geologic record. The results confirm a
previously observed large range in K-Ar and Rb-Sr dates (>500 to <400 Ma),
but based on the timing of magmatism, peak metamorphism, and the age of
the Silurian unconformity in the Connemara region, only those dates
between c. 475 and 443 Ma are permissible cooling ages. Variations in
40Ar/ 39Ar muscovite and biotite cooling ages at a single locality are only small
(<10 Ma) implying relatively rapid cooling (>6-26*C/Ma) following highA distinct cooling age
temperature metamorphism and deformation;
variation (>15 Ma) occurs on the regional-scale between northern (c. 475 to
465 Ma) and southern (c. 460-450 Ma) Connemara, consistent with spatial and
temporal differences in the metamorphic and magmatic evolution across the
Connemara region. We infer that this cooling record was related to a lateral
thermal gradient in an evolving continental magmatic arc, rather than to
differential unroofing of the orogen. Older (>475 Ma) 4 0Ar/ 3 9Ar dates are
attributed to excess 40Ar contamination, whereas younger (< 443 Ma)
44Ar/ 39Ar dates are related to thermal resetting or fluid infiltration related to
emplacement of a major post-orogenic granitic batholith (c. 420 to < 400 Ma).
The results of this study imply that the large (>100 Ma) spread in
thermochronometers commonly observed in orogenic belts does not
automatically translate into a protracted cooling history, but that only a small
ages.
are permissible
cooling
number
of
thermochronometers
125
5.2. Introduction
Studies of the thermal histories of ancient metamorphic terrains using
thermochronologic techniques frequently yield large ranges of apparent ages,
sometimes spanning hundreds of millions of years (Onstott & Peacock 1987;
Mezger et al. 1991; Hodges & Bowring 1995). Such ranges are often interpreted
in terms of a protracted cooling history, assuming reasonable closure
temperatures for mineral-isotopic systems (e.g. Cliff 1985; Heaman & Parrish
1991; Hodges 1991). However, recent studies of the mechanisms by which
radiogenic isotopes are lost from mineral systems (e.g. Lee 1995) suggests that
assigning an exact closure temperature to any particular mineral sample is
not entirely straightforward. One could imagine, for example, ranges of dates
provided by a single mineral-isotopic system that reflected local differences in
deformational history or mineral chemistry rather than protracted cooling.
One fruitful approach to assessing the importance of this problem is to
compare thermochronologic data for minerals of known composition with
independent constraints on the thermal and deformational history of the
metamorphic terrain from which they were collected.
In this paper, we present thermochronologic data for a syn-tectonic,
amphibolite-facies metamorphic belt, the Caledonian Connemara terrane, for
which a protracted cooling history spanning at least 75 million years has been
inferred from earlier Rb-Sr and K-Ar results (e.g. Elias et al. 1988).
Connemara
provides
a
special
opportunity
to
compare
the
thermochronologic record as recorded in metamorphic rocks with very tight
constraints on the thermal and deformational history of the area provided by
the U-Pb geochronology of synmetamorphic intrusive igneous rocks of the
Connemara complex (Chapters 2 and 3) We employed the "Ar/ 39Ar method
on muscovite, biotite, and phlogopite from metapelitic and calcsilicate rocks
sampled at different metamorphic grades, ranging from staurolite to upper
126
sillimanite grade. The results were then evaluated against independent
constraints. We found a substantial spread in
40Ar/ 39 Ar
ages, largely
consistent with the results of previous studies, but the distribution of ages
seems less a function of regional slow cooling than a function
of
polymetamorphism, complex variations in the thermal history of the region
during metamorphism, the variable incorporation of excess
40Ar
in mineral
systems, and low-temperature resetting of the 40Ar/ 39 Ar systematics.
5.3. Geologic Setting
The Connemara region of the Irish Caledonides (Figure 5.1) exhibits
the thermal
and
deformational
characteristics
of
a continental
arc
environment (Leake & Tanner 1994). The arc itself is represented by mafic to
felsic plutons of the Connemara complex (Leake 1896, 1989), one of several
tholeiitic to calc-alkaline magmatic provinces that formed during the middle
Ordovician Grampian-Taconian orogeny (c. 474-463 Ma) along the eastern
Laurentian margin (e.g. Friedrich et al. in review, van Staal et al. submitted,
Chapter 2). At Connemara, the country rocks for the Connemara complex are
metasedimentary units of the Dalradian Supergroup (Harris 1994). The
outcrop pattern of abundant marker horizons in this stratigraphy delineate a
complicated deformational history (Leake & Tanner 1994; Tanner &
Shackleton 1979). The overall structure is dominated by the shallowly eastplunging, range-scale Connemara antiform, which exposes a spectacular 3-D
cross section through the Dalradian fold and thrust belt and the Connemara
complex (Figures 5.1 and 5.2; e.g. Leake & Tanner 1994). The deepest
structure, in the subvertical southern limb of the Connemara antiform, is the
Mannin thrust (Figure 5.1); it carries the Dalradian metasedimentary package
and the Connemara complex in its hanging wall, and juxtaposes these units
with greenschist-facies metarhyolites
in the footwall (Figure 5.1). The
shallowest structures are normal faults (e.g. the Renvyle Bofin Slide) that
127
occur in the shallowly northward-dipping northern limb of the Connemara
antiform (Figure 5.1). The topology of the structural package is such that
deeper structural levels are exposed to the west and shallower structural
levels to the east and to the north.
At Connemara, the Dalradian units experienced
two phases of
metamorphism: an early, low- to high-pressure amphibolite-facies event
(M2) and a late, high-temperature, medium- to low-pressure event (M3;
Figure 5.3; e.g. Yardley et al. 1987). The M2 event was similar to that recorded
by metamorphic assemblages in age-equivalent rocks elsewhere in the Irish
and British Caledonides (e.g. Yardley et al. 1980; Leake & Tanner 1994; Yardley
et al. 1987; Cliff et al. 1996). However, the M3 event is unique to Connemara
and the distribution of M3 isograds suggests that this event was related to
intrusion of the Connemara complex (e.g. Yardley et al. 1987). Despite the
structural complexity of the region, these isograds strike generally east-west,
transecting all major structures, suggesting a simple thermal evolution
during the late stages of M3. However, the relative age relationships between
deformational structures, intrusive phases of the Connemara complex, and
M2 and M3 metamorphic assemblages provide evidence for a more
complicated thermal history overall.
5.3.1. Deformation History
Detailed descriptions of the structural geology of Connemara have been
presented by Tanner & Shackleton (1979), Tanner (1990), Leake & Tanner
(1994), Wellings (1998). The oldest preserved fabric (Si) occurs only as
inclusion trails in garnet porphyroblasts with unknown tectonic significance
(Figures 5.3 and 5.4.a;
e.g. Leake & Tanner 1994). Two main regional
deformation phases (D2 and D3) can be defined in outcrops of the Dalradian
sequence (Figure 5.3). The earliest macroscopic folds in Connemara (F2) are
East-West-trending open to isoclinal. They are associated with an axial planar
schistosity (S2), defined by muscovite and biotite, which is mainly preserved
in northern Connemara (Figure 5.4b). There the S2 schistosity was deformed
128
by minor folds during a D3 event, which was accompanied by retrograde
muscovite + chlorite growth (Figure 5.4c, e.g. Boyle & Dawes 1991; Cruse &
Leake 1968). Farther south, the S2 schistosity was deformed by small F3 folds
(Figure 5.4d) and large-scale F3-fold and thrust nappes, which plunge ENE
and ESE up to 35*. Especially in pelitic lithologies, S2 was overprinted by a
penetrative S3 schistosity defined by biotite intergrown with fibrolite or
prismatic sillimanite (MS3; Figure 5.4e). In southern Connemara, where M3
temperatures were high enough to cause partial melting in the Dalradian
sequence, evidence for syn-anatectic deformation demonstrates a clear
temporal relationship between D3 and M3.
In this region, F3 folds were
disrupted by syndeformational anatectic melting, indicating that anatexis
followed the main D3 deformation (Figure 5.4.). The Mannin thrust, which
places the Connemara complex and its country rocks over metarhyolites of
the Delaney Dome formation, is regarded as a late D3 structure (e.g. Tanner e t
al. 1989).
Fabrics and structures of D3 age were deformed on a regional scale by
the F4 Connemara antiform, described above. The fact that M3 metamorphic
isograds cut across D4 structural trends requires that the M3 metamorphic
peak stretched over the D3-D4 deformational interval. The youngest
structures in the region may be extensional shear zones exposed in northern
Connemara. One, described by Williams & Rice (1989), occurs directly below
the Silurian unconformity (Figure 5.1); the nature of its hanging wall is
largely concealed by post-deformational Silurian strata. The second - referred
to as the Renvyle Bofin slide - corresponds to the northern outcrop boundary
of the Dalradian Supergroup at Connemara and has a demonstrated top-tothe-north normal shear sense (Figure 5.1; Bolye & Dawes 1991; Wellings
1998).
129
5.3.2. Intrusive History
of
Intrusions
the
Connemara
complex
either
were
broadly
oldest plutons,
synchronous with or postdate crustal shortening. The
consisting of ultramafic and gabbroic rocks, occur in both northern (DawrosCurrywongaun-Doughruagh complex, Figure 5.1) and southern Connemara
(e.g. Cashel-Lough Wheelaun intrusion, Roundstone complex, e.g. Leake &
Tanner 1994). A younger intrusive suite, which consists mainly of quartz
diorite and granite, intruded older mafic plutons only
Connemara.
The
gabbros
of
northern
Connemara
in southern
were
emplaced
syntectonically into the same tectonostratigraphic level (the Kylemore Fm) as
the southern Connemara gabbros (Leake & Tanner 1994; Chapter 2). Wellings
(1998)
recently
suggested that
Dawros-Currywongaun-Doughruagh
the
complex intruded during D2 deformation. If he is correct, a recent U-Pb
zircon age for a gabbro from the complex constrains the age of D2 in northern
Connemara as 474 Ma (Chapter 2). The Cashel-Lough Wheelaun intrusion of
southern Connemara intruded just prior to or early during D3 deformation
(Tanner 1990) at 470 Ma (U-Pb zircon age; Chapter 2). A minimum age for
both D3 and D4 deformation is provided by the undeformed Oughterard
granite (Leake, 1986; Tanner et al.
1997), which has a U-Pb xenotime
crystallization age of 463 Ma (Chapter 2).
5.3.3. Metamorphic History
From north to south across Connemara, Grampian metamorphic
conditions
increases
progressively
from
upper
greenschist
to
upper
amphibolite facies (Figures 5.1 and 5.3). Four major zones, trending east-west,
have been identified based on the distribution of prograde pelitic mineral
assemblages (Yardley, 1976; Barber & Yardley 1985; Boyle & Dawes 1991):
* Garnet-staurolite.
Distinguished
only in the Dalradian units
of
northernmost Connemara, this zone is characterized by staurolite ± garnet
130
+ muscovite ± kyanite or andalusite + quartz + plagioclase + biotite
chlorite.
* Staurolite-sillimanite transition.
South of a well-defined sillimanite-in
isograd, the metapelitic assemblage is characterized by prismatic or
fibrolitic sillimanite + staurolite ± garnet + muscovite
+ quartz +
plagioclase + biotite.
" Sillimanite-muscovite. An isograd corresponding to the last appearance of
staurolite marks the beginning of a zone defined by sillimanite + garnet +
muscovite + quartz + plagioclase + biotite.
e
Sillimanite-K-feldspar. In southern Connemara,
near
the principal
outcrop region of quartz-diorites of the Connemara igneous complex,
muscovite is no longer stable and the diagnostic metapelitic assemblage
becomes sillimanite + biotite + cordierite + K-feldspar + quartz +
plagioclase. Many metasedimentary outcrops are migmatitic in the
southernmost parts of this zone, suggesting that temperatures were high
enough to promote widespread anatexis. Andalusite has been identified in
some migmatized rocks.
Despite the simplicity of the metamorphic pattern at Connemara,
textural relationships suggest that the observed assemblages grew during two
prograde events. Minerals diagnostic of the garnet-staurolite assemblage help
define the S2 schistosity in northern Connemara, whereas sillimanitemuscovite and sillimanite-K-feldspar assemblages help define the S3
schistosity in central and southern Connemara (Yardley 1976; Barber &
Yardley 1985, Yardley et al. 1987, Boyle & Dawes 1991; Fergusen & Al-Ameen
1986). This has led previous workers to define an M2 event on the basis of the
northern Connemara assemblages and an M3 event on the basis of central
and southern Connemara assemblages. Estimated peak M2 conditions in
northern Connemara were 550 ± 50*C and 6-8 kb (Figure 5.3; Boyle & Dawes
1991). Peak M3 temperatures were at least 650'C in the middle of the
sillimanite-K-feldspar zone, increasing to -750*C in southernmost Dalradian
131
outcrops (Barber & Yardley 1985). Near the main outcrop area of the quartz
diorites, M3 pressures during anatexis reached ~5.5 kb, but may have declined
to <3 kb during crystallization of the leucosome.
Indirect constraints on the age of M2 and M3 are provided by the U-Pb
ages of synmetamorphic intrusive rocks. Based on the work of Wellings
(1998), S2 and the M2 assemblages that define it are at least partly coeval with
intrusion of the Currywongaun gabbro at 474 Ma (Chapter 2). The M3 event
overlapped in time with emplacement of the bulk of the Connemara complex
(Yardley et al. 1987), which occurred during the 470-463 Ma interval (Chapters
2 and 3).
While
textural
evidence
suggests that amphibolite
facies
metamorphism should be attributed to two separate events at Connemara,
the two were separated in time by no more than a few million years and the
prograde M2-M3 transition is probably best regarded simply as a temperature
increase during progressive unroofing of the region.
5.3.4.
Post-Grampian Sedimentation, Igneous Activity, and Late-
Stage Fluid Flow
The high-grade metasedimentary rocks of eastern Connemara are
overlain unconformably by unmetamorphosed shallow marine sedimentary
rocks of lower Silurian age (Figure 5.1; e.g. Graham et al. 1989). The age of this
unconformity (c. 443 Ma; Tucker & McKerrow 1995) marks the end of the
Grampian orogeny in the geologic record at Connemara. By Upper Silurian
to Devonian time, the Connemara metamorphic complex was buried again
under c. 3-4 km of marine sedimentary rocks (Graham et al. 1989). At this
time, a major post-Grampian granitic intrusion -
the Galway batholith
-
intruded older plutons of the Connemara igneous complex (e.g. Leake 1978;
Leake & Ahmed Said 1993).
The thermal pulse associated with batholith
intrusion led to the development of a regionally extensive hydrothermal
132
system (Jenkin et al. 1992) which may be responsible for local retrogression of
M2-M3 metamorphic assemblages throughout Connemara (cf. Chapter 4).
5.4. Previous Thermochronology
Previous geochronological investigations at Connemara
yielded a
range of K-Ar and Rb-Sr mineral dates between 490 and 415 Ma, which was
interpreted as representing a prolonged period of unroofing (e.g. Giletti et al.
1961, Dewey et al. 1970; Elias et al. 1988) or local resetting related to
hydrothermal alteration (e.g. Leggo et al. 1966; Miller et al. 1991). A discordant
U-Pb zircon date of c. 490 Ma for the Cashel-Lough Wheelaun gabbro (Jagger
et al. 1988), intruded just before or during the early stages of D3, has been
used to suggest that M3 metamorphism began at about that time (Cliff et al.
1996).
This interpretation gained support from slightly younger K-Ar
homblende dates from some outcrops (Elias et al. 1988). Mica K-Ar and Rb-Sr
cooling ages clustered between 460 and 450 Ma in southern Connemara,
leading previous workers to suggest two important episodes of cooling in the
region - one between 490 and 480 Ma, and the other between 460 and 450 Ma,
separated by an interval of relatively slow cooling (e.g. Elias et al. 1988;
Dempster 1985). However, not all early thermochronologic data fit into this
simple model. One K-Ar hornblende date of 420 Ma was reported from
northern Connemara, and several southern Connemara samples yielded
mica ages in the 420-410 Ma age range (e.g. Elias et al. 1988). Such anomalies
led to the development of complex models for regional cooling involving
differential uplift along high-angle faults and localized reheating events
related to hydrothermal activity (Elias et al. 1988).
An alternative model was proposed by Miller et al. (1991). These
authors determined K-Ar dates for 21 hornblendes and several micas from
across Connemara, obtaining a large range of dates similar to that reported by
Elias et al. (1988).
In one 30 m-wide quarry in southern Connemara,
133
hornblende dates for amphibolites containing abundant quartz + epidote
veins ranged over 70 million years. Miller et al. (1991) found a weak positive
correlation between K-Ar hornblende dates and 8D values for the same
amphiboles, suggesting that the range of homblende dates represented
variable amounts of Ar loss related to hydrothermal activity. In this scenario,
K-Ar dates provide little constraint on the thermal evolution of the
Connemara region.
New U-Pb geochronologic constraints on intrusive rocks in the
Connemara region help constrain the range of viable interpretations of
existing K-Ar and Rb-Sr thermochronologic data. For example, a more
precise redetermination of the age of the Cashel-Lough Wheelaun gabbro and
a new determination of the age of the Currywogaun gabbro of southern
Connemara demonstrate that peak M2-M3 metamorphism is no older than
475-470 Ma (Chapters 2 and 3), such that older published K-Ar hornblende
dates must represent excess 4 Ar. Moreover, given the very brief duration of
Grampian orogenesis at Connemara (Chapter 2), we would expect that all
amphibole and mica cooling dates would cluster within a few million years,
between perhaps 460 and 450 Ma (e.g. Friedrich et al. in press; Chapter 3). In
this paper, we report new
4
Ar/ 39Ar geochronologic data obtained in an
attempt to better understand the extent and cause of the large range of K-Ar
ages obtained by previous workers.
5.5. Methods
We collected muscovite, biotite and phlogopite from metapelitic and
calcsilicate rocks from small subareas within each metamorphic zone (Figure
5.5). We began with the assumption that the effective closure of a mineralisotopic system is a volume diffusion process, governed by effective diffusion
dimension and chemical composition (McDougall & Harrison 1988).
Although recent studies suggest that fast diffusion pathways can complicate
134
the geometry and rate of diffusion in minerals (Lee 1995), the effective
diffusion dimension of mica crystals appears to be similar to the physical
grain size (Hames & Bowring 1994). In the hope of recovering a larger portion
of the cooling history at each locality, we concentrated on analyzing micas
with a range of composition and grain size.
Muscovites, biotites, and
phlogopites were separated from crushed or uncrushed rock and purified
following standard procedures, details of which can be found in Appendix 1
and Hodges et al. (1994).
We performed both high-precision incremental heating analyses of
large
samples
(several
milligrams)
with
a resistance
furnace,
and
microanalysis of between one and 10 individual crystals with a laser
(Appendix 1). In order to control the effects of sample composition and grain
size, we determined
the
compositions
of between
three
and
five
representative crystals of each analyzed sample, and measured the grain sizes
of each crystal analyzed (Tables 5.1, 5.2, 5.3). The grain sizes of biotite,
muscovite, and phlogopite from northern Connemara are relatively uniform
and small (typically 300 to 500 gm in diameter). More significant grain size
variations are found in southern Connemara, with the largest grains (>6000
gm diameter) occurring in some muscovite-bearing pegmatites.
All age calculations were based on assuming an initial
4 0Ar / 3 6Ar
ratio
of modern atmosphere (295.5). Although previous work suggests that excess
44Ar
contamination
does exist in
Connemara
minerals,
particularly
hornblende, attempts to isolate such components and correct for them using
inverse isotope correlation diagrams were hampered by uniformly high
radiogenic Ar yields for all samples. Fortunately, this characteristic means
that calculated dates are relatively insensitive to our choice of
4
initial
Ar/ 3 9Ar. We interpret the dates reported here as cooling ages, but the
presence of small amounts of excess 4 0Ar may make some of the dates close
overestimates of the true cooling ages.
135
3 9Ar
weighted mean ages and their 2a errors were calculated for all
total fusion and incremental release analyses. These ages represent the
39
Ar-
volume averaged total gas age of a sample. For incremental heating analyses,
a plateau age is reported if at least three consecutive steps overlap within 27
error and contain over 50% of the
39Ar
released. For total fusion analyses,
results from multiple analyses of an individual sample were combined
statistically to give an apparent age distribution for the sample. These are
shown as normalized
probability
distribution plots (e.g. Figure 5.7).
Interpretation of the total fusion analyses was guided by the shape of the
probability distribution. If the distribution showed a single mode, without
secondary peaks or significant skewness, we assumed that the
39
Ar weighted
mean date of all analyses represents the cooling age of the sample. If the
apparent age distribution is multi-modal or skewed, the
39
Ar weighted mean
age may be geologically meaningless, and we interpret distinct peaks as the
best indicator of age components represented by the fusion
analyses.
Skewness of peaks toward older ages suggests contamination by excess "Ar,
assuming that the excess component is distributed inhomogeneously such
that multiple total fusion analyses of a sample will yield ages with variable
excess. In this case, we regard the sample mode as a close minimum estimate
of a sample's cooling age (e.g. Figure 5.7).
Following Dodson (1973), we calculated closure temperatures for micas
from existing constraints for Ar diffusivity and grain size by assuming a
cooling rate (Table 5.4). We relied on diffusion data for Ar from the following
sources: phlogopite - Giletti (1974); biotite - Grove & Harrison (1996); and
muscovite - Hames & Bowring (1993). Estimated closure temperatures are
shown in Table 5.5.
136
5.6. "Ar/ 9 Ar Results and Interpretation
The data collected in this study are discussed in geographic order, from
the garnet-staurolite zone in northern Connemara to the sillimanite-Kfeldspar zone in southern Connemara (Figures 5.5, 5.8, and 5.9). In general,
4 0Ar/ 3 9Ar
ages of biotite and muscovite are the oldest in northern Connemara
(Figures 5.6 and 5.7; Table 5.6; Appendix 4), younger in central Connemara
(Figures 5.6 and 5.7; Table 5.7, Appendix 4), and the youngest in the
migmatitic rocks of southern Connemara (Figures 5.6 and 5.7; Table 5.8,
Appendix 4) where they also show the largest age range.
5.6.1. Northernmost Garnet-Staurolite Zone
To test the interpretation of Boyle & Dawes (1991), based on the Elias et
al. (1988) K-Ar data, that cooling from -500-300 *C in northern Connemara
occurred over the 470-430 Ma interval, we analyzed coexisting muscovite
and biotite from two garnetiferous rocks from northernmost Connemara.
Furnace incremental heating experiments for muscovite (X,
=0.73) and
biotite (Xn = 0.47) from sample AF16 yield essentially flat spectra but no
statistically defined plateaus (Figure 5.6; Table 5.6). The weighted mean
40Ar/ 39Ar
dates for the flat segments of these spectra are 471.0 ± 1.0 and 462.5
± 1.1 Ma for muscovite and biotite, respectively. Total fusion results for larger
muscovite (X,
= 0.67) and biotite crystals from metapelite AF7 show
symmetric apparent-age distributions with weighted mean dates of 476.4 ± 3.7
and 468.4 ± 3.1 Ma for these minerals, respectively (Figure 5.7; Table 5.6). W e
interpret the
4
Ar/ 3 9Ar dates of all four analyses as cooling ages. The age
differences between the four minerals are consistent with differences in
closure temperatures related to differences in grain size (Table 5.5 and 5.6).
