AN ABSTRACT OF THE THESIS OF
Robert J. Walker for the degree of Doctor of Philosophy in Geosciences
presented on November 3, 1989.
Title:
Ma matism and Mineralization of the Ash Peak Area
Arizona:
Petrochemical Interpretations
Redacted for privacy
Abstract approved:
Andesitic and rhyolitic magmatism was active during the midTertiary (Early Miocene) of the Ash Peak area, southeastern Arizona.
Andesitic magmas of similar composition both preceded and followed the
low- and high-silica rhyolitic magmas.
The changes from andesitic to
rhyolitic and back to andesitic volcanism is postulated to be the result
of local variations in the tectonic regime.
Parental basalts formed by either processes of subduction or
extension may have ascended through the crust at differing rates in
response to the tectonic regime.
Under pre-extensional and post-
extensional conditions, parental basaltic magmas may have ascended
slowly relative to extensional conditions.
Loss of heat would induce
crystallization of the basalt and drive the composition toward
intermediate compositions.
Elevated abundances of trace elements in the
andesitic rocks suggest that the fractionating magma underwent cycles of
fractionation punctuated by replenishment of parental basaltic magma.
Extensional tectonic conditions may have allowed the parental
basaltic magmas to ascend rapidly to the crustal level at which they
became bouyantly compensated.
They may then have acted as sources of
heat and volatiles for the partial melting of the crust to produce
parental rhyolitic magmas similar in composition to biotite rhyolite.
Crystal fractionation models provide a reasonable representation of the
observed petrochemical abundances of the later rhyolites.
Petrochemical
abundances of the rhyolites suggest that the suites of biotite rhyolite
to biotite tuff/crystal-rich rhyolite to Ash Peak Glass follows a
crystal Fractionation trend.
The crystal-poor rhyolites are modelled by
mixing parental magma into the Ash Peak Glass magma chamber followed by
crystal fractionation.
Porphyritic rhyolites were modelled as hybrid
magmas formed by the mixing of either biotite rhyolite or upper andesite
magmas with crystal-poor rhyolites and compensating for the observed
phenocryst assemblage.
Abundances and patterns of trace elements in ore and gangue
minerals of gold-silver-carbonate-silica and carbonate-manganese oxide
epithermal vein deposits of Ash Peak suggest that they crystallized from
a common hydrothermal fluid.
The fluid is proposed to be an aqueous
phase that separated from the biotite rhyolite magma.
Oxygenated
meteoric waters increasingly depleted the hydrothermal fluid of Ce as it
ascended to higher levels.
Magmatism and Mineralization of the Ash Peak Area, Arizona:
Petrochemical Interpretations
by
Robert James Walker
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Completed November 3, 1989
Commencement June, 1990
APPROVED:
Redacted for privacy
RerfilWo
e scienc s in
,rge of mjor
Redacted for privacy
rfLIACOT ceo
sc
Redacted for privacy
Dean of Gradua'
trloot
Date thesis is presented November 3, 1989
Typed by Robert J. Walker
ACKNOWLEDGEMENT
This study could not have been completed without the support of
many people.
I would like to take this opportunity to thank all of my
friends, colleagues, and family for the help, patience, and
encouragement you have given me over the last few years.
I would like to thank my committee for the help they have
provided during my Ph.D. years.
A special thank you to Roman Schmitt
for teaching me about INAA and Cy Field for getting me through.
Financial and analytical support from the U.S. Geological Survey,
Branch of Central Mineral Resources is greatly appreciated.
A sincere
thank you to Chuck Thorman who provided the means and Don Richter who
showed me the way.
The majority of the INA analyses were performed at
the Oregon State University Radiation Center; I am grateful for the
support provided by directors Chih Wang, Clifford Smith, and especially
Art Johnson.
Phelps Dodge Corporation and Arizona Flux Mines
Incorporated allowed unrestricted access to the Ash Peak mine and
surrounding areas, without their support the study of the ore deposits
would not have been possible.
For your time and consideration, thank
you to Jerry Waegli, Mike Pawlowski, Rick Preace, and Fred Menzer of
Phelps Dodge Corporation and Les Billingsley of Arizona Flux Mines
Incorporated.
My second biggest thank you is to my friends for things you said
and did to help me and for all of the little things you did without
knowing.
Scott, Vivian, Jim, Mike, Jay, Julie, Don, and especially the
Radiation Safety group, Kay, Gordon, Rainier, and Dan, thank you all.
Finally, my biggest thank you to my family, Belinda, Sean, and
Amanda.
Words will never be enough.
TABLE OF CONTENTS
INTRODUCTION
1
REGIONAL GEOLOGY AND TECTONICS
PRECAMBRIAN
PALEOZOIC
MESOZOIC
CENOZOIC
DISCUSSION
8
VOLCANIC ROCK UNITS
ANDESITIC VOLCANIC ROCKS
Lower Andesites
RHYOLITIC VOLCANIC ROCKS
Biotite Rhyolite
Biotite Tuff/Crystal-Rich Rhyolite
Pyroclastic Rocks
Ash Peak Glass
Crystal-Poor Rhyolite
Porphyritic Rhyolite
Upper Andesitic Volcanics
SUMMARY
23
24
24
28
29
33
PETROCHEMISTRY OF THE VOLCANIC ROCKS
ANDESITIC VOLCANIC ROCKS
RHYOLITIC VOLCANIC ROCKS
Biotite Rhyolite
Biotite Tuff/Crystal-Rich Rhyolite
Ash Peak Glass
Crystal-Poor Rhyolite
Porphyritic Rhyolite
SUMMARY
56
60
65
65
68
73
78
82
82
PETROGENESIS OF THE VOLCANIC ROCKS
PETROGENESIS OF ANDESITIC VOLCANIC ROCKS
PETROGENESIS OF RHYOLITIC VOLCANIC ROCKS
Biotite Rhyolite Petrogenesis
Biotite Tuff/Crystal-Rich Rhyolite Petrogenesis
Ash Peak Glass Petrogenesis
Discussion
Crystal-Poor Rhyolite Petrogenesis
Porphyritic Rhyolite Petrogenesis
SUMMARY
88
88
GEOCHEMISTRY AND GENESIS OF THE MINERAL DEPOSITS
GOLD-SILVER-CARBONATE-SILICA VEINS
CARBONATE-MANGANESE OXIDE VEINS
GEOCHEMISTRY
INTERACTION OF MAGMATISM AND MINERALIZATION
128
129
138
140
145
SUMMARY AND CONCLUSIONS
155
9
14
16
20
20
36
38
44
48
51
54
92
93
101
103
105
105
117
121
REFERENCES
159
APPENDIX 1
Energy Dispersive X-Ray Fluorescence Data for
Andesites and Rhyolites Associated with the Ash
Peak
Rhyolite Peak Eruptive Complex,
Southeastern Arizona
168
APPENDIX 2
Normalization Factors, Partition Coefficients,
and Formulae for Crystal Fractionation Models
180
LIST OF FIGURES
Page
Figure
1.
Location map for the Ash Peak area
2
2.
General geology of the Ash Peak area
4
3.
Tripartite division of the Older Precambrian
of Arizona
10
Abundances of major oxides of the
rhyolitic rocks
41
5.
Rock classification diagrams
57
6.
Normalized abundances of andesites
63
7.
Normalized abundances of biotite
rhyolite
67
Normalized abundances of biotite tuff/
crystal-rich rhyolites
71
Normalized abundances of biotite rhyolite
and biotite tuff/crystal-rich rhyolite
72
Normalized abundances of biotite rhyolite,
biotite tuff/crystal-rich rhyolite, and
crystal-poor rhyolite, APEC
74
Normalized abundances of biotite rhyolite,
biotite tuff/crystal-rich rhyolite, and
Ash Peak Glass
76
Normalized abundances of biotite rhyolite,
biotite tuff/crystal-rich rhyolite, Ash
Peak Glass, and crystal-poor rhyolites
77
Normalized abundances of crystal-poor
rhyolites
81
Normalized abundances of porphyritic
rhyolites
83
Normalized abundances of biotite rhyolite,
porphyritic rhyolite, and crystal-poor
rhyolite
86
Normalized abundances of biotite rhyolite
and lower andesite
95
4.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Page
Figure
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
Variation diagrams of andesitic and
rhyolitic rocks
96
Crystal fractionation model for biotite
rhyolite and biotite tuff/crystal-rich
rhyolite
104
Crystal fractionation model for biotite
tuff/crystal-rich rhyolite and Ash Peak
Glass
106
Variation diagrams of Nb, Ta, Nb/Ta, and
Rb versus Th of rhyolitic rocks
108
Variation diagrams of REE versus Th of
rhyolitic rocks
110
Variation diagrams of Zr, Hf, and Zr/Hf
versus Th of rhyolitic rocks
113
Variation diagrams of Ba, Sc, and Yb/Sc
versus Th of rhyolitic rocks
115
Crystal fractionation model for biotite
tuff/crystal-rich rhyolite and crystalpoor rhyolite, APEC
118
Magma mixing model of biotite rhyolite
and crystal-poor rhyolite, RPEC for
porphyritic rhyolites
122
Magma mixing model of upper andesite
and crystal-poor rhyolite, RPEC for
porphyritic rhyolites
124
Normalized abundances of major oxide
and trace elements of altered andesites
from the Ash Peak mine
135
Normalized abundances of carbonates
from the Ash Peak vein
143
Normalized abundances of carbonates
from the Thurston-Hardy deposits
144
Normalized abundances of manganese oxides
from the Thurston-Hardy and Rattlesnake
Pit deposits
147
Normalized abundances of average rock and
mineral types from the Ash Peak, ThurstonHardy, and Rattlesnake Pit deposits
150
LIST OF TABLES
Table
Page
1.
Pre-Oligocene stratigraphic column
21
2.
Abundances of major oxides of lower andesites
26
3.
Petrography of rhyolites
31
4.
Abundances of major oxides of biotite rhyolite
32
5.
Abundances of major oxides of biotite tuff/
crystal-rich rhyolites
34
6.
Abundances of major oxides of Ash Peak Glass
40
7.
Abundances of major oxides of crystal-poor
rhyolites, APEC
46
Abundances of major oxides of crystal-poor
rhyolites, RPEC
47
Abundances of major oxides of porphyritic
rhyolites
50
10.
Abundances of major oxides of upper andesites
53
11.
Average chemical analyses of andesitic and
rhyolitic litho-chemical groups
58
8.
9.
12.
Chemical analyses of intermediate volcanic
rocks
61
Representative chemical analyses of intermediate
and silicic volcanic rocks
64
14.
Chemical analyses of biotite rhyolite
66
15.
Chemical analyses of biotite tuff/crystalrich rhyolites
70
16.
Chemical analyses of Ash Peak Glass
75
17.
Chemical analyses of crystal-poor rhyolites
79
18.
Chemical analyses of porphyritic rhyolites
84
19.
Chemical analyses of altered andesites from
the Ash Peak mine
134
13.
Table
20.
21.
22.
Page
Chemical analyses of carbonates from the Ash
Peak and Thurston-Hardy deposits
142
Chemical analyses of manganese oxides from the
Thurston-Hardy and Rattlesnake Pit deposits
146
Element mobility in response to differing
hydrothermal conditions
152
LIST OF APPENDIX TABLES
Table
A.
Page
Chemical analyses (XRF) of intermediate volcanic
rocks
169
B.
Chemical analyses (XRF) of biotite rhyolite
172
C.
Chemical analyses (XRF) of biotite tuff/crystalrich rhyolite
173
D.
Chemical analyses (XRF) of Ash Peak Glass
174
E.
Chemical analyses (XRF) of crystal-poor rhyolites
from the Ash Peak eruptive center
175
Chemical analyses (XRF) of crystal-poor rhyolites
from the Rhyolite Peak eruptive center
178
Chemical analyses (XRF) of porphyritic
rhyolites
179
H.
Non-volatile Cl chondrite normalization values
181
I.
Partition coefficients used in crystal fractionation models
182
Analytical uncertainties associated with the
data
184
K.
Recalculation of Fe0 and Fe203
185
L.
Formula used by Magma86 in crystal fractionation
models.
186
F.
G.
J.
MAGMATISM AND MINERALIZATION OF THE ASH PEAK AREA, ARIZONA:
PETROCHEMICAL INTERPRETATIONS
INTRODUCTION
Ash Peak is the remnant of a pyroclastic breccia cone constructed
over one of two rhyolite-producing volcanic vents identified within the
study area.
The other volcanic vent is located approximately ten
kilometers northwest of Ash Peak near Rhyolite Peak.
These two
eruptive centers were active during mid-Tertiary time, and have been
collectively designated the Ash Peak
by Richter and others (1981).
Rhyolite Peak eruptive complex
This complex is located in the northern
Peloncillo Mountains of southeastern Arizona, as shown in Figure 1,
along the north-south boundary between Graham and Greenlee Counties.
Silicic volcanic rocks as manifest by the eruption of rhyolite at Ash
Peak, which was both preceded and followed by more voluminous
outpourings of andesite and basaltic andesite, represents an anomalous
change in magma chemistry,
Located between the two eruptive centers is
the Ash Peak mining district.
Mineralization in the district is
defined by veins of gold-silver-calcite-silica and manganese-calcite
that are similar in mineralogy, alteration, and structural association
to those of other districts hosting Tertiary epithermal vein deposits
in the southwestern United States.
The objectives of this investigation, based on the determination
of major, minor, and trace element abundances in the silicic and
intermediate volcanic rocks of this area, have been to define
petrochemical changes in the magma chamber(s) from which these rocks
were erupted, and to develop geologic models that explain the changes
that may have taken place within the magma chamber(s) to produce the
2
tforenel
MNALFAOMM
Peak
SUW
Area
GAUURO MTS.
PELONCILLO
SANTA CATALINA MIS.
Cochlse County
4 ...
DOS CABEZAS
Bisbee
Douglas
Figure 1.
Location map for the Ash Peak area, Arizona (modified from
Schumacher, 1978).
3
volcanic rocks found at Ash Peak.
Additionally, trace element contents
of the mineral deposits within the Ash Peak area have been determined.
Trace element abundances of ore and gangue have allowed processes of
ore formation to be modelled within the context of changes postulated
for the rhyolite producing magma chamber(s).
The attainment of these
goals has furthered our understanding of geologic processes operable in
the formation of silicic magmas and provided information concerning the
role of silicic magmatism in mineralization phenomena.
Ash Peak, as depicted in Figures 1 and 2, is located
approximately 40 kilometers east of Safford, Arizona, along U.S. Route
70 which crosses the study area two to three kilometers above the
southern boundary.
The northern boundary was selected to be U.S. Route
666 which connects Safford with the towns of Clifton and Morenci,
Arizona.
The area of study was kept relatively small (approximately 25
square kilometers) and roughly centered on the Ash Peak mine (Fig. 2)
in order to avoid possible overlap with other igneous systems near Ash
Peak.
During two field seasons, Fall of 1983 and Spring of 1984,
approximately 300 samples of silicic and intermediate volcanic rock
were gathered for petrochemical analyses.
Detailed geologic mapping
was performed, where necessary, in conjunction with sample collection
to supplement the maps published by Richter and others (1981, 1983).
These geologic maps (1:48000) were prepared by the U.S. Geological
Survey as part of the Silver City project, a two degree CUSMAP
(Conterminous United States Mineral Assessment Program).
The majority of rock samples collected were prepared and analyzed
in the laboratories of the Branch of Central Mineral Resources, U.S.
4
Undifferentiated Tertiary and
Quaternary Alluvium
Upper Andesite
Porphyritic Flow
11111111111111
Porphyritic Dome
Porphyritic Intrusive
Crystal-Poor Rhyolite
Ash Peak Glass
Pyroolastics
Biotite Tuff / Crystal-Rich Rhyolite
SEIR
Biotite Rhyolite
Lower Andesite
Figure 2.
General geology of the Ash Peak area, Arizona (modified from
Richter and others, 1981 and 1983).
.
e..
,
A
IIP
.4 ,:
:it f...:::.
NI*)*****-1::::::::::4.
v "I1:4::
. ..-.:!=.
.
::::::::::.
4..N.
4'it
.0:::::.:
4.
k
-.:;;;;e
I
,
6
Geological Survey, Denver, Colorado.
Abundances of K20, CO, total Fe
as FeO, 1102, Rb, Sr, Y, Zr, Nb, Ba, La, and Ce were determined using
energy-dispersive X-ray fluorescence methods of analysis.
following X-ray sources were used:
The
109Cd for total Fe as FeO, Rb, Sr,
Y, Zr, and Nb; 241Am for Ba, La, and Ce; and 55Fe for K2O, CaO, and
T102.
Scheduling constraints of the XRF equipment restricted the
number of samples analyzed; 291 samples were analyzed using the 109Cd
source and 271 samples by 109Cd and 241Am.
analyzed by all three sources.
are presented in Appendix 1.