Collectively, the data are consistent with cooling at a moderate rate over the ~
137
476-463 Ma interval, and we found no evidence to support protracted cooling
in northernmost Connemara.
Garnet-Staurolite and Staurolite-Sillimanite Transition
5.6.2.
Zones
Previously published K-Ar ages from these zones range between 450
Ma to 400 Ma for biotite and between 470 and 430 Ma for muscovite (Elias et
al. 1988). To evaluate the significance of this spread in dates, we analyzed
muscovites, biotites and phlogopites from a variety of metasedimentary rocks
(Figures 5.6 and 5.7). Most 40Ar/ 39Ar analyses yield 3 9Ar weighted mean dates
older than 470 Ma, but a few muscovites, one biotite and one of the
phlogopites yield
4
Ar/ 39Ar dates that are significantly younger.
We first
report the >470 Ma dates, and then discuss the significance of the younger
results.
Total fusion analyses of biotite (X,
= 0.41) from one staurolite-garnet
schist (AF12), biotites from three staurolite-garnet-sillimanite schists (AF35 X., = 0.37, AF70 -X,
= 0.43 , AF79 -XA
calcsilicate rocks (AF37 -X,
= 0.45), and phlogopites from two
= 0.06 and AF40 -X.
(Figure 5.7, Table 5.6). Biotite AF12 has a
3 9Ar
= 0.01) yielded similar dates
weighted mean date of 478.0
± 5.1 Ma, which broadly overlaps with the sample mode of 471-483 Ma. Total
fusion analyses of the other micas yield
39Ar
weighted mean dates of 472.2
± 4.3 Ma (AF35), 474.6 ± 5.4 Ma (AF70), 476.3 ± 4.5 Ma (AF37), and 472.9 ± 2.4
Ma (AF40). More precise incremental release analyses of biotites AF70 and
AF79 yielded 39Ar weighted mean dates of 466.8 ± 1.0 Ma and 473.1 ± 1.1 Ma
for c. 70 % of the released gas. In all cases except the AF40 phlogopite, these
dates broadly overlap with the sample modes of the respective probability
distributions, so we interpret them to yield approximate cooling ages (cf. Table
5.6). In contrast, the probability distribution of phlogopite AF40 shows
distinctive modes at 455 and 495 Ma, inconsistent with the
138
39Ar
weighted
mean age, indicating
that the sample
is probably contaminated
by
heterogeneously distributed excess * Ar.
Younger * Ar/ 39Ar dates were obtained for micas in samples collected
in or near fault zones. Muscovite AF15 from a mylonitic metasedimentary
rock collected within the Renvyle Bofin Slide
(Figures 5.5, 5.8, 5.9).
Conventional furnace analysis of this sample yielded a plateau date of 452.5 ±
1.4 Ma. Given the probability that the mylonitic fabric developed at low
temperature (for example, there is no evidence for plastic deformation of
feldspars in thin section) and the fact that the muscovite helps define the
mylonitic fabric, we interpret 452 Ma as a close estimate of the time of
mylonitization. Two additional samples were collected near smaller faults
that offset the Renvyle Bofin Slide (Figure 5.5). The incremental release
spectra of muscovite (Xms =0.6) and biotite (X, =0.39) from a strongly foliated
garnet schist rock (AF49) displayed flat segments with mean dates of 452.3
± 1.0 Ma and 458.1 ± 2.3 Ma, respectively (Figure 5.6, Table 5.6). Muscovite
from another garnet schist (AF67 - Xm, =0.64) has a plateau date of 448.7 ± 1.8
Ma. The similar muscovite dates suggest an important phase of post-M3
deformation and dynamic recrystallization at c. 450 Ma. The older AF49
biotite date probably reflects excess 4 0Ar.
Another anomalously young date was provided by phlogopite from a
calcsilicate rock (AF66). Its spectrum displayed a flat segment with a 39Ar
weighted date of 418.5 ± 1.6 Ma. This date is similar to many K-Ar dates
obtained by Elias et al. (1988) from the same region. Most likely, it reflects
local resetting by hydrothermal fluids related to intrusion of the post-orogenic
Galway batholith.
5.6.3. Sillimanite-K-feldspar Zone
In this zone, we concentrated
on biotites and phlogopites from
metasedimentary samples collected near Derryclare Lough area in central
Connemara (Figures 5.5, 5.6, 5.7; Table 5.7). Biotite AF34 (X, =0.29) from a
139
sillimanite-K-feldspar-biotite gneiss has a plateau date of 448.5 ± 2.0 Ma.
Phlogopite AF38 (XA
=0.04) from a metasomatic diopside rock yields a
plateau date of 455.8 ± 1.3 Ma, consistent with a U-Pb date of c. 462 Ma for
titanite from the same sample (Chapter 4). Phlogopite AF52 (XA
=0.02) from
another metacarbonate rock has a more complicated release spectrum with a
flat segment corresponding to a date of about 457 Ma.
We interpret the
plateau and flat segment dates of these three samples as cooling ages.
These cooling ages are significantly younger than those of trioctahedral
micas with similar compositions and grain sizes from the garnet-staurolite
and staurolite-sillimanite transition zones of northern Connemara (Figure
5.8; Tables 5.6 and 5.7). The boundary between the cooling age provinces
broadly coincides with the muscovite-breakdown isograd and the crest of the
Connemara antiform.
additional samples from
Several
yielded 4"Ar/
39 Ar
unambiguously.
the sillimanite-K-feldspar
zone
data that were complicated and difficult to interpret
The frequency distribution of laser fusion
phlogopite AF27 (X.
dates for
= 0.05) shows distinctive modes at 479 and 443 Ma.
Petrographically, this sample contains phlogopites of two different grain sizes,
one somewhat larger than 500 pm and one significantly smaller. It may be
that the two modes in the release spectrum correspond to different cooling
ages for the two phlogopites. However, the c. 479 Ma date seems inconsistent
with other data from the area, particularly U-Pb titanite ages that record
cooling through a much higher closure temperature (- 650*C; Chapter 4)
roughly fifteen million years later. We suspect that this older mode reflects
excess 4"Ar contamination.
A similar interpretation can be proposed for data from another
phlogopite separate (AF32 - X,
=0.03; Figure 5.7); although its laser fusion
dates have a 39Ar weighted mean of 473.7 ± 2.3 Ma, the frequency distribution
is positively skewed away from a single mode between 460 and 465 Ma.
Biotite AF28 (X,
=0.42) from a biotite-sillimanite-garnet schist has a
140
39A r
weighted mean age of 453.9 ± 21.7 Ma, but its population distribution displays
three modes. The youngest, at 442-447 Ma, is similar to the young mode for
the AF27 phlogopite, and this range may reflect localized resetting of Ar
systematics by hydrothermal fluid interactions or dynamic recrystallization
during deformation.
5.6.4. Migmatitic Portions of the Sillimanite-K-feldspar Zone
4
Ar/ 3 'Ar research on migmatitic rocks of the sillimanite-Kfeldspar zone was designed to augment data presented elsewhere (Friedrich e t
al. in press; Chapter 3) pertinent to the detailed thermal evolution of
Our
southern Connemara. Our earlier work suggested relatively rapid cooling
over the 460-377'C temperature interval between 460 and 454 Ma near the
northern distribution limit of anatectic melt products in the sillimanite-Kfeldspar zone. As described in the following paragraphs, the 4 Ar/ 3 'Ar dates
for samples collected closer to the main outcrop region of the Connemara
igneous complex are notably younger than the mica ages reported by Friedrich
et al. (in press; Chapter 3) and much younger than the crystallization ages of
igneous phases in the Connemara complex.
Previous workers found abundant evidence that the K-Ar hornblende
chronometer cannot be applied with confidence to amphibolites from
southern Connemara (Miller et al. 1991). The most commonly proposed
explanation for the tremendous range in hornblende apparent ages was the
disturbance of isotopic systematics by hydrothermal fluids related to intrusion
of the c. 400 Ma Galway batholith and related "post-orogenic" plutons (Leggo
et al. 1966). In order to seek additional support for this hypothesis, we
analyzed micas from metapelitic schists and older granitic dikes collected at
various distances from late plutons. In general, the muscovites yielded
relatively simple release spectra with plateaus or flat segments, whereas the
biotite results were much more complex.
141
Biotite AF22 is from a schist collected ten kilometers away from the
post-orogenic Roundstone granite (Figure 5.5). Petrographic analysis provided
no evidence of hydrothermal alteration (Figure 5.9). Laser fusion analyses
yielded symmetric frequency distributions with 3'Ar weighted mean dates of
453 and 440 Ma for the c. 400 gm and c. 150 pm size fractions, respectively
(Table 5.7), consistent with a lower closure temperature for the smaller size
fraction. Substantially older than the 420 Ma crystallization age of the granite,
these dates probably represent cooling after M3 metamorphism.
Collected 5 km south of the AF22 outcrop, AF18 is a garnet-K-feldspar
schist containing biotite (XA
= 0.53), secondary muscovite (Xm. = 0.74) that
replaces K-feldspar, and chlorite that fills fractures in garnet (Figure 5.5).
Incremental heating analysis of coarse (900 gm) muscovite yielded a plateau
date of 456.5 ± 2.2 Ma, whereas a finer-grained fraction (450 pm) had a plateau
date of 453.6 ± 0.9 Ma (Figure 5.7). Laser fusion analyses of fragments of a 450
pm single crystal of this muscovite had a 3'Ar weighted mean date of 457.4
±3. Ma, which lies within the uncertainty of the two other analyses. The
similarity of these ages with those obtained for M3 micas farther north in the
"migmatite" zone of Connemara suggests that retrogression in AF18 occurred
at a relatively high temperature. Laser total fusion analyses for two aliquots
of biotite
(Xan=
0.53) from this sample have "Ar weighted mean ages of 489.1
± 3.3 and 275.6 ± 5.1 Ma (Figure 5.7, Table 5.8).
complex behavior, we also performed
To better understand this
a furnace
incremental
heating
experiment on a 10 mg biotite aliquot with the resistance furnace, and a laser
incremental heating experiment on a single 750 gm crystal (Figure 5.6 and
Table 5.8). Both experiments yielded similar 3'Ar
weighted mean dates of
446.8 ± 2.3 and 445.3 ± 2.1 Ma, respectively. In both cases, the spectra displayed
similar stair-step shapes with age components of c. 508 and c. 365 Ma (Figure
5.6). Given U-Pb constraints that M3 occurred over the 460-474 Ma interval,
the 508 Ma component must reflect contamination with excess 4 Ar. The c.
365 Ma component is much younger than other biotite cooling ages from this
142
area and does not correspond to a previously identified thermal event. This
**Ar/ 3'Ar date, however may correspond to one of several low-temperature
fluid alteration
events
that
affected
the
Connemara
region
after
emplacement of the Galway granites (cf. Jenkin et al. 1992).
We also analyzed micas from two samples from a small area less than 5
km north of the Roundstone granite (Figure 5.5). Sample AF45 is from the
paleosome of an anatectic pelitic gneiss that surrounds pegmatitic leucosome
AF44. Furnace incremental heating analysis of the biotite (XA.=0. 4 9 ) from the
leucosome yields a plateau date of 418.4 ± 6.5 Ma. Given the crystallization age
of this pegmatite (467 ± 2 Ma - U-Pb zircon; Chapter 3), this 40Ar / 39 Ar date
reflects complete resetting during intrusion of the Roundstone
granite, and possibly minor resetting at c. 350 Ma or younger.
probably
The
40Ar/ 39Ar
systematics of the biotite (X,
= 0.49) in AF45 are more
complex (Figures 5.6 and 5.7). Laser fusion analyses of aliquots of up to seven
grains have a 39Ar weighted mean date of 434.5 ± 3.1 Ma. However, frequency
distribution plots show three distinct modes at c. 420 Ma, 437 Ma and 463 Ma
(Figure 5.7). Laser incremental heating of another aliquot yielded a stair-step
profile with a flat segment at 420.1 ± 6.5 Ma and a minimum age of 327 ± 20
Ma (Figure 5.6). The simplest interpretation of this spectrum is that the
sample suffered a major Ar re-equilibration event at c. 420 Ma followed by a
second, less profound event at 330 Ma. Muscovite (X, = 0.63) in AF45
occurs as fine-grained pinnate and as large (up to 5000 gm), randomly
oriented, secondary muscovite books (Figure 5.9). Laser incremental heating
analyses of different size fractions yielded simple plateaus or flat segments
corresponding to dates between 439 and 429 Ma (Figure 5.6 and Table 5.8).
Large (700 pm) crystals gave the oldest dates, whereas the smallest crystals (200
gm) gave the youngest dates. Despite the positive correlation between grain
size and apparent age, these cannot be closure ages related to simple cooling
after M3 metamorphism because AF45 is from a structural level just below
143
the older Silurian unconformity (Figure 5.5). Conceivably, the young ages
imply partial resetting associated with intrusion of the Roundstone granite.
5.7. Interpretation of "Ar/ 9Ar Results
The "Ar/
39 Ar
mica dates presented here and elsewhere (Figure 5.10;
Friedrich et al. in press; Chapter 3) fall into four interpretive groups:
Category 1
Ages
representing
simple
cooling
after
the
Grampian
metamorphic event (Figure 5.10; AF7 muscovite and biotite;
AF12 biotite; AF16 muscovite and biotite; AF18 muscovite; AF22
biotite; AF34 biotite; AF35 biotite; AF37 phlogopite; AF38
phlogopite; AF52 phlogopite; AF70 biotite; AF79 biotite).
Category 2
Ages representing isotopic resetting during late Grampian
deformation (Figure 5.10; AF15 muscovite; AF49 muscovite;
AF67 muscovite).
Category 3
Ages representing isotopic resetting during post-Grampian
hydrothermal activity (Figure 5.10; AF18 biotite, AF27 phlogopite
(?); AF28 biotite (?); AF44 biotite; AF45 biotite
(?); AF66
phlogopite).
Category 4
Dates with no obvious geologic significance (Figure 5.10; AF18
biotite; AF27 phlogopite; AF32 phlogopite; AF40 phlogopite;
AF45 muscovite; AF49 biotite).
The last category can be defined because the c. 400-420 Ma post-orogenic
granites are the youngest well-dated potential resetting source, and because
dates much older than c. 475 Ma are inconsistent with U-Pb zircon
constraints on the age of the early Grampian Cashel-Lough Wheelaun gabbro
(Friedrich et al. in review; Chapter 2).
144
5.7.1. Significance of Category 1 Ages
Mica ages that record the thermal relaxation of the Grampian orogen
show a geographic pattern opposite that proposed by Elias et al. (1988) (Figures
5.8 and 5.10). The oldest reliable Category 1 ages - ranging from 478 to 467 Ma
-
were
obtained
for garnet-staurolite
zone
and staurolite-sillimanite
in northern Connemara. To the south,
toward the Connemara igneous complex, Category 1 biotite ages decrease to
as young as 453-440 Ma in the highest temperature portions of the
sillimanite-K-feldspar zone. The only complication in this pattern occurs
transition zone samples collected
along the extreme northern margins of the Dalradian outcrop region, where
biotite ages from staurolite zone rocks are slightly younger (468-463 Ma) than
those a few kilometers to the southeast in the staurolite-sillimanite transition
zone (478-467 Ma). One possible explanation for the relatively young ages in
northernmost Connemara is limited Ar degassing related to late extensional
deformation (Boyle & Dawes 1991).
Our sampling density was not sufficient to ascertain if this geographic
trend is continuous or if there are abrupt transitions. We can say that there is
no significant discontinuity in the pattern at the sillimanite isograd (Figures
5.8 and 5.10), which has been suggested by some authors as corresponding to
the effective northern distribution limit of M3 prograde metamorphic
assemblages (Yardley 1976; Tanner & Shackleton 1979). In fact, we found no
obvious correspondence between cooling age and whether or not a mineral
grew as part of the M2 or M3 assemblage. Even if the northern limit of
prograde M3 metamorphism extends only as far north as sillimanite-in
isograd, as has been proposed elsewhere (Yardley et al. 1987), the thermal
effects of M3 extended throughout Connemara (cf. Boyle & Dawes 1991).
Thus, Category 1 ages represent cooling subsequent to the M3 event (Figures
5.10 and 5.11).
Three explanations could satisfy the general north-to-south younging
of Category 1 ages. The first, which is consistent with some U-Pb titanite and
145
monazite dates of less than 470 Ma in central and southern Connemara (Cliff
et al. 1996; Friedrich et al. in press; Chapters 3 and 4), is that the older
40Ar/ 3 9Ar
dates of northern Connemara represent contamination by excess
'"Ar. We cannot disprove this interpretation with the data at hand; our
samples were so radiogenic that the use of isotope correlation diagrams to
evaluate this problem was impractical. However, the consistency of most
northern Connemara 4'Ar/3Ar ages from a single subarea is not a pattern
typical of pervasive excess
40Ar
contamination.
Most well-documented
examples of sample suites contaminated with excess 40Ar are characterized by
irreproducible results for multiple aliquots of the same sample, wide
variations in dates from single outcrops, and/or complex release spectra
(Harrison & McDougall 1981; von Blanckenburg & Villa 1988; Arnaud and
Kelley 1995). Because many of the young titanites found in Connemara are
thought to have grown during late- or post-M3 metasomatic events (Chapter
4) and because we cannot preclude a similar origin for the few young
monazites dated in the region, we see no reason to reject the
40Ar/ 3 'Ar
evidence for the M3 metamorphism in northern Connemara at c. 470 Ma.
A second possible explanation for the N-S age trend is that the region
was unroofed diachronously -
trending Connemara antiform.
first in the north -
about the East-West
This interpretation is consistent with the
coincidence of the abrupt break in cooling ages with the axial trace of this
antiform, but inconsistent with evidence from paleomagnetic studies that
northern Connemara had already cooled to below c. 510-580*C before folding
(Morris & Tanner 1977; Robertson 1984). Furthermore, structural, petrologic,
and thermobarometric constraints on paleodepth trends during M3 suggest
synchroneity across the Connemara region.
We
favor
a third
explanation
consistent
with
all
available
geochronologic and structural data: that intrusion of intermediate to felsic
phases of the Connemara igneous complex over the c. 468-463 Ma interval
maintained M3 temperatures high in southern Connemara while regional
cooling occurred in northern Connemara.
146
Textural relations in dated
metamorphic rocks further suggest that structures and fabrics commonly
attributed to D3 are older in the north than in the south; for example, micas
that define the S3 schistosity in northern Connemara had cooled well below
-400*C while synkinematic micas were still growing at upper amphibolite
facies in southern Connemara. While this model requires high lateral
temperature gradients, we suggest that such
a thermal regime would be
consistent with a continental arc setting like that of Connemara.
5.7.2. Significance of Category 2 Ages
Category 2 ages may provide close minimum estimates on the age of
the Renvyle Bofin slide and related late structures in central and northern
Connemara (Figures 5.8 and 5.10). If this interpretation is correct, extensional
deformation in this part of the Grampian orogen was taking place no more
than about fifteen million years after peak metamorphism associated with
large-scale crustal shortening in the continental arc environment.
5.7.3 Significance of Category 3 Ages
Most Category 3 ages record resetting of the Ar systematics of micas
throughout much of Connemara at c. 420 Ma. Occurring roughly twenty
million years after Dalradian units had been overlapped unconformably by
passive margin sediments at Connemara, this event cannot be regarded as
part of the Grampian orogeny (Figure 5.10). Instead, its age suggests a causal
relationship with the intrusion of post-orogenic granites along the southern
margin of the older Connemara igneous complex (the Galway Batholith; e.g.
Leggo et al. 1966; Friedrich unpublished data). If this interpretation is correct,
the thermal effects of the post-orogenic magmatic event extended far beyond
southern Connemara, perhaps through a regional metasomatic mechanism
similar to that invoked for older c. 462 Ma metasomatic deposits in
Connemara (Yardley et al. 1991; Cliff et al. 1993). Such a large regional extent
of hydrothermal activity related to emplacement of the Galway has been
147
suggested
by
Jenkin et al. (1992) for retrograde alteration in southern
Connemara.
Two other anomalous dates may be related to similar alteration events.
The frequency distribution plots for AF27 phlogopite and AF28 biotite display
modes between 450 and 440 Ma. These dates are significantly younger than
those obtained for compositionally similar minerals at the same structural
level in the sillimanite-K-feldspar
evidence of post-metamorphic
zone. Neither AF27 or AF28 show
dynamic recrystallization, but irregularly
spaced c. 1-3 mm quartz-filled fractures occur orthogonal to the S3 foliation
defined by biotite and phlogopite in these samples. Therefore, we tentatively
attribute the 450-440 Ma dates to retrograde resetting. An extremely young
near-plateau segment on the incremental heating spectrum for AF45 biotite
(c. 330 Ma) suggests that some alteration may have occurred long after
intrusion of the Galway batholith and related plutons. However, the AF45
biotite incremental heating analysis was poor, with very large uncertainties,
and laser analyses of the same sample yielded discrepant results.
5.8. Toward a Comprehensive Model of the Thermal and
Tectonic Evolution of Connemara
The "Ar/
39 Ar
data presented in this manuscript, along with other
geochronologic data presented in other chapters, suggest a new thermal and
tectonic model of the Grampian orogen at Connemara. In presenting this
model, we refer to three simplified developmental cross sections from
northern to southern Connemara (Figure 5.12). The construction of these
cross sections is described in Appendix 3.
The oldest section, drawn to represent the structural pattern at c. 474470 Ma, is dominated by a regional-scale recumbent fold of D2 age (Figure
5.12a). The D2 event, responsible not only for the fold nappe but also for an S2
148
schistosity throughout Connemara, was at least in part synchronous with M2
metamorphism. Although direct constraints on the initiation age and
duration of D2 and M2 are limited, Wellings (1998) suggested that the c. 474
Ma Currywongaun gabbro intruded during the regional D2 and M2.
Numerous
cooling ages for M3 sillimanite-grade micas in northern
4'Ar/3Ar
Connemara are within uncertainty of this age or only slightly younger at 475
Ma, so we infer that the transition from D2/M2 to D3/M3 in the northern
half of the Connemara region occurred in no more than a few million years.
On a regional scale, this transition may simply represent a deformational
continuum at progressively higher temperatures.
We believe that both D3 and M3 as defined by F3 folds and S3 axial
planar schistosity in Dalradian rocks affected all of Connemara. Both D3 and
M3 appear
to have been very short-lived
phenomena
in northern
Connemara (c. 470 Ma), but may have lasted 7 to 8 million years in southern
Connemara as a result of more prolonged and more voluminous magmatic
activity (470-463 Ma).
M3 and D3 reached their peak intensities at 468-466 Ma in southern
Connemara, coincident with widespread quartz diorite magmatism and
anatexis of metasedimentary rocks (Figure 5.12b). At that time, a very high
thermal gradient developed across Connemara, with temperatures of >750*C
in the southernmost sillimanite-K-feldspar zone and <300*C in northern
Connemara (Figure 5.12b). D3 structures of this age include an intense
foliation in anatectic metapelite rocks and wide ductile zones, especially in
southwestern Connemara above the Mannin thrust, both of which contrast
significantly with the older F3 folds and tectonite fabrics that developed at
lower temperatures throughout Connemara.
Figure 5.12c represents the regional structure after development of the
D4 Connemara antiform. The 4"Ar/ 3 9Ar cooling ages in this paper, considered
in the light of existing geophysical data, provide tight constraints on the age of
the antiform.
Paleomagnetic analyses of mafic rocks indicate that the
149
northern Connemara gabbros cooled below -510-580*C, the nominal range
for the Curie temperature of magnetite, before folding of the Connemara
antiform, whereas the southern Connemara gabbros cooled below this
temperature range after development of the antiform (Morris & Tanner 1977,
Robertson 1988). The north-south variation in "Ar/"Ar mica ages thus
brackets formation of the Connemara antiform to between c. 470 Ma and c.
463-463 (Figure 5.11). Additional confirmation of the lower age bracket comes
from the U-Pb crystallization age of 462.5 Ma for the post-F4 Oughterard
granite. The persistence of "late" M3 high temperatures
in southern
Connemara during development of the F4 Connemara antiform may help
explain the asymmetry in deformational style across the crest of the antiform,
with more intense deformation and shearing in the southern limb (locally
referred to as 'the steep belt', e.g. Leake & Tanner 1994). Formation of the
antiform was linked kinematically with ductile deformation of the quartz
diorites and gabbros of southern Connemara along an incipient Mannin
thrust-shear zone that eventually resulted in formation of the brittle Mannin
thrust.
At c. 462 Ma, the waning stages of Connemara igneous complex activity
maintained temperatures in the southern part of the region relatively high,
perhaps more than 150*C higher than temperatures in northern Connemara.
However, extensive hydrothermal systems extended northward from the
igneous complex, leading to local resetting of mica chronometers and the
crystallization of metasomatic
assemblages such as the diopside rocks
described by Yardley et al. (1991).
Similar
episodes metasomatic
or
hydrothermal alteration events appear to have occurred more recently in the
history of the region.
The youngest structures for which we have age constraints are the
extensional faults of northern Connemara. Movement along the Renvyle
Bofin slide occurred at c. 452 Ma based on the
4 Ar/ 3'9Ar
muscovite
crystallization age of mylonite AF15. This age is consistent with the recent
150
reinterpretation of the structural age of this fault to be post-D3 (Wellings
1998). It seems likely that extensional faulting entirely postdates contractional
deformation at Connemara.
5.9. Conclusions
Our "0Ar/ 3 9 Ar data supports the previously reported large spread in
mineral dates from the Connemara region. However, consideration of our
data in the context of other petrographic, geochronologic and stratigraphic
constraints, suggests that some dates represent regional cooling subsequent to
metamorphism, others record episodes of dynamic recrystallization, still
others are reflect alteration episodes, and a few are so contaminated by excess
"Ar as to be geologically meaningless. The regional cooling ages display
patterns that can be linked directly to temporal and spatial variations in
deformation, metamorphism, and magmatism. Northern Connemara cooled
below c. 300*C by c. 470 Ma, whereas southern Connemara was not at such
low temperatures until c. 450 Ma. This phenomenon probably was related to
large lateral thermal gradients in an evolving continental arc setting. One
important implication of the Connemara study is that regional trends in
cooling ages may reflect regional variations in the orogenic temperature
structure rather than regional variations in the unroofing history, as is
commonly assumed.
The significance of the geologic record to proper interpretation of
mineral cooling ages cannot be overemphasized. If we knew less about the
stratigraphic, intrusive, and structural history of Connemara, it would be a
simple matter to interpret the broad spread in mineral ages from this region
in terms of protracted cooling over nearly one hundred million years. This
realization is a reminder that geochronology is of limited value in regions
that have not been characterized adequately through careful field mapping
and structural analysis.
151
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transactions of the Royal Society, London, A, 321, 243-270.
157
Appendix 5.1: Sample description
Northern Connemara
AF7 two mica-garnet schist from N shore. Minerals in this rock include
garnet + biotite + muscovite + quartz + plagioclase + chlorite. The foliation is
defined by muscovite and biotite. Chlorite occurs associated with muscovite,
in garnet pressure shadows, and associated with veins orthogonal to the
foliation plane.
AF12 garnet-staurolite schist from Tully Mountain (south of the RBS).
Foliation plane is defined by biotite + staurolite + quartz + plag. Quartz also
occurs in pressure shadows around garnet. Biotite + staurolite are wrapped
around the garnet. Inclusions in garnet are zircon (near veins), rutile, and
quartz. Chlorite fills veins.
AF16 garnet-schist from N of Tully Mountain. Minerals in this rock
include garnet + biotite + muscovite + quartz + plagioclase + monazite.
Alternating layers of small recrystallized quartz and foliated muscovite and
biotite layers, and large quartz crystals that are partially boudinaged. Quartz
replaces garnet.
AF35 garnet-staurolite schist from the staurolite-sillimanite transition
zone. Minerals in this rock are garnet + staurolite + biotite + quartz +
plagioclase + ilmenite + tourmaline + chlorite. Segregated layering with
highly foliated biotite-rich layers and garnet-quartz-plagioclase rich layers.
Garnet, staurolite, tourmaline and ilmenite grow over biotite and other
minerals with sharp grain boundaries and no pressure shadows.
AF49 garnet-schist from the staurolite/sillimanite transition zone. A
highly foliated rock (weak S-C fabric), defined by biotite, muscovite and
quartz. Muscovite occurs especially around quartz and in some places seems
to have overgrown biotite. Quartz replaces garnet and fills pressure shadows.
158
The mineral assemblage includes garnet + muscovite+ biotite + quartz +
plagioclase + tourmaline. Chlorite occurs in and near fabric cutting veins.
AF67
Garnet-staurolite-biotite-muscovite
schist.
Primary
compositional layering indicated by large quartz bands. A mylonitic texture in
this rock includes long biotite crystals and fine grained quartz crystals.
Inclusions within garnet are oriented parallel to the main foliation in this
rock.
AF70 Biotite-sillimanite schist. Biotite and sillimanite define the
foliation plane. Biotite occurs as large crystals that are only weakly foliated.
Other minerals are quartz, plagioclase and garnet.
AF79 staurolite-garnet-sillimanite schist from Cur Hill. Biotite defines
a foliation plane and is wrapped around garnet. Fibrolite occurs in pressure
shadows of the garnet and with biotite in the foliation plane. Staurolite is
abundant in this rock and grew statically over foliated biotite. Segregation
layering of biotite and quartz + plagioclase. Accessory minerals include
tourmaline, opaque mineral, and monazite.
Central Connemara
AF34. K-feldspar-biotite-quartz gneiss from the upper sillimanite zone.
This rock is only weakly foliated but contains quartz+K-feldspar-rich layers
alternating with biotite rich layers. Most garnet is replaced by quartz +
cordierite (?) and K-feldspar. This rock also contains apatite and opaques.
AF27 Calcsilicate rocks from the upper sillimanite zone. This rock
contains clinopyroxene ± quartz + phlogopite + titanite + unidentified
alteration products. Many small quartz veins cut across all minerals.
AF28 Biotite-Sillimanite-K-feldspar gneiss. Garnet is almost completely
replaced by fibrolite, sillimanite and quartz, surrounded by randomly oriented
biotite. In other areas of the thin section, biotite and a quartz+K-feldspar
159
intergrowth forms segregation layering. This rock also contains minor
chlorite, apatite, and monazite.
AF32 and AF38 Diopside rocks from central Connemara. These rocks
are the metasomatic diopside rocks described by Yardley et al. (1993), which
also contain phlogopite.
AF52 Calcsilicate rock from upper sillimanite zone, north side of
Lissoughter Hill, Connemara Marble Formation. With randomly oriented
phlogopite.
Southern Connemara
AF18 garnet-migmatite gneiss. Migmatite zone. The garnets in this
rock are porphyroblastic and are highly fractured. The fractures are filled with
chlorite. Muscovite occurs around altered K-feldspar and overgrowing biotite
and quartz.
AF22 Biotite-sillimanite-garnet gneiss. Centimeter-size
garnets are
highly fractured and partly replaced by biotite and sillimanite. Other minerals
in this rock are quartz, plagioclase, minor muscovite (secondary?) and Kfeldspar. Only weakly foliated.
AF44 Granitic pegmatite. Plagioclase and K-feldspar in this sample are
seritized. Biotite is pleochroic green/brown.
AF45 Banded gneiss. Centimeter thick leucosome layers alternate with
restitic layers. The leucosome contains garnet xenocrysts and quartz and
plagioclase. The restite contains cordierite that is altered to pinite, and some
of the layers contain large randomly oriented secondary muscovite crystals.
160
Appendix 5.2: Analytical Methods
We obtained muscovite, biotite and phlogopite separates by standard
crushing, sieving, Wilfley, magnetic, and heavy liquid separation. Two
representative crystals of each sample were used to determine the major
element compositions using a JEOL 733 electron microprobe operating with a
beam current of 10nA and an accelerating voltage of 15kV. T remaining
separates were cleaned ultrasonically with distilled water, and wrapped in
aluminium foil before irradiation at the McMaster University Reactor in
Hamilton, Ontario.
hornblende
Fast neutron flux was monitored
(520.4 Ma, Samson
and Alexander
using Mmhb-
1987). Corrections
for
interfering reactions were accomplished by including K2 SO 4 and CaF2, and
KCl salts in the irradiation packages. All samples were analyzed by a MAP
215-50 gas source mass spectrometer at MIT.
Gas was extracted from the
sample in three ways: by furnace incremental heating, laser incremental
heating, and laser fusion. The first method involved experiments on 5 to 50
mg samples in a double-vacuum resistance furnace for a period of 10 minutes
per increment. Laser incremental heating was performed with a defocused,
Coherent Ar-ion laser on one to 10 crystal aliquots. Progressive heating was
accomplished by firing the laser for two-minute intervals at successively
higher power levels until each sample melted. Laser total fusion of between
one and 30 crystals was achieved by firing a defocused laser beam for 30
seconds at high power (typically >20 W). Representative systems blanks for
laser microanalysis for the M/e 40, 39, 38, 37, 36 (moles) were 9 x 10-16, 2 x10-1,
8 x 1018, 1 x 10-17, and 8 x 1018. Blanks varied in the furnace as a function of
temperature and loading cycle, but were typically at least an order of
magnitude higher than those for laser analyses. All analyses were corrected
for blanks, interfering isotopes, and mass discrimination.
161
For incremental heating analyses, plateaus were defined as 3 or more
consecutive release steps with a total of >50 % of the
3 9Ar
released that are
overlapping within 2ay uncertainties irrespective of the contribution of the
value uncertainty. For samples not yielding plateaus, total gas ages or
J-
39Ar
weighted mean ages are reported. To determine the best age and assess the
uncertainties of total fusion analyses, we used a Monte Carlo approach which
is described in detail by Hodges and Bowring (1995).
162
Appendix 5.3: Construction of Schematic Cross Sections
The Connemara antiform plunges about 10 degrees eastward over a
distance of 80 km distance along trend, the structure exposes a roughly 10 kmthick crustal section. This allows great insight into the 3-D structure of the
Connemara antiform and the gabbro and gneiss complex.
Cross section
restoration prior to 460 Ma involved several assumptions. The Silurian
unconformity is parallel to the structurally highest unit, rather than cutting
across previously folded layers. This implies that the unconformity was tilted
together with the Dalradian block sometime after the Caledonian orogeny.
We assumed that offset on the Renvyle Bofin slide of northern Connemara
was negligible because there is no significant break in metamorphic grade
across the fault. The geometry of unfolding of the F4 Connemara antiform
was based on the paleomagnetic data which required a 20 degree southward
tilt of the F3 fold axes prior to F4 (Robertson 1988, Morris and Tanner 1977).
Paleomagnetic constraints on the pre-F4 orientation of regional F3 fold axes
and geometric considerations require restoration of the Mannin thrust late
during the development of the Connemara antiform, a relationship shown
schematically by a steep incipient Mannin thrust (Figure 5.12). The present
day surface trace is shown on each cross section as a reference line.
163
Appendix 5.4. 0 Ar
39 Ar
36Ar/4"Ar
analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
( 'A40Aror asAr/ 40Ar
3
Ar/40 Ar)...,.
3
9
ArK moles Cum.%
Ar 4"Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
39
t
**
Sample AF7b-c48-1 @@
Laser Laser Total Fusion Analysis
1
3.54E-05
2
1.12E-05
3
5.26E-05
4
1.80E-05
5
0.OOE+00
4.43E-09
6
7
1.35E-05
8
0.OOE+00
3.56E-05
9
10
1.27E-04
21
2.97E-06
22
0.OOE+00
23
1.35E-05
2.31E-05
24
25
2.82E-06
26
5.58E-06
27
3.76E-07
28
1.97E-04
29
1.15E-05
30
7.18E-06
47
5.17E-06
48
2.90E-07
49
5.49E-07
50
7.67E-07
4.31E-05
3.91E-05
8.49E-05
4.64E-05
4.88E-05
6.85E-05
6.88E-05
9.23E-05
5.33E-05
5.59E-05
5.91 E-05
7.95E-05
5.19E-05
5.09E-05
4.53E-05
4.59E-05
8.29E-05
5.18E-05
4.45E-05
4.60E-05
5.98E-05
5.41E-05
9.49E-05
1.73E-04
4.65E-02
4.50E-02
4.41 E-02
4.57E-02
4.52E-02
4.43E-02
4.56E-02
4.50E-02
4.58E-02
4.46E-02
4.60E-02
4.63E-02
4.49E-02
4.44E-02
4.53E-02
4.46E-02
4.62E-02
4.56E-02
4.42E-02
4.46E-02
4.51E-02
4.51E-02
4.49E-02
4.55E-02
5.22E-04
4.13E-04
7.19E-04
3.84E-04
3.20E-04
5.82E-04
5.36E-04
5.73E-04
3.44E-04
5.09E-04
4.90E-04
6.23E-04
4.29E-04
5.98E-04
5.11E-04
4.22E-04
4.86E-04
4.40E-04
6.59E-04
5.40E-04
4.32E-04
8.01E-04
8.15E-04
9.96E-04
1.29E-14
1.32E-14
6.07E-15
1.10E-14
1.28E-14
8.72E-15
9.05E-15
7.83E-15
1.15E-14
1.28E-14
1.09E-14
1.13E-14
1.13E-14
1.34E-14
1.34E-14
1.28E-14
7.85E-15
1.40E-14
1.31E-14
1.26E-14
8.90E-15
9.88E-15
5.46E-15
3.26E-15
98.6
99.3
98.1
99.1
99.7
99.7
99.3
99.7
98.6
95.9
99.6
99.7
99.3
99.0
99.6
99.5
99.6
93.9
99.3
99.5
99.5
99.7
99.6
99.6
455.7
472.0
475.5
465.2
471.6
479.5
466.0
473.0
461.9
461.5
464.3
461.9
472.5
476.0
470.5
476.4
462.3
443.7
479.4
476.6
471.5
472.5
474.7
468.4
468.4
468.4
14.1
14.1
18.0
14.2
14.4
16.4
15.9
17.9
14.4
15.0
15.1
16.7
14.8
15.4
14.6
14.6
16.6
14.2
15.4
14.9
15.2
16.2
18.9
26.2
92.9
3.1
(6.9)
(6.1)
(12.7)
(6.6)
(6.7)
(10.2)
(9.7)
(12.5)
(7.2)
(8.4)
(8.4)
(11.0)
(7.5)
(8.5)
(7.3)
(6.9)
(10.9)
(7.4)
(8.4)
(7.6)
(8.3)
(9.9)
(14.0)
(22.9)
99.6
99.6
98.3
99.3
99.5
98.4
99.6
99.6
97.4
99.0
62.7
465.3
492.1
470.3
468.4
482.9
498.6
474.9
473.7
476.9
476.7
321.6
14.9
16.1
15.8
14.0
14.2
15.5
28.1
14.6
22.5
15.6
535.5
(8.1)
(9.2)
(9.4)
(6.0)
(5.8)
(7.8)
(25.1)
(7.2)
(18.5)
(8.9)
(535.5)
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF7m-c48-1 §§
Laser Laser Total Fusion Analysis
11
3.25E-07
4.38E-06
12
13
4.50E-05
1.36E-05
14
5.06E-06
15
16
4.53E-05
17
9.65E-07
2.38E-06
18
7.58E-05
19
2.22E-05
20
31
1.26E-03
5.99E-05
5.07E-05
6.30E-05
3.96E-05
3.46E-05
4.45E-05
1.87E-04
4.66E-05
1.26E-04
5.99E-05
3.86E-03
4.59E-02
4.30E-02
4.47E-02
4.54E-02
4.39E-02
4.18E-02
4.48E-02
4.49E-02
4.36E-02
4.43E-02
4.35E-02
3.96E-04
6.52E-04
5.66E-04
3.89E-04
3.96E-04
4.99E-04
1.04E-03
4.60E-04
9.81E-04
5.13E-04
6.12E-03
8.23E-15
1.06E-14
7.92E-15
1.28E-14
1.38E-14
1.10E-14
2.69E-15
1.04E-14
3.91E-15
8.27E-15
1.48E-16
Appendix 5.4. continued.4 Ar 39Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
4
36
Ar/ *Ar ( 36Ar/ 4*Ar)er,o, 39Ar/ 4*Ar 39Ar/ 4*Ar),,ror 39ArK moles Cum.% 39Ar 4*Ar*% Age (Ma) Age Error (w J) Age Error (w/o J)
Increment
t
**
Continued: Sample AF7m
32
33
34
35
36
37
38
39
40
1.19E-04
5.81E-05
3.25E-05
7.13E-05
4.74E-05
9.90E-05
3.48E-05
3.80E-05
6.47E-05
1.18E-04
6.03E-05
4.62E-05
6.27E-05
8.36E-05
1.04E-04
5.28E-05
4.61E-05
8.69E-05
4.48E-02
4.41E-02
4.39E-02
4.48E-02
4.49E-02
4.35E-02
4.33E-02
4.50E-02
4.38E-02
7.24E-04
5.96E-04
4.99E-04
5.91E-04
7.85E-04
7.25E-04
5.28E-04
2.16E-04
6.87E-04
5.06E-15
9.34E-15
1.22E-14
9.41 E-15
7.51E-15
5.41E-15
1.07E-14
1.29E-14
8.08E-15
96.2
98.0
98.7
97.6
98.3
96.8
98.7
98.5
97.8
460.6
475.1
479.7
466.5
468.6
475.2
485.7
468.3
476.7
476.4
476.4
20.4
15.9
14.9
15.8
17.9
19.7
15.6
14.0
18.1
73.4
3.7
(16.1)
(9.5)
(7.5)
(9.5)
(12.6)
(15.0)
(8.5)
(6.0)
(12.8)
98.0
98.9
99.2
99.4
98.8
98.9
99.3
99.7
99.1
453.5
489.0
469.3
492.9
474.8
494.2
494.8
538.2
470.5
478.0
478.0
12.9
13.8
15.1
16.2
13.4
14.7
40.3
52.8
12.8
34.6
5.1
(6.7)
(7.3)
(10.0)
(11.1)
(7.0)
(8.8)
(38.5)
(51.3)
(5.8)
53.9
86.0
68.6
95.4
99.2
98.9
99.0
99.5
99.4
98.3
100.0
367.0
417.8
486.8
447.3
451.7
451.5
453.5
453.2
452.2
447.7
452.6
88.8
36.0
30.7
7.0
2.5
2.6
3.0
2.8
3.0
3.4
22.3
(88.8)
(35.9)
(30.6)
(6.6)
(.4)
(.9)
(1.7)
(1.5)
(1.7)
(2.4)
(22.2)
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF12b-c48-1 @@
Laser Total Fusion Analysis
5.52E-05
61
2.59E-05
62
1.62E-05
63
1.09E-05
64
3.02E-05
65
2.50E-05
66
1.46E-05
67
8.90E-08
68
6g
2.08E-05
4.75E-05
4.92E-05
7.52E-05
8.06E-05
4.91 E-05
6.17E-05
2.85E-04
3.60E-04
3.93E-05
4.65E-02
4.30E-02
4.52E-02
4.28E-02
4.44E-02
4.25E-02
4.26E-02
3.88E-02
4.50E-02
3.95E-04
3.73E-04
4.28E-04
3.98E-04
3.69E-04
3.66E-04
1.17E-03
1.12E-03
3.51E-04
1.12E-14
1.04E-14
7.58E-15
6.19E-15
1.12E-14
8.34E-15
1.75E-15
1.23E-15
9.79E-15
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF15m-cl67-7§@
Furnace Furnace Furnace Temperature [K]
4.87E-04
1.56E-03
923
2.78E-04
4.75E-04
973
1.64E-04
1.06E-03
1023
5.29E-05
1.56E-04
1123
1.11E-06
2.74E-05
1223
6.54E-06
3.82E-05
1273
1.28E-05
3.41E-05
1323
1.1OE-05
1.78E-05
1373
1.33E-05
2.03E-05
1423
1.70E-05
5.87E-05
1473
1.83E-04
4.65E-08
1523
2.37E-02
3.27E-02
2.20E-02
3.36E-02
3.46E-02
3.45E-02
3.43E-02
3.45E-02
3.46E-02
3.46E-02
3.48E-02
3.76E-04
3.76E-04
2.52E-04
9.59E-05
3.OOE-05
4.52E-05
6.71E-05
5.38E-05
4.74E-05
1.05E-04
4.18E-04
1.02E-14
1.15E-14
1.76E-14
6.74E-14
8.38E-13
3.72E-13
2.14E-13
2.67E-13
2.76E-13
4.10E-14
1.23E-14
0.5
1.0
1.8
5.0
44.1
61.5
71.5
84.0
96.8
98.8
99.3
Appendix 5.4. continued.'' Ar/ -9Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
4
36
Ar/ *Ar ( 36Ar/*Ar)rw 3*Ar/4*Ar 3 Ar/4*Ar)r, "Ar, moles Cum.% *Ar 4*Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
Increment
t
**
Continued: Sample AF15m
1973
9.94E-05
1.10E-04
2.89E-02
1.79E-04
1.44E-14
97.1
100.0
Total Gas Age tt:
"Ar Wtd. Mean Age
518.5
452.1
452.0
3.47E-04
6.21E-05
3.02E-05
2.61E-05
1.57E-05
4.02E-05
1.05E-05
1.34E-05
3.28E-05
5.76E-06
2.30E-05
6.02E-05
1.01E-03
1.51F,-04
2.90E-02
6.13E-02
2.71E-02
3.27E-02
3.38E-02
3.37E-02
3.38E-02
3.29E-02
3.35E-02
3.39E-02
3.39E-02
3.38E-02
3.36E-02
3.30E-02
5.39E-04
3.51 E-04
9.72E-05
5.61 E-05
7.64E-05
8.23E-05
8.63E-05
8.24E-05
6.93E-05
6.67E-05
1.26E-04
2.60E-04
8.47E-04
3.86E-04
8.01E-15
3.31E-14
3.89E-14
8.81E-14
9.08E-14
8.96E-14
1.03E-13
1.25E-13
1.29E-13
1.26E-13
9.09E-14
2.62E-14
6.56E-15
1.49E-14
36.0
0.8
88.9
4.2
80.0
8.3
95.4
17.4
98.5
26.7
98.2
36.0
98.9
46.6
99.1
59.4
97.6
72.7
99.5
85.7
99.6
95.1
99.5
97.8
81.4
98.5
100.0
100.0
Total Gas Age tt:
"Ar Wtd. Mean Age:
210.0
242.9
463.4
459.7
458.3
459.1
460.8
472.7
459.2
462.3
462.5
462.6
388.9
475.3
452.9
452.4
56.6
5.1
5.4
4.1
3.2
5.6
2.9
3.2
4.8
2.7
4.0
8.3
128.9
19.4
2.5
1.5
(56.6)
(4.9)
(4.8)
(3.4)
(2.1)
(5.0)
(1.7)
(2.0)
(4.1)
(1.1)
(3.2)
(7.9)
(128.9)
(19.2)
5.81E-04
8.30E-04
2.53E-04
3.76E-04
9.72E-05
2.39E-06
4.75E-06
2.38E-05
1.52E-05
1.03E-05
1.07E-05
9.35E-05
1.44E-04
1.47E-04
2.01E-02
3.85E-02
1.59E-02
1.92E-02
3.23E-02
3.29E-02
3.33E-02
3.32E-02
3.32E-02
3.35E-02
3.32E-02
3.42E-02
3.34E-02
2.84E-02
7.86E-04
1.24E-03
4.99E-04
3.00E-04
2.38E-04
6.42E-05
7.13E-05
1.04E-04
1.81E-04
1.31E-04
8.44E-05
1.04E-04
4.79E-04
1.65E-04
4.29E-15
3.40E-15
5.81E-15
1.08E-14
2.24E-14
2.90E-13
4.29E-13
1.53E-13
6.99E-14
1.07E-13
1.48E-13
5.28E-14
2.02E-14
2.25E-14
63.8
99.8
62.7
48.2
99.7
98.8
99.6
96.3
100.0
100.0
100.0
100.0
99.9
75.6
495.7
414.6
597.4
402.9
483.5
472.2
471.3
457.7
474.0
470.3
473.6
460.8
471.3
424.7
117.7
91.9
62.8
83.6
12.8
2.4
2.5
3.9
3.7
3.1
2.9
11.5
18.8
22.0
2.7
1.5
(117.7)
(91.9)
(62.7)
(83.6)
(12.6)
(.9)
(1.1)
(3.2)
(3.0)
(2.1)
(1.7)
(11.3)
(18.6)
(21.9)
(15.4)
Sample AF16b-cl67-7@@
Furnace Furnace Temperature [K]
2.17E-03
923
3.75E-04
973
6.78E-04
1023
1.55E-04
1073
4.95E-05
1123
5.98E-05
1173
3.67E-05
1223
2.85E-05
1273
CI
1323
8.08E-05
1373
1423
1473
1523
1923
1.61E-05
1.32E-05
1.55E-05
6.28E-04
0.OOE+00
Sample AF16m-cl67-7@@
Furnace Furnace Temperature [K)
1.22E-03
923
7.85E-06
973
1.26E-03
1023
1.75E-03
1073
1.11E-05
1123
4.01E-05
1173
1.27E-05
1223
1.26E-04
1273
6.16E-07
1323
4.80E-07
1373
3.27E-07
1423
8.30E-07
1473
2.67E-06
1523
8.27E-04
1973
0.3
0.6
1.0
1.8
3.5
25.2
57.2
68.6
73.8
81.8
92.9
96.8
98.3
100.0
Total 3as Age tt:
"Ar Wtd. Mean Age:
469.2
469.2
Appendix 5.4. continued.