A total of 204 samples were
The data acquired by these procedures
Analytical facilities at the Oregon State
University Radiation Center, Corvallis, Oregon were used to perform
sequential instrumental neutron activation analyses (INAA) of selected
Using INAA procedures developed by Professor Roman A. Schmitt
samples.
of the OSU Radiation Center, total Fe as FeO, Na2O, K2O, Sc, Cr, Co,
Rb, Sr, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Zr, Hf, Ta, Th, and U
abundances for 79 rock, mineral, and hydrothermally mineralized samples
were determined.
Partial evaluations of 15 of these samples were made
using the INAA laboratory of the U.S. Geological Survey in Denver,
Colorado.
The INAA data were supplemented by major oxide analyses
performed by Analytical Laboratories of the U.S. Geological Survey in
Denver using standard analytical procedures (predominantly X-ray
fluorescence).
Thin section petrography was used to identify the
minerals and textures in 96 of the samples of igneous rock and to
establish possible crystallization histories for the various rock
types.
This information was used to constrain liquidus mineralogy and
mineral-liquid proportions in the crystal fractionation models
subsequently developed for petrogenetic interpretations of the Ash Peak
7
volcanic rocks.
Previous work at Ash Peak has been primarily restricted to
studies of the mineral deposits of this area.
Regional geologic
interpretation is based on geologic mapping by Richter and others
(1981, 1983).
Reconnaissance age determinations, petrographic
examinations, and petrochemical analyses were included in Richter and
others (1981, 1983) and have proved to be an excellent foundation for
the current investigation.
One important objective of the U.S. Geological Survey's mission
is to locate reserves of strategic metals within the boundaries of the
United States.
Pursuant to this, the Branch of Central Mineral
Resources, U.S. Geological Survey, provided support for this
investigation in an effort to more completely understand the
mineralization at Ash Peak and its possible relationship to the
rhyolitic magmatism.
8
REGIONAL GEOLOGY AND TECTONICS
Hypotheses relating to magma genesis and the spatial-temporal
evolution of igneous rocks as derived from petrochemical data require
some knowledge or reasonable estimate of rock types and their
compositions at depth in the terrane under consideration.
By
reconstructing the local stratigraphic column, it is possible to
speculate on the particular source rocks that may have undergone
partial melting to produce magmas or to identify rock types that may
have acted as contaminants once a magma was formed.
For such reasons
it was considered appropriate to compile such information for rocks
underlying the Ash Peak
Rhyolite Peak eruptive complex to provide
constraints for the petrogenetic interpretation of these data.
Although the lithic record of this terrane is excellent for the
interval between mid-Tertiary and Holocene time, equivalent information
for older rocks at depth can only be assembled from published
investigations performed elsewher6 in southeast Arizona and southwest
New Mexico.
Fortunately, the lithologic continuity of the rocks
exposed around Ash Peak and the degree of detail to which they have
been studied is such that geologic extrapolation of pre-Oligocene
events at Ash Peak is possible.
Investigations throughout Arizona have
demonstrated that the southern Cordillera, including Ash Peak, has
undergone a complex tectonic history.
The tectonic region, roughly
encompassing the states of Arizona, New Mexico, Sonora and Chihuahua,
Mexico, is unique in the Western Cordillera in that its history has
been shaped by events that occurred on both the Atlantic and Pacific
margins of the North American craton.
Geologic studies over the last
85 years, beginning with the pioneering work of Ransome (1903, 1904),
9
have done much to decipher the Precambrian and Phanerozoic history of
the Southwest.
PRECAMBRIAN
Understanding of the Precambrian geology of Arizona has been
greatly enhanced in recent years by studies in the Chiricahua Mountains
(Cooper, 1959), Dragoon Mountains (Gilluly, 1956), Dos Cabezas
Mountains (Erickson, 1968), and Little Dragoon Mountains (Silver, 1978)
all located in southeastern Arizona (see Fig. 1).
Butler and Wilson
(1938) divided the Precambrian rocks of Arizona into Older Precambrian
and Younger Precambrian based on conspicuous differences in lithology,
age, and tectonic fabric.
Metamorphosed intrusive, sedimentary, and
volcanic rocks generally older than 1,420 m.y. comprise the Older
Precambrian which is separated from the unmetamorphosed sedimentary
rocks and locally abundant diabase sills (Silver, 1978) of the Younger
Precambrian by a pronounced unconformity.
Titley (1982) has proposed a
threefold division of the Older Precambrian rocks of Arizona as shown
in Figure 3.
Each division covers approximately one-third of the state
and is based on northeast-trending belts which are comprised of rocks
that are different in age and lithology.
In addition, there are
anomalous changes in magnetic susceptibility coincident with the
lithologic boundaries between the Older Precambrian belts.
These
magnetic features persist to the northeast and thus indicate their
continuation in the basement beneath the Phanerozoic cover of the
Colorado Plateau.
Rocks occupying the area northwest of the line
labeled Bright Angel-Mesa Butte Anomaly on Figure 3 are generally the
oldest of the three regions.
They are characterized by 1,800 m.y.
10
UTAH
<c>-
CO
vy
:;14,V.
GNEISS-
ey
METAVOLCANIa\,. -7 +4,
\\900/0
TERRAIN
CA 1800 my?
0
Flagstaff
GRANITEPrescott
YAVAPAI SERIES
CA 1760-1820 my?
LL
2
GRANITE-PINAN
SCHIST
Phoenix
CA 1650 -1700
my?%%,,%,
Morenci
Safford
Ajo
'Tucson
Nogales
Bisbee
Figure 3. Tripartite division of the Older Precambrian of Arizona
(modified from Titley, 1982).
11
gneisses and metavolcanics of the Vishnu schist, and are best exposed
in the Grand Canyon region.
The central belt of Older Precambrian
rocks lies between the Bright Angel-Mesa Butte Anomaly and the Holbrook
Line (Fig. 3) and is composed of volcanic and metavolcanic rocks of the
1,760 to 1,820 m.y. Yavapai Series.
The overlap in ages of the Yavapai
Series rocks and those to the northwest has been attributed to suturing
of allochthonous terrines of similar age (Karlstrom and others, 1987).
The Central Belt rocks are best exposed and have been most thoroughly
studied in the Jerome-Prescott area where they are associated with
volcanogehic massive sulfide deposits (Lindberg and Jacobson, 1974).
Interlayered flows of dacite and rhyolite, pyroclastic rocks, and
clastic sedimentary rocks in excess of 6,700 meters dominate the
Yavapai Series.
Based on composition, the Yavapai Series has been
compared to and correlated with greenstone belts of other areas such as
the older Superior Province of central Canada (Anderson and Silver,
1976).
The belt southeast of the Holbrook Line is composed of
metasedimentary rocks of the 1,680 to 1,700 m.y. Pinal schist
(Anderson, 1966).
Extrapolation based on compilations and work by Anderson (1966),
Silver (1978), and Titley (1982) suggest that the basement underlying
the Ash Peak
Rhyolite Peak eruptive complex and vicinity is most
probably the middle Proterozoic Pinal schist.
According to Silver
(1978), the Pinal schist in the Johnny Lyon Hills and adjacent Little
Dragoon Mountains, approximately 110 km southwest of Ash Peak, consists
predominantly of quartz-muscovite schist, fine-grained arkose, and
quartzites with only minor and thin layers of volcanics.
Despite
subsequent metamorphism, the presence of graded beds, rip ups, and
12
other sedimentary features indicate that the precursor greywacke, subgreywacke, conglomerate and shale of the Pinal schist is of turbidite
affinity (Cooper and Silver, 1964).
The Pinal schist in the Little
Dragoon Mountains has been described by Cooper and Silver (1964) as
containing schists and slates derived from greywacke, shale, siltstone,
and minor small lenses of conglomerate.
Other exposures of the Pinal
schist elsewhere in the Southwest have been described as sericitechlorite-quartz schist and phyllite and chlorite-oligoclase-microclinequartz schist.
Although the base is not exposed, estimates of
thickness for the Pinal schist in Cochise County (south of Ash Peak)
range from more than 2,750 to as much as 6,100 meters, and are
interpreted to represent sedimentation in a classic eugeoclinal
environment (Silver, 1978).
Accumulation of this thick sedimentary
sequence was followed by regional metamorphism (1,625 to 1,680 m.y.)
that formed the schists and related metamorphic rocks.
Post-kinematic
batholithic rocks were emplaced after this metamorphic event (Silver,
1978).
The geologic record in southeastern Arizona for the interval
from 1,625 to approximately 1,420 m.y. is missing.
However, batholiths
of granite were again emplaced into the Older Precambrian metamorphics
at about 1,420 m.y.
The volume of granitic material emplaced at this
time must have been large because Anderson (1966) has estimated that
the basement of southeast Arizona is now composed of 20 percent
residual roof pendants of schist enclosed in a sea of gneiss and
granite.
Quiescence followed the emplacement of these later batholiths
with erosion prevailing over much of the land surface.
The resultant
pronounced unconformity is present at many localities in the Southwest;
the most noted of which is found in the Grand Canyon between the Older
13
Precambrian Vishnu schist and the overlying Younger Precambrian Unkar
Group.
In contrast to the metamorphic and igneous rocks that comprise
the Older Precambrian, sedimentary rocks of the Apache Group, which are
possibly correlative to those of the Unkar Group in the Grand Canyon,
dominate the Younger Precambrian.
Younger Precambrian sedimentary
rocks throughout southeastern Arizona lie unconformably on an extensive
erosion surface incised into the crystalline basement.
The
conglomerates, sandstones, and shales that comprise the Apache Group
probably were deposited in a shallow marginal or interior basin
(Silver, 1978).
The age of the Apache Group has been bracketed between
the emplacement of the last Older Precambrian granitic bodies at 1,420
m.y. and the injection of diabase sills into the Apache Group 1,100
m.y. ago.
The appreciable range in these ages results from the unknown
duration of the post-Older Precambrian erosional event, which may have
been protracted.
The top of the Apache Group is defined by an
unconfcrmity upon which the Middle Cambrian Bolsa Quartzite was
deposited.
The pre-Paleozoic tectonic history of Ash Peak and much of the
southern Cordillera is largely unknown.
Voluminous granitic
batholiths, widespread calc-alkaline volcanics, and associated
metamorphic belts that comprise this Proterozoic terrane suggest, in
terms of modern plate tectonics theory, that subduction mechanisms may
have been prevalent (Dickinson, 1981).
Reconstructions of the
Precambrian by Stewart (1976) place the North American craton within a
large continental mass that may have formed during the interval 1,700
to 850 m.y. ago.
The age of rifting of this proto-continent and the
14
concomitant definition of the North America craton is unknown.
Stewart
(1976) has speculated that rifting to form the North American craton as
an independent landmass was active approximately 850 m.y. ago.
The
tectonic history of the southern Cordillera from latest Precambrian
through most of Paleozoic time can be reconstructed with reasonable
certainty from the available record and was largely uneventful as may
be deduced from the absence of angular unconformaties, volcanics and
batholithic intrusions, and the products of metamorphic events.
PALEOZOIC
Speculations with respect to the composition of Paleozoic rocks
beneath the Ash Peak
Rhyolite Peak eruptive complex are derived from
published sedimentary stratigraphic columns for the Dos Cabezas,
central Peloncillo, and Chiricahua Mountains south of Ash Peak and in
the Clifton-Morenci area to the north.
Information about the Paleozoic
strata and their genesis were gathered from the investigations of
Gilluly, Cooper, and Williams (1954), Gillerman (1958), Moolick and
Dureck (1966), Peirce (1976), Armstrong and Mamet (1978), Hayes (1976),
The
Mayer (1978), Ross (1978), Schumacher (1978), and Titley (1982).
Paleozoic rocks of southeastern Arizona are entirely of sedimentary
origin; igneous rocks of this era have not been recognized to date.
Although these strata are dominated by limestones and dolomites of
great diversity, a variety of siliciclastic sedimentary rocks is also
present.
This sequence essentially records repeated cycles of
transgression and regression of the Paleozoic seas.
Inundation of the
craton/continent proceeded from west to east during Cambrian and early
Ordovician times and from southeast to northwest during late
15
Ordovician, Carboniferous and Permian times.
Earlier formed strata
were eroded during periods of emergence (e.g. Lower Devonian
regression) and thus sizeable portions of the geologic record were
destroyed in some locales.
Faulting and folding produced
topographically high areas that profoundly effected the local character
of sedimentation.
For example, during the Upper Devonian
transgression, a southwestward extension of the Defiance Positive
(Peirce, 1976) caused shoaling of the carbonate platform that had
developed in most of the Southwest and produced local shallow water
lagoon, beach, fluvial, and tidal flat deposits.
The southern Cordillera throughout much of Paleozoic time was a
stable cratonic shelf (Peirce, 1976) upon which thick sequences of
carbonates and volumetrically minor elastic sediments accumulated.
According to Coney (1978), southeast Arizona was part of a
southwestward extension or protrusion of the North American craton
during Paleozoic time.
Dickinson (1981) proposed the existence of the
Paleozoic Transcontinental Basement Arch, a stable feature that limited
tectonic activity in southeastern Arizona to epeirogenic adjustments.
Subsidence in the Cordilleran miogeocline beginning 600 to 650 m.y. ago
was marked by marine transgression across Arizona (Stewart and Suczek,
1977).
Although predominantly marine, sedimentary facies and
thicknesses were locally modified depending on relationships with
epeirogenic basins and highlands.
It is perhaps remarkable that
Arizona remained tectonically- quiescent during mid-Paleozoic time when
the margins of the North American plate to the west and southeast were
concurrently undergoing profound orogenic disruptions.
Collision of
the African and South American plates is recorded by the intracratonic
16
uplift blocks and associated basins of the Ancestral Rocky Mountains.
Collision-related intraplate deformation in southeastern Arizona is
recorded in the development of the Pedrogosa Basin.
Plate collision
during Pennsylvanian-Permian time culminated in the formation of the
Appalachian-Ouachita-Marathon orogen which was a segment of the
Hercynian suturing of Pangea (Dickinson, 1981).
The western margin of
the North American plate was extended by arc-continent collisions of
the Late Devonian Antler orogeny and the Late Permian-Early Triassic
Sonoma orogeny.
At these times, miogeoclinal sediments, oceanic crust,
and volcanic arc material were accreted to or thrust on the continental
margin, resulting in the westward expansion of the plate edge.
MESOZOIC
Igneous activity dominated the Mesozoic era of southeastern
Arizona and resulted in the accumulation of volcanic and plutonic rocks
that greatly exceeds the thickness of sedimentary rocks preserved from
the Paleozoic.
Hayes and Drewes (1978) report Paleozoic thicknesses in
southeastern Arizona of approximately 1,500 meters whereas Mesozoic
rocks have a composite maximum thickness in excess of 12,000 meters.
The reconstruction of the Mesozoic section that may underlie Ash Peak
is based on studies in the Mule and Huachuca Mountains (Hayes, 1970a),
the Patagonia Mountains (Simons, 1972), and a compilation of studies
from throughout southeastern Arizona (Hayes and Drewes, 1968, 1978).
Erosion of Paleozoic sedimentary rocks began at the close of the
Permian and continued into the Middle Triassic when volcanism
commenced.
Preserved on the sedimentary erosion surface is the 220
m.y. old Mount Wrightson Formation.
This unit, exposed in the Santa
17
Rita Mountains southwest of Ash Peak (Fig. 1), is composed of a basal
andesite 750 m in thickness, an intermediate rhybdacite 1,500 m in
thickness, and an upper andesite 750 m in thickness (Drewes, 1971).
Correlative but thinner sections of the Mount Wrightson Formation are
reported from the Sierrita Mountains (Cooper, 1971) and the Patagonia
Mountains (Simons, 1972).
Volcanic sedimentary rocks of Lower Jurassic age (approximately
190 m.y%) are more widespread in southeastern Arizona than are those of
Middle Triassic age.
Stratigraphic sections composed of thick-bedded
to massive mudstone and siltstone derived from underlying volcanics and
Paleozoic sedimentary rocks often with intercalated sandstone,
conglomerate, and volcanic flows have been described in the Huachuca
and Patagonia Mountains by Hayes (1970a, 1970b) and Simons (1972) and
in the Tucson, Sierrita, Santa Rita, Empire, and Dragoon Mountains by
Hayes and Drewes (1968, 1978).
The volcanic component of the Lower
Jurassic rocks consists of up to 1,200 meters of quartz latite to
rhyodacite tuff with minor felsite; flow-banded, sparsely porphyritic
rhyolite; rhyolitic flow rocks with minor beds of interlayered tuff;
and massive, densely welded, crystal-rich, rhyolitic tuff (Hayes,
1970a,b).
However, volcanic and volcaniclastic rocks of Triassic-
Jurassic age are lacking in the central Peloncillo Mountains
(apkoximately 100 km south of Ash Peak),
Instead, sedimentary rocks
of Late Early Cretaceous age rest unconformably on the Paleozoic
section (Gillerman, 1958).