Increment
Ar/ * Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
36
Ar/4*Ar ( *Ar/4"Ar).,, 39Ar/ 4*Ar 39Ar/ 40Ar).rm 39ArK moles Cum.% 9Ar 40Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
40
t
**
Sample AF18b.51-cl37-3 @@
Laser Tube Current [A]
10
11
12
13
14
15
16
17
18
19
20
21
5.31E-04
6.87E-05
4.25E-05
4.21E-05
3.80E-05
2.79E-05
1.14E-05
9.41E-06
6.52E-05
2.83E-05
4.69E-05
5.38E-05
2.48E-05
5.52E-06
1.11 E-05
1.08E-05
1.81 E-05
2.09E-05
2.39E-05
1.99E-05
7.91E-05
9.67E-05
3.68E-04
1.63E-04
3.84E-02
3.72E-02
3.40E-02
3.30E-02
3.27E-02
3.21 E-02
3.18E-02
3.28E-02
3.33E-02
3.35E-02
3.47E-02
3.33E-02
4.32E-04
2.43E-04
4.20E-04
6.41 E-04
3.37E-04
6.34E-04
5.84E-04
5.95E-04
1.02E-03
1.22E-03
1.62E-03
1.46E-03
1.30E-14
1.29E-14
7.11E-15
4.45E-15
3.10E-15
3.48E-15
2.30E-15
2.97E-15
9.39E-16
6.17E-16
4.72E-16
1.17E-15
84.1
97.7
98.5
98.5
98.6
98.9
99.4
99.5
97.8
98.9
98.4
98.2
369.9
434.6
474.1
486.8
491.3
500.2
507.6
493.3
480.6
481.7
465.8
482.0
446.0
445.3
5.2
3.5
5.9
8.8
5.6
9.4
9.1
8.6
16.6
19.8
49.2
27.9
27.9
2.1
(4.8)
(2.6)
(5.3)
(8.4)
(5.0)
(9.0)
(8.7)
(8.2)
(16.4)
(19.6)
(49.1)
(27.8)
0.0
0.0
15.0
51.1
37.9
73.9
56.9
69.3
71.9
76.1
87.4
0.0
0.0
198.3
342.2
238.5
276.5
262.1
269.5
265.2
318.4
312.3
276.2
275.6
353.3
142.8
47.6
21.2
36.2
9.3
12.7
11.8
11.4
9.1
14.7
19.1
5.1
(353.3)
(142.8)
(47.6)
(21.1)
(36.2)
(9.1)
(12.6)
(11.7)
(11.3)
(8.9)
(14.6)
69.6
97.6
98.4
97.4
96.1
99.7
99.7
363.4
455.2
465.3
474.6
477.6
507.1
485.3
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF18b.a-cl37-3 §@
Laser mapping
1
2
3
4
5
6
7
8
9
10
11
3.47E-03
3.44E-03
2.88E-03
1.65E-03
2.1OE-03
8.75E-04
1.45E-03
1.03E-03
9.43E-04
8.02E-04
4.16E-04
2.37E-04
2.88E-04
1.27E-04
1.06e-04
1.98E-04
7.05E-05
8.74E-05
9.14E-05
1.04E-04
6.08E-05
1.02E-04
3.98E-03
1.13E-02
1.34E-02
2.54E-02
2.78E-02
4.63E-02
3.78E-02
4.47E-02
4.71 E-02
4.09E-02
4.80E-02
9.38E-04
1.85E-03
5.48E-04
7.31E-04
1.29E-03
9.96E-04
9.08E-04
1.14E-03
8.04E-04
7.90E-04
1.77E-03
5.60E-17
9.29E-17
1.011E-15
1.06E-15
1.06E-15
2.41 E-15
2.07E-15
1.98E-15
3.89E-15
2.95E-15
1.38E-15
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF18b.new-cl37-3 @@
Furnace Temperature [K]
1000
1050
1100
1150
1200
1250
1300
1.02E-03
7.35E-05
4.65E-05
8.05E-05
1.24E-04
5.43E-07
2.21E-07
2.62E-05
2.88E-05
5.87E-05
7.54E-05
5.95E-05
8.53E-05
3.09E-05
3.24E-02
3.53E-02
3.47E-02
3.36E-02
3.29E-02
3.19E-02
3.35E-02
6.34E-05
1.10E-04
9.54E-05
2.05E-04
1.54E-04
6.72E-05
1.03E-04
9.19E-14
7.60E-14
5.06E-14
3.43E-14
3.79E-14
6.85E-14
6.05E-14
18.2
33.3
43.4
50.2
57.7
71.3
83.3
(3.7)
(3.7)
(7.3)
(9.9)
(7.9)
(11.2)
(4.1)
Appendix 5.4. continued. 40 Ar 39 Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
36
Ar/*Ar ( 36Ar/4*Ar).rro, 39Ar/ 4 Ar 39Ar/4 Ar).,,, 39ArK moles Cum.% 3*Ar 4 Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
Increment
t
**
Continued: Sample AF18b.new
1350
1600
1900
7.82E-05
1.25E-04
1.34E-06
4.62E-05
1.98E-04
4.62E-05
3.36E-02
3.52E-02
4.52E-02
1.86E-04
2.51E-04
1.85E-04
2.21E-14
2.03E-14
4.17E-14
474.4
449.9
371.7
447.57
446.85
6.7
24.4
5.2
98.6
92.0
94.9
97.7
94.9
96.8
96.4
98.3
94.0
91.6
501.0
490.0
480.6
469.7
458.8
451.3
573.1
515.7
524.8
464.1
489.7
489.4
11.0
9.1
8.2
13.2
11.2
7.7
13.8
8.4
19.8
22.3
38.6
3.3
(9.2)
(6.9)
(5.8)
(11.9)
(9.7)
(5.5)
(12.0)
(5.7)
(18.8)
(21.6)
99.4
98.8
99.6
99.2
99.3
99.7
99.5
98.5
99.4
99.3
454.8
470.2
458.6
456.4
461.0
474.8
461.8
438.5
454.7
459.0
457.5
457.4
7.6
12.9
8.2
9.0
9.7
9.0
7.6
7.3
8.8
9.2
50.8
2.7
(5.2)
(11.6)
(6.0)
(7.1)
(7.9)
(7.0)
(5.1)
(5.0)
(6.9)
(7.3)
95.0
454.7
8.8
(8.5)
87.7
97.4
96.0
91.7
100.0
99.6
Total Gas Age:
"Ar Wtd. Mean Age:
(6.3)
(24.3)
(4.8)
2.45
2.33
Sample AF18b. 1-c/48-1 §@
Laser Total Fusion
71
72
73
74
75
76
77
78
79
80
3.62E-05
2.62E-04
1.63E-04
6.61 E-05
1.60E-04
9.63E-05
1.14E-04
4.92E-05
1.94E-04
2.75E-04
4.75E-05
3.48E-05
2.70E-05
9.15E-05
5.34E-05
3.29E-05
6.22E-05
2.21 E-05
9.54E-05
1.29E-04
4.17E-02
3.99E-02
4.21E-02
4.45E-02
4.44E-02
4.62E-02
3.49E-02
4.02E-02
3.77E-02
4.23E-02
6.47E-04
4.64E-04
4.58E-04
3.79E-04
7.64E-04
4.33E-04
5.40E-04
4.32E-04
1.08E-03
1.38E-03
6.49E-15
6.28E-15
8.15E-15
3.17E-15
7.07E-15
7.69E-1 5
2.81E-15
1.15E-14
2.08E-15
1.77E-15
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF18m. 1-c48-1 §@
Laser Total Fusion
51
52
53
54
55
56
57
58
59
60
1.01E-05
3.OOE-05
3.97E-07
1.52E-05
1.22E-05
2.95E-07
6.47E-06
3.72E-05
8.65E-06
1.08E-05
2.18E-05
8.27E-05
4.30E-05
4.22E-05
4.35E-05
3.55E-05
2.82E-05
2.43E-05
3.96E-05
4.28E-05
4.69E-02
4.49E-02
4.66E-02
4.67E-02
4.62E-02
4.48E-02
4.62E-02
4.85E-02
4.70E-02
4.65E-02
5.24E-04
5.97E-04
3.61 E-04
5.81 E-04
6.71 E-04
5.87E-04
4.36E-04
5.16E-04
5.84E-04
6.OOE-04
1.36E-14
3.94E-15
7.27E-15
9.44E-15
8.75E-15
7.93E-15
1.17E-14
1.18E-14
7.61E-15
7.16E-15
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF18m8.2. 1-c/37-3 @@
Laser Tube Current [A]
13.5
1.62E-04
3.37E-05
3.44E-02
6.31E-04
3.01E-15
6.6
Appendix 5.4. continued. 4" Ar! " Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
3*Ar/4*Ar 36 4
Increment
( Ar/ "Ar)r, 39Ar/4*Ar 39Ar/ 4 Ar) , 39ArK moles Cum.% 39Ar 4*Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
t
S
A
Continued; Sample AF18m8.2. 1
9.16E-06
1.19E-05
2.46E-05
7.41 E-05
5.26E-06
1.23E-04
1.35E-04
4.01 E-05
2.33E-05
3.39E-04
2.81E-04
1.72E-04
3.61 E-02
3.56E-02
3.58E-02
3.57E-02
3.71E-02
3.66E-02
9.67E-04
8.22E-04
7.08E-04
1.80E-03
2.98E-03
1.23E-03
7.72E-16
3.31E-15
3.96E-15
3.13E-16
2.11E-16
5.61E-16
81.7
99.5
88.9
99.4
97.6
99.0
98.3
97.5
98.8
99.6
100.0
96.1
Total Gas Age:
"Ar Wtd. Mean Age:
454.1
458.9
455.8
450.0
443.6
434.7
456.5
456.5
19.5
10.8
8.8
45.5
45.5
24.3
26.1
2.2
(19.3)
(10.5)
(8.5)
(45.4)
(45.5)
(24.2)
2.57E-02
3.05E-02
3.06E-02
3.04E-02
3.49E-02
3.50E-02
3.57E-02
3.58E-02
3.60E-02
3.59E-02
3.54E-02
3.41E-02
1.24E-03
2.31E-03
1.54E-03
9.79E-04
5.69E-04
4.64E-04
1.58E-04
2.38E-04
2.57E-04
1.30E-04
2.82E-04
9.80E-04
1.72E-16
1.15E-16
1.72E-16
7.03E-16
1.99E-15
5.60E-15
6.56E-15
7.48E-15
1.76E-14
1.43E-14
9.69E-15
7.34E-16
0.3
0.4
0.7
1.8
4.8
13.4
23.5
35.0
62.1
84.0
98.9
100.0
Total Gas Age:
"Ar Wtd. Mean Age:
71.5
99.2
60.7
89.0
96.4
98.6
98.5
99.0
98.8
99.5
99.7
95.7
456.6
523.3
336.7
476.7
453.9
461.0
452.5
453.6
451.5
455.2
461.0
459.9
455.2
455.1
134.2
125.4
150.5
41.3
17.8
13.6
12.9
12.5
12.2
12.1
12.7
4.4
0.0
4.1
(133.6)
(124.6)
(150.3)
(39.5)
(13.2)
(6.5)
(5.2)
(3.9)
(3.1)
(2.5)
(4.1)
(40.7)
4.89E-02
4.90E-02
4.90E-02
4.82E-02
4.27E-02
4.71E-02
4.59E-02
4.30E-02
4.67E-02
4.59E-04
4.12E-04
4.47E-04
7.16E-04
3.34E-04
6.47E-04
4.51 E-04
4.79E-04
7.82E-04
1.10E-14
1.34E-14
1.06E-14
9.66E-15
5.86E-15
7.23E-15
1.23E-14
4.71E-15
8.29E-15
98.5
98.3
98.1
97.2
90.2
97.8
95.0
93.2
98.0
441.1
438.9
438.3
441.4
459.7
452.6
451.9
470.0
457.2
447.6
447.6
20.3
17.8
20.9
22.9
22.4
19.6
14.7
28.9
19.0
51.0
6.6
(17.5)
(14.6)
(18.4)
(20.5)
(19.8)
(16.6)
(10.4)
(26.9)
(15.9)
Sample AF18mf.38-cl37-3 @@
Laser Tube Current [A]
11
11.5
12
12.5
13
13.5
14
14.5
15
16
18
20
9.61E-04
2.11E-05
1.33E-03
3.66E-04
1.14E-04
3.80E-05
4.16E-05
2.36E-05
3.08E-05
7.11E-06
1.90E-06
1.38E-04
7.94E-04
8.87E-04
1.00E-03
2.67E-04
9.37E-05
3.02E-05
4.06E-05
2.4 1E-05
1.05E-05
1.73E-05
2.05E-05
3.12E-04
Sample AF22b. 15-40-cl48-1 §§
Laser Total Fusion (planchette #)
15
3.69E-05
16
4.69E-05
17
5.1BE-05
18
8.18E-05
3.24E-04
6.41 E-05
1.57E-04
2.22E-04
5.47E-05
1.47E-04
1.22E-04
1.54E-04
1.66E-04
1.47E-04
1.30E-04
7.77E-05
2.02E-04
1.19E-04
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF22b.81-90-cl48-1 @@
Laser Total Fusion
Appendix 5.4. continued.
Increment
3*Ar/44Ar
Ar! " Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
39ArK moleS CUm.% 39Ar 4oAr*% Age (Ma) Age Error (w J)Age Error (w/o J)
( 36Ar/**Ar)e..o 39Ar/*Ar 39Ar/4"Ar),
40
t
**
Continued: Sample AF22b.81-90
2.29E-07
2.91E-07
2.90E-06
4.45E-07
3.53E-05
3.65E-07
2.64E-07
8.23E-06
3.45E-06
9.01 E-08
5.96E-05
7.OOE-05
6.27E-05
1.35E-04
9.93E-05
8.71E-05
6.80E-05
6.71E-05
9.42E-05
6.77E-05
4.78E-02
4.57E-02
4.66E-02
4.53E-02
4.66E-02
4.55E-02
4.72E-02
4.64E-02
4.74E-02
4.72E-02
4.54E-04
1.93E-04
4.01 E-04
6.97E-04
5.31E-04
4.47E-04
4.84E-04
6.23E-04
4.95E-04
3.53E-04
1.43E-14
1.17E-14
1.31E-14
6.05E-1 5
9.73E-15
9.21E-15
1.23E-14
1.39E-14
9.03E-15
1.24E-14
99.6
99.7
99.6
99.6
98.6
99.6
99.6
99.4
99.5
99.6
448.9
467.3
459.0
470.5
454.5
468.6
453.9
459.6
452.2
453.6
458.0
458.0
9.6
10.4
10.0
18.5
13.9
12.7
10.6
11.2
13.1
10.2
54.7
3.6
(8.0)
(8.7)
(8.3)
(17.7)
(12.8)
(11.4)
(9.1)
(9.7)
(11.9)
(8.6)
54.3
85.8
99.4
98.8
90.6
96.1
100.0
99.6
91.4
438.2
462.5
455.2
443.1
447.7
486.9
480.9
493.5
479.5
468.4
468.3
32.3
9.1
5.4
6.4
7.8
7.5
6.3
4.4
8.6
25.1
2.5
(32.2)
(8.7)
(4.9)
(5.9)
(7.4)
(7.0)
(5.7)
(3.6)
(8.2)
81.5
99.8
99.8
100.0
100.0
100.0
99.8
100.0
100.0
100.0
402.2
452.6
464.4
450.0
457.2
446.9
445.9
442.2
454.5
476.4
453.9
108.8
5.2
5.3
5.9
6.6
5.0
5.1
3.2
6.6
4.5
29.4
(108.8)
(4.6)
(4.7)
(5.4)
(6.2)
(4.4)
(4.5)
(2.2)
(6.2)
(3.7)
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF27p-c/67-5 §§
J Value
9.93E-03
±
5.93E-05
1.96E-02
2.92E-02
3.44E-02
3.53E-02
3.20E-02
3.08E-02
3.25E-02
3.15E-02
2.98E-02
6.54E-04
3.92E-04
3.21 E-04
4.06E-04
5.09E-04
3.78E-04
3.97E-04
1.23E-04
4.18E-04
1.88E-15
4.26E-15
7.52E-1 5
6.41 E-15
6.14E-15
5.07E-15
9.01E-15
7.50E-15
6.54E-15
Laser Total Fusion Analysis
1.55E-03
4.81E-04
2.03E-05
3.98E-05
3.17E-04
1.32E-04
0.OOE+00
1.29E-05
2.92E-04
1.39E-04
4.81E-05
2.59E-05
3.23E-05
3.04E-05
3.56E-05
2.OOE-05
2.48E-05
4.21E-05
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF28b-cl67-5 @@
Laser Total Fusion Analysis
6.25E-04
21
5.1OE-06
22
7.62E-06
23
9.68E-07
24
5.29E-07
25
5.98E-07
26
5.72E-06
27
6.18E-07
28
6.94E-07
29
1.09E-06
30
7.28E-04
1.1OE-05
9.76E-06
2.24E-05
1.93E-05
1.29E-05
1.77E-05
1.32E-05
1.11 E-05
2.14E-05
3.24E-02
3.48E-02
3.38E-02
3.51E-02
3.44E-02
3.53E-02
3.54E-02
3.58E-02
3.47E-02
3.29E-02
4.74E-03
3.87E-04
3.75E-04
4.19E-04
4.90E-04
3.68E-04
3.59E-04
1.42E-04
5.21 E-04
2.OOE-04
1.29E-16
9.14E-15
9.80E-15
5.60E-15
6.98E-15
9.39E-15
8.73E-15
8.94E-1 5
7.86E-1 5
6.67E-15
Total Gas Age:
Ar!/ Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
3Ar/4*Ar ( seAr/*Ar).,, 39Ar/ 4 Ar 39Ar/ 4*Ar),, 39ArK moles Cum.% "Ar 4*Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
Appendix 5.4. continued.
Increment
4
**
t
Continued: Sample AF28b
453.9
"Ar Wtd. Mean Age:
Sample AF32p-c167-6 @@
Laser Total Fusion Analysis
6.75E-05
51
3.78E-07
52
7.79E-05
53
7.89E-05
54
3.52E-05
55
5.37E-05
56
2.46E-05
57
1.36E-05
58
3.71E-05
59
5.72E-05
60
2.10E-05
2.04E-05
2.99E-05
2.24E-05
3.72E-05
3.83E-05
2.33E-05
2.76E-05
1.95E-05
2.13E-05
3.33E-02
3.37E-02
2.92E-02
3.10E-02
3.09E-02
3.41E-02
3.42E-02
3.35E-02
3.31E-02
3.36E-02
5.31 E-04
5.48E-04
2.92E-04
2.58E-04
7.15E-04
4.70E-04
5.70E-04
5.17E-04
5.30E-04
3.94E-04
98.0
100.0
97.7
97.7
99.0
98.4
99.3
99.6
98.9
98.3
Total Gas Age:
"Ar Wtd. Mean Age
464.3
467.1
518.6
493.4
499.5
455.9
458.9
468.6
470.5
461.5
473.8
473.7
7.4
7.5
6.6
5.2
11.5
7.6
7.7
7.6
7.4
5.9
32.7
6.55E-15
5.82E-15
4.27E-15
7.31E-15
5.02E-15
6.72E-15
6.76E-15
4.98E-15
7.64E-15
8.06E-15
(7.0)
(7.1)
(6.1)
(4.6)
(11.2)
(7.2)
(7.3)
(7.2)
(7.1)
(5.4)
2.3
Sample AF34b8.53- c137-3 §@
Laser Tube Current [A]
10.5
11
11.5
12
12.5
13
13.5
14
15
16
18
20
6.28E-04
9.33E-05
6.19E-05
3.47E-05
3.01E-05
2.17E-05
1.89E-05
1.81E-05
2.19E-07
2.07E-07
4.07E-05
3.94E-05
8.73E-05
7.65E-05
6.14E-05
3.68E-05
2.96E-05
2.65E-05
2.19E-05
2.41 E-05
2.28E-05
2.59E-05
2.48E-04
1.46E-03
4.65E-02
3.83E-02
3.67E-02
3.63E-02
3.62E-02
3.62E-02
3.64E-02
3.59E-02
3.63E-02
3.56E-02
3.49E-02
3.68E-02
9.06E-04
7.81E-04
6.55E-04
5.37E-04
4.62E-04
5.66E-04
3.96E-04
4.29E-04
3.04E-04
3.32E-04
5.64E-04
2.52E-03
2.23E-15
2.02E-15
2.61 E-15
4.07E-15
5.08E-15
5.99E-15
6.69E-15
6.26E-15
6.98E-15
7.06E-15
3.02E-15
5.51 E-16
4.3
81.2
97.0
8.1
97.9
13.0
98.7
20.8
98.8
30.4
99.1
41.8
99.2
54.6
99.2
66.5
99.7
79.8
99.7
93.2
98.5
99.0
98.6
100.0
Total Gas Age:
"Ar Wtd. Mean Age:
300.8
421.4
441.3
448.3
450.6
450.7
448.8
454.7
452.2
460.1
463.1
442.4
445.1
444.9
10.5
11.9
10.3
7.7
6.6
7.4
5.6
6.1
4.9
5.5
31.1
172.9
24.6
2.9
(10.4)
(11.6)
(10.1)
(7.3)
(6.2)
(7.0)
(5.0)
(5.6)
(4.3)
(4.9)
(31.0)
(172.9)
2.13E-04
9.18E-04
2.39E-04
3.49E-04
1.99E-04
2.01E-04
2.48E-04
4.84E-02
4.68E-02
4.73E-02
4.61 E-02
4.65E-02
4.68E-02
4.72E-02
3.67E-04
1.83E-03
3.55E-04
5.53E-04
6.10E-04
3.01 E-04
4.45E-04
8.40E-15
1.91E-15
7.47E-15
4.88E-15
8.63E-15
8.61E-15
7.20E-15
99.4
99.6
99.2
99.6
99.6
99.6
99.6
448.4
463.0
456.5
469.0
466.0
462.6
459.3
27.3
112.4
30.6
44.2
26.9
26.6
31.8
(25.3)
(111.9)
(28.7)
(42.9)
(24.8)
(24.4)
(30.0)
Sample AF34b1-c/48-2 @@
Laser Total Fusion Analysis
8.60E-06
1
1.56E-06
2
1.62E-05
3
6.05E-07
4
3.47E-07
5
3.50E-07
6
4.21E-07
7
Ar 39 Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
36Ar/ 4*Ar ( 36Ar/4 *Ar),,, 3'Ar/**Ar 3'Ar/ 4*Ar),,.., 3 9Ar moles Cum.% 3'Ar 44Ar*% Age (Ma) Age Error (w J) Age Error (w/o J)
Appendix 5.4. continued.
Increment
40
t
**
Continued: Sample AF34b1
2.62E-07
1.62E-05
1.46E-05
2.90E-05
3.67E-05
3.85E-05
1.73E-05
1.50E-04
1.39E-04
1.17E-04
1.78E-04
1.31E-04
1.71E-04
1.61E-04
4.70E-02
4.62E-02
4.77E-02
4.62E-02
4.69E-02
4.74E-02
4.72E-02
4.50E-04
5.61 E-04
5.63E-04
4.09E-04
4.29E-04
7.11E-04
4.48E-04
1.16E-14
1.23E-14
1.51E-14
8.59E-15
1.19E-14
9.21E-15
9.92E-15
99.6
99.2
99.2
98.8
98.6
98.5
99.1
461.3
466.0
453.5
465.1
457.6
453.1
457.6
459.3
459.3
21.2
20.6
18.0
24.4
19.3
23.7
22.3
62.5
6.9
(18.4)
(17.6)
(14.7)
(22.0)
(16.3)
(21.3)
(19.7)
98.0
97.7
99.7
98.3
98.6
97.9
98.1
97.7
99.6
99.1
98.2
97.8
98.5
98.8
98.5
98.5
98.5
98.5
98.5
99.6
99.6
99.1
99.4
99.5
99.6
99.6
98.4
99.6
98.9
455.9
449.0
473.6
482.8
478.6
452.0
457.5
465.2
483.5
468.5
462.4
473.2
479.3
482.8
492.5
466.0
471.6
466.4
456.5
466.8
472.0
466.2
451.8
484.6
461.6
471.4
472.9
471.9
499.2
24.8
36.6
25.8
26.4
24.9
24.0
25.9
25.1
25.6
24.5
25.2
26.1
26.0
26.0
26.4
24.4
26.3
25.0
24.2
25.3
25.3
24.1
23.7
25.4
24.2
24.7
25.5
24.4
409.8
(8.6)
(28.5)
(9.2)
(9.8)
(5.2)
(6.4)
(11.1)
(8.3)
(7.3)
(5.7)
(9.0)
(10.1)
(9.1)
(8.8)
(8.7)
(5.5)
(10.8)
(7.9)
(6.6)
(8.6)
(7.9)
(3.8)
(5.2)
(6.6)
(5.7)
(5.9)
(8.3)
(4.3)
(409.0)
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF35b-c/48-2 @@
Laser Total Fusion Analysis
41
5.67E-05
42
6.66E-05
2.11E-07
43
4.53E-05
44
45
3.74E-05
46
5.91E-05
47
5.31E-05
48
6.49E-05
49
1.09E-06
2.05E-05
50
52
4.87E-05
6.45E-05
53
4.1OE-05
54
3.05E-05
55
3.94E-05
56
3.79E-05
57
4.1OE-05
58
4.05E-05
59
3.96E-05
60
6.79E-07
61
4.95E-07
62
1.91 E-05
63
7.21E-06
64
7.26E-06
65
3.04E-07
66
3.93E-07
67
4.37E-05
68
4.51E-07
69
70
2.73E-05
6.22E-05
2.25E-04
6.96E-05
5.83E-05
3.84E-05
4.94E-05
7.58E-05
5.97E-05
5.02E-05
4.38E-05
7.13E-05
6.97E-05
6.49E-05
6.14E-05
4.84E-05
4.30E-05
7.58E-05
6.00E-05
5.25E-05
5.48E-05
4.85E-05
2.12E-05
1.96E-05
2.38E-05
2.16E-05
1.69E-05
6.08E-05
2.80E-05
3.11E-03
4.68E-02
4.75E-02
4.56E-02
4.40E-02
4.46E-02
4.73E-02
4.67E-02
4.56E-02
4.45E-02
4.59E-02
4.62E-02
4.48E-02
4.44E-02
4.42E-02
4.31 E-02
4.59E-02
4.53E-02
4.59E-02
4.70E-02
4.64E-02
4.58E-02
4.62E-02
4.80E-02
4.43E-02
4.70E-02
4.58E-02
4.51 E-02
4.58E-02
4.26E-02
4.86E-04
1.11E-03
3.54E-04
6.59E-04
1.98E-04
2.89E-04
7.21 E-04
4.31 E-04
3.98E-04
2.21 E-04
2.47E-04
5.47E-04
4.24E-04
4.29E-04
6.11E-04
1.56E-04
5.78E-04
3.22E-04
2.17E-04
6.13E-04
5.64E-04
3.1OE-04
5.60E-04
6.12E-04
5.89E-04
6.09E-04
3.78E-04
2.81 E-04
6.35E-03
7.31E-15
2.24E-15
8.17E-15
8.92E-15
1.06E-14
8.83E-15
9.19E-15
9.08E-15
9.61E-15
9.60E-15
1.1OE-14
1.11E-14
1.19E-14
1.29E-14
1.29E-14
1.54E-14
8.87E-15
1.15E-14
1.38E-14
8.OOE-15
1.09E-14
1.40E-14
1.21E-14
1.31E-14
1.26E-14
1.37E-14
1.09E-14
1.19E-14
1.76E-16
Appendix 5.4. continued.
Increment
Ar/ I Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
36Ar/*Ar ( 3sAr/*Ar).,r, 39Ar/4*Ar 3sAr/*Ar).,o, 39ArK moles Cum.% 39Ar 44Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
4*
t
**
Continued: Sample AF35b
1.15E-05
4.15E-05
1.09E-05
5.61E-06
3.65E-07
2.96E-04
7.68E-07
1.44E-05
8.14E-05
6.01E-05
7.73E-05
1.47E-03
9.94E-05
1.95E-03
2.02E-04
4.55E-03
4.50E-02
4.40E-02
4.27E-02
4.12E-02
4.27E-02
4.33E-02
4.13E-02
3.23E-02
5.42E-04
5.86E-04
4.03E-04
2.45E-03
5.02E-04
4.72E-03
1.17E-03
6.01E-03
7.44E-15
9.14E-15
6.85E-15
3.53E-16
5.37E-15
2.95E-16
2.60E-15
9.15E-17
99.3
98.4
99.4
99.5
99.7
91.0
99.7
99.3
477.4
483.6
500.7
516.9
501.6
456.9
515.9
635.3
472.2
472.2
26.8
26.3
27.5
198.9
28.9
259.9
39.4
730.0
96.5
4.3
(11.3)
(9.5)
(10.8)
(197.2)
(13.9)
(258.9)
(29.6)
(729.4)
100.0
99.3
100.0
100.0
99.8
99.4
100.0
99.9
100.0
99.4
485.3
499.9
432.8
495.0
468.2
485.1
455.9
441.8
495.7
483.5
476.4
476.3
10.4
20.1
39.2
16.8
10.2
13.2
11.1
19.4
8.3
20.8
24.2
4.5
(10.1)
(19.9)
(39.1)
(16.6)
(9.9)
(13.0)
(10.9)
(19.3)
(7.9)
(20.7)
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
99.9
100.0
473.7
461.7
473.9
460.4
499.7
468.0
428.9
478.2
450.4
471.3
466.9
466.9
3.9
7.8
10.0
6.4
6.8
8.1
63
7.8
8.0
5.5
30.2
2.2
(3.2)
(7.5)
(9.7)
(6.0)
(6.4)
(7.8)
(5.9)
(7.4)
(7.7)
(5.0)
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF37p-c67-6 @@
Laser Total Fusion Analysis
0.OOE+00
61
2.47E-05
62
0.OOE+00
63
3.30E-07
64
65
5.01E-06
66
1.91 E-05
0.OOE+00
67
3.56E-06
68
0.OOE+00
69
2.13E-05
70
6.39E-05
1.11 E-04
2.73E-04
1.1OE-04
6.05E-05
7.05E-05
6.40E-05
1.12E-04
5.29E-05
1.36E-04
3.23E-02
3.1OE-02
3.68E-02
3.16E-02
3.36E-02
3.21 E-02
3.47E-02
3.59E-02
3.15E-02
3.22E-02
4.69E-04
9.72E-04
2.28E-03
6.36E-04
5.41E-04
7.11E-04
6.69E-04
1.32E-03
3.OOE-04
8.82E-04
3.96E-15
2.27E-15
1.16E-15
2.44E-15
4.54E-15
3.75E-15
4.17E-15
2.88E-15
4.56E-15
1.93E-15
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF38p-c167-6 @@
Laser Total Fusion Analysis
4.91E-07
71
7.11E-07
72
9.79E-07
73
8.36E-07
74
1.1OE-06
75
6.42E-08
76
1.22E-06
77
8.66E-07
78
1.36E-06
79
9.72E-07
80
2.06E-05
2.54E-05
2.46E-05
1.87E-05
3.15E-05
2.40E-05
3.06E-05
2.03E-05
3.60E-05
2.70E-05
3.32E-02
3.42E-02
3.32E-02
3.43E-02
3.12E-02
3.37E-02
3.71 E-02
3.28E-02
3.51 E-02
3.34E-02
1.54E-04
5.74E-04
7.33E-04
4.71 E-04
3.50E-04
5.87E-04
4.64E-04
5.47E-04
5.62E-04
3.06E-04
7.26E-15
5.22E-15
6.21E-15
7.43E-15
4.74E-15
6.34E-15
5.33E-15
6.36E-15
4.51E-15
5.13E-15
Total Gas Age:
"Ar Wtd. Mean Age:
Appendix 5.4. continued. 40 Ar 39 Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
3
4
*Ar/ 4Ar ( 38Ar/4"Ar).,,, 39Ar/*Ar 3*Ar/44Ar),, 3 9ArK moles Cum.% 39Ar 44Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
Increment
t
§
#
**
Sample AF38p 12.f-cl67-6@@
Temperature [T]
973
1073
1123
1173
1223
1298
1323
1348
1378
1423
1473
1523
1573
1673
1923
1.89E-03
1.25E-03
7.34E-05
4.68E-05
6.84E-06
5.88E-08
0.OOE+00
0.OOE+00
1.84E-06
0.OOE+00
3.25E-05
0.OOE+00
0.OOE+00
0.OOE+00
1.45E-03
5.57E-05
6.89E-05
9.08E-05
7.18E-06
1.23E-05
8.44E-06
4.08E-06
7.86E-06
1.71E-06
1.38E-06
6.45E-05
8.78E-05
4.36E-05
2.91E-04
3.83E-04
4.09E-02
5.39E-02
3.38E-02
3.18E-02
3.23E-02
3.42E-02
3.43E-02
3.47E-02
3.46E-02
3.47E-02
3.53E-02
3.60E-02
3.48E-02
3.38E-02
2.40E-02
2.51 E-04
6.1BE-04
8.30E-05
1.02E-04
7.77E-05
6.82E-05
4.76E-05
5.11E-05
5.63E-05
6.46E-05
1.24E-04
1.90E-04
3.80E-04
3.21 E-04
9.90E-04
9.30E-14
1.36E-13
6.49E-14
1.42E-13
2.22E-13
3.60E-13
4.93E-13
7.24E-13
1.06E-12
1.14E-12
1.10E-13
2.60E-14
1.86E-14
1.99E-14
3.79E-15
2.0
44.1
5.0
63.1
6.4
97.8
9.5
98.6
14.3
99.8
100.0
22.1
100.0
32.8
48.5
100.0
71.5
99.9
100.0
96.1
98.5
99.0
100.0
99.1
100.0
99.5
100.0
99.9
100.0
57.2
Total Gas Age:
"Ar Wtd. Mean Age:
184.0
199.2
457.8
486.0
485.1
461.9
459.9
455.1
456.3
456.1
445.3
440.5
454.3
466.2
384.9
446.7
445.8
6.8
6.7
11.4
2.9
3.0
2.6
2.4
2.5
2.3
2.3
8.0
10.6
7.1
35.6
70.1
4.1
0.8
(6.7)
(6.7)
(11.1)
(1.7)
(1.9)
(1.3)
(.8)
(1.1)
(.7)
(.8)
(7.7)
(10.3)
(6.8)
(35.6)
(70.1)
3.42E-05
4.19E-05
2.56E-05
2.50E-05
2.22E-05
2.86E-05
2.69E-05
1.67E-05
2.82E-05
3.01 E-02
3.12E-02
3.46E-02
3.52E-02
3.39E-02
3.09E-02
3.49E-02
3.36E-02
3.17E-02
6.61 E-04
2.17E-04
3.47E-04
4.05E-04
5.97E-04
3.30E-04
6.49E-04
4.79E-04
3.08E-04
4.39E-15
4.70E-15
7.15E-15
6.41E-15
7.28E-15
6.15E-15
6.30E-15
8.55E-15
5.70E-15
100.0
100.0
99.7
98.5
99.9
99.4
100.0
100.0
100.0
514.6
498.8
454.3
442.7
463.0
500.0
451.5
467.1
492.2
473.0
472.9
11.2
6.7
5.6
5.9
8.0
6.5
8.4
6.7
6.1
29.5
2.4
(10.8)
(6.2)
(5.1)
(5.4)
(7.7)
(6.0)
(8.1)
(6.2)
(5.5)
80.7
97.8
97.0
97.6
350.0
412.9
422.2
421.9
7.1
9.9
20.9
29.7
(6.0)
(8.9)
(20.4)
(29.3)
Sample AF40p-cl67-5 §§
Laser Total Fusion Analysis
42
6.06E-07
0.OOE+00
43
9.01 E-06
44
5.12E-05
45
2.27E-06
46
2.15E-05
47
0.OOE+00
48
3.22E-08
49
0.OOE+00
50
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF44b2. 100-c/37-1 @
Laser Tube Current [A]
1
2
3
4
6.46E-04
6.30E-05
9.33E-05
7.02E-05
3.97E-05
5.30E-05
1.25E-04
2.24E-04
3.90E-02
3.94E-02
3.81E-02
3.84E-02
4.64E-04
7.05E-04
1.47E-03
1.49E-03
7.92E-15
3.91 E-15
1.60E-15
8.42E-16
44.7
66.8
75.9
80.6
Appendix 5.4. continued. ' Ar! I Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
3
3
39Ar/ 4*Ar 39Ar/ 4 Ar) , 3 9Ar moles Cum.% 3*Ar 4"Ar*%
Increment
6Ar/*Ar ( *Ar/Ar).r,
Age (Ma) Age Error (w J)Age Error (w/o J)
**
t
Continued: Sample AF44b2. 100
5
6
7
8
9
10
11
1.28E-04
3.14E-05
1.21E-04
3.35E-05
1.17E-05
1.45E-04
2.71E-03
2.54E-04
3.66E-04
2.67E-04
2.64E-04
5.96E-04
5.08E-04
4.97E-03
3.84E-02
3.80E-02
3.70E-02
3.72E-02
3.81E-02
3.73E-02
4.11E-02
1.70E-03
1.93E-03
2.08E-03
1.78E-03
2.26E-03
1.89E-03
4.78E-03
7.76E-16
5.19E-16
6.95E-16
6.86E-16
3.16E-16
3.65E-16
7.52E-17
96.0
85.0
87.9
98.8
91.9
96.2
95.7
98.7
97.5
99.4
99.6
95.5
100.0
19.8
Total Gas Age:
"Ar Wtd. Mean Age:
415.8
430.6
430.3
438.6
431.7
424.4
88.1
386.8
387.6
33.7
46.4
38.3
36.2
71.7
62.5
635.3
18.8
5.9
(33.4)
(46.1)
(38.0)
(35.9)
(71.6)
(62.3)
(635.3)
6.52E-05
3.92E-05
2.21E-05
1.24E-05
2.99E-05
2.25E-05
1.37E-05
3.01E-05
4.71E-05
3.32E-05
5.45E-02
5.59E-02
5.35E-02
4.79E-02
5.13E-02
5.25E-02
5.13E-02
4.85E-02
5.11E-02
5.48E-02
8.07E-04
5.85E-04
5.12E-04
2.81E-04
5.55E-04
2.91E-04
3.85E-04
6.37E-04
7.35E-04
3.65E-04
2.91E-15
6.81E-15
1.OOE-14
1.38E-14
9.02E-15
1.10E-14
1.46E-14
9.37E-15
7.45E-15
8.76E-15
99.5
99.6
99.6
99.5
99.6
99.6
99.6
99.8
98.9
99.6
413.3
404.4
420.8
463.4
436.6
427.3
436.2
459.7
435.4
411.5
434.6
434.5
12.0
9.6
9.2
9.2
10.0
8.8
9.0
10.9
11.4
9.0
44.5
3.1
(9.0)
(5.7)
(4.4)
(2.8)
(5.4)
(3.3)
(3.3)
(6.4)
(7.8)
(4.4)
62.3
96.9
97.2
96.9
99.2
99.2
99.3
99.6
99.6
99.1
99.4
99.4
327.4
340.8
353.8
390.0
408.5
402.3
409.0
408.8
418.8
420.1
417.2
386.5
400.9
400.8
19.1
17.1
15.2
9.9
10.0
7.2
7.1
8.3
74.1
6.5
9.8
74.4
23.9
2.9
(18.8)
(16.7)
(14.7)
(9.0)
(9.0)
(5.7)
(5.6)
(7.1)
(73.9)
(4.6)
(8.7)
(74.3)
Sample AF45b 1-c/48-3 §@
Laser Total Fusion
21
22
23
24
25
(A
26
27
28
29
30
1.88E-06
0.OOE+00
5.53E-08
5.39E-06
0.OOE+00
0.OOE+00
0.OOE+00
0.OOE+00
2.51E-05
0.OOE+00
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF45b8.16-c137-1 @@
Laser Tube Current [A]
10.5
11
11.5
12
12.5
13
13.5
14
15
16
18
20
1.27E-03
9.33E-05
8.26E-05
9.63E-05
1.66E-05
1.69E-05
1.46E-05
3.85E-06
4.42E-06
1.91E-05
1.19E-05
1.09E-05
1.11E-04
1.49E-04
1.28E-04
7.87E-05
6.24E-05
3.77E-05
4.52E-05
4.49E-05
6.22E-04
2.85E-05
4.11 E-05
5.73E-04
3.24E-02
4.83E-02
4.65E-02
4.16E-02
4.05E-02
4.12E-02
4.04E-02
4.06E-02
3.95E-02
3.92E-02
3.96E-02
4.31E-02
1.09E-03
1.39E-03
1.12E-03
3.75E-04
6.54E-04
4.56E-04
2.99E-04
5.71 E-04
2.86E-03
3.52E-04
7.84E-04
5.57E-03
1.92E-15
1.84E-15
2.14E-15
3.06E-15
4.38E-15
6.33E-15
5.99E-15
5.16E-15
4.47E-16
1.06E-14
2.49E-15
1.84E-16
4.3
8.5
13.3
20.2
30.0
44.2
57.7
69.3
70.3
94.0
99.6
100.0
Total Gas Age:
"Ar Wtd. Mean Age:
increment
3
Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
36Ar/ 4*Ar ( 3*Ar/4*Ar),,o, 39Ar/ 4*Ar 39Ar/4*Ar).,ror 39ArK moles Cum.% 39Ar 4"Ar*% Age (Ma) Age Error (w J) Age Error (w/o J)
Appendix 5.4. continued.