The cessation of Triassic-Jurassic volcanic activity was followed
by broad regional uplift along west- and northwest-trending normal
faults of large displacement. Erosion that ensued led to the formation
18
of extensive alluvial fans composed of repeated sequences of
feldspathic sandstones that grade upward into massive siltstone and
mudstone (Bilodeau, 1978).
Thereafter, southeastern Arizona began to
subside following this uplift, erosion, and alluvial fan formation.
These clastic rocks were then overlain by marine limestones deposited
during transgression of the Cretaceous sea (Hayes, 1970a,b, Drewes,
1971, Bilodeau, 1978, Hayes and Drewes 1978).
Regression toward the southeast followed limestone deposition and
sediment was deposited as deltas prograded seaward.
Uplift was
simultaneous with regression and some of the clastic rocks were soon
stripped away.
Basal conglomerates were then deposited on the erosion
surface and these in turn were succeeded by fluvial sands and shales.
Late Cretaceous andesitic to dacitic volcanic rocks in excess of 1,500
m complete the Mesozoic sequence and may possibly represent the
surficial expression of Laramide plutonism (Hayes, 1970a,b).
Thus, plutonic rocks of monzonitic to granitic composition were
emplaced concurrently with much of the volcanism and possibly the
sedimentation of Mesozoic time.
This intrusive magmatism was episodic.
The earliest plutons are Jurassic (165 to 160 m.y.) in age and are
represented by those of the Tucson, Sierrita, Santa Rita, Patagonia,
Huachuca, and Mule Mountains (Hayes and Drewes (1978).
Plutons of
Middle to Late Cretaceous (110 to 80 m,y.) age are at least two orders
of magnitude more volumetrically abundant than those formed by either
earlier or later magmatic events.
Cretaceous
Porphyritic intrusions of the Late
Early Tertiary Laramide orogeny (about 72-56 m.y.) are
well known hosts of porphyry Cu-Mo and related skarn mineralization.
They are generally smaller, but widely distributed, as these Laramide
19
plutons were emplaced in a broad northwest-trending swarm across
southern Arizona, New Mexico, and northern Sonora (Gilluly 1963, as
cited in Anderson, 1966, and Titley, 1982).
Paleozoic marine sedimentation terminated and Mesozoic volcanism
commenced in response to the breakup of Pangea between Late Triassic
and Late Jurassic time (Dickinson, 1981).
This event marked the
initiation of subduction and the creation of a magmatic arc along the
western margin of the North American craton.
The position of the
magmatic arc has been determined by the location of thick accumulations
of Late Triassic volcanic rocks and Jurassic plutons.
Early Cretaceous
backarc extension created topographic highs that were quickly eroded
forming alluvial fans.
Subsidence created the Chihuahua Trough within
which Early Cretaceous limestones and siliciclastic sediments
accumulated over the alluvial fans (Dickinson, 1981).
Beginning in Early Cretaceous time, the position of the magmatic
arc retreated from the western margin of the craton eastward away from
the subduction zone.
This eastward movement of the locus Of magmatism
is thought to have been induced by a decrease in the angle of dip of
the subducted plate (Lipman and others, 1972 and Dickinson, 1981).
Comparisons of modern arc-trench systems have shown that the magmatic
arc is usually located above the subducted slab when it has reached a
depth of between 90 and 125 km (Gill, 1981).
Thus, slabs having a
shallow dip will not reach the critical depth for the generation of
magmas until having moved appreciably inland of the trench.
Volcanic
and plutonic rocks of northern Mexico exhibit a progressive decrease in
age from approximately 130 to 60 m.y.- as the distances increase from
about 50 km to 600 km, respectively, from the trench (Damon and others,
20
1981, p. 145).
CENOZOIC
Eastward migration of the arc continued until it was 950 km from
the trench approximately 40 m.y. ago.
Volcanism was dominated by
magmas of intermediate composition, and Early Tertiary (pre-Oligocene)
basaltic andesite and andesite accumulations in excess of 1,500 m are
reported near Ash Peak in the central Peloncillo Mountains (Gillerman,
1958), northern Pyramid Mountains (Thorman and Drewes, 1978), and
Little Hatchet Mountains (Zeller, 1970).
In Oligocene and early
Miocene time the position of the magmatic arc shifted rapidly westward
and by about 20 m.y. ago was within 150 km of the trench.
Eruption of
silicic lava and caldera formation characterized this magmatism during
the westward migration of the arc (Deal and others, 1978) which
resulted from a steeper angle of subduction.
Mid Tertiary volcanic
rocks at Ash Peak are primarily rhyolite flows and domes with
volumetrically minor pyroclastic rocks, whereas ash-flow tuffs were the
dominant product of volcanism to the east and southeast.
Subduction of
the offshore spreading center in late Cenozoic time ended arc magmatism
and created the San Andreas transform fault and Basin and Range
extensional tectonics (Dickinson, 1981).
DISCUSSION
The stratigraphic column for the Ash Peak area is summarized in
Table 1 and is based on the geologic studies cited above.
21
Table 1.
Pre-Oligocene Stratigraphic Column at Ash Peak, Arizona.
Age
Thickness (m)
Dominate Lithology
Cenozoic
(preOligocene)
1,500
basaltic andesite and andesite
volcanic and volcanoclastic rocks of
Mesozoic
12,000
intermediate to felsic composition
intruded by monzonite to granite plutons
Paleozoic
1,500
limestone and dolomite with minor
siticiclastic rocks
arkosic quartzites, siliceous mudstones,
Upper Proterozoic
300
(Younger Precambrian)
diabase intrusions
Middle Proterozoic
(Older Precambrian)
comglomerates, limestone, with numerous
schists and slates of Pinal Schist,
>=6,000
gneiss, and granite
22
Rhyolites of Miocene age erupted at Ash Peak were probably
derived from parental magma(s) formed by partial melting of crustal
rocks.
The presence of abundant large xenocrysts of plagioclase
feldspars within porphyritic rhyolite suggests that sources of the
parental magma were igneous rocks of mafic to intermediate composition.
Petrographic examination implies that the progenitor liquid may have
been preserved as quenched droplets that encase the xenocrysts or as
globules that occur without plagioclase xenocrysts.
The exact
petrochemistry of the proposed parent liquid is riot known but an
intermediate composition is inferred from this observation.
A high
degree of partial melting is implied for the trapped liquid; magmas
produced by melting small amounts of mafic to intermediate parent rocks
would yield felsic melts.
Major and trace element abundances of the
trapped liquid will be important aspects of future investigations at
Ash Peak.
Igneous rocks of mafic to intermediate composition are not
present in the Paleozoic and Precambrian stratigraphic column under Ash
Peak (see Table 1).
Cenozoic basaltic andesites are not considered
possible source rocks because of their shallow depth.
Upon reaching
the pre-Oligocene rocks, mafic magmas that are proposed as the heat
source for melting would presumably continue to the surface.
Mesozoic
volcanic and volcaniclastic rocks of intermediate composition are the
most probable source rocks for partial melting and formation of the Ash
Peak rhyolites.
23
VOLCANIC ROCK UNITS
Volcanic activity was prevalent at Ash Peak and much of
southeastern Arizona during middle Tertiary time (16 to 30 m.y.; Damon
and others, 1981).
At least five rhyolite-producing eruptive complexes
within a 35 km radius of Ash Peak were active during this period.
Rhyolitic volcanism at Ash Peak took place between 24 and 21 m.y. ago
in a geographical age progression from older eruptive complexes to the
south to younger ones north of Ash Peak (Richter and others, 1981,
1983).
Volcanic rocks of the Ash Peak
Rhyolite Peak eruptive complex
were initially divided into nine rock types based on textural and
mineralogical features readily identifiable in hand specimens.
These
were designated andesite, biotite rhyolite, biotite tuff, pyroclastic
rocks, crystal-rich rhyolite, crystal-poor rhyolite, spherulitic
rhyolite, rhyolite glass, and porphyritic rhyolite.
However, chemical
data subsequently obtained for samples comprising these nine original
divisions indicated that only seven petrochemically unique groupings
were present, of which most corresponded to the original rock type
designations.
Based on trace element abundances, the biotite tuff and
the crystal-rich rhyolite are essentially identical.
In addition,
samples of glass- and spherulite-bearing phases of rhyolite were found
not to be unique groups, but can be chemically correlated to associated
crystal-poor, crystal-rich, or porphyritic rhyolites.
The Ash Peak
Glass, a thick vitric unit of controversial origin, was defined as a
new litho-chemical group on the basis of unusually low concentrations
of light rare earth elements (LREE).
Thus, mineralogical, textural,
and chemical criteria have been used to define seven litho-chemical
24
groups into which all of the Ash Peak volcanic rocks have been
assigned.
They are the andesitic volcanic rocks which occur above and
below the stratigraphically intermediate rhyolitic volcanic rocks and
include:
biotite rhyolite, biotite tuff/crystal-rich rhyolite,
pyroclastic rocks, Ash Peak Glass, crystal-poor rhyolite, and
porphyritic rhyolite.
Several subdivisions have been distinguished
within the major groups based on diagnostic petrographic and (or)
chemical features.
The geologic map (Fig. 2) illustrates the
distribution of these litho-chemical groups within the Ash Peak area.
ANDESITIC VOLCANIC ROCKS
The eruption of andesitic lavas both preceded and followed the
rhyolitic episode associated with the Ash Peak
complex.
Rhyolite Peak eruptive
These andesitic rocks have been designated the lower andesite
and the upper andesite, respectively.
Samples of andesitic volcanic
rock collected within the Ash Peak area vary from basaltic andesite
(53-57 weight % S02) to andesite (57-63 % Si02).
Small amounts of
basalt (45-53 % Si02) are reported from the region north of route US
666 (Richter and others, 1983).
Lower Andesites
Andesitic volcanic rocks that underlie the rhyolites of the Ash
Peak
Rhyolite Peak eruptive complex were originally designated the
"old amygdaloidal flows" by Richter and others (1981) and were later
termed the "lower andesite flows" by Richter and others (1983).
These
lower andesites are composed of basaltic andesites and andesites that
represent the oldest rocks exposed in the Ash Peak area.
They are
25
exposed in the east central and northern parts of the Ash Peak area
(Fig. 2).
Outcrops are generally subdued and identification is often
based on the presence of a dark reddish-brown soil that is favored by
prickly pear cactus.
Hand specimens vary in color from brown or
greyish-brown to brick-red or reddish-brown, and often contain
In addition
conspicuous white amygdules of quartz and (or) chalcedony.
to these minerals, Richter and others (1981, 1983) report chlorite,
zeolites (clinoptilolite and heulandite), and clays.
Veinlets of
calcite +/- chlorite are commonly present in specimens collected near
hydrothermal veins.
Individual flows 2 to 10 m thick comprise a total
thickness of at least 100 m (Richter and others, 1983), although a
lower contact has not been located within the Ash Peak area.
The upper
contact is generally concordant with the rhyolitic volcanic rocks
although locally there are intercalations of biotite rhyolite.
Lower andesites are typically microporphyritic and contain one to
three percent phenocrysts of olivine that average less than 1 mm in
diameter which are altered to iddingsite and iron oxides.
The
groundmass consists primarily of an intergranular to pilotaxitic
aggregate of plagioclase feldspar microlites, dark glass or
cryptocrystalline material, and opaque minerals.
Representative major oxide analyses for samples of the lower
andesites are listed in Table 2.
Abundances of Fe0 and Fe2O3 were
calculated using the method outlined in Appendix 2 of Le Maitre
(1976a).
The lower andesites are hypersthene normative and the
normative corundum calculated for sample AP84114 suggests these rocks
may have lost alkalis during alteration.
Abundances of major oxides
for the lower andesite samples deviate noticeably from respective
26
Table 2.
Representative Major Oxide Analyses of Andesitic Volcanic
Rocks Associated with the Ash Peak
Rhyolite Peak Eruptive
Complex, Lower Andesite.
Sample H
bsttc and
and
and
AP83040
AP84114
AP84181
Average High-K
Average High-K
bsttc and of
andesite of
Gill, 1981
Gill, 1981
Standard Major Oxide Analyses
Si02
(%)
53.6
58.2
60.1
54.6
59.4
TiO2
1.62
1.34
0.86
0.91
0.73
41203
17.6
17.9
16.4
17.7
16.9
Fe203
9.71
6.90
4.51
3.6
2.9
Fe0
0.22
n.a.
0.72
4.2
3.3
Mn0
0.09
0.19
0.08
0.18
0.12
Mg0
0.99
1.06
2.63
3.9
3.1
Ca0
6.25
3.55
4.75
7.6
6.0
Na20
4.13
5.06
3.94
3.3
3.3
K20
3.84
3.56
3.23
2.1
2.5
P20
0.78
0.91
0.35
0.30
0.24
H20.
0.26
n.a.
0.87
H20-
0.38
n.a.
1.40
1.2
1.3
CO2
0.42
n.a.
<0.01
Total
99.9
98.6
99.8
99.6
99.8
sum H2O
Recalculated Major Oxide Analyses, 100% volatile free
Si02
(5)
54.5
59.2
61.7
55.5
60.3
1102
1.65
1.36
0.88
0.92
0.74
41203
17.9
18.2
16.8
18.0
17.2
Fe203
4.82
3.13
2.55
3.7
2.9
Fe0
4.76
3.54
2.62
4.3
3.4
Mn0
0.09
0.19
0.08
0.18
0.12
MgO
1.01
1.08
2.70
4.0
3.1
Ca0
6.36
3.61
4.88
7.7
6.1
Na20
4.20
5.15
4.05
3.4
3.4
K20
3.91
3.62
3.32
2.1
2.5
P205
0.79
0.93
0.36
0.30
0.24
C1PW Normative Mineralogical Analyses
0
(X)
3.5
8.3
12.3
Or
23.0
21.5
19.6
Ab
35.4
43.6
34.2
An
18.3
11.9
18.0
C
1.4
Ac
..
Di
--
4.4
3.1
wo
2.2
1.6
en
1.1
1.2
fs
1.1
wo
0.2
--
Hy
2.7
3.6
6.5
en
1.4
2.7
5.5
fs
1.3
0.9
1.0
Mt
6.9
5.1
3.8
11
3.1
2.6
1.7
Ap
1.7
2.0
0.8
Cc
1.0
._
n.a.
"--.
not analyzed
indicates that the mineral is not present based on the chemical analysis
27
"average" orogenic basaltic andesite and andesite (Gill, 1981).
Abundances of TiO2 are somewhat high in basaltic andesites compared to
orogenic andesites and may reflect an extensional source for the lower
andesites (Lipman and others, 1989).
Iron is almost exclusively in the
trivalent state, 0.22/9.71 (% Fe0/Fe203) for basaltic andesites and
0.72/4.51 for andesites which deviates considerably from the "average"
of 4.2/3.6 and 2.9/3.3 respectively, and probably reflects postemplacement oxidation.
Both rock types are markedly depleted in Mg0
(0.99 vs. 3.9 % Mg0 for basaltic andesite and 1.06 and 2.63 vs. 3.33 %
Mg0 for andesite) and Ca0 (6.25 vs. 7.6 % Ca0 and 3.55 and 4.75 vs. 6.0
% CO) relative to the "average" orogenic basaltic andesite or
andesite.
In addition, both types are strongly enriched in the alkali
elements, especially K20, relative to their "average" composition (3.84
vs. 2.1 and 3.56 and 3.23 vs. 2.5 % K20).
The low ferrous/ferric ratio accompanied by the enrichment of the
alkali elements suggests that the chemical constituents of the lower
andesites have been mobilized and (or) modified.
This may have been
due to hydrothermal alteration associated with the formation of the Ash
Peak vein deposits or by processes of weathering.
Thus, although
chemically the andesite samples classify as trachyandesites, on the
basis of total alkali contents (total Na20 + K20 values of 8.56 and
7.17 %), this is unlikely to reflect their primary composition.
Lower andesites have an apparent age of late Oligocene based on
stratigraphic position and radiometric dating of similar rocks outside
of the Ash Peak area.
Although a mean whole rock K-Ar age
determination of 22.2 +/- 0.05 m.y. (early Miocene) was obtained on
samples collected from three localities near Ash Peak (Richter and
28
others, 1981, 1983).
These age determinations are probably erroneous
because the major oxide data suggests that the whole rock chemistry,
including K20, of the lower andesites has been modified by hydrothermal
activity and (or) processes of weathering.
A more reliable estimate of
age may be provided by radiometric ages of similar but less altered
andesites collected from south of Ash Peak that are slightly older
(25.6 +/- 0.05 m.y., late Oligocene) according to Richter and others
(1981).
RHYOLITIC VOLCANIC ROCKS
The petrogenetic interpretation of the rhyolitic volcanic rocks
produced by the Ash Peak
main focuses of this study.
Rhyolite Peak eruptive complex is one of the
Their areal distribution, lithologic and
petrographic description, and stratigraphic succession are therefore
important aspects of the Ash Peak investigation.