40 Ar!
t
**
Sample AF45mf.4-cl37-1 @@
Laser Tube Current [A]
13.5
13.75
14
14.25
14.5
14.75
15
15.5
16
17
18
20
5.47E-05
2.40E-04
8.50E-05
1.24E-04
1.39E-04
1.04E-04
3.85E-05
9.83E-05
1.27E-04
1.69E-05
7.50E-04
1.90E-04
1.14E-05
9.20E-05
2.66E-05
4.61 E-05
7.1OE-05
6.03E-05
3.04E-05
6.49E-05
7.44E-05
6.73E-05
3.40E-03
4.82E-04
3.86E-02
3.50E-02
3.71 E-02
3.65E-02
3.59E-02
3.61 E-02
3.56E-02
3.75E-02
3.65E-02
3.58E-02
3.94E-02
3.77E-02
5.81E-04
6.31E-04
1.16E-03
1.31E-03
1.27E-03
1.67E-03
1.08E-03
1.14E-03
1.01E-03
1.09E-03
4.51E-03
1.26E-03
4.98E-15
7.35E-16
2.86E-15
1.38E-15
9.10E-16
8.53E-16
2.OOE-15
1.22E-15
8.65E-16
1.65E-15
1.61E-16
1.12E-15
98.1
26.6
92.7
30.5
97.2
45.8
96.1
53.1
95.6
58.0
96.7
62.5
98.6
73.2
96.8
79.7
84.4
96.0
99.2
93.1
77.6
94.0
100.0
94.1
Total Gas Age:
"'Ar Wtd. Mean Age:
421.4
437.6
433.1
435.2
439.9
441.5
455.0
427.9
435.1
454.9
335.0
414.6
433.1
433.1
7.4
14.2
13.3
15.7
16.9
20.0
13.7
14.6
14.6
15.4
396.7
57.4
20.1
5.4
(5.8)
(13.5)
(12.5)
(15.0)
(16.2)
(19.5)
(12.8)
(13.9)
(13.9)
(14.6)
(396.7)
(57.2)
6.38E-06
1.03E-05
4.57E-05
6.77E-05
3.07E-05
1.66E-05
3.22E-05
1.21E-05
3.59E-02
3.60E-02
3.69E-02
3.74E-02
3.66E-02
3.79E-02
3.68E-02
3.75E-02
2.02E-04
3.95E-04
9.69E-04
3.26E-04
1.02E-03
6.26E-04
4.98E-04
6.64E-04
1.03E-14
8.36E-15
1.53E-15
1.76E-15
2.48E-15
5.66E-15
3.14E-15
5.49E-15
98.1
96.8
98.6
97.1
97.6
98.2
97.9
98.0
449.8
443.4
440.7
429.9
440.3
428.8
439.3
432.3
440.2
440.2
5.4
6.5
12.5
9.7
12.5
8.0
8.0
8.4
22.9
2.7
(2.4)
(4.5)
(11.6)
(8.5)
(11.5)
(6.6)
(6.5)
(7.0)
74.2
91.1
89.1
93.1
97.2
98.1
98.4
98.8
98.4
97.6
352.0
399.7
420.6
457.2
441.5
445.8
426.5
434.1
426.2
431.6
78.9
69.5
34.8
18.3
9.9
7.3
6.0
6.7
6.8
9.1
(78.8)
(69.4)
(34.5)
(17.6)
(8.7)
(5.5)
(3.9)
(4.8)
(5.1)
(7.8)
Sample AF45m.b-cl37-1 §@
Laser Mapping
1
5.47E-05
9.80E-05
3.95E-05
8.90E-05
7.13E-05
5.11 E-05
6.34E-05
5.84E-05
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF45m8.8-c37-1 §§
Laser Tube Current [A]
11.5
12
12.25
12.5
13
13.5
14
15
16
8.67E-04
2.93E-04
3.60E-04
2.25E-04
8.70E-05
5.51 E-05
4.55E-05
3.20E-05
4.39E-05
7.23E-05
5.80E-04
5.84E-04
2.66E-04
1.11 E-04
6.82E-05
2.82E-05
1.72E-05
1.83E-05
2.45E-05
5.28E-05
3.57E-02
3.81E-02
3.52E-02
3.34E-02
3.63E-02
3.63E-02
3.82E-02
3.76E-02
3.83E-02
3.74E-02
3.11E-03
1.62E-03
9.28E-04
8.55E-04
2.89E-04
3.98E-04
3.42E-04
4.18E-04
4.27E-04
4.76E-04
3.54E-16
3.78E-16
7.20E-16
1.55E-15
2.89E-15
7.22E-15
1.17E-14
1.13E-14
8.28E-15
3.77E-15
0.7
1.4
2.7
5.6
11.0
24.4
46.2
67.2
82.6
89.6
Appendix 5.4. continued.
Ar! * Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
36Ar/ 4 Ar ( 3
39 4
Increment
6Ar/**Ar).,,
Ar/ *Ar 39Ar/ 4*Ar),,, 39ArK moles Cum.% aAr 4*Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
**
t
Continued: Sample AF45m8.8
2.34E-05
1.32E-05
40
3.53E-05
1.91E-04
3.80E-02
3.61E-02
5.51E-04
1.05E-03
4.76E-15
8.30E-16
98.5
99.0
100.0
99.3
Total Gas Age:
"Ar Wtd. Mean Age:
430.9
452.2
432.8
432.7
8.3
25.9
27.0
2.6
(6.9)
(25.5)
8.74E-03
1.70E-02
2.26E-02
2.52E-03
2.76E-02
3.67E-02
3.62E-02
3.37E-02
3.58E-02
3.75E-02
3.66E-02
3.71E-02
3.83E-02
3.76E-02
3.69E-02
3.75E-02
3.91 E-02
4.23E-03
2.44E-02
2.36E-02
1.04E-02
2.84E-02
9.59E-03
3.18E-03
1.95E-03
2.57E-04
2.98E-04
5.37E-04
6.01 E-04
5.65E-04
3.35E-04
3.68E-04
1.07E-03
9.11E-03
1.43E-17
5.24E-18
1.02E-17
1.65E-18
9.26E-18
5.13E-17
1.70E-16
4.90E-16
1.04E-14
8.68E-15
5.15E-15
3.82E-15
6.86E-15
9.11E-15
7.46E-15
1.29E-15
7.20E-17
0.0
0.0
0.1
0.1
0.1
0.2
0.5
1.4
20.7
37.0
46.6
53.7
66.5
83.5
97.5
99.9
100.0
Total Gas Age:
"Ar Wtd. Mean Age:
98.2
90.4
93.2
95.4
90.9
97.6
99.1
99.5
98.0
99.7
99.7
99.7
99.7
99.7
99.7
99.6
98.2
1390.7
789.9
640.4
2871.3
528.6
439.0
450.2
481.0
450.2
438.8
448.3
442.8
431.0
437.7
445.2
439.2
416.6
442.9
442.7
630.3
1917.2
1608.3
6225.8
1366.6
307.2
75.8
36.7
5.7
5.8
7.9
8.8
7.5
6.0
6.5
15.5
197.2
24.9
2.2
(630.2)
(1,917.2)
(1,608.3)
(6,225.7)
(1,366.6)
(307.1)
(75.7)
(36.3)
(3.0)
(3.4)
(6.3)
(7.5)
(6.0)
(3.7)
(4.4)
(14.8)
(197.2)
3.41 E-02
3.40E-02
3.23E-02
3.63E-02
3.78E-02
3.78E-02
3.75E-02
3.73E-02
3.79E-02
3 65E-02
3.80E-02
3.80E-02
2.53E-03
1.45E-03
8.42E-04
2.16E-03
9.OOE-04
4.61E-04
3.74E-04
3.57E-04
4.47E-04
9.91 E-04
1.71E-03
1.73E-03
2.15E-16
4.OOE-16
4.19E-16
4.69E-16
1.47E-15
8.91E-15
8.95E-15
1.07E-14
1.56E-14
1.52E-15
5.66E-16
3.75E-16
81.7
96.0
98.0
99.5
99.6
99.2
98.4
98.6
99.0
99.7
99.5
99.4
399.5
463.5
492.7
450.9
436.1
433.5
434.4
437.0
432.6
449.2
433.6
432.4
58.2
66.2
48.5
34.6
13.7
6.7
6.2
6.1
6.5
14.1
24.8
33.2
(58.0)
(66.0)
(48.2)
(34.3)
(12.9)
(4.9)
(4.1)
(3.9)
(4.6)
(13.3)
(24.4)
(32.9)
Sample AF45m11. 13-c137-1 @@
Laser Tube Current [A)
11
11.5
12
12.25
12.5
13
13.5
14
15
15.1
15.25
15.5
16
16.5
17
18
20
6.02E-05
3.20E-04
2.25E-04
1.55E-04
3.03E-04
7.12E-05
2.14E-05
7.OOE-06
5.95E-05
4.22E-07
7.05E-07
9.89E-07
5.40E-07
4.03E-07
5.01E-07
2.90E-06
5.28E-05
1.44E-03
8.06E-03
8.83E-03
4.33E-03
8.62E-03
2.46E-03
5.66E-04
2.15E-04
8.65E-06
1.11 E-05
2.07E-05
3.37E-05
1.60E-05
1.20F,-05
1.63E-05
8.43E-05
1.59E-03
Sample AF45m1 1. 12--cI37-1 @@
Laser Tube Current [A]
11
12
12.25
12.5
12.75
13.25
13.75
14.25
15.25
15.75
16
16.5
6.12E-04
1.26E-04
5.90E-05
7.44E-06
2.60E-06
1.70E-05
4.31E-05
3.75E-05
2.37E-05
2.29E-06
6.44E-06
9.97E-06
3.97E-04
5.08E-04
3.62E-04
2.09E-04
7.80E-05
1.05E-05
1.11E-05
9.23E-06
7.27E-06
6.62E-05
1.49E-04
2.45E-04
0.4
1.2
2.1
3.0
5.9
23.8
41.6
63.0
94.3
97.3
98.4
99.2
Ar! 39 Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
36Ar/ 40Ar ( 3
aAr/ 40Ar)e,,o, 39Ar/40 Ar 39Ar/4 *Ar).,,,, 39ArK moles Cum.% 39Ar 4OAr*% Age (Ma) Age Error (w J) Age Error (w/o J)
Appendix 5.4. continued.
Increment
40
t
$
#
Continued: Sample AF45m11.12
17
18
1.63E-05
4.43E-04
**
4.95E-04
1.13E-03
3.76E-02
3.71 E-02
3.49E-03
2.46E-03
2.35E-16
1.73E-16
99.2
99.7
100.0
86.7
Total Gas Age:
"Ar Wtd. Mean Age:
435.8
391.0
435.3
435.3
67.5
137.6
30.9
2.5
(67.3)
(137.6)
3.08E-04
6.85E-04
2.48E-04
8.1OE-05
5.25E-05
1.37E-04
5.35E-05
1.50E-04
1.46E-05
4.75E-05
2.06E-04
9.19E-03
6.41 E-02
4.06E-02
2.14E-02
3.27E-02
3.45E-02
3.43E-02
3.41E-02
3.40E-02
3.42E-02
2.24E-02
6.90E-04
1.71 E-03
3.45E-04
2.99E-04
1.74E-04
2.49E-04
2.15E-04
1.23E-04
1.79E-04
1.09E-04
4.91 E-04
2.35E-15
7.52E-15
1.30E-14
1.81E-14
4.50E-14
3.14E-14
3.04E-14
3.51 E-14
7.32E-14
1.45E-13
5.07E-15
31.5
0.6
84.3
2.4
91.4
5.6
67.8
10.1
92.1
21.2
98.9
28.9
100.0
36.4
97.6
45.0
99.0
63.1
100.0
98.8
95.4
100.0
Total Gas Age:
"Ar W td. Mean Age:
530.4
222.2
365.5
496.2
447.1
453.4
460.3
452.6
459.4
461.6
639.6
455.2
454.7
137.3
50.5
26.7
16.7
7.3
16.8
7.3
18.3
3.6
6.3
36.5
2.4
3.3
(137.2)
(50.5)
(26.6)
(16.5)
(7.0)
(16.6)
(6.9)
(18.2)
(2.8)
(5.9)
(36.4)
4.50E-04
2.51 E-04
1.12E-04
1.20E-04
4.82E-05
1.66E-05
5.95E-06
7.08E-06
2.21E-05
1.08E-05
5.25E-06
3.09E-05
9.42E-05
3.06E-02
4.55E-02
2.28E-02
2.97E-02
3.50E-02
3.44E-02
3.44E-02
3.43E-02
3.45E-02
3.45E-02
3.43E-02
3.48E-02
3.22E-02
3.59E-04
6.72E-04
2.48E-04
1.91 E-04
3.32E-04
1.07E-04
3.66E-05
5.67E-05
1.10E-04
1.08E-04
7.95E-05
1.12E-04
2.99E-04
8.75E-15
1.20E-14
1.48E-14
2.25E-14
3.85E-14
1.53E-13
3.48E-13
1.83E-13
9.82E-14
1.19E-13
1.61E-13
7.01E-14
2.68E-14
64.3
0.7
91.5
1.7
63.5
2.8
4.6
83.2
97.4
7.7
98.8
19.8
99.3
47.6
99.3
62.1
97.9
70.0
98.9
79.5
99.3
92.3
99.0
97.9
99.2
100.0
Total Gas Age:
"Ar Wtd. Mean Age:
341.2
327.8
439.7
442.4
439.7
452.2
454.1
454.8
446.7
451.5
455.2
448.2
480.9
450.8
450.7
64.4
24.7
20.9
17.0
7.2
3.4
2.6
2.7
3.8
3.0
2.7
4.6
12.7
2.6
1.2
(64.4)
(24.6)
(20.7)
(16.8)
(6.8)
(2.3)
(.8)
(1.1)
(2.9)
(1.8)
(1.1)
(3.9)
(12.5)
Sample AF49b- c167-7 @@
Furnace Temperature [K]
900
950
1000
1050
1100
1150
1200
1250
1300
1550
1973
Sample AF49mFurnace Temperature [K]
923
973
1023
1073
1123
1173
1223
1273
1323
1373
1423
1523
1923
2.32E-03
5.31E-04
2.90E-04
1.09E-03
2.66E-04
3.54E-05
2.18E-07
7.92E-05
3.45E-05
8.57E-08
1.56E-04
c167-7 §@
1.21E-03
2.86E-04
1.23E-03
5.69E-04
8.78E-05
4.16E-05
2.45E-05
2.47E-05
7.19E-05
3.58E-05
2.36E-05
3.34E-05
2.50E-05
Appendix 5.4. continued. '* Ar/ * Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
Increment
asAr/ 4*Ar ( 36Ar/ 4 Ar).,, 3*Ar/*Ar 3*Ar/4*Ar)., 3 "r moles Cum.% asAr 4"Ar*% Age (Ma) Age Error (w J)Age Error (w/o J)
**
t
Sample AF52p- c/67-7 @@
Furnace Temperature [K]
2.50E-03
923
0.OOE+00
973
1.85E-03
1023
1.48E-03
1073
2.57E-04
1123
5.95E-05
1173
4.27E-05
1223
1.40E-05
1273
6.59E-06
1373
6.37E-06
1423
9.21E-06
1523
5.29E-04
1973
3.1 1E-04
1.89E-03
2.53E-04
1.53E-04
1.97E-04
6.34E-05
6.47E-06
3.1OE-06
4.71E-06
5.29E-06
5.88E-06
7.04E-05
5.04E-02
1.83E-01
4.04E-02
3.18E-02
5.83E-02
3.16E-02
3.12E-02
3.20E-02
3.37E-02
3.42E-02
3.43E-02
3.15E-02
5.87E-04
1.42E-03
3.64E-04
2.58E-04
1.91E-04
1.28E-04
8.08E-05
4.09E-05
2.35E-05
4.31E-05
2.87E-05
1.75E-04
1.26E-14
2.12E-14
1.60E-14
2.39E-14
4.30E-14
3.35E-14
1.13E-13
2.36E-13
5.52E-13
8.OOE-13
6.95E-13
3.17E-14
2.06E-04
2.88E-04
1.79E-04
7.246-05
4.98E-05
2.98E-05
3.12E-05
2.92E-05
3.93E-05
4.91E-05
2.59E-04
2.94E-04
1.06E-03
6.68E-04
9.44E-03
8.54E-02
2.33E-02
2.98E-02
3.78E-02
3.78E-02
3.67E-02
3.60E-02
3.69E-02
3.77E-02
3.72E-02
4.01E-02
3.94E-02
1.90E-02
2.30E-04
1.00E-03
2.94E-04
1.25E-04
1.56E-04
1.87E-04
1.37E-04
1.34E-04
2.12E-04
3.29E-04
4.84E-04
5.26E-04
2.51E-03
1.31E-03
7.03E-15
1.91E-14
1.92E-14
3.82E-14
5.16E-14
6.18E-14
7.17E-14
7.70E-14
6.40E-14
4.27E-14
1.86E-14
5.67E-15
2.01E-15
2.68E-15
0.4
26.1
1.2
100.0
1.7
45.4
56.3
2.5
92.4
4.0
98.2
5.1
9.0
98.7
17.1
99.6
99.8
47.8
99.8
75.1
99.7
98.9
84.4
100.0
Total Gas Age:
"Ar Wtd. Mean Age:
90.2
95.0
190.2
291.4
263.2
484.5
492.7
484.6
464.5
458.2
456.7
424.3
455.6
454.8
31.0
51.6
29.8
21.7
15.6
8.7
3.0
2.7
2.6
2.6
2.6
9.8
2.8
1.0
(31.0)
(51.6)
(29.8)
(21.7)
(15.5)
(8.3)
(1.4)
(.7)
(.6)
(.8)
(.8)
(9.6)
5.8
57.7
53.0
73.3
95.1
96.8
98.1
97.9
97.6
96.4
92.9
92.0
76.5
99.4
106.4
117.6
369.3
395.4
403.9
410.4
426.7
432.6
422.4
409.6
401.4
371.6
319.2
756.5
109.3
16.9
33.6
10.7
6.1
4.3
4.4
4.3
5.4
6.7
30.0
32.1
120.8
129.9
(109.3)
(16.9)
(33.5)
(10.5)
(5.8)
(3.8)
(3.8)
(3.7)
(5.0)
(6.4)
(30.0)
(32.1)
(120.8)
(129.8)
31.0
58.4
41.7
(30.9)
(58.3)
(41.6)
Sample AF66p- c167-6 §§
Furnace Temperature [KI
923
973
1023
1073
1123
1173
1223
1273
1323
1373
1423
1473
1523
1973
3.19E-03
1.43E-03
1.59E-03
9.02E-04
1.67E-04
1.09E-04
6.25E-05
7.OBE-05
7.96E-05
1.22E-04
2.38E-04
2.72E-04
7.96E-04
1.89E-05
1.5
5.4
9.4
17.4
28.1
40.9
55.8
71.8
85.1
94.0
97.9
99.0
99.4
100.0
Total Gas Age:
400.5
'Ar Wtd. Mean Age.
399.0
Sample A F67m-cl67-5 @@
Furnace Temperature
973
1023
1073
IKI
8.92E-04
1.39E-03
1.17E-03
1.94E-04
2.54E-04
2.01E-04
3.19E-02
1.74E-02
1.98E-02
1.55E-03
2.53E-04
4.12E-04
5.47E-15
5.51E-15
9.69E-15
1.2
2.5
4.6
73.6
590
65.5
372.6
523.3
512.0
39
Appendix 5.4. continued. 40 Ar
Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
Increment
Ar/4*Ar ( 3aAr/4"Ar)e,,,
36
39Ar/**Ar
Ar/ 4"Ar)e,,,or 39ArK moles Cum.%
39
39Ar 4"Ar*%
t
Age (Ma) Age Error (w J)Age Error (w/o J)
**
Continued: Sample AF67m
1123
1173
1223
1273
1323
1373
1423
1473
1523
1973
1.60E-04
4.69E-05
5.31 E-05
6.32E-05
7.38E-05
7.61E-05
5.44E-05
1.51E-04
5.63E-04
2.65E-04
3.99E-05
1.14E-05
2.09E-05
4.12E-05
7.54E-05
4.05E-05
7.08E-05
1.59E-04
8.88E-04
3.92E-04
3.46E-02
3.45E-02
3.49E-02
3.44E-02
3.44E-02
3.47E-02
3.45E-02
3.38E-02
3.15E-02
1.74E-02
2.27E-04
1.62E-04
1.27E-04
1.93E-04
3.11E-04
2.21E-04
1.99E-04
7.02E-04
1.60E-03
6.43E-04
1.96E-14
1.38E-13
1.12E-13
5.22E-14
3.02E-14
3.12E-14
2.39E-14
1.24E-14
3.34E-15
3.74E-15
9.0
39.9
64.9
76.5
83.3
90.3
95.6
98.4
99.2
100.0
Total Gas Age:
"Ar Wtd. Mean Age:
95.3
98.6
98.4
98.1
97.8
97.7
98.4
95.5
83.4
92.2
436.5
451.1
446.1
450.5
449.4
445.0
449.9
446.2
420.8
764.5
452.4
452.1
5.9
3.3
3.7
5.9
10.0
5.9
9.1
21.2
119.7
81.8
2.6
2.4
(5.5)
(2.3)
(2.9)
(5.4)
(9.8)
(5.5)
(8.8)
(21.1)
(119.6)
(81.7)
6.64E-05
1.88E-05
9.37E-06
3.22E-06
1.53E-06
3.99E-06
3.30E-06
2.17E-06
3.58E-06
2.72E-06
8.27E-06
4.58E-06
1.44E-05
3.22E-02
4.80E-02
3.27E-02
3.38E-02
3.39E-02
3.40E-02
3.29E-02
3.29E-02
3.33E-02
3.35E-02
3.40E-02
3.36E-02
3.40E-02
1.19E-04
1.04E-04
1.48E-05
1.79E-05
1.65E-05
4.90E-05
3.83E-05
3.83E-05
4.35E-05
4.68E-05
5.20E-05
4.93E-05
3.97E-05
7.09E-14
1.95E-13
3.70E-13
9.39E-13
1.02E-12
6.09E-13
7.84E-13
1.40E-12
1.50E-12
1.30E-12
2.72E-13
3.09E-13
8.15E-14
0.8
50.0
85.8
3.0
7.2
91.6
17.8
97.5
99.4
29.4
99.2
36.3
99.2
45.1
99.5
60.9
99.8
77.8
92.5
99.6
99.5
95.6
99.4
99.1
99.9
100.0
Total Gas Age:
'Ar Wtd. Mean Age:
258.8
294.3
442.2
454.5
460.3
458.8
472.0
472.6
469.0
466.4
459.9
464.1
461.3
459.3
459.0
9.6
2.5
2.7
2.5
2.5
2.6
2.6
2.6
2.6
2.6
2.7
2.6
3.1
4.3
0.7
(9.5)
(1.9)
(1.2)
(.5)
(.3)
(.8)
(.6)
(.6)
(.7)
(.7)
(1.2)
(.8)
(1.8)
6.1 1E-05
8.98E-05
8.20E-05
6.93E-05
5.64E-05
5.22E-05
7.38E-05
6.41 E-05
4.53E-02
4.64E-02
4.50E-02
4.81E-02
4.64E-02
4.75E-02
4.71E-02
4.53E-02
7.85E-04
3.71 E-04
6.03E-04
2.95E-04
5.36E-04
3.12E-04
3.26E-04
6.81E-04
1.22E-14
1.OOE-14
9.10E-15
1.19E-14
1.32E-14
1.62E-14
1.03E-14
1.19E-14
97.9
97.6
98.7
98.5
98.7
99.4
98.9
98.5
479.3
468.0
485.5
457.0
472.9
465.9
468.0
482.5
22.9
23.0
23.7
21.3
21.8
20.9
22.0
22.8
(10.6)
(11.6)
(11.9)
(8.7)
(8.5)
(6.9)
(9.5)
(10.3)
Sample AF70b -c167-7 §@
00
Furnace Temperature (K]
1.69E-03
923
4.78E-04
973
2.83E-04
1023
1073
8.28E-05
1.91E-05
1123
2.52E-05
1223
2.63E-05
1273
1.81E-05
1323
7.28E-06
1373
1.38E-05
1423
1.65E-05
1473
2.02E-05
1523
2.85E-06
1923
Sample AF70b-cl67-7 §@
Laser Total Fusion Analysis
5.94E-05
71
6.97E-05
72
3.44E-05
73
3.99E-05
74
3.29E-05
75
9.17E-06
76
2.46E-05
77
78
4.01E-05
Ar/ * Ar analytical data for biotite, muscovite, phlogopite from Connemara
Atomic ratios
39
4
39 4
4
40
Ar/ *Ar). " 3 9Ar moles CUm.% 39Ar 4OAr*% Age (Ma) Age Error (w J) Age Error (w/o J)
3Ar/ Ar ( "Ar/ "Ar) "'0, Ar/ *Ar
Appendix 5.4. continued.
Increment
40
t
**
Continued: Sample AF70b
4.12E-05
3.68E-05
4.14E-07
5.95E-06
3.24E-07
2.84E-07
2.53E-07
5.02E-07
3.94E-07
6.10E-05
8.18E-05
8.19E-05
5.20E-05
6.34E-05
5.55E-05
4.99E-05
1.02E-04
7.85E-05
4.63E-02
4.65E-02
4.67E-02
4.60E-02
4.50E-02
4.67E-02
4.62E-02
4.80E-02
4.58E-02
4.85E-04
7.28E-04
1.78E-04
4.06E-04
5.01E-04
2.66E-04
5.00E-04
4.29E-04
6.55E-04
1.22E-14
9.89E-15
1.06E-14
1.65E-14
1.31E-14
1.56E-14
1.73E-14
8.98E-15
1.10E-14
98.4
98.6
99.6
99.5
99.7
99.6
99.6
99.6
99.6
472.4
471.1
474.3
479.4
489.7
473.8
479.2
463.2
482.1
474.6
474.6
21.8
23.3
22.5
21.6
22.7
21.3
21.7
23.5
23.4
87.2
5.4
(8.8)
(12.0)
(10.2)
(7.5)
(9.4)
(7.2)
(7.7)
(12.9)
(11.5)
97.1
95.1
99.9
99.7
99.9
98.6
99.8
79.3
90.8
100.0
486.9
441.7
409.1
471.3
441.1
447.2
418.6
426.5
447.8
430.5
441.4
441.2
9.1
4.8
7.1
5.8
7.8
6.6
7.9
8.1
6.1
6.0
±
±
(8.7)
(4.2)
(6.8)
(5.3)
(7.4)
(6.2)
(7.6)
(7.7)
(5.7)
(5.5)
(29.2)
(2.2)
99.9
100.0
80.9
98.2
99.5
99.7
99.6
99.6
99.8
99.7
81.5
408.9
878.5
436.0
445.1
464.7
470.2
486.8
476.1
470.5
472.8
456.6
473.5
19.4
21.9
3.4
2.5
2.6
2.8
2.7
2.7
2.6
4.2
9.3
3.4
(19.2)
(21.5)
(2.4)
(.5)
(.8)
(1.3)
(.8)
(.9)
(.7)
(3.3)
(8.9)
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF73b-cl67-5 @§
Laser Total Fusion Analysis
9.75E-05
81
1.66E-04
82
1.48E-06
83
8.59E-06
84
1.66E-06
85
4.57E-05
86
7.86E-06
87
7.00E-04
88
3.11E-04
89
8.19E-07
90
2.20E-05
2.05E-05
2.52E-05
3.14E-05
2.88E-05
3.13E-05
2.70E-05
3.80E-05
2.10E-05
1.48E-05
3.11E-02
3.41E-02
3.90E-02
3.32E-02
3.58E-02
3.48E-02
3.80E-02
2.95E-02
3.20E-02
3.69E-02
5.99E-04
2.96E-04
6.63E-04
2.88E-04
6.06E-04
4.33E-04
7.05E-04
4.31E-04
4.01E-04
5.09E-04
4.98E-15
5.96E-15
5.29E-15
4.84E-15
4.23E-15
5.41E-15
5.10E-15
5.67E-15
7.77E-15
8.28E-15
Total Gas Age:
"Ar Wtd. Mean Age:
Sample AF79b-cl67-759
Furnace Temperature [K]
1.99E-06
935
3.04E-07
973
6.47E-04
1023
6.02E-05
1123
1.80E-05
1173
8.39E-06
1223
1.37E-05
1323
1.17E-05
1373
5.38E-06
1423
8.63E-06
1523
6.25E-04
1973
7
6.60E-05
1.57E-05
1.65E-05
1.98E-06
4.12E-06
9.17E-06
4.55E-06
5.07E-06
1.91E-06
2.56E-05
5.93E-05
3.89E-02
1.58E-02
2.93E-02
3.48E-02
3.36E-02
3.32E-02
3.19E-02
3.27E-02
3 32E-02
3 30E-02
2.81E-02
1.90E-03
4.82E-04
4.43E-05
3.76E-05
5.00E-05
4.65E-05
3.83E-05
5.19E-05
4.99E-05
7.63E-05
1.42E-04
3.34E-14
3.17E-14
1.51E-13
4.45E-13
2.88E-13
2.70E-13
7.12E-13
2.92E-13
1.25E-13
1.05E-13
2.69E-14
1.4
2.6
8.7
26.7
38.3
49.1
77.9
89.6
94.7
98.9
100.0
Total Gas Age:
Appendix 5.4. continued. "IAr! I Ar analytical data for biotite, muscovite, phiogopite from Connemara
Continued: Sample AF79b
'Ar Wtd. Mean Age:
472.8
1.0
All uncertainties indicate 2 a errors in individual measurements: t number of moles of K-derived "Ar ("ArK) released during each heating step. § cumulative percentage
of "ArK after each heating increment. # percentage of radiogenic "Ar ("Ar*) in the total "Ar for each analysis. ** Uncertainties, quoted at 2 a, include propagated error
in the irradition parameter J. Uncertainties in parantheses indicate the contribution of analytical error to the overall uncertainty. tt Total gas ages calculated by summing
40
for each increment. Assigned uncertainties (2 a) include the propagated errors in J and isotopic measurements. Age calculations are based on the decay
Ar* and "OArK
3
40
constants of Steiger and Jager (1977). Typical system blanks are M/e Ar and Ar (in moles) 1.3 x 10 ' and 1.0 x 10 '* for the furnace and 5.6 x 10" and 4 x 10 lB
for the laser.
§§ The irradiation parameter J for eac sample aliquot is listed below.
Irradiation
J-value
error (1a)
0.010329
0.010349
6.23E-05
3.09E-05
0.013501
0.013689
0.014043
2.07E-04
3.96E-04
3.37E-04
0.009933
0.009967
0.009911
2.97E-05
2.74E-05
3.01 E-05
CLAIR37
c137-1
c137-3
CLAIR 48
c148-1
c148-2
c148-3
CLAIR 67
00
c167-5
c167-6
c167-7
Table 5.1. Electron microprobe analyses of biotite from Connemara
AF18
AF16
AF12
Si02
TiO 2
A120 3
FeOT
MnO
MgO
CaO
Na 20
K 20
NiO
TOTAL
35.907
1.51
19.73
18.85
0.05
9.88
0.05
0.42
8.65
0.11
95.16
(0.28)
(.06)
(.12)
(.28)
(.03)
(.15)
(.04)
(.09)
(.15)
(.03)
(0.33)
34.91
1.85
18.61
21.08
0.09
7.95
0.00
0.06
9.32
0.06
93.92
(0.44)
(.34)
(.52)
(.67)
(.05)
(.24)
(.00)
(.05)
(.20)
(.04)
(1.08)
34.872
2.84
20.44
24.09
0.29
5.21
0.05
0.06
3.37
0.03
91.25
(1.95)
(.01)
(1.81)
(1.40)
(.01)
(.75)
(.02)
(.04)
(1.35)
(.04)
(3.01)
35.05
3.28
18.60
21.65
0.32
7.13
0.02
0.28
8.44
0.02
94.79
(0.14)
(.08)
(.07)
(.19)
(.12)
(.06)
(.00)
(.02)
(.07)
(.01)
(0.16)
35.17
3.05
19.94
18.54
0.27
8.06
0.00
0.27
9.66
0.10
95.05
AF35
AF34
AF28
AF24
(0.22)
(.12)
(.21)
(.29)
(.04)
(.10)
(.01)
(.12)
(.17)
(.05)
(0.73)
44.23
1.98
23.93
12.50
0.14
5.77
0.05
0.10
7.97
0.03
96.70
(10.16)
(1.9)
(5.3)
(9.0)
(.1)
(2.8)
(.1)
(.1)
(.9)
(.0)
(0.63)
35.28
1.60
19.01
18.23
0.08
12.48
0.00
0.12
6.78
0.02
93.59
Cations per 11 oxygen atoms
2.77
0.09
1.79
1.21
0.00
1.14
0.00
0.06
0.42
0.01
2.78
0.11
1.75
1.40
0.01
0.94
0.00
0.01
0.47
0.00
2.75
0.17
1.90
0.00
1.59
0.02
0.61
0.00
0.01
0.17
2.76
0.19
1.72
1.42
0.02
0.84
0.00
0.04
0.42
0.00
2.74
0.18
1.83
1.21
0.02
0.94
0.00
0.04
0.48
0.01
3.13
0.11
1.99
0.74
0.01
0.61
0.00
0.01
0.36
0.00
2.72
0.09
1.73
1.17
0.00
1.43
0.00
0.02
0.33
0.00
2.77
1.23
2.78
1.22
2.75
1.25
2.76
1.24
2.74
1.26
3.13
0.87
2.72
1.28
Als
FeT
Mg
Mn
Ti
sum
0.56
1.21
1.14
0.00
0.09
3.00
0.53
1.40
0.94
0.01
0.11
2.99
0.65
1.59
0.61
1.59
0.17
4.61
0.48
1.42
0.84
0.02
0.19
2.95
0.57
1.21
0.94
0.02
0.18
2.90
1.12
0.74
0.61
0.01
0.11
2.58
0.44
1.17
1.43
0.00
0.09
3.15
Xa.n= Fe/(M site)
Na / (Na + K)
(Fe + Mg) / Al,
Si,, / (Si,, + Al,)
0.41
0.13
0.47
0.02
0.53
0.05
0.49
0.09
0.42
0.08
0.29
0.04
0.37
0.05
4.22
4.46
3.40
4.72
3.79
0.69
0.70
0.69
0.69
0.68
1.20
0.78
5.88
0.68
Si
Ti
Al
FeT
Mn
Mg
0
Ca
Na
K
Ni
T site
Sil
Ali
M site
(0.87)
(.12)
(.25)
(.91)
(.03)
(.42)
(.03)
(.16)
(.67)
(.04)
(0.80)
Table 5.1 continued. Electron microprobe analyses of biotite from Connemara
AF73
AF70
AF45
AF49
AF39
34.45
37.41 (0.29) 35.23 (0.16)
34.37 (0.29)
36.13 (0.31)
Si0 2
2.95 (.07)
3.01
2.38 (.17)
1.82 (.08)
1.91 (.11)
TiO 2
19.34
19.52 (.10)
19.86 (.31)
19.52 (.25)
21.95 (.45)
A1203
21.19
17.52 (.37)
18.93 (.21)
22.03 (.42)
17.91 (.80)
FeOT
0.11
0.16 (.03)
0.13 (.03)
0.11 (.02)
0.28 (.03)
MnO
6.15
7.95 (.06)
8.61 (.32)
6.76 (.14)
7.18 (.44)
MgO
0.00
0.00 (.00)
0.03 (.02)
0.00 (.00)
0.00 (.01)
CaO
0.24
0.19 (.03)
0.16 (.02)
0.20 (.10)
0.09 (.05)
Na20
7.76
9.06 (.06)
8.54 (.14)
8.78 (.25)
8.85 (.22)
K2O
0.04
0.03 (.03)
0.01 (.01)
0.06 (.06)
0.02 (.02)
NiO
94.01 (0.44)
92.29
93.70 (0.99)
94.32 (1.57)
94.60 (0.33)
TOTAL
Cations per 11 oxygen atoms
Si
Ti
Al
FeT
Mn
Mg
Ca
Na
K
Ni
AF79
(0.68)
(.16)
(.43)
(.49)
(.09)
(.16)
(.00)
(.04)
(.35)
(.04)
(0.23)
33.18
1.93
18.50
20.52
0.03
9.18
0.00
0.32
8.08
0.00
91.75
2.78
0.11
1.99
1.15
0.02
0.83
0.00
0.01
0.43
0.00
2.72
0.14
1.85
1.46
0.01
0.80
0.00
0.03
0.44
0.00
2.89
0.11
1.78
1.13
0.01
0.99
0.00
0.02
0.42
0.00
2.76
0.17
1.80
1.24
0.01
0.93
0.00
0.03
0.45
0.00
2.76
0.18
1.83
1.42
0.01
0.73
0.00
0.04
0.40
0.00
2.69
0.12
1.77
1.39
0.00
1.11
0.00
0.05
0.42
0.00
2.78
1.22
2.72
1.28
2.89
1.11
2.76
1.24
2.76
1.24
2.69
1.31
0.78
1.15
0.83
0.02
0.11
2.88
0.57
1.46
0.80
0.01
0.14
2.97
0.66
1.13
0.99
0.01
0.11
2.90
0.57
1.24
0.93
0.01
0.17
2.92
0.58
1.42
0.73
0.01
0.18
2.93
0.46
1.39
1.11
0.00
0.12
3.08
0.40
0.03
2.55
0.70
0.49
0.06
3.97
0.68
0.39
0.06
3.20
0.72
0.43
0.06
3.84
0.69
0.49
0.09
3.68
0.69
0.45
0.11
5.48
0.67
T site
Sii,
Ali,
M site
Al
FeT
Mg
Mn
Ti
sum
X ann
Na / (Na + K)
(Fe + Mg) / Ali,
Si,, / (Sii, + Ali)
(0.02)
(.07)
(.46)
(.57)
(.01)
(.01)
(.00)
(.10)
(.01)
(.00)
(0.91)
Table 5.2. Electron microprobe analyses of phlogopite from Connemara
AF38
AF27
AF32
AF37
42.77 (.93)
40-97
43.98 (2.25) 41.14 (.32)
SiO 2
0.45 (.02)
0.40 (.17)
0.26
TiO 2
1.39 (.06)
14.55 (.90)
13.75 (.55)
16.56
16.84 (.86)
A120 3
1.45 (.07)
2.98 (.22)
1.84
FeO T
2.40 (.08)
0.01 (.02)
0.21 (.04)
0.13
0.05 (.09)
MnO
24.09 (1.37)
20.18 (2.94)
27.73 (.45)
24.84
MgO
0.00 (.00)
0.01 (.02)
0.00 (.01)
0.02
CaO
0.07 (.02)
0.72 (.06)
0.08
0.07 (.08)
Na2O
6.99 (.39)
9.85 (.19)
10.33 (.33)
10.62
K20
0.07
0.01 (.01)
0.00 (.01)
0.00 (.01)
NiO
95.39
95.42 (0.93)
92.25 (0.40)
94.77 (1.94)
TOTAL
Cations per 11 oxygen atoms
Si
Ti
Al2
FeT
Mn
Mg
Ca
Na2
AF40
(.13)
(.18)
(1.25)
(.16)
(.05)
(.76)
(.04)
(.02)
(.18)
(.02)
(0.76)
44.60
0.47
14.11
0.43
0.00
24.68
0.00
0.09
9.96
0.02
94.36
AF52
(.77)
(.02)
(.43)
(.09)
(.00)
(.56)
(.00)
(.03)
(.21)
(.01)
(0.83)
42.95
0.87
16.27
1.23
0.01
24.56
0.01
0.41
9.20
0.01
95.52
3.12
0.07
1.41
0.14
0.00
2.13
0.00
0.01
0.45
0.00
2.96
0.02
1.17
0.09
0.00
2.98
0.00
0.10
0.32
0.00
3.06
0.02
1.23
0.18
0.01
2.57
0.00
0.01
0.47
0.00
2.93
0.01
1.39
0.11
0.01
2.65
0.00
0.01
0.48
0.00
3.16
0.02
1.18
0.03
0.00
2.61
0.00
0.01
0.45
0.00
3.01
0.05
1.34
0.07
0.00
2.57
0.00
0.06
0.41
0.00
Al,
3.12
0.88
2.96
1.04
3.06
0.94
2.93
1.07
3.16
0.84
3.01
0.99
M site
Al,
FeT
Mg
Mn
Ti
sum
0.52
0.14
2.13
0.00
0.07
2.88
0.13
0.09
2.98
0.00
0.02
3.22
0.28
0.18
2.57
0.01
0.02
3.07
0.32
0.11
2.65
0.01
0.01
3.11
0.34
0.03
2.61
0.00
0.02
2.99
0.35
0.07
2.57
0.00
0.05
3.04
0.05
4.34
0.78
0.03
23.73
0.74
0.06
9.64
0.76
0.04
8.50
0.73
0.01
7.80
0.79
0.02
7.46
0.75
K2
Ni
T site
Si,
XA =
Fe/(Fe+Mg+Mn+Ti+Al")
(Fe + Mg) / Al,
Si, / (Si, + AI)
185
(.30)
(.09)
(.51)
(.07)
(.02)
(.84)
(.01)
(.06)
(.16)
(.02)
(0.97)
Table 5.3. Electron microprobe analyses of muscovite from Connemara
AF45
AF18
AF16
AF7
Si0
2
TiO 2
A1
203
FeO T
MnO
MgO
CaO
Na20
K20
TOTAL
AF67
AF49
49.73
(1.32)
46.65
(.44)
48.08
(1.35)
46.29
47.68
(1.65)
0.27
(.07)
0.58
(.07)
0.10
0.17
(.14)
0.26
35.47
(.32)
35.35
35.33
(.67)
30.58
(2.17)
1.63
0.02
0.64
0.00
2.59
0.00
0.76
0.04
(1.93)
2.24
0.02
1.54
0.00
(.14)
(.02)
(.23)
(.00)
34.03 (1.85)
1.43 (.11)
0.00 (.00)
0.52 (.06)
0.00 (.00)
(.05)
47.34
(1.69)
0.52
(.07)
35.59
(.49)
0.75
0.01
0.52
0.01
(.12)
(.02)
(.13)
(.01)
1.54
0.00
0.42
0.01
(.11)
(.02)
(.06)
(.03)
0.95
(.15)
0.52
(.08)
0.53
0.69
(.08)
0.73
(.09)
1.21
(.40)
9.95
(.18)
9.67
(.42)
10.91
7.70
(.51)
9.08
(.28)
9.25
(.24)
95.27
(0.65)
94.83
95.19
(1.88)
Cations per 11 oxygen atoms
Si
Ti
Al2
FeT
Mn
Mg
Ca
Na2
K2
(2.11)
95.48
(0.57)
94.95
(.00)
(.49)
(.04)
(1.04)
94.17
(1.20)
3.18
0.03
2.81
0.04
0.00
0.05
0.00
3.25
0.03
2.71
3.15
0.01
2.84
3.19
0.01
2.78
3.38
0.01
2.45
0.08
0.00
0.05
0.00
0.07
0.09
0.00
0.07
0.00
0.07
0.14
0.00
0.08
0.00
0.09
0.13
0.00
0.16
0.00
0.10
0.42
0.47
0.33
0.39
0.40
3.15
3.38
3.18
0.84
0.85
0.62
0.82
1.99
1.99
1.83
1.99
0.09
0.04
0.01
2.13
0.09
0.07
0.01
2.15
0.13
0.16
0.01
2.13
0.04
0.05
0.03
2.11
0.47
0.39
0.40
0.07
0.10
0.16
0.54
0.49
0.55
K/(K+ Na + Ca)
XA=Al /(Al,+FeT +Mg+Mn+Ti)
0.87
0.80
0.72
0.92
0.86
0.94
XKM.
0.74
0.60
0.64
Si, / (Si, + Al.)
0.79
0.85
0.79
0.16
T site
Si,
Al,
M site
Al,,
FeT
Mg
Ti
sum
3.16
A site
K2
Na2
sum
XK=
186
Table 5.4. "Ar Diffusion data for mica.
Mineral
Xn
Do
EA
2sec-1]
biotite
muscovite
phlogopite
0.54
0.71
0.04
[cm
0.077
0.4
0.00039
0.75
Reference
mol-1 K]
[kcal
47.1 ± 1.5
50.5 ± 2.2
43
57.9 ± 2.6
Harrison et al. (1985)
Grove and Harrison (1996)
Hames and Bowring (1993)
Giletti (1974)
Table 5.5. Closure Temperatures for mica from Connemara
Sample
Mineral
Grain Radius Cooling Rate
Closure Temperature
[Im]
[*C/My]
[*C]
AF7
biotite
300
6 to 20
322 - 340
AF7
muscovite
300
6 to 20
372 - 394
AF12
AF16
biotite
biotite
200
150
6 to 20
3 to 20
311 - 328
294 - 320
AF16
muscovite
150
3 to 20
318-349
AF35
AF37
AF70
AF79
AF38
biotite
phlogopite
biotite
biotite
phlogopite
200
125
200
200
600
6 to 26
6 to 26
6 to 26
6 to 26
6 to 26
311 - 332
392 - 414
311 - 332
AF52
AF18
AF18
AF22
phlogopite
muscovite
muscovite
biotite
200
225
450
100
6 to 26
7 to 14
7 to 14
7 to 14
AF22
biotite
325
7 to 14
406 - 429
344 - 356
368 - 381
294 - 302
327-336
320 - 348
376 - 402
Note: Closure temperatures for a mineral pair from the same sample (mucovite-biotite) or
from the same locality (phlogopite and biotite) were calculated assuming internally
consistent cooling rates. In all calculations we assumed that a cylinder best describes the
mica diffusivities. The closure temperature equation is from Dodson (1973).
General Uncertainites are ±50 *C.