The Ash Peak
rhyolitic volcanic rocks are dominated by high-silica (>75 % Si02)
rhyolite flows and domes with relatively minor volumes of pyroclastic
material.
Thus, in terms of the major oxide abundances and style of
emplacement, volcanism of the Ash Peak
Rhyolite Peak eruptive complex
is inferred to be similar to high-silica rhyolite systems associated
with extensional tectonics reported in the literature from the Coso
volcanic field, California (Bacon and others, 1981), Twin Peaks, Utah
(Crecraft and others, 1981), and the Medicine Lake volcanic field,
California (Grove and Donnelly-Nolan, 1986).
29
Biotite Rhyolite
Biotite rhyolite, the earliest rhyolitic rock type erupted within
the Ash Peak area, can be easily distinguished from overlying biotitebearing rhyolites on the basis of a less evolved chemical composition
and glassy to pumiceous appearance.
These rocks were designated
"biotite rhyolite flows" by Richter and others (1981) in the southern
part of the Ash Peak area, but were included with other early-erupted
rhyolites and termed "moderately crystal rich rhyolite flows" in the
northern part (Richter and others, 1983).
Two sub-types of biotite
rhyolite have been identified within the Ash Peak area on the basis of
mineralogy and major and trace element contents.
Sub-type I consists
of low-silica rhyolite (>70 % Si02) and sub-type II consists of highsilica rhyolite.
As depicted in Figure 2, isolated outcrops of biotite
rhyolite (sub-type 1) are located in the extreme southern part of the
Ash Peak area and in a nearly continuous band (sub-type II) in the
center of the study area north of route US 70.
Outcrops of sub-type I
biotite rhyolite are subdued and often restricted to ephemeral stream
beds and pediments.
The band of biotite rhyolite north of Ash Peak
consists of low rounded hills covered with abundant sandy soil.
Biotite rhyolite forms light grey to dark olive-green pumiceous or
dense flows containing conspicuous phenocrysts of alkali and
plagioclase feldspar and biotite.
Individual flows are usually less
than 3 m thick with a maximum exposure of approximately 70 m (Richter
and others, 1983), although a basal contact was not observed within the
Ash Peak area.
Locally, thin flows of lower andesite can be found
stratigraphically above biotite rhyolite (Richter and others, 1981).
The results of petrographic examination of representative biotite
30
rhyolite samples are presented in Table 3.
Plagioclase feldspar,
alkali feldspar, biotite, and clinopyroxene characterize the phenocryst
assemblage of sub-type I with sub-type II dominated by alkali feldspar
and quartz with minor biotite.
A typical biotite rhyolite consists of
phenocrysts of sodic plagioclase feldspar, alkali feldspar, and biotite
set in a groundmass of dense to pumiceous glass.
Plagioclase feldspar
(An15_25) is either unzoned or normally zoned, and forms the cores of
some alkali feldspar phenocrysts.
Alkali feldspar is commonly
microperthitic but not to the extent it is in the later rhyolites (i.e.
porphyritic rhyolite).
Corroded phenocrysts of hornblende are
occasionally present in the somewhat perlitic cryptofelsitic to
spherulitic groundmass.
Rounded zircons (up to 0.2 x 0.05 mm)
represent the most abundant accessory phase, and may be associated with
opaques, isolated in the glassy groundmass, or in biotite-plagioclasealkali feldspar glomerocrysts.
Small crystals of prismatic apatite and
subhedral allanite have been identified in, or adjacent to, some of the
larger feldspar phenocrysts.
The compositions of biotite rhyolite are presented in Table 4.
Sub-type I is similar to the 'average" rhyolite of Le Maitre (1976b).
Sub-type II has higher Si02 content and lower abundances of all other
major oxides relative to sub-type I.
K-Ar ages were determined for alkali feldspar separates and
biotite separates from each of two locations within the Ash Peak area.
The mineral separates yielded'concordant ages of 23.1 +/- 0.05 m.y.
(early Miocene) from the sample location north of Ash Peak and 23.2 +/0.05 m.y. south of Ash Peak (Richter and others, 1981, 1983).
31
Table 3.
Petrography of Rhyolitic Volcanic Rocks from Ash Peak,
Arizona.
Litho - Chemical
Group
Biotite Rhyolite
Biotite
Crystal-Rich Rhyolite
Porphyritic Rhyolite
Tuff
Sub-type
Sample el
Flow
li
I
AP83067 AP84200
AP83038
Dome
Intrusive
Dome
Flow
AP84066
AP84073
AP84163
AP84166
AP84185
Alkali
Feldspar
4.1
7.0
5.7
10.3
6.6
23.2
8.2
15.0
Feldspar
6.2
1.4
1.3
n.o.
1.3
1.2
0.4
1.0
Biotite
2.1
0.6
0.4
n.o.
0.7
n.o.
n.o.
n.o.
Clinopyroxene
0.5
n.o.
n.o.
n.o.
n.o.
n.o.
0.9
n.o.
n.o.
0.4
n.o.
n.o.
n.o.
n.o.
n.o.
Quartz
0.4
2.8
0.3
n.o.
n.o.
n.o.
n.o.
n.o.
Opaque
0.5
<0.1
<0.1
0.3
0.1
1.0
0.7
1.0
xenoliths
n.o.
n.o.
n.o.
n,o.
n.o.
n.o.
2.4
6.5
Groundmass
86.2
88.2
91.9
89.4
91.3
74.6
88.3
75.6
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
Plagioclase
Hornblende
Total
n
n.o.= not observed
Modal mineralogical analyses are based on point counts of standard petrographic
thin sections with at least 2000 determinations per thin section.
32
Table 4. Representative Major Oxide Analyses of Rhyolitic Volcanic
Rocks Associated with the Ash Peak
Rhyolite Peak Eruptive
Complex, Biotite Rhyolite.
Average
sub-type 1
Sample #
AP83067
rhyolite of
sub-type II
AP84031
AP84023
AP84200
LeMaitre
(1976b)
Standard Major Oxide Analyses
Si02
(%)
70.5
70.2
71.7
70.1
TiO2
0.20
0.22
0.10
0.11
0.28
A1203
13.8
13.6
12.4
12.2
13.27
fe203
0.82
1.21
0.89
0.88
1.48
FeO
0.96
0.65
0.20
0.26
1.11
Mn0
0.05
0.05
0.03
0.03
0.06
Mg0
0.30
0.28
0.17
0.40
0.39
Ca0
0.97
1.13
0.88
1.68
1.14
Na20
3.70
4.10
3.72
2.41
3.55
72.82
K20
4.32
4.29
3.77
3.93
4.30
P205
0.07
<0.05
<0.05
<0.05
0.07
14204
2.80
3.49
4.72
5.26
1.10
1420-
0.50
0.59
1.04
2.94
0.31
CO2
<0.01
<0.01
<0.01
<0.01
0.08
99.0
99.8
100.2
100.0
Total
99.6
Recalculated Major Oxide Analyses, 100% volatile free
Si02
(%)
73.7
73.3
76.4
76.2
Ti02
0.21
0,23
0.11
0.12
0.28
11203
14.4
14.2
13.2
13.3
13.48
73.95
Fe203
0.81
0.82
0.50
0.57
1.50
FeO
1.07
1.11
0.64
0.64
1.13
Mn0
0.05
0.05
0.03
0.03
0.06
MO
0.31
0.29
0.18
0.43
0.40
Ca0
1.01
1.18
0.94
1.83
1.16
Ma20
3.86
4.29
3.96
2.62
3.61
K20
4.51
4.48
4,02
4.27
4.37
P205
0.07
n.d.
n.d.
n.d.
0.07
CIP41 Normative Mineralogical Analyses
0
(Z)
31.3
28.2
35.6
39.9
Or
26.7
26.5
23.8
25.3
Ab
32.7
36.2
33.5
22.2
An
4.6
5.9
4.7
9.1
C
1.5
0.1
0.6
1.0
Ac
--
--
Di
wo
YO..
en
fs
My
,....
1.1
1.0
0.7
1.5
en
0.8
0.7
0.5
1.1
fs
0.4
0.3
0.3
0.4
1.6
1.6
0.9
0.9
0.4
0.4
0.2
0.2
Mt
11
Ap
0.2
....
Cc
n.a. = not analyzed
"--" indicates that the mineral is not present based on the chemical analysis
33
Biotite Tuff/Crystal-Rich Rhyolite
Biotite tuff and crystal-rich rhyolites are stratigraphically
above the biotite rhyolites.
As mentioned previously, although biotite
tuff and crystal-rich rhyolite are lithologically dissimilar they were
combined into a single litho-chemical group because of the close
similarities of their major and trace element contents.
Biotite tuff was an early product of pyroclastic volcanism and
forms the platform upon which the Ash Peak pyroclastic breccia cone was
later constructed.
It was mapped as upyroclastic cone crater breccia"
by Richter and others (1981) and is composed of rhyolite tuff breccias.
Biotite tuff is exposed in the interior of the dissected pyroclastic
breccia cone associated with the Ash Peak eruptive center (Fig. 2).
The unit is structureless, massive and consists of pale yellow,
slightly collapsed pumice lapilli, and ash that is locally altered to
clinoptilolite.
Hand specimens contain readily distinguishable
crystals of alkali feldspar, hornblende, biotite, and minor lithic
fragments in a fine-grained to cryptocrystalline matrix.
The maximum
exposed thickness of 15 m is located near the center of the Ash Peak
pyroclastic cone,..
Biotite tuff contains alkali feldspar, microperthitic alkali
feldspar, subhedral plagioclase feldspar (An20_25), resorbed quartz,
altered biotite and hornblende, opaques, and accessory zircon in a
compact cryptocrystalline matrix (Table 3).
The modal mineralogy of
biotite tuff is very similar to biotite rhyolite sub-type II, with the
exception of quartz which is greatly reduced.
Representative major oxide analyses of biotite tuff and crystalrich rhyolite are presented in Table 5 along with recalculated
34
Table 5.
Representative Major Oxide Analyses of Rhyolitic Volcanic
Rocks Associated with:the Ash Peak
Rhyolite Peak Eruptive
Complex, Biotite Tuff/Crystal-Rich.Rhyplite.
Average
rhyolite of
biotite tuff
Sample A
AP83038
AP84076
leMaitre
crystal-rich rhyolite
AP83032
AP83052
AP83045
AP84073
AP84124
AP84131
(1976b)
Standard Major Oxide Analyses
Si02
67.6
76.4
76.5
74.5
75.0
76.1
76.1
75.3
TiO2
0,09
0.08
0.09
0,08
0.11
0.08
0.15
0.07
0.28
A1203
12.1
13.2
11.7
13.0
13.2
11.6
12.9
12.4
13.27
Fe203
1.24
0.90
1.37
1.29
1.42
1.25
1.00
1.71
1.48
Fe°
0.07
n.a.
<0.01
0.05
0.05
0.09
n.a.
0.07
1.11
Mn0
0.02
0.09
0.07
0.03
0.03
0.07
0.05
0.03
0.06
Mg0
0.11
<0.10
0.11
0.23
0.11
0.23
<0.10
<0.10
0.39
Ca0
0.37
0.52
0.54
0,62
0.52
1.13
0.47
0.29
1.14
Na20
1.79
4.48
3.31
3.89
3.76
3.43
4.41
3.69
3.55
K20
7.45
3.56
4.39
4.58
5.27
4.85
5.58
5.42
4.30
P205
<0.05
0.03
0.05
0.10
<0.05
0.06
0.04
<0.05
0.07
1120+
4.30
n.a.
0.65
0.17
0.36
0.31
n.a.
0.31
1.10
N20-
4.86
n.e.
0.65
0,52
0.45
0.77
n.a.
0.45
0.31
CO2
<0.01
n.a.
<0.01
0.01
<0.01
0.10
n.a.
<0.01
0.08
!oral
100.0
99.3
99.4
99.1
100.3
100.1
100.6
99.7
100.0
73.95
(%)
72.82
Recalculated Major Oxide Analyses, 100% volatile free
Si02
74.5
77.0
78.0
75.8
75.4
77.0
75.6
76.1
TiO2
0.10
0.08
0.09
0.08
0.11
0.08
0.15
0.07
0.28
A1203
13.3
13.3
11.9
13.2
13.3
11.7
12.8
12.5
13.48
1.50
(%)
Fe203
0.58
0.35
0.55
0.53
0.56
0.53
0.35
0.67
Fe0
0.81
0.51
0.78
0.78
0.87
0.78
0.61
1.06
1.13
MnU
0.02
0.09
0.07
0.03
0.03
0.07
0.05
0.03
0.06
mg0
0.12
n.d.
0.11
0.23
0.11
0.23
n.d.
n.d.
0.40
Ca0
0.41
0.52
0.55
0.63
0.52
1.14
0.47
0.29
1.16
Na20
1.97
4.51
3.38
3.96
3.78
3.47
4.38
3.73
3.61
K20
8.21
3.59
4.48
4.66
5.30
4.91
5.55
5.48
4.37
P205
n.d.
0.03
0.05
0.10
n.d.
0.06
0.04
n.d.
0.07
CIPW Normative Mineralogical Analyses
0
(5)
30.4
35.9
39.9
33.5
31.8
36.1
29.0
32.8
Or
48.5
21.2
26.5
27.6
31.4
29.0
32.8
32.4
Ab
16.7
38.2
28.5
33.4
32.0
29.3
34.7
31.5
An
2.0
2.4
2.4
2.4
2.6
1.9
C
0.4
1.1
0.6
0.8
0.4
--
--
Ac
-,
Of
wo
en
--
1.8
1.9
0.9
0.2
1.0
0.4
0.1
0.3
0.5
0.1
0.2
0.4
0.6
fs
to
--
Hy
1.2
--
--
0.6
0.3
0.6
0.8
0.5
en
0.3
--
0.3
0.6
0.3
fs
0.2
--
0.3
0.3
0.4
0.3
0.2
Mt
1.2
0.7
1.1
1.1
1.3
1.1
Ii
0.2
0.2
0.2
0.2
0.2
0.2
0.3
AP
0.1
0.1
0.2
0.1
0.1
Cc
--
0.0
0.2
0.2
1.5
n.a. = not analyzed
"--" indicates that the mineral is not present based on the chemical analysis
0.1
35
abundances and normative determinations.
Major oxide abundances of
typical biotite tuff may be represented by samples AP83032 and AP83052
which have relatively low H20+ values.
In which case, biotite tuff is
enriched in Si02 and possibly K20, and depleted in Ti02, A1203, FeO,
Fe203, MgO, and Ca0 relative to the "average" rhyolite of Le Maitre
(1976b).
The high water and K20 content of sample AP83038 are strong
indicators of element mobilization possibly due to hydrothermal
activity.
Abundances of major oxides and the modal mineralogy of
biotite tuff, exhibit close affinities with biotite rhyolite sub-type
II (Table 4) which may provide information regarding their
consanguinity.
Field relationships indicate that crystal-rich rhyolites were
erupted contemporaneously with the development of the Ash Peak breccia
cone.
Hand specimens were classified as crystal-rich rhyolite if they
contained 3-5 percent readily observable crystals, in contrast to those
of porphyritic rhyolites which typically contain approximately 20
percent crystals.
Crystal-rich rhyolites are high-silica rhyolites
that although designated simply "rhyolite" by Richter and others (1981)
were described as "moderately crystal rich" and constituted a mappable
unit.
Crystal-rich rhyolites include flows, domes, and the breccia
cone vent plug, and like biotite tuff, they are restricted to the Ash
Peak eruptive center (Fig 2).
They vary in color from light-purple to
dark brownish-red or brick-red and from dense compact masses to
laminated bodies with abundant cavities (lithophysae?) containing
tridymite.
Locally, chatoyant sanidine with subordinate biotite and
quartz comprise the phenocryst assemblage of most samples.
Thin
spherulitic layers that alternate with cryptocrystalline layers of
36
various color combinations, such as red on grey, produce dramatic flowlaminated specimens.
Domal rocks are usually brown or grey with a
slight bluish tinge and commonly contain irregular-shaped spherules
encircling the phenocrysts.
They exhibit diagnostic intrusive
structures such as extensive vertical flow banding and
distortion/deformation of layers that they crosscut.
The vent plug in
the dissected pyroclastic breccia cone east of Ash Peak consists of
pinkish- to reddish-grey, laminated, and brecciated rhyolite with
abundant alkali feldspar crystals.
Cavities within the vent plug
contain tridymite which probably formed by vapor-phase alteration.
In both domal and flow phases, alkali feldspar up to 2 mm in
length is the most abundant phenocryst, with sodic plagioclase
feldspar, biotite, quartz (often resorbed), and opaques present as
minor mineral phases (Table 3).
The phenocrysts are set in a
cryptofelsitic to spherulitic groundmass of alkali feldspar and quartz.
Abundances of major oxides of the crystal-rich rhyolites are
similar to biotite tuff with affinities to biotite rhyolite sub-type
11.
Thus, they are depleted in Ti02, FED, Fe203, MgO, and Ca0 and
enriched in SiC2 and the alkali elements relative to "average"
rhyolite.
Ppociastic Rocks
Pyroclastic rocks are volumetrically significant in the early
stages of rhyolitic volcanism at both eruptive centers when large
pyroclastic breccia cones were constructed.
These rocks were
designated "pyroclastic breccia cone and related deposits" by Richter
and others (1981) in the area south of Ash Peak.
They were grouped
37
into the more general category "pyroclastic deposits, undivided" by
Richter and others (1983) for their mapping north of Ash Peak.
In both
areas they are composed of pumice-lithic-crystal pyroclastic breccias.
Pyroclastic rocks are located in the northern part of the Ash Peak area
(Fig. 2) associated with the Rhyolite Peak eruptive center (RPEC) and
in the southeast at the Ash Peak eruptive center (APEC).
Although
vents for the biotite rhyolites have not been identified, the locations
of the vents that erupted the bulk of the pyroclastic rocks at the Ash
Peak and Rhyolite Peak centers are fairly well constrained.
The Ash
Peak cone, now mostly eroded, was centered at lat 32°44'46" N., long
109°15'37" W., where the vent plug is now exposed on the eastern flank
of Ash Peak.
The cone at Rhyolite Peak is speculatively placed at lat
32°49'20" N., long 109°18'26" W., where only a circular fringe of
pyroclastic deposits remain.
Reconstructions using slope angles and
the basal diameter suggest that the Ash Peak pyroclastic breccia cone
was at least 250 m high.
Thick accumulations of pyroclastic rocks (at
least 200 m exposed) that mark the former site of the breccia cone thin
and disappear to the south and west of Ash Peak, and have been totally
removed to the north and east.
A pronounced angular discordance within
the breccia cone clearly shows that the vent site migrated at least
once during the construction of the cone.
Pyroclastic deposits are composed of layers of coarse breccia
<0.5 to 3 m in thickness that are yellow to orange in color.
These
breccias are composed of pumice, lithic, and crystal fragments and are
locally interlayered with fine-grained ash, accretionary lapilli, thin
ash flow sheets, and epiclastic beds (Richter and others, 1981).
Pumice fragments are nondeformed, largely altered to clinoptilolite
38
(Richter and others, 1981), and range from block to lapilli in size.
Block-sized fragments commonly display impact structures in the
underlying matrix.
Lithic fragments occasionally constitute nearly 35
percent cf the breccia and are dominated by crystal-poor rhyolite and
lesser quantities of andesite or basaltic andesite.
Field
relationships and trace element abundances indicate that crystal-poor
rhyolites postdate the formation of the pyroclastie cone.
Thus the
Presence of apparently similar rocks within the pyroclastic breccias
raises a problem that is as yet unresolved.
Crystals of angular quartz
and alkali feldspar are supported in a cryptocrystalline matrix of ash
and pumice.
Petrographic and petrochemical analyses of the pyroclastic
depo its of the Ash Peak area were not performed for this study.
Ongoing investigations of the Ash Peak
Rhyolite Peak eruptive complex
and other eruptive complexes located in the Peloncillo and Whitlock
Mountains will be concerned with the petrographic and petrochemical
characterization of these rocks.
Ash Peak Glass
The Ash Peak Glass was defined during the present study on the
basis of the unusually low LREE abundances of these rocks.
This litho-
chemical group is typified by the large vitric unit (Ash Peak glass) of
high silica rhyolite that covers the west side of the Ash Peak
pyro lastic cone (Fig. 2).
Included in the group are high- silica
rhyolite flows possessing the same trace element abundances as the Ash
Peak glass.
Ash Peak Glass, like biotite tuff and crystal-rich
rhyolite, is restricted to the Ash Peak eruptive center.
The vitric
39
unit forming the west and north flanks of Ash Peak has a maximum
thickness of 50 meters and can be divided into three parts.
The bottom
eight to ten meters and the top five meters are composed of grey-green
to black glass that is aphyric to crystal-poor.
The interior of the
Ash Peak glass is composed of large (to 7 cm) red to pink spherules in
a vitric groundmass.
The Ash Peak glass extends from the summit of Ash
Peak a distance of 1.2 km to the west and 1.8 km to the southwest where
it has thinned to between four and five meters.
Representative thin sections of Ash Peak glass consist of
moderately perlitic, aphyric glass.
The glass has partially
devitrified to spherules of delicate hair-like crystals of what is
probably alkali feldspar and cristobalite.
Relative to the average rhyolite listed in Table 6, Ash Peak
Glass is depleted in all major oxides except Si02, K20, and possibly
Na20.
At 77.4 percent, Ash Peak Glass has the highest Si02 content of
any rhyolite group analyzed within the Ash Peak area (Table 6, Fig. 4).
Several petrochemical trends are suggested by the relative abundances
of the major oxides of the rhyolitic volcanic rocks.
The Si02 content
increases and 1102, A1203, total FeO, and Mg0 are depleted in an
apparent progression from biotite rhyolite to biotite tuff/crystal-rich
rhyolite to Ash Peak Glass.
The abundances of Na20 and K20 portrayed
in Figure 4 are not conclusive but suggest that the alkali element
content of the magmas remained approximately constant between the
litho-chemical groups.
The origin of the Ash Peak glass has been the subject of
considerable controversy,
The thickness of the Ash Peak glass, its
glassy character, and the large area it covers suggest that it was not
40
Table 6.
Representative Major Oxide Analyses of Rhyolitic Volcanic
Rocks Associated with the Ash Peak
Rhyolite Peak Eruptive
Complex, Ash Peak Glass.
Average
rhyolite of
LeMaitre
Sample #
AP83063
AP84139
AP84204
AP84056
(1976b)
Standard Major Oxide Analyses
5IO2
(%)
73.6
73.8
72.6
76.7
TiO2
0.04
0.04
0.04
0.04
0.28
A1203
12.1
12.0
11.7
12.3
13.27
Fe203
0.76
0.60
1.09
1.06
1.48
Fe0
0.30
0.46
<0.01
0.12
1.11
Mn0
0.04
0.04
0.03
0.03
0.06
Mg0
<0.10
<0.10
<0.10
<0.10
0.39
Ca0
0.64
0.58
0.80
0.36
1.14
Na20
3.50
3.96
3.26
3.38
3.55
K20
72.82
4.31
4.01
4.35
4.97
4.30
P205
<0.05
<0.05
<0.05
<0.05
0.07
N20+
3.27
3.67
3.78
0.52
1.10
N20-
0.85
0.63
1.39
0.50
0.31
<0,01
<0.01
<0.01
<0.01
0.08
99.4
99.8
99.0
100.0
100.0
CO2
Total
Recalculated Major Oxide Analyses, 100% volatile free
Si02
(%)
77.2
77.3
77.4
77.5
TiO2
0.05
0.04
0.04
0.04
0.28
A1203
12.7
12.6
12.5
12.4
13.48
1.50
73.95
Fe203
0.46
0.47
0.48
0.46
Fe0
0.63
0.65
0.63
0.69
1.13
Mn0
0.04
0.04
0.03
0.03
0.06
Mg0
n.d.
n.d.
n.d.
n.d.
0.40
Ca0
0.67
0.61
0.85
0.36
1.16
Na20
3.67
4.14
3.47
3.42
3.61
K20
4.52
4.20
4.64
5.02
4.37
P205
n.d.
n.d.
n.d.
n.d.
0.07
C1PW Normative Mineralogical Analyses
0
(X)
37.0
35.7
37.5
37.6
Or
26.8
24.8
27.4
29.7
Ab
31.1
35.0
29.4
28.9
An
3.3
3.0
4.2
1.8
C
0.5
0.1
0.2
0.7
Ac
--
--
Di
MO
en
fs
Wo
--
Ny
0.3
0.3
0.4
0.3
0.3
0.3
0.4
0.3
0.9
0.9
0.9
1.0
0.1
0.1
0.1
0.1
en
.-
fs
Mt
It
Ao
--
Cc
n.a.
not analyzed
"--" indicates that the mineral is not present based on the chemical analysis
41
0.3
0 22.
Porphyritic
0.1
A
CrystalPoor
vv
Ar
Ash Peak Glass
Biotite Tuff/CrystalRich
Iry
16
Biotite Rhyolite
0.0
70
72
74
76
78
80
6.0
15.014.0 -
1.3.0 -
A
Porphyritic
CrystalPoor
1?.0 -
Ash Peak Glass
A
Biotite Tuff/CrystalRich
11.0 -
Biotite Rhyolite
10.0
70
I
72
74
76
78
80
Si02 (wt. %)
Figure 4.
Abundances of major oxides of the rhyolitic volcanic rocks.
42
3.5
oX,3
3.0 -
0
2.5
2.0
xv
cn
V
1.5
0
-I-,
7'..
A V
A
Porphyritic
7.0
gli
CrystalPoor
A
Ash Peak Glass
0.5
V
A
A Biotite Tuff/CrystalRich
0
Biotite Rhyolite
00
70
72
74
76
78
80
0. 4 -
c\z)
0.3
0CP
0.2
V.
Porphyritic
0. 7 -
v
CrystalPoor
Ash Peak Glass
A Biotite Tuff/CrystalRich
Biotite Rhyolite
00
72
74
S102 (wt.
Figure 4.
(continued).
76
%)
78
80
43
6.0
5.0 24
4.0 -
A
e
A
.
v IN
A
4.
A
3.0
2.0
Porphyritic
1.0
Ash Peak Glass
CrystalPoor
A
Biotite Tuff/CrystalRich
Biotite Rhyolite
00
70
72
I2
7 74
7
76
78
80
78
80
6.0
AA
5.0 A
4.0 -
ye
di
le
3.0 -
0
C\I
2.0 -
Porphyritic
CrystalPoor
1.0-
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
0.0
70
72
74
76
7
Si02 (wt. %)
Figure 4.
(continued).
44
the product of a typical rhyolite flow or dome building eruption.
Richter (personal communication, 1987) considers the Ash Peak glass to
be an agglutinate formed by a Strombolian-type eruption of silicic
magma following the construction of the pyroclastic cone.
Other
geologists that have examined the unit have determined that the
textures are diagnostic of an ash-flow tuff or a large rhyolite flow.
Crystal-Poor Rhyolite
Crystal-poor rhyolites are the most abundant rhyolitic rocks
associated with the Ash Peak
Rhyolite Peak eruptive complex.
These
rocks were termed "massive, crystal-poor rhyolite flows" by Richter and
others (1981) and were reclassified as "crystal-poor rhyolite domes and
flows, undivided' by Richter and others (1983).
Crystal-poor rhyolites
from both eruptive centers are high-silica rhyolites.
They are exposed
in a NW-SE trending band south of the Rhyolite Peak eruptive center and
in a brdad swath west of the Ash Peak eruptive center (Fig. 2).
At
both eruptive centers, crystal-poor rhyolites form steep-sided hills
and cliffs on the upper parts of nearly all topographicly high areas.
They are aphyric to crystal-poor, usually flow-laminated and display
wide range of colors:
yellow.
a
purple, grey, pink, red, brown, tan, orange, and
Individual flows tend to be short and thick, and thus it is
difficult to impossible to formulate regional correlations.
Single
flows of crystal-poor rhyolite associated with the Rhyolite Peak center
may be as thick as 190 m with a total aggregate thickness of 300 m
(Richter and others, 1983).
At the Ash Peak center, the flows tend to
be thinner and the total thickness probably does not exceed 180 m
(Richter and others, 1981), although capping units have been removed by
45
erosion.
Locally there are conspicuous pods, masses, and layers of
grey-green to black glass, usually <1 m thick but in some instances >7
m thick, less commonly vitrophyre is present.
Lava flows often possess
brecciated bases with the spaces between the fragments usually filled
by rhyolite.
At the Rhyolite Peak eruptive center, ramp structures are
well-developed in crystal-poor rhyolite flows.
Microphenocrysts of quartz, alkali feldspar, biotite,
clinopyroxene, and opaque minerals constitute less than one percent of
the rock (Richter and others, 1981, 1983).
Thin (usually 0.5 to 1 mm
thick) layers composed of cryptocrystalline quartz and alkali feldspar
alternate with layers of spherules to form the groundmass which has
been flow deformed into complex undulating patterns.
The entire rock
may be dusted with minute rods and needles of opaque minerals or,
locally, hematite.
Representative major oxide analyses, non-volatile recalculations,
and CIPW normative determinations for crystal-poor rhyolites from the
Ash Peak and Rhyolite Peak eruptive centers are presented in Tables 7
and 8, respectively.
The rock types discussed previously contain, with
exceptions, large amounts of adsorbed water (H20 +) which has a
significant effect on the recalculated data.
Examination of Tables 7
and 8 and Figure 4 must be carried out with prudence when evaluating
the major oxide data for the crystal-poor rhyolites.
In general, the
analyses suggest that abundances of Si02 in the crystal-poor rhyolites
are lower and Ti02, total FeO, and Mg0 are higher than in other
rhyolites.
The A1203 content decreases and CaO, Na20, and K20 remained
the same with increasing Si02.
46
Table 7.
Representative Major Oxide Analyses of Rhyolitic Volcanic
Rocks Associated with the Ash Peak
Rhyolite Peak Eruptive
Complex, Crystal-Poor Rhyolites from the Ash Peak Eruptive
Center.
Average
rhyolite of
LeMaitre
Sample N
AP83036
AP83050
AP83056
AP83058
AP83062
AP84066
AP84084
AP64085
(1976b)
72.82
Standard Major Oxide Analyses
Si02
(%)
73.1
74.4
75.5
75.4
76.6
76.4
75.1
76.6
TiO2
0.07
0.06
0.09
0.07
0.06
0.07
0.06
0.06
0.28
A1203
11.9
11.1
12.3
12.2
12.0
12.2
12.5
11.7
13.27
Fe203
0.78
1.48
1.76
1.54
1.52
1.38
1.64
1.48
1.48
Fe0
0.72
0.01
0.06
0.04
0.07
0.04
0.04
0.06
1.11
Mn0
0.04
0.04
0.06
0.04
0.03
0.03
0.04
0.03
0.06
MgO
<0.10
0.23
<0.10
0.11
<0.10
0.11
<0.10
0.12
0.39
Ca0
0.51
2.66
0.34
0.42
0.27
0.38
0.13
0,31
1.14
Na20
3.93
3.73
4.67
4.08
4.11
3.76
4.22
3.84
3.55
K20
4.10
4.49
4.58
4.62
4.22
5.08
4.75
4.53
4.30
P205
<0.05
<0.05
<0.05
0.05
<0.05
0.09
<0.05
<0.05
0.07
H20
3.39
0.46
0.14
0.54
0.42
0.34
0.36
0.34
1.10
H20-
0.71
0.40
0.31
0.58
0.40
0.44
0.33
0.60
0.31
CO2
<0.01
1.11
<0.01
0.01
<0.01
<0.01
0.01
0.05
0.08
99.3
100.2
99.8
99.7
99.7
100.3
99.2
99.7
100.0
73.95
Total
Recalculated Major Oxide Analyses, 100% volatile free
Si02
76.8
75.8
76.0
76.5
77.5
76.8
76.3
77.6
TiO2
0.07
0.06
0.09
0.07
0.06
0.07
0.06
0.06
0.28
A1203
12.5
11.3
12.4
12.4
12.1
12.3
12.7
11.9
13.48
Fe203
0.67
0.60
0.68
0.61
0.62
0.53
0.64
0.60
1.50
Fe0
0.92
0.86
1.09
0.93
0.93
0.84
1.00
0.90
1.13
Mn0
0.04
0.04
0.06
0.04
0.03
0.03
0.04
0.03
0.06
Mg0
n.d.
0.23
n.d.
0.11
n.d.
0.11
n.d.
0.12
0.40
Ca0
0.54
2.71
0.34
0.43
0.27
0.38
0.13
0.31
1.16
Na20
4.13
3.80
4.70
4.14
4.16
3.78
4.29
3.89
3.61
K20
4.31
4.58
4.61
4.69
4.27
5.11
4.83
4.59
4.37
P205
n.d.
n.d.
n.d.
0.05
n.d.
0.09
n.d.
n.d.
0.07
(X)
CIPW Normative Mineralogical Analyses
0
1%)
35.0
33.9
30.8
33.6
36.3
34.5
32.5
36.6
Or
25.5
26,8
27.3
27.7
25.3
30.2
28.5
27.1
Ab
34.9
31.8
37.9
35.0
35.2
32.0
36.2
32.9
An
2.7
0.3
--
1.3
1.4
1.3
0.6
1.2
C
0.1
0.2
0.0
0.2
0.0
Ac
--
Di
1.4
0.3
--
1.0
0.7
0.2
--
en
0.6
--
Hy
fs
0.1
0.4
0.8
1.6
0.0
--
..