187
Sample
Table 5.6: Summary of *Ar/ 39Ar analyses for muscovite, biotite, and phlogopite from northern Connemara
plateau or
Mode
Grainradius number of 39Ar wtd 2 a
Mineral
Locality
Rocktype
flat segment
(range)
error
grains per mean
(range)
(Natl. Grid
analysis
age
pm
Reference)
[Ma]
Garnet-Staurolite
Zone
AF7
metapelite
693.636
AF16
metapelite
670.637
AF12
AF15
AF40
metapelite
mylonite
calcsilicate
663.607
AF49
metapelite
804.574
662.583
5.3 mg
3.7
3.1
1.5
1.5
4.3
1.1
2.4
125-150
3.0 mg
450.7
1.2
452.3 ± 1.0
125-150
0.4 mg
454.7
3.3
459.8 i 2.5
250-355
250-355
125-150
125-150
125-250
1 to 4
1 to 5
3.3 mg
4.0 mg
1 to 5
125-250
Staurolite/Sillimanite
Zone
metapelite
AF35
calcsilicate
AF37
849.538
842.540
biotite
phlogopite
150-250
100-150
1 to 7
4.7 mg
472.2
476.3
4.3
4.5
AF66
calcsilicate
760.554
125-150
2.1 mg
399.0
3.0
AF67
AF70
semipelite
metapelite
914.508
phlogopite
(w/altered)
muscovite
biotite
biotite
125-250
125-250
28.2 mg
452.1
459.0
474.6
2.4
0.7
5.4
AF73
AF79
metapelite
metapelite
biotite
biotite
200-350
150-300
1-2
441.2
472.8
1.0
915.508
471-476
469-476
476.4
468.4
469.2
452.4
481.5
452.0
472.9
muscovite
biotite
muscovite
biotite
biotite
muscovite
phlogopite
(w/altered)
muscovite
(crappy)
biotite
471.0 ± 1.0
462.5 ±1.1
471-483
452.5 ± 1.4
455 & 495
466-474
483
0.7
470-481
(477)
443 & 473
418.5 i 1.6
(432)
448.7 ± 1.8
466.8 i 1.0
473 ± 1
Sample
rocktype
Table 5.7. Summary of 4"Ar/"Ar analyses for biotite and phlogopite from central Connemara
39
Locality
Mineral
grainradius
number of
number of
Ar wtd.
2o
Mode
AF27
AF28
AF32
AF34
(Nat'l. Grid
Ref.)
calcsilicate
835.490
calcsilicate
836.492
calcsil'icate
811.500
metapelite
806.501
AF38
calcsilicate
AF52
calcsilicate
718.531
856.505
phlogopite
biotite
phlogopite
biotite
biotite
phlogopite
phlogopite
phlogopite
(range)
Im
250-350
225-450
300
300-400
150-350
400-800
500-1250
125-250
grains per
analysis
3 to 6
4
1 to 4
furnace
1 to 5
furnace
1
9.4 mg
analyses
9
10
10
14
10
mean age
[Ma]
468.3
453.9
473
444.9
459.3
445.8
466.9
454.8
error
2.5
1.
2
2.9
6.9
.8
2.2
1.0
(range)
[Ma]
443 & 479
442, 447, 472
460-465
plateau or
near plateau
[Ma]
448.5 ± 2.0
460-465
455.8 ± 1.3
472
456.7 ± 2.6
Table 5.8. Summary of "ArPAr analyses for muscovite and biotite from southern Connemara
Mineral
Sample
Rocktype
Locality
Migmatitic SillimaniteK-feldspar Zone
AF18
metapelite
833.415 biotite
biotite
AF18
metapelite
AF22
metapelite
AF44
AF45
pegmatite
metapelite
AF45
metapelite
Label of grain radius number of
grains per
(range)
Analysis
analysis
gm
number of
analyses
39
Ar wtd. 2 a error Mode
(range)
mean date [Ma]
[Mal
[Ma]
mapping
1 to 3
11
10
276.5
489.4
5.1
3.3
1
laser step
445.3
2.1
250-750
10.3 mg
furnace
446.8
2.3
ml
8.2.1
mf.38
b81-90
bl5-20
200-250
400-500
200-250
250 - 400
75-125
1
3 grains
2
1 to 2
15-20
10
laser step
laser step
10
10
2.7
2.2
2
3
biotite
814.410 biotite
814.410 biotite
biotite
b36-40
2.100
b1
b8.16
200- 400
ca. 300
125-150
400-500
1-3
3
1 to 7
1
5
laser step
10
laser step
457.4
456.5
455
458.0
calc sep/
447.6
calc sep.
387.6
434.5
400.8
814.410 muscovite
muscovite
muscovite
muscovite
ml 1.13
m.b
m8.8
mf.4
700
2500
400-500
ca. 200
3
mapping
4
8
laser step
8
laser step
laser step
442.7
440.2
432.7
433.1
2.2
2.7
2.6
5.4
b.a
b1
> 1000
150-350
biotite
b.51
375
biotite
b.furn
833.415 muscovite
muscovite
muscovite
823.443 biotite
biotite
6.6
5.9
3.1
2.9
plateau or
flat
segment
[Ma]
270, 320
453, 485,
515
445 2 inv. saddle
(370-507)
447 2 inv. saddle
(363-507)
459
456 ± 2
453 ± 1
459
440
446
453
418 ± 6
437, 463
stair step
(327-420)
439 3
440-450
430 ± 3
429 ± 5
Figure Captions
Figure 5.1. Simplified geologic map of Connemara. The insert shows the
location of the Connemara region within the Irish Caledonides. The
Connemara terrane is separated from the rest of the former Laurentian
margin by Ordovician island-arc and forearc terranes (e.g. South Mayo). The
Ordovician terrane boundary is mostly concealed beneath Silurian rocks. The
Silurian rocks rest unconformably on the high-grade metamorphic rocks of
the Dalradian Supergroup. This figure shows the approximate location of
metamorphic isograds, the best recognized of which is the sillimanite- in
isograd. The metamorphic isograds are associated with the Connemara
complex, which consists of a range of mafic to felsic intrusions, the Silurian
unconformity, late orogenic extensional faults (e.g. the RBS - Renvyle Bofin
Slide), major F4 folds, such as the Connemara antiform, and the Mannin
thrust which emplaces the Connemara complex and the Dalradian country
rocks over low-grade metamorphic rocks of the Delaney Dome Fm (DDF).
Complex of northern
Dawros--Currywongaun-Douruagh
DCDC Connemara. C-LWG - Cashel-Lough Wheelaun Gabbro.
Figure 5.2. Summary of the deformation and structure of the Connemara
region. The location of cross section A-A' is marked in Figure 5.1. The
dominant schistosities and fold axes were defined specifically in the Dalradian
fold and thrust belt (e.g. Tanner and Shackleton 1979). It is generally assumed
that these deformation phases can be recognized in all of Connemara.
Figure 5.3. Pressure-temperature path for the Connemara region. This figure
is modified after Yardley et al. (1987) and emphasizes the regional differences
in metamorphic grade.
Figure 5. 4. Field photographs and photo micrographs of the deformation
sequence in Connemara. (A) S1-schistosity is only preserved as inclusion
trails within garnet. (B) The S2 schistosity is the dominating penetrative
fabrics at the low-metamorphic zones of northern Connemara, but a weak F3
overprint is usually observed. Exposure from the shore at Tully Mountain.
(C) Photo micrograph of sample AF17, a muscovite-garnet metapelite in
which muscovite + chlorite define a retrograde mineral assemblage, probably
equivalent to the M3 assemblages farther south. (D) Field photograph of the
impure marble of the Lakes Marble Formation. This picture is from Cur Hill,
at the staurolite-sillimanite transition zone, and shows the typical F3 folds
that occur at a variety of scales throughout Connemara. F3 folds refold minor
F2 fold axes. (E) Photomicrograph of sample AF79, a staurolite-sillimanite-
191
garnet schist from the staurolite-sillimanite transition zone at Cur Hill. This
thin section shows biotite and sillimanite defining the main S3 foliation
plane. In other areas, fibrolite grows axial planar with F3 fold axial traces. (F)
Outcrop picture of 'migmatitic ' metasedimentary rocks c. 50 m NE of the
parking lot at the Alkock and Brown landing site memorial. The field
relationships show the S2 schistosity folded around an F3 fold axis in a
metasedimentary rock. This block has been disrupted subsequent to F3
folding, as indicated by its random orientation relative to the strong anatectic
foliation defined within the anatectic metapelite rock. The foliation (S3') is
defined by alignment of paleosome and leucosome. (G) Outcrop picture of F2
to F4 fold generations. F2 and F3 fold axial traces are subparallel, whereas F4
folds are almost orthogonal to the older folds. F4 folds are accompanied by
axial planar granitic pegmatites. Outcrop at the head of Errislanaan peninsula,
c. 150 m SW of the lighthouse.
Figure 5.5. Sample locality map.
Figure 5. 6. 40Ar/ 39 Ar furnace and laser incremental release spectra. The
results are shown as a function of metamorphic grade. All diagrams are
plotted at the same scale for comparison purposes. Size dimension is reported
as the grain diameter. The data are listed in tables in Appendix 5.4.
Figure 5.7. Results of 4 0Ar/ 39 Ar laser total fusion analyses. This figures shows
the frequency distributions of each total fusion analysis, weighted by their 2 s
analytical error. The 39Ar weighted mean age of each analysis is reported in
the lower left corner for most analyses. "n" is the number of total fusion
analyses. The sample Mode is reported near the peak of the frequency
distribution. The size dimension of samples is reported as their average grain
diameter.
Figure 5.8. Map of the 40Ar/ 39Ar dates as a function of metamorphic grade and
distance from major intrusions. Only those results have been shown that can
be represented by a single date. Note that not all of the dates shown are
interpreted as cooling ages (see text).
Figure 5.9. Photomicrographs of selected samples. See sample description in
Appendix 1. The long dimension of the photographs is c. 3 mm.
Figure 5.10. Interpretative time-distance diagram of 4 Ar/ 3 9Ar dates for the
Connemara region. The shaded area contains all data that are permissible
cooling ages as constrained by the age of the magmatic arc, peak
192
metamorphism and the Silurian unconformity. The north-south axis is not
drawn to scale.
Figure 5.11. Temperature-time diagram showing 40Ar/ 39Ar muscovite biotite,
and phlogopite cooling ages.
Figure 5.12. (A-C) Schematic Cross sections of the thermal and structural
evolution of the Connemara region. See text for discussion.
193
194
90 25'
10" 15'
Figure 5.1
196
"D1 Deformation"
Inclusion trails in porphyroblasts (Si)
S1
"D2 Deformation"
One recumbent fold (F2), minor folds
and penetrative schistosity (S2)
"D3 Deformation"
Open to isoclinal folds (F3).
S3 schistosity in sillimanite-grade rocks
S2
"D3' Deformation"
Gneissic foliation (S3') in
anatectic restite and quartz diorite.
Mannin thrust.
"D4 Deformation"
Major upright antiforms and minor
crenulation cleavage.
S3
u F4 - - - - - X
*
F3
-
-
L F2
A
- -
North
5-
I
thrust
A'
South
- 5
--
trace
km.
0
5 km
Dalradian fold belt
Connemara Complex
Figure 5.2
197
km
-,,
198
Ii
North
(Garnet zone)
M2
South
(" IMigmatite" zone)
M3
cib
04
2-
300
700
500
Temperatui re
Figure 5.3
199
OC
I
200
S1
quartz
3 mm
Pq
ca. 1.5 m
Figure 5.4
201
I
202
5 mm
S2 .
F3
-
fold axis
foliated --
metapelitic
restite
18 cm
F2-F4: fold axial traces
granitic .
pegmatite
Figure 5.4 continued
203
I
204
-1100 15'
1K.
RBS
LO
Antiform
................ Mayo South
Normal fault
Unconformity
Galway granite
Silurian and
Ordovician rocks
Mannin thrust
Quartz-diorite gneiss
Mafic intrusions
0
km
Fold belt in Dalradian
Supergroup
10
sill-in
Sillimanite-in isograd; lable is
on side of sillimanite-grade rocks
staur-ouI
Staurolite-out isograd; lable is
on side of staurolite-free rocks
"migmatite"
Anatectic metapelite rocks on
south side of this lable
Figure 5.5
38*
Sample number
and locality
206
Garnet-Staurolite Zone
250-300
Muscovite AF15
Muscovite AF1 6
Biotite AF16
5
AF1Smif
AF16m
555 250-300 sm
AF16b
Ism
14-52
1465
Ma
462.5± 1.1
375
1285
375375
fla5tiesegment
% WANmlased
Cumnidins
0
285
15
I.
20
100
m'sed
Cumua65 %3GArK
0
Biotite AF49
1.4 Ma
452.5
195
100
25.-300
465
i
a
12W5-
steps 3 -12
15
1
471
Cumlab
%3ArKmleased
100
Muscovite AF49
AF49b
51
AF49m
250-300 50m
s.
146
452.3 ± 1.0 Ma
459.8 ± 2.5 Ma
5375
39
sepse6-9
78%
AK
76%3PArK
S
isleased
0
Cumulabne%30RArK
195
10
fat segment
0
Cwnstee%
3
100
ArK'eleased
Staurolite-Sillimanite Transition Zone
Muscovite AF67
Ca550 Lm
ss
Biotite AF70
AF87m
43St
± 1.8 Ma
!S448.7
25
1255p5.11
8%2Ag
195
3
CuMav. % ArKmleed
0
AF70b
055 Co. 500 pm
41
47
33W
442
steps5-14
0.8Ma
464.4
195
100
3
0
CUrmb
Biotite AF79
miewd
100
%3ArK
Phlogopite AF66
ca 550 p
555,250-300
1465-
AF56p
pum
465-
375
'48713/
436 3 W
375
3
473 : 4 W
128S
395±11
43
VA
Steps 1-11-
195
466.07± 0 9 Ma
mlased
Cumubw%3PArK
0
419 ± 2 Ma
meanage
195
100
0
3
100
r.5.ased
Cumubas% SArK
Sillimanite Zone
Phlogopite AF52
Phlogopite AF38
Biotite AF34
AF34b5.53
600-800 pm
4e5
aw.,800-1200
AF3p12.f1
5pm
125-250
5
455 8* 1.3 Ma
t
375
376.
375
128s
26
215
0
satseau
steps
7%3k
3
% ArKmleased
CumulavO
0
492.7 ±30 Ma
4567
2 6 Ma
plateau!
steps
8-10
%39Ar
F[3
100
AF52p
46
21a5
448.5 ± 2.0 Ma
191
1m
Cumul,
%39r
mlsd
Figure 5.6
207
100
0
Cumulabv % 39A
meiased
100
208
"Migmatite" Zone
Muscovite AF45
AF45mf.4
400 pm
555.
ia5-
Biotite AF18
Muscovite AF18
AF1b.newbt
500-1500 pm
375-
433.1 t5.4 Ma
AFlamf38
400-5W pm
285-
555. 900-1000
363±4Ma
AF18.51
(100% ArK)
AF18m821
30-1000pm
pm
AF45m8
7465-
plateau
3
750 pm
steps 1-6
(63%39Ar
195"
453.6 ± 0.9 Ma
375.
432.7 ± 2.6 Ma
steps37-12
(y8
456.2 ± 2.2 Ma
370Mm±5 Ma
flat msgment
3
(100% 9ArK
3
Cmuamwe%
1
9ArKmlased
Cumulabw%3ArNmleased
155
442.7 ± 2.2 Ma
285
plateau
steps 12-17
3
(53% 9SAK)
100
Cumulative%39Arkreleasd
195-
Biotite AF45
Biotite AF44
555
AF18b21
Lj1285-
375-37
418.4 I 6.5 Ma
1285
3507
plateu
Mm
195
195
3
0
Cumuinieb% Akreeaed
0
100
Cumulatw%39Arkreleased
100
0
CunmuaV %3 A
Galway Batholith
Errisbeg-Townland Granite
K-feldspar AF57
Roundstone Granite
Ana
K-feldspar AF54
260Ma
290 ± 2 k
'221 ±6Mat
Biotite AF57
247±:,2MW
AFSnl-1
278±i2 Wm
Biotite AF54
A54*1".l
417± 1 Ma
401 ± 1 Ma
tatsegmntd
aflit
segmtent
100%
steps33-13
91% 9Ar
3ArK
CumulsVm
%39%islesd
1C55
%39AKneased
CumuaWv
Figure 5.6 continued.
209
~
9A4
AF45mi.13
1400pm
1C0
miesamed
210
Garnet-Staurolite-Sillimanite Zones
Total fusion Analyses - Frequency Distributions
1.4
biotite AF7
1.2
500-710 pm
1.0
1.4
1.2
469 Ma
1.0
C
0.8
5
0.8
O
0.6
g
0.6
.n = 23
39
Ar wtd. mean
468.4 ± 3.1 Ma
500-710 pm
471 Ma
n= 19
Ar wtd. mean
-476.4 3.7 Ma
39
150 200 250 300 350 400 450
age [Ma]
150 200 250
500
300 350 400 450
age [Ma]
500
phlogopite AF37
biotite AF12
250-300 jm
,
muscovite AF7
200-300 pm
471 Ma
483 Ma
0.8
0.8
r 0.6
0.6
.n=8
39
Ar wtd. mean
478.0 ± 5.1 Ma
150 200 250
39
Ar wtd. mean
476.3 ± 4.5 Ma
300 350 400 450
age [Ma]
150 200
500
500
phlogopite AF40
biotite AF35
300-500 jm
250 300 350 400 450
age [Ma]
250-500 pm
466-474 Ma
5 M 495 Ma
0.8
0.8
0.6
0.6
.n=9
. 39Ar wtd. mean
472.9 ± 2.4 Ma
n =37
39
Ar wtd. mean
472.2 ± 4.3 Ma
150 200 250 300 350 400 450
age [Ma]
150 200
500
400-700 jm
477 Ma
443 Ma
473 Ma
.n = 10
n = 17
39
Ar wtd. mean
441.2 2.0 Ma
39Ar wtd. mean
474.6 5.4 Ma
150 200 250
500
biotite AF73
biotite AF70
250-500 pm
250 300 350 400 450
age [Ma]
300 350 400 450
age [Ma]
150 200 250
500
Figure 5.7
211
300 350 400 450
age [Ma]
500
212
Upper Sillimanite Zone
Total Fusion Analyses - Frequency Distributions
Phlogopite AF38
Phlogopite AF27
500-700
|650 srm x 500 jim
im
5 analyses
443 Ma
479 Ma
<500 sm/
n= 9
39Ar wtd.
mean
468.3 ±.2.5 Ma
150 200
500
250 300 350 400 450
age [Ma]
age [Ma]
Biotite AF34b
Phlogopite AF28
500-900
=
srm
442 Ma
47 Ma
0.8
472M
0.6
42 Ma
39Ar
wtd. mean
453.9 ± 1.7 Ma
150
200 250 300 350 400 450
age [Ma]
Phlogopite AF32
1.4
600 pm
1.2
460-465 Ma
1.0
0.8
0.6
0.4
0.2
n = 10
39
Ar wtd. mean
473 .7± 2.3 Ma
150 200 250 300 350
age [Ma]
400 450
Figure 5.7 continued.
213
500
214
"Migmatite" Zone:
Total Fusion Analyses - Frequency Distributions
Biotite AF22
biotite AF 8b1
1.4
1.4
500-800 pm
AF22b.81-90
1.2
1.2
459
1.0
1.0
0.8
0.8
0.6
r 0.6
0.4
0.4
0.2
0.2
n =10
39Ar
wtd. mean
.458.0 ±3.6 Ma
150
200
1.4
1.2
459
ca. 2000 Am
-laser mapping
1.0
0.8
n = 10
wtd. mean
150
500
300 350 400 450
age [Ma]
1.0
S 0.8
1.4
AF22b.15-40
446
-
440 N
0.2
0.6
0.4
Ar wtd. mean
0.2
.447.6 ±6.6 Ma
200
250
500
-250-300 jm
437
1 463
0.8
200-400pm
1
=9
.n
39
150
300 350 400 450
age [Ma]
1.0
0.6
0.4
250
1.2
75-125gm
-
200
Biotite AF45b1
Biotite AF22
-150-800 pm
286±4 Ma
0.2
457.4 ± 2.7 Ma
1.2
n=11
Ar wtd. mean
39
320
0.4
39Ar
1.4
500
270
0.6
250
300 350 400 450
age [Ma]
Biotite AF 8b.a
Muscovite AF 8m1
150 200
250
.n = 10
39
Ar wtd. mean
434.5 ±3.1 Ma
150
300 350
age [Ma]
200
Muscovite AF45m.b
5000 pm
450
laser mapping
n= 8
39Ar
wtd. mean
-440.2 ±2.7 Ma
150
200
250
300 350 400 450
age [Ma]
Figure 5.7 continued.
215
500
250
300 350 400 450
age [Ma]
50
I
216
Figure 5.8
218
AF7iu 5.9
Figure 5.9
219
I
220
Figure 5.9 continued.
221
222
The Geologic Record
Starolte/
Sillimanite
Zone
Sillimanite-
K-feldspar
Zone
Migmatitic Sillimanite-
K-feldspar
Zone
40Ar/ 39Ar
dates
umber
sampi
muscovite
) blotite
phlogopite
Grain Radius
< 250 pm
MO
HO
~li
*250-500 pm
>500 pm
CATEGORY 1
Permissible Cooling Ages
CATEGORY 2
late Grampian deformation
CATEGORY 3
Post-Gampian hydrothermal activity
and thermal resetting related to
Galway Batholith
CATEGORY4
no obvious
geologica significance
North
increasing metamorphic grade
* South
Figure 5.10
Chapter 3
Chapter 2
Chapter 4
I
224
800-
700-
600.
0
CL
500-
E
0)
400-
300.
200
Figure 5.11
Time [Ma]
226
(A) ca. 474-470 Ma
A
Regional Contact Metamorphism (MP3)
D3 deformation
intrusion of gabbros
zircon 474.s*1.0 Ma
U-Pb
North
a
Cashel- Lough
Wheeteon
gebbro
(U-Pbzircon470.1 ± 1.4Ma)
.sufacetree
South
03' deformation
intrusion of quartz diorites
done!
(B) ca. 467 Ma I
imitedextend
temperature
ca.750*C
temperature
ca.300*C
"Anatexis"
(-750 *C)
40Ar/39Ar muscovite and
biotite cooing ages
North
paleosome
(U-Pbmonazite 468 ± 2Ma)
A6i
(UP icn48i2a)
South
-'
~A
fluid infiltration
metasomatism
(C) ca. 462 Ma
North
Oughterard Granite
U-Pbxenotime
462.s 1.2 Ma
cooling
thermal aureole
cold
temperature u300 *C
0
A
U-Pb sphene dates
of fluid infiltrationt()
South
Metasomatic Diopsidle Rock
A'
(u-Pb spbene 4622 IMa)
/
km
Hi
~A'
S5
s0uretQ4T5rar
La=0.6 cnrrt
km
-
0
0
footwall
orn
5
lop
Figure 5.12
227