0.4
0.4
en
Ap
--
1,9
fs
11
--
wo
Wo
Mt
1.6
0.1
--
0.3
0.5
0.3
--
0.2
0.4
0.2
0.3
0.2
0.3
0.3
0.6
0.3
0.3
1.3
1.2
0.8
1.4
1.4
1.2
1.5
1.3
0.1
0.1
0.2
0.1
0.1
0.1
0.1
0.1
0.0
0.1
._
0.1
Cc
2.5
0.0
0.2
--
n.a. = not analyzed
indicates that the mineral is not present based on the chemical analysis
47
Table 8.
Representative Major Oxide Analyses of Rhyolitic Volcanic
Rocks Associated with the Ash Peak
Rhyolite Peak Eruptive
Complex, Crystal-Poor Rhyolites from the Rhyolite Peak Eruptive
Center.
Average
rhyolite of
LeMaitre
Sample a
083001
AP83002
AP83021
AP83028
AP84171
(1976b)
72.82
Standard Major Oxide Analyses
Si02
(X)
71.7
74.7
77.1
74.9
70.8
1102
0.09
0.08
0.10
0.09
0.08
0.28
A1203
12.3
12.6
11.3
12.7
12.2
13.27
Fe203
0.93
1.98
1.81
Fe0
0.79
0.04
<0.01
MnO
0.05
0.02
Mg0
<0.10
Ca0
Na20
K20
1.97
1.14
1.48
<0.01
0.64
1.11
0.04
0.05
0.06
0.06
.0.10
<0.10
<0.10
0.17
0.39
0.51
0.63
0.35
0.28
0.94
1.14
4.56
4.22
2.84
4.33
3.07
3.55
3.35
4.32
4.69
4.82
3.80
4.30
P205
<0.05
0.10
0.10
<0.05
<0.05
0.07
H20+
4.10
0.26
0.60
0.13
4.08
H20-
0.45
0.38
0.51
0.41
1.71
0.31
<0.01
<0.01
<0.01
<0.01
<0.01
0.08
98.8
99.3
99.4
99.7
98.7
100.0
CO2
Total
1.10
Recalculated Major Oxide Analyses, 100% volatile free
Si02
(X)
76.0
75.8
78.5
75.6
76.2
TiO2
0.09
0.08
0.10
0.09
0.09
0.28
41203
13.0
12.8
11.5
12.8
13.1
13.48
Fe203
0.78
0.79
0.73
0.74
0.86
1.50
Fe0
1.05
1,17
1.03
1.17
1.03
1.13
Mn0
0.05
0.02
0.04
0.05
0.06
0.06
73.95
Mg0
n.d.
n.d.
n.d.
n.d.
0.18
0.40
Ca0
0.54
0.64
0.36
0.28
1.01
1.16
Na20
4.84
4.28
2.89
4.37
3.31
3.61
K20
3.55
4.38
4.77
4.87
4.09
4.37
P205
n.d.
0.10
0.10
n.d.
n.d.
0.07
CIPW Normative Mineralogical Analyses
0
32.9
32.9
42.8
30.9
38.6
Or
21.0
25.9
28.2
28.8
24.2
Ab
40.9
36.2
24.4
16.9
27.9
An
2.7
2.5
1.1
1.0
5.0
C
0.2
0.1
1.2
( X )
Ac
..
1.4
..
Di
0.4
wo
0.2
en
fs
0.2
No
Hy
0.5
0.4
0.4
0.1
1.2
en
.-
fs
0.5
0.4
0.4
0.1
1.5
1.7
1.5
1.7
1.5
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.5
Mt
IL
AP
0.7
Cc
n.a. = not analyzed
"--" indicates that the mineral is not present based on the chemical analysis
48
Porphyritic Rhyolite
Porphyritic rhyolite represents the last silicic volcanic
activity identified within the Ash Peak area before magmatism reverted
to andesitic compositions.
Richter and others (1983) designated three
mappable subdivisions of porphyritic rhyolite:
"porphyritic low-silica
rhyolite and dacite flows, porphyritic low-silica rhyolite dome, and
porphyritic low-silica rhyolite shallow intrusives ".
For brevity in
this report, these subdivisions were termed porphyritic flows, dome,
and intrusive respectively.
Porphyritic rhyolites analyzed in this
study are low-silica rather than high-silica rhyolites.
The location
of the Rhyolite Peak eruptive center (Fig. 2) is coincident with
outcrops of the intrusive and domal subdivisions; distributed to the
west and east of the center are porphyritic flows.
The porphyritic
rhyolite domal phase envelops the intrusive mass and also occurs as
scattered proximal outcrops.
Porphyritic rhyolites are easily eroded
and form low rounded hills unless capped by upper andesites, in which
case they form steep sided knolls.
Unlike rhyolites previously
described, these rocks are conspicuously porphyritic with large (2-8
mw) phenocrysts of alkali feldspar forming 20-25 percent of the rock.
Hand specimens range in color from purple to yellow and include brown,
grey, pink, and tan varieties.
Intrusive rocks are holocrystalline,
domal and flow rocks possess cryptocrystallire groundmasses.
Rounded
xenoliths of mafic material are often prevalent and may constitute 1 to
3 percent of the rock.
Porphyritic flows are very similar in hand
specimen to intrusive and domal varieties but exhibit well-developed
extrusive features such as brecciated tops and bottoms, pods and lenses
of glass, and flew textures.
They tend to be thick (to 100 m),
49
massive, and have a composite areal extent comparable to crystal-poor
rhyolites (Richter and others, 1983).
Total porphyritic flow
thicknesses may be as much as 150 m and are terminated by a prominent
erosion surface upon which rest the upper andesites (Richter and
others, 1983).
Porphyritic rhyolites are composed of alkali feldspar
microperthite with subordinate plagioclase feldspar (Ani5), biotite,
clinopyroxene (often altered), and opaque minerals in a groundmass of
granular alkali feldspar and quartz (intrusive) or cryptocrystalline
material (Table 3).
Plagioclase feldspar forms the cores of some
alkali feldspar phenocrysts or exists as small subhedral crystals.
In
addition to the minerals identified in this study, Richter and others
(1983) report the presence of clinopyroxene (2-5 percent), hypersthene
locally altered to fibrous amphibole (0-5 percent), sphene, and a trace
of olivine.
Globular xenoliths that have been interpreted as quenched
liquid are most prevalent in domal and flow subdivisions but are also
significant in the intrusive subdivision.
They occasionally contain
plagioclase feldspar crystals that possess textures interpreted as
melting features.
Representative major oxide analyses, non-volatile recalculations,
and CIPW normative determinations for porphyritic rhyolites are
presented in Table 9.
Abundances of major oxides of the porphyritic
rhyolites do not continue the trends of the previous rhyolitic volcanic
rocks.
The Si02 content is lower and T102, A1203, total FeO, MgO, and
Ca0 content are greater than those of the earlier rhyolites.
Porphyritic rhyolites are similar in composition to "average" rhyolite
with the exception of lower Mg0 (0.25 vs. 0.39 %) and Ca0 (0.67 vs.
50
Table 9.
Representative Major Oxide Analyses of Rhyolitic Volcanic
Rocks Associated with the Ash Peak
Rhyolite Peak Eruptive
Complex, Porphyritic Rhyolite.
Average
rhyolite of
Sample 9
intrus
dome
flow
flow
flow
AP84163
AP84166
AP84025
AP84185
AP84199
(1976b)
72.82
LeMaitre
Standard Major Oxide Analyses
Si02
70.6
74.7
71.7
72.9
69.7
TiO2
0.31
0.24
0.25
0.28
0.31
0.28
A1203
14.3
13.3
14.1
14.3
14.8
13.27
Fe203
2.57
1.80
2.51
2.20
2.25
1.48
Fe0
0.33
n.a.
0.12
n.a.
0.44
1.11
KnO
0.03
0.03
0.05
0.05
0.03
0.06
MgO
0.36
0.20
0.26
0.20
0.24
0.39
Ca0
0.72
0.60
0.68
0.66
0.65
1.14
Na20
4.51
4.01
4.58
4.40
4.31
3.55
K20
5.05
4.74
4.94
4.82
5.25
4.30
P205
0.08
0.06
0.05
0.08
0.15
0.07
820
0.48
n.a.
0.33
n.a.
0.33
1.10
H20-
0.76
11.8 .
0.24
n.a.
0.46
0.31
(X)
CO2
<0.01
n.a.
<0.01
n.a.
<0.01
0.08
Total
100.1
99.8
99.8
99.8
98.9
100.0
73.95
Recalculated Major Oxide Analyses, 100X volatile free
Si02
71.5
75.0
72.3
73.1
71.1
TiO2
0.32
0.24
0.25
0.28
0.31
0.28
A1203
14.5
13.4
14.2
14.3
15.1
13.48
Fe203
1.13
0.69
1.01
0.84
1.07
1.50
Fe0
1.72
1.04
1.55
1.28
1.60
1.13
Mn0
0.03
0.03
0.05
0.05
0.03
0.06
Mg0
0.36
0.20
0.26
0.20
0.24
0.40
Ca0
0.73
0.60
0.69
0.66
0.66
1.16
Na20
4.57
4.02
4.62
4.41
4.40
3.61
K20
5.11
4.76
4.98
4.83
5.35
4.37
P205
0.08
0.06
0.05
0.08
0.15
0.07
(X)
CIPW Normative Mineralogical Analyses
0
(X)
23.4
32.0
24.6
27.4
23.6
Or
30.2
28.1
29.5
28.6
31.7
Ab
38.6
34.0
39.0
37.3
37.2
An
3.1
2.6
3.1
2.8
2.3
C
0.3
0.7
0.1
0.8
1.2
Ac--
--
--
Di
Wo.._
MO
en
fs
Hy
en
fs
Mt
IL
Ap
--
1.1
0.6
0.9
0.6
0.8
0.9
0.5
0.7
0.5
0.6
0.2
0.1
0.2
0.1
0.2
2.5
1.5
2.3
1.9
2.3
0.6
0.5
0.5
0.5
0.6
0.2
0.1
0.1
0.2
0.3
Cc
--
n.a. = not analyzed
"--" indicates that the mineral is not present based on the chemical analysis
51
1.14 %) and higher Na20 (4.31 vs. 3.55 %) and K20 (4.70 vs. 4.30 %)
abundances.
Upper Andesitic Volcanics
Following the cessation of rhyolitic volcanism within the Ash
Peak area, lavas of andesitic composition were once again extruded.
These rocks were designated "basaltic andesite and andesite flows" by
Richter and others (1981) in the southern part of the area and "upper
andesite flows" (Richter and others, 1983) for exposures north of Ash
Peak.
Whereas upper andesites within the Ash Peak area are classified
as basaltic andesites, Richter and others (1983) report that the
majority of the upper andesite rocks outside the Ash Peak area are
olivine-clinOpyroxene andesites, with minor quantities of olivine
andesite, basaltic andesite; and two pyroxene andesite.
Although
restricted to the extreme northern and southern regions of the study
area (Fig. 2), upper andesites are very extensive.
in contrast to
exposures of the lower andesites, upper andesites form striking
outcrops often capping the hills in the northern part of the area.
They are porphyritic or microporphyritic, grey to black in color with
a
range of brownish variations, and generally have empty vesicles within
a fine-grained groundmass.
Individual flows vary in thickness from 2
to 20 m, with typically scoiaceous tops and bottoms, and comprise an
aggregate total thickness Of more than 400 m (Richter and others,
1983)
The lower contact of the upper andesites in the northern part
of the Ash Peak area rests on a prominent soil horizon developed on the
underlying porphyritic rhyolites.
Petrographic examinations support the earlier descriptions of
52
Richter and others (1981, 1983), and show that phenocrysts of the upper
andesites consist chiefly of large crystals of plagioclase feldspar
(An45_55) up to 5 mm in length together with subordinate amounts of
clinopyroxene, orthopyroxene, and olivine.
The phenocrysts are
contained in a pilotaxitic, intergranular, or intersertal groundmass of
plagioclase feldspar microlites, clinopyroxene, minute opaque minerals,
and dark cryptocrystalline or glassy material.
Major oxide concentrations of upper andesite samples are
presented in Table 10.
They are similar to those of the lower
andesites (Table 2) and thus they deviate from "average" basaltic
andesite in several important elements.
Upper andesites have TiO2
contents similar to the lower andesites and presumably this also
reflects a rift environment for the origin of these rocks.
Iron is
more abundant than "average" basaltic andesite and ferric iron is
dominate over ferrous iron (0.48/10.27 vs. 4.2/3.6 % Fe0/Fe203).
Upper
andesites are depleted in Mg0 (2.97 and 1.65 vs. 3.9 % Mg0) and Ca0
(7.12 and 4.87 vs. 7.6 % Ca0) relative to the "average" but not to the
extent seen in the lower andesites (0.99 vs. 3.9 % Mg0 and 6.25 vs. 7.6
% Ca0).
Abundances of Na20 are only slightly elevated and K20 values
are essentially the same as "average" basaltic andesite.
The behavior
of the alkali elements suggests that, unlike the lower andesites, these
rocks were not affected by hydrothermal alteration nor substantial
weathering.
A whole-rock K-Ar age determination of 19.4 +/- 0.4 m.y. was
obtained for a sample of an upper andesite flow 6.5 km northwest of the
Ash Peak area (Richter and others, 1983).
53
Table 10.
Representative Major Oxide Analyses of Andesitic
Volcanic Rocks Associated with the Ash Peak
Rhyolite Peak
Eruptive Complex, Upper Andesite.
Average HighK
bsltc and bsltc and
Sample A
AP84008
bsltc and of
AP84183
Gill, 1981
Standard Major Oxide Analyses
Si02
54.6
53.4
54.6
TiO2
1.14
1.88
0.91
A1203
17.8
16.3
17.7
Fe203
8.50
10.27
3.6
FeO
n.a.
0.48
4.2
Mn0
0.13
0.19
0.18
MgO
2.97
1.65
3.9
Ca0
7.12
4.87
7.6
5a20
4.01
5.04
3.3
K20
2.50
2.73
2.1
P20
0.50
0.97
0.30
H20.
n.a.
0.06
H20-
n.a.
0.83
CO2
n.a.
0.02
Total
99.3
98.7
(%)
sum H2O
1.2
99.6
Recalculated Major Oxide Analyses
Si02
(X)
55.3
54.9
55.5
TiO2
1.15
1.93
0.92
A1203
18.0
16.8
18.0
Te203
4.32
5.35
3.7
FeO
3.81
5.17
4.3
Mn0
0.13
0.20
0.18
Mg0
3.01
1.70
4.0
Ca0
7.21
5.01
7.7
Na20
4.06
5..18
3.4
K20
2.53
2.81
2.1
P205
0.51
1.00
0.30
CIPW Normative Mineralogical Analyses
o
(X/
4.0
3.3
Cr
15.0
16.6
Ab
34.3
43.8
An
23.5
14.2
7.2
3.3
Ac
Di
MO
en
fs
3.8
1.7
2.5
1.0
1.0
0.6
7.2
5.3
5.1
3.3
2.1
2.1
Wo
Hy
en
fs
Mt
fl
Ap
5.5
7.8
2.2
3.7
1.1
2.2
Cc
0.1
n.a. = not analyzed
"--" indicates that the mineral
is not present based on the chemical analysis
54
SUMMARY
Silicic volcanism was active within the Ash Peak area during the
interval 24 to 21 m.y. ago and was preceded and followed by the
eruption of intermediate composition lavas.
Peak
Volcanic rocks of the Ash
Rhyolite Peak eruptive complex have been divided into seven
litho - chemical groups based on hand specimen characteristics and the
abundances of major and trace elements.
Extrusion of biotite rhyolite
followed and was locally contemporaneous with the eruption of lower
andesite.
Large pyroclastic breccia cones were constructed at both of
the Ash Peak and Rhyolite Peak eruptive centers.
Crystal-poor rhyolite
flows and domes succeeded pyroclastic activity and are the most
voluminous silicic rocks within the area.
The eruption of porphyritic
rhyolite associated with the Rhyolite Peak eruptive center represented
the final phase of silicic volcanism at Ash Peak.
Upper andesites were
extruded following a hiatus protracted enough for the development of
soils on the silicic volcanics.
Volcanic features produced by different eruption modes of silicic
and intermediate composition lavas are readily observable within the
Ash Peak
Rhyolite Peak eruptive complex.
Structures typical of
fissure eruptionS of moderately fluid lava are preserved in
intermediate volcanic rocks, including movement during cooling as
complex flow lines and patterns of vesicles and amygdules.
Textbook
examples of ramp structures and flow brecciated tops and bottoms can be
seen in domes or thick flows of what was highly viscous silicic lava.
The internal structure of a pyroclastic breccia cone and associated
vent plug are remarkably exposed at one of the principle Ash Peak
eruptive sites.
Pyroclastic breccia textures, bomb impact structures,
55
base surge deposits, and epiclastic beds formed by deposit reworking
between eruptions are spectacularly displayed within the cone.
Vulcanian processes dominated construction of the breccia cone until
the formation of the Ash Peak Glass by Strombolian eruption toward the
culmination of pyroclastic activity.
56
PETROCHEMISTRY OF THE VOLCANIC ROCKS
Various classification schemes have been proposed in the past
several decades to categorize volcanic rocks.
One of the earliest and
most commonly used schemes employs silica content (S102 in weight
percent) to subdivide volcanic or plutonic rocks into ultramafic,
mafic, intermediate, and silicic clans.
Tbday, a combination of Si02
and K20 content is used to discriminate in more detail various types of
volcanic rocks (Peccerillo and Taylor, 1976; Gill, 1981; the Basaltic
Volcanism Study Project, 1981).
The recommended method of classifying
volcanic rocks, as proposed to the IUGS SubCommission on Systematics of
Igneous Rocks (Le Maitre, 1984), uses the total alkali silica (TAS)
diagraM.
Compositional trends for volcanic rock samples of the Ash
Peak area are plotted on the TAS diagram given in Figure 5.
Richter
and others (1983) applied a modified version of the K20 versus Si02
scheme of Peccerillo and Taylor (1976) to the volcanic rocks of Ash
Peak.
This classification scheme is also illostrated in Figure 5 and
includes some of the samples analyzed for this study.
The possibility
of alkali metasomatism by weathering and (or) hydrothermal alteration
has been alluded to previously and these effects should be considered
in any classification system used at Ash Peak.
This study follows the
K20 versus Si02 system of Richter and others (1983) in combination with
lithologic criteria and the elemental abundances considered least
susceptible to mobilization (e.g., REE, Nb, Ta).
An overall summary of
the average elemental abundances and: petrochemical trends of the litho-
chemical groups at Ash Peak are given in Table 11.
Chemical analyses
of individual samples are tabulated in the discussions of the
appropriate litho-chemical section and in Appendix 1.
57
16
14
12
3>
CD
Alkali
Rhyolite
N
10
CN
N
Goo
N
8
Ak-
(11 creir
114.
6
0
N
Rhyolite
Andesite
4
Basaltic
C)
Andesite
2
0
37
45
41
49
53
57
Basaltic
Andesite
6
Andesite
Dacite
61
65
69
73
LowSilica HighSilica
Rhyolite
Rhyolite
ill,
o
5
O
ill
o
0
crn
4,
. 4.11
AA
()R
77
A-. 4I
4
i
3
CD
y
2
1
0
----
0
50
55
60
65
70
75
80
S102 wt.%
Figure 5.
Rock classification diagrams and the position of
representative samples from the Ash Peak
Rhyolite Peak Eruptive
Complex. The upper diagram is the total alkali silica (TAS)
diagram of Le Maitre (1984). The lower diagram from Peccerillo
and Taylor (1976) was modified by Richter and others (1983).
58
Table 11.
Average Chemical Analyses of Litho-Chemical Groups
Associated with the Ash Peak
Rhyolite Peak Eruptive Complex.
Litho-Chem
Andesitic Volcanic Rocks
Group
Lower
Variety
BasAnd
Lower
And
Upper
BasAnd
Biotite Tuff/Crystal-Rich
Biotite Rhyolite
Sub I
Sub II
Rhyolite
BioTuff Xtal-rich Average
Number of Samples
6
2
2
7
2
4
Si02
(X) (1)
54.5
60.5
55.1
73.5
76.3
76.3
76.0
76.2
TiO2
(X)
1.58
1.12
1.54
0.21
0.11
0.09
0.10
0.10
A1203 (%)
17.9
17.5
17.4
14.3
13.2
13.0
12.6
12.8
Fe203 (%) (2)
4.82
2.84
4.83
0.81
0.53
0.50
0.53
0.51
Fe0
(%)
4.76
3.08
4.49
1.09
0.64
0.72
0.83
0.77
FeO
(X) (3)
8.36
5.61
8.87
1.76
1.11
1.15
1.27
1.21
14n0
(X)
0.09
0.14
0.16
0.05
0.03
0.05
0.05
0.05
Mg0
(X)
1.01
1.89
2.35
0.30
0.31
0.12
0.09
0.10
Ca0
(%)
5.18
4.24
6.11
1.03
1.38
0.53
0.61
0.57
Na20
(%)
4.55
4.60
4.62
3.83
3.29
3.45
3.84
3.65
K20
(X)
3.38
3.47
2.67
4.33
4.15
5.23
5.31
5.27
8205
(%)
0.79
0.64
0.75
0.04
0.00
0.05
0.03
0.04
Sc (ppm)
15.7
11.1
16.9
2.9
1.3
1.7
1.8
1.8
Cr (ppm)
52
55
14
4
3
2
10
6
19.3
12.4
22.6
2.8
0.9
0.7
3.3
2.0
Co (ppm)
4
Rb (ppm)
77
83
59
162
198
216
184
200
Cs (ppm)
0.9
0.6
0.3
1.5
3.0
2.8
2.1
2.4
Sr (ppm)
582
458
610
94
98
16
16
16
Ba (ppm)
1098
1133
921
683
560
116
145
130
La (ppm)
60.2
53.3
44.9
43.5
35.0
36.3
41.8
39.0
Ce (ppm)
128.0
109.4
92.4
85.8
70.0
75.3
86.8
81.0
Nd (ppm)
60.7
51.9
52.3
31.0
23.1
31.3
33.7
32.5
Sm (ppm)
12.02
9.43
9.56
5.44
4.14
6.44
6.92
6.68
Eu (ppm)
3.33
2.43
2.81
0.91
0.59
0.34
0.35
0.35
Tb (ppm)
1.53
1.39
1.36
0.91
0.70
0.96
1.43
1.20
(ppm)
48
44
42
32
26
36
46
41
Yb (ppm)
4.31
3.42
3.67
3.49
3.33
4.16
4.55
4.35
Lu (ppm)
0.29
0.50
0.56
0.48
0.44
0.58
0.66
0.62
Y
2r (ppm)
402
368
309
226
122
130
160
145
0 (ppm)
10.0
8.4
7.4
6.9
4.5
5.5
6.5
6.0
Nb (ppm)
19
20
22
19
18
25
28
26
Ta (ppm)
1.7
1.8
1.9
2.5
2.4
2.7
4.2
3.5
Th (ppm)
7.1
6.6
4.0
12.0
17.0
15.7
16.8
16.3
U
2.1
1.5
1.0
2.2
2.2
3.3
4.7
4.0
40.1
43.6
42.1
32.8
27.1
23.9
24.7
24.3
3.5
4.4
3.9
5.4
8.0
5.1
3.6
4.1
0.14
0.15
(ppm)
Zr/Hf
Th/U
Eu /Eu'
0.89
0.80
0.92
0.50
0.42
0.16
la/K
17.8
15.4
16.8
10.1
8.4
6.9
7.9
7.4
Cs/K
0.3
0.2
0.1
0.3
0.7
0.5
0.4
0.5
(1)
(2)
(3)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1 for complete analyses
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
59
Table 11.
(continued).
Litho-Chem
Group
Ash Peak
Crystal-Poor Rhyolite
Porphyritic Rhyolite
Glass
Variety
Number of Samples
5
Ash Peak
Rhyo Pk
AP-RP
EC
EC
Average
10
5
Intrus
Dome
Flow
6
3
6
Si02
(X) (1)
77.4
76.7
76.5
76.6
TiO2
71.5
(%)
75.0
72.2
0.04
0.07
0.09
0.08
A1203 (%)
0.32
0.24
0.28
12.5
Fe203 (X) (2)
12.2
12.5
12.3
14.5
13.4
14.5
0.47
0.62
0.76
0.67
Fe0
1.13
0.69
(%)
0.97
0.65
0.93
1.10
0.99
Fe0
1.72
1.04
(%) (3)
1.48
1.05
1.46
1.75
1.56
Mn0
2.42
(%)
1.63
2.28
0.04
0.04
0.04
0.04
Mg0
(%)
0.03
0.03
0.04
<0.10
<0.10
<0.10
<0.10
Ca0
(X)
0.36
0.20
0.24
0.62
0.64
0.45
0.58
0.73
0.60
0.67
4.44
Na20
(%)
3.71
4.11
4.10
4.11
K20
(X)
4.30
4.05
4.37
4.62
4.39
4.55
P205
4.66
(%)
4.51
4.83
<0.05
<0.05
<0.05
<0.05
0.08
0.06
0.09
3.7
Sc (ppm)
1.3
1,0
1.4
1,1
4.1
Cr (ppm)
3.5
4
s
6
6
4
6
4
2.6
2.0
2.4
2.3
4,1
3.4
Co (ppm)
2.1
Rb (ppm)
223
278
259
272
Cs (ppm)
202
227
216
4.3
3.9
3.3
3.7
2.4
2,8
2.9
11
13
5
10
88
68
67
36
49
61
53
334
189
344
Sr (ppm)
8a (ppm)
La (ppm)
19.0
53.1
76.0
60.8
Ce (ppm)
77.4
72.2
76.7
49.9
131.3
172.3
144.9
149.7
160.8
165.4
Nd (ppm)
21.6
47.4
69.8
54.9
51.9
Sm (ppm)
52.0
S8.0
6.06
11.20
13.51
11.97
11.28
11.94
11.63
Eu (ppm)
0.15
0.17
0.17
0.17
0.75
lb (ppm)
0.51
0.64
1.38
2.05
2.19
2.10
1.60
2.03
1.86
92
89
91
63
95
65
(ppm)
I
51
Yb (ppm)
5.83
8.34
8.19
8.29
6.81
Lu (ppm)
10.02
7.03
0.82
1.12
1.12
1.12
0.95
1.38
1.00
Zr (ppm)
103
237
298
257
ilf (ppm)
381
290
362
5.6
10.3
11.7
10.7
11.2
11.0
11.7
Nb (ppm)
30
55
52
54
Ta (ppm)
35
42
39
4.2
5.8
5.0
5.5
2.9
3.2
19.9
3.5
23.1
24.2
23.5
17.9
22.1
20.7
7.2
7.6
9.3
8.1
3.2
4.4
6.1
18.2
22.9
25.5
23.8
34.0
26.4
31.0
2.8
3.1
2.6
2.9
5.6
5.1
3.4
0.04
0.04
0.04
0.21
0.13
0.17
Th (ppm)
U
(ppm)
2r/Hf
Th/U
Eu /Eu'
0.07
La/K
4.3
11.5
17.3
13.4
Cs/K
16.6
16.0
15.9
1.0
0.9
0.8
0.8
0.5
0.6
0.6
(1)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1 for complete analyses
(2)
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
(3)
60
ANDESITIC VOLCANIC ROCKS
Basaltic andesites and andesites have been recognized within the
Ash Peak area (Fig. 5).
The Si02 determinations of the andesitic
volcanic rocks were limited to five samples (Table 12) thus other
criteria are needed to distinguish andesite from basaltic andesite in
the samples analyzed by energy-dispersive XRF.
The data for rocks of
intermediate composition that have been more completely analyzed
suggest that Ash Peak basaltic andesite is higher in total Fe than that
of Ash Peak andesite (Table 12).
Using this criteria, the samples not
analyzed for silica were classified as basaltic andesite if their total
Fe contents (as Fe0) were greater than seven weight percent, and as
andesite if they were less.
By this scheme, basaltic andesite and
andesite are present in the lower andesites and only basaltic andesite
is present in the upper andesites.
It should be reiterated that
Richter and others (1983) have reported that the upper andesites beyond
the Ash Peak area are dominated by olivine-clinopyroxene andesites.
Basaltic andesites exhibit higher average contents of Ca0 (5.18
and 6.11 vs. 4.24 %) and TiO2 (1.58 and 1.54 vs. 1.12 %) than andesite,
but the compositional variability between individual samples is large.
Determination of the relative abundance of Ca0 is hampered by the
possibility of contamination by the inclusion of amygdaloidal calcite
in the analyses.
It was previously mentioned that abundances of TiO2
are close to the mean for that of "volcanic arc" andesites erupted in
continental setting (Chayes, 1965, as cited in Barker, 1983, p. 335).
The alkali elements, especially K, may have been added to the lower
andesites by metasomatism and thus a comparison with other rocks of
intermediate composition reported in the literature cannot be made.
a
61
Table 12.
Chemical Analyses of Andesitic Volcanic Rocks Associated
with the Ash Peak
Rhyolite Peak Eruptive Complex, Lower
Andesite.
Lower Andesites
Upper Andesites
basaltic andesite
Sample #
AP83040A AP830408
Si02
(%) (1)
54.5
TiO2
(%)
1.65
A1203 (%)
17.9
Fe203 (X) (2)
4.82
Fe0
(%)
4.76
FeO
(%)
(3)
9.11
Mn0
(%)
0.09
MgO
(X)
1.01
1.52
8.54
Ca0
(%)
6.36
6.08
Na20
(%)
4.20
3.92
K20
(%)
3.91
3.32
P205
(%)
AP83065
AP84193
AP84111
1.78
1.47
1,49
8.94
5.61
8.60
7.55
AP84137
7.45
basaltic andesite
AP84114
AP84181
AP84008
AP84183
59.2
61.7
55.3
54.9
1.36
0.88
1.15
1.93
18.2
16.8
18.0
16.8
3.13
2.55
4.32
5.35
3.54
2.62
3.81
5.17
6.32
4.91
7.74
10.00
0.19
0.08
0.13
0.20
1.08
2.70
3.01
1.70
3.61
4.88
7.21
5.01
3.88
3.95
3.87
5.29
5.00
5.01
5.15
4.05
4.06
5.18
2.90
2.93
3.11
4.13
3.62
3.32
2.53
2.81
0.93
0.36
0.51
1 00
14.2
11.3
11.0
15.4
18.3
5
8
101
23
4
9.9
8.0
16.7
27.8
17.5
0.79
Sc (ppm)
17.5
16.5
16.8
Cr (ppm)
100
98
55
23.3
27.2
34.2
CO (ppm)
andesite
15.7
13.7
4 <1.1E+01
12.3
8.7
Rb (ppm)
87
88
63
56
62
103
75
Cs (ppm)
91
1.9
56
61
1.4
0.4
0.4
0.5
0.7
0.6
0.7
0.3
0.4
Sr (ppm)
752
721
698
414
446
463
Ba (ppm)
435
481
1201
732
487
1090
1054
1083
1054
1104
1193
1073
834
1009
49.3
La (ppm)
74.4
63.8
63.6
55.5
51.1
52.8
Ce (ppm)
52.3
54.3
40.4
155
132
128
123
111
120
105
114
77
108
56.9
61.9
55.8
59.4
65.0
38.8
46.1
58.5
Nd (ppm)
66.2
63.9
Sm (ppm)
13.2
11.8
11.5
12.9
10.9
11.8
Eu (ppm)
11.2
7.7
3.07
7.1
2.87
12.0
3.08
3.67
3.53
3.74
3.14
1.72
2.11
3.50
1.72
Tb (ppm)
1.58
1.35
1.42
1.71
1.50
1.63
(ppm)
1.83
0.94
54
1.00
47
42
50
48
45
Yb (ppm)
57
30
3.73
33
3.85
50
3.91
5.22
4.42
4.70
4.07
Lu (ppm)
2.76
0.54
2.49
0.20
4.84
0.25
0.26
0.26
0.63
0.37
0.37
0.75
Zr (ppm)
413
383
363
441
397
413
10 (ppm)
389
346
9.8
258
359
9.5
9.2
11.4
9.9
10.4
Nb (ppm)
8.4
8.5
5.9
15
8.9
11
15
28
26
21
25
14
Is (ppm)
1.3
19
24
1.2
1.4
2.2
2.1
1.8
2.3
1.3
1.7
2.1
Y
Th (ppm)
U
(ppm)
Zr /Hf
0.25
12.2
10,8
6.0
4.4
4.4
4.7
4.5
8.8
2.8
4.3
2.7
3.6
1.3
1.2
1.8
2.6
1.4
1.5
1.0
1.1
40.5
42.4
40.3
39.$
38.7
40.1
39.7
46.3
40.9
4.4
43.6
4.0
4.6
3.7
2.4
1.8
Eu/Eu
3.2
5.7
0.78
4.5
0.83
3.3
0.89
0.92
1,03
1.01
0.85
La/K
0.74
19.1
0.95
19.2
0.92
21.9
18.9
16.4
12.8
Cs/K
14.4
16.4
0.48
16.0
17.6
0.42
0.14
0.14
0.16
0.17
0.15
0.22
0.11
0.13
Th/U
(1)
(2)
(3)
Major element oxides represent anhydrous
recalculation to 100%, see Appendix 1 for complete analyses
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
62
The Na20 content is approximately 25 percent higher than "normal"
basaltic andesite (4.2 vs. 3.3 %) and 50 percent higher than andesite
(5.2 vs. 3.4 %).
Moreover, K20 contents of the andesites are two to
three times the values reported by Nockolds (1954, as cited in
Carmichael and others, 1974, 3.62 vs. 1.11 %), Chayes (1969, as cited
in Barker, 1983, 3.62 vs. 1.61 %), and Gill (1981, 3.62 vs. 2.1 %).
One of the most noteworthy petrochemical features of the volcanic
rocks of Ash Peak is the nearly identical trace element contents of the
andesitic volcanic rocks.
The coincidence of trace element abundances
between the upper and lower andesite units is best displayed by the REE
as illustrated in Figure 6.
The abundances of U and Th, however,
exhibit markedly different concentrations between the upper and lower
suites.
The REE abundances of the andesitic volcanic rocks of Ash Peak
are two to thrO times higher than rocks of intermediate composition
reported in the literature (Table 13) and are uniformly high for all of
the samples examined (Table 12).
Analyses of basaltic andesite from
the upper and lower groups, in comparison to those of andesite, have
higher abundances of Sc (16.9 and 15.7 vs. 11.1 ppm), Co (22.6 and 19.3
vs. 12.4 ppm), and Sr (610 and 582 vs. 458 ppm) and lower Ba (921 and
1098 vs. 1133 ppm).
The relatively high abundance of Cs in the lower
andesites is likely the result of K metasomatism.
The origin of the K
metasomatism may have been hydrothermal alteration related to the
formation of the Ash Peak vein deposits or an unidentified regional
event.
63
1000.0
100.0
10.0 -
1.0
0 Lower Basaltic Andesite
Lower Andesite
Upper Basaltic Andesite
0.1
Cs Rb K Th U Sr Ba La Ce
Figure 6.
Nd
SmEuGd*Tb Y
Yb Lu Sc Zr Hf Nb Ta
Normalized elemental abundances of the andesitic volcanic
rocks associated with the Ash Peak
Rhyolite Peak eruptive
complex.
64
Table 13.
Representative Chemical Analyses of Other Intermediate and
Silicic Volcanic Rocks.
Sample B
(1)
(2)
(3)
(4)
(4)
(6)
(5)
(7)
Si02
(X)
49.07
55.5
60.3
70.0
73.8
TiO2
12.1
76.3
77.0
(X)
1.72
0.92
0.74
0.41
0.28
0.43
0.17
0.06
A1203 (X)
17.04
18.0
17.2
15.2
14.0
13.5
12.7
Fe203 (X)
12.6
5.11
3.7
2.9
3.0
1.9
1.3
0.9
0.5
6.49
4.3
3.4
1.3
0.3
0.5
Fe0
(X)
Mn0
(X)
0.15
0.18
0.12
MgO
(X)
6.64
4.00
Ca0
(X)
10.17
7.70
2.73
Na20
(X)
K20
(X)
P205
(X)
Sc (ppm)
0.05
0.03
0.04
0.01
0.02
3.10
1.07
0.34
0.51
0.18
0.04
6.10
2.51
1.27
1.82
1.06
3.40
0.40
3.40
4.05
3.93
3.68
3.11
4.43
0.67
2.10
2.50
3.64
4.38
5.18
5.11
4.42
0.21
0.30
0.24
0.10
<0.05
0.12
0.04
0.02
31.0
16.0
7.3
4.6
6.5
1.9
1.0
220
213
9
1
4
2
29.0
16.0
5.6
2.0
Cr (ppm)
Co (ppm)
Rb (ppm)
68
120
119
152
119
172
324
8.0
10.2
0.9
1.8
7.0
190
196
118
202
94
3
310
795
850
1433
204
7
Cs (ppm)
Sr (ppm)
583
Ba (ppm)
670
0.1
La (ppm)
15.0
19.0
21.2
22.3
Ce (ppm)
81.0
45.6
17.1
33.0
38.0
43.2
43.8
166.7
98.4
38.9
20.3
18.7
66.9
39.8
23.5
4.23
4.00
9.74
5.90
6.97
0.79
0.61
1.59
0.67
0.03
1.13
0.68
1.61
Nd (ppm)
Sm (ppm)
Eu (ppm)
Tb (ppm)
Y
(ppm)
Yb (ppm)
22
1.60
1.90
23
25
29
26
73
2.47
2.50
4.77
3.92
7.40
0.39
0.40
0.69
0.56
0.99
213
228
303
120
100
5.2
5.6
9.4
5.0
5.9
24.0
16.0
75.2
Lu (ppm)
Zr (ppm)
Hf (ppm)
Nb (ppm)
111
2,3
4.6
7.0
Ta (ppm)
Th (ppm)
U
(ppm)
5.3
5.8
1.3
1.9
11.5
14.2
1.4
1.7
7.1
15.7
22.3
34.7
2.1
3.2
12.7
(1) Average basalt of Basaltic Volcanism Study Project, 1981
(2) Average high potassium basaltic andesite of Gill, 1981
(3) Average high potassium andesite of Gill, 1981
(4) Average rhyolite from Medicine Lake
volcanoe, California, Grove and Donnelly-Nolan, 1986
(5) Average rhyolite from Twin Peaks, Utah, Nash and Crecraft, 1985
(6) Average high-silica rhyolite from Twin Peaks, Utah, Nash and Crecraft, 1985
(7) Average high-silica rhyolite from
Coso volcanic field, California, Bacon and others, 1981
65
RHYOLITIC VOLCANIC ROCKS
Interpretations of the chemical data gathered for this study
required consideration of the possibility of element mobility.
Although some of the samples display alteration textures (e.g., vaporphase alteration), most appear to be unaltered.
However, hydrothermal
systems were active in the area, probably after eruption of the
rhyolites and definitely after the emplacement of the lower andesites;
thus, some of the samples may be altered despite the absence of
macroscopic evidence to the contrary.
The results of this study
confirm for the most part the classical theories regarding element
mobility (Pearce, 1983 and Henderson, 1984).
Specifically, elements
with low ionic potential (Z/r, where Z is the ionic charge and r is the
ionic radius) such as the alkali and alkaline earth elements may be
mobile under hydrothermal, alteration, and diagenetic conditions,
especially if a fluid phase is present (Humphris and Thompson, 1978b).
Thus, hypotheses based on patterns and trends using these mobile
elements should be considered suspect unless substantiated by other
evidence.
In contrast, the transition and rare earth elements are
generally less mobile or immobile in these environments (Humphris and
Thompson, 1978b).
Biotite Rhyolite
Nine samples of biotite rhyolite, the first erupted silicic
volcanic unit, were among the 52 samples of rhyolite given more
complete analyses.
The results of these analyses are provided in Table
14 and Figure 7 and they indicate that two sub-types of biotite
rhyolite are present, low-silica rhyolite (sub-type I) and high-silica
66
Table 14. Chemical Analyses of Biotite Rhyolites Associated with the
Ash Peak
Rhyolite Peak Eruptive Complex.
Sub-Type I
Sample >X
AP83067
AP84031 083039L
AP84012
Sub-Type II
AP84022
AP84094
AP84095
5102
(%)
TiO2
(%)
0.21
73.3
0.23
A1203 (%)
14.4
14.2
Fe203 (%) (2)
0.81
0.82
Fe0
(X)
1.07
1.11
FeO
(%) (3)
1.77
1.82
Mr
(X)
0.05
0.05
Mo0
(%)
0.31
0.29
CaO
(%)
1.01
1.18
0.91
Na20
4.29
3.07
4.40
3.70
3.99
3.51
5.48
3.71
3.97
3.78
4.35
(1)
73.7
0.20
1.75
1.84
1.69
1.74
(X)
3.86
K20
(X)
4.51
4.48
P2O5
(X)
0.07
<0.05
Sc (ppm)
2.7
2.6
3.0
3.2
2.9
3.0
Cr (ppm)
3
4
4
4
2
2
2.6
2.5
3.1
4.4
2.6
Rb (ppm)
124
168
168
174
Cs (Wm)
1.5
1.6
0.7
1.9
Co (ppm)
1.69
AP84023
AP84200
76.4
76.2
0.11
0.12
13.2
13.3
0.50
0.57
0.64
0.64
1.07
1.14
0.03
0.03
0.18
0.43
0.94
1.83
3.96
2.62
4.02
4.27
<0.05
<0.05
2.7
1.3
1.3
11
3
3
2.8
1.5
0.5
1.4
186
183
133
212
184
1.6
1.8
1.2
3.4
2.6
Sr (ppm)
72
102
63
101
94
122
106
95
100
Ba (ppm)
733
765
665
667
617
709
627
532
588
La (ppm)
43.0
42.0
45.2
45.6
44.4
44.5
39.7
35.0
Ce (ppm)
35.0
92.0
89.0
85.6
88.0
83.4
85.9
76.7
70.0
70.0
33.3
31.7
31.7
27.4
32.3
37.6
22.9
22.9
23.3
5.38
5.48
5.43
5.65
5.36
5.52
5.28
4.29
3.99
0.89
0.89
0.91
1.03
0.89
0.92
0.83
0.57
0.60
1.25
1.18
0.81
0.82
0.79
0.80
0.74
0.73
0.66
Nd (PPRO
Sm (ppm)
Eu (ppm)
Tb (ppm)
(PM)
26
31
32
33
36
29
27
24
Yb (ppm)
3.66
3.46
3.86
3.41
3.53
3.36
3.17
3.56
Lu (ppm)
3.10
0.48
0.48
0.47
0.53
0.49
0.45
0.46
0.44
0.44
Zr (ppm)
213
238
209
237
248
226
210
126
118
RI (ppm)
7.2
7.2
6.9
7.1
6.8
6.9
6.2
4.5
4.5
Y
34
Nb (ppm)
19
20
17
21
23
17
14
20
15
Ta (ppm)
2.5
2.4
2.8
2.8
2.5
2.9
1.5
2.2
2.7
12.4
12.1
12.4
12.4
11.8
12.0
10.9
17.0
17.1
2.4
2.0
2.2
2.3
2.4
2.3
2.1
2.4
1.9
29.6
33.1
30.3
33.4
36.5
32.8
33.9
28.0
26.2
5.3
5.9
5.6
5.4
4.9
5.2
5.2
7.0
9.0
0.45
0.45
0.52
0.57
0.52
0.52
0.50
0.39
0.45
Th (ppm)
U
(ppm)
Zr /Hf
Th/U
Eu/Eu*
La/K
9.5
9.4
8.2
12.3
11.2
11.8
9.1
8.7
Cs/K
8.2
0.34
0.36
0.13
0.51
0.40
0.48
0.28
0.86
0.61
(1)
(2)
(3)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1
for complete analyses
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
67
1000.0i
100.0
10.0
Biotite Rhyolite
1.0
0
0.1
glITITIIIITTTIIIIIIIIIIIII1
Cs Rb K Th U Sr Ba La Ce
Figure 7.
0 Sub type I
Subtype II
Nd
SmEuGdTb Y
Yb Lu Sc Zr Hf Nb To
Normalized elemental abundances of biotite rhyolite
associated with the Ash Peak
Rhyolite Peak eruptive complex.
68
rhyolite (sub-type II).
The proposed sub-type I of the biotite
rhyolite unit is composed of the samples listed in Table 14.
Samples
representative of sub-type I were collected in the southern portion of
the Ash Peak area south of the Ash Peak eruptive center.
Samples
representative of sub-type II (AP84023 and AP84200) are from the middle
portion of the study area north of Ash Peak and on the northeast flank,
respectively.
Outcrops of sub-type II biotite rhyolite are the most
plentiful within the study area but the amount of volcanic cover in the
areas containing sub-type I biotite rhyolite is greater.
Analyses given in Table 11 show that biotite rhyolite, in
comparison to the other rhyolitic volcanic rocks of the area, has the
lowest abundances of incompatible elements (Rb, REE except Eu, Nb, Hf,
Ta, and Th) and the highest abundances of compatible elements (MgO,
CaO, Sr, Ba, and Eu).
The mean concentration for Ba is nearly six
times larger than the Ba concentrations of the other rhyolitic lithochemical groups.
The REE patterns depicted in Figure 7 are relatively
flat with only minor negative Eu anomalies (Eu/Eu*= 0.49).
Cotton and
Wilkinson (1980) point out that Y and Dy display similar chemical
behavior in most environments, thus, for this study Y is used as a
proxy for Dy.
Sub-type I is distinguished from sub-type II by the markedly
higher abundances of Ti02, total FeO, Na20, Sc, Ba, Eu, Zr, and Hf and
lower abundances of Th, Rb, and Si02 (Table 14).
Biotite Tuff, /Crystal -Rich Rhyolite
Only biotite tuff, of the two groups of pyroclastic rocks
identified within the Ash Peak area, was chemically analyzed.
The
69
pumice-lithic-crystal pyroclastic units were considered too
heterogeneous to provide meaningful chemical information.
A follow-up
study of the Ash Peak area that is currently underway will include
chemical analyses of the pumice and crystal fractions of these rocks,
but they are not considered in the present study.
In contrast to the inherent heterogeneity of the pumice-lithiccrystal pyroclastic rocks, samples of biotite tuff from throughout the
study area are lithologically and chemically homogeneous.
Petrochemical patterns of crystal-rich rhyolite are also similar to
those of biotite tuff (Table 15 and Fig. 8), and therefore the two rock
types were combined into a single litho-chemical group.
Samples of biotite tuff/crystal-rich rhyolite are enriched
relative to those of biotite rhyolite (see Table 11 and Fig. 9) in
S102, K20, Rb, middle and heavy rare earth elements (MREE and HREE,
respectively), Nb, Ta, and Th.
In addition, they are depleted in
A1203, T102, total FeO, MgO, CaO, Sr, Ba, Eu and slightly depleted in
Zr.
They have a more pronounced negative Eu anomaly (Eu/Eu*= 0.15 vs.
0.49).
The Zr/Hf and Th/U ratios of biotite tuff/crystal-rich rhyolite
are lower than those of biotite rhyolite (24 vs. 33 and 4.1 vs. 5.9,
respectively).
Trends toward decreasing ratios continue with the
successively younger rhyolitic volcanic rocks except for the latest
phase, the porphyritic rhyolites.
Biotite rhyolite and biotite
tuff/crystal-rich rhyolite have similar abundances of the LREE, Sc, Hf,
Ta, Cr, and Co although the last two elements have such low abundances
overall that they cannot be used to recognize changes between any of
the rhyolitic litho-chemical groups.
Biotite tuff/crystal-rich
rhyolites exhibit medial abundances of trace elements and patterns with
70
Table 15.
Chemical Analyses of Biotite Tuff/Crystal-Rich Rhyolite
Associated with the Ash Peak
Rhyolite Peak Eruptive Complex.
Biotite Tuff
Sample /4
AP83038
AP84076
Crystal-Rich Rhyolites
AP83032
AP83052
AP83045
AP84073
AP84124
AP84131
Si02
(X) (1)
74.5
77.0
78.0
75.8
75.4
77.0
75.6
76.1
TiO2
(X)
0.10
0.08
0.09
0.08
0.11
0.08
0.15
0.07
A1203 (X)
13.3
13.3
11.9
13.2
13.3
11.7
12.8
12.5
Fe203 (X) (2)
0.58
0.35
0.55
0.53
0.56
0.53
0.35
0.67
Fe0
(X)
0.81
0.51
0.78
0.78
0.87
0.78
0.61
1.06
Fe0
(X) (3)
1.31
0.82
1.26
1.23
1.34
1.23
0.89
1.63
Mn0
(X)
0.02
0.09
0.07
0.03
0.03
0.07
0.05
0.03
Mg0
(X)
0.12
<0.10
0.11
0.23
0.11
0.23
<0.10
<0.10
Ca0
(X)
0.41
0.52
0.55
0.63
0.52
1.14
0.47
0.29
Na20
(X)
1.97
4.51
3.38
3.96
3.78
3.47
4.38
3.73
K20
(X)
8.21
3.59
4.48
4.66
5.30
4.91
5.55
5.48
P205
(X)
<0.05
0.03
0.05
0.10
<0.05
0.06
0.04
<0.05
Sc (ppm)
2.2
1.4
1.7
1.6
2.1
1.6
2.1
1.5
Cr (ppm)
0
3
3
2
3
16
9
10
Co (ppm)
0.4
0.3
1.7
0.5
3.0
0.9
5.8
3.6
Rb (ppm)
273
200
185
204
195
172
174
194
Cs (ppm)
3.2
4.7
1.3
1.9
1.5
2.1
0.5
4.3
Sr (ppm)
15
24
16
8
18
35
5
5
Ba (ppm)
114
94
145
112
145
121
136
177
La (ppm)
36
38
34
37
48
34
47
38
Ce (ppm)
79
72
77
73
91
75
97
84
Nd (ppm)
28.8
36.0
28.6
31.9
30.5
30.5
35.3
38.6
Sm (ppm)
5.42
6.60
6.26
7.46
6.75
5.93
7.20
7.81
Eu (ppm)
0.38
0.29
0.34
0.36
0.44
0.30
0.45
0.23
Tb (ppm)
0.78
1.12
0.93
1.02
1.29
1.59
1.32
1.52
(ppm)
34
32
34
42
52
39
43
50
Yb (ppm)
3.80
3.45
4.24
5.14
4.07
4.37
4.25
5.51
0.52
0.50
0.63
0.66
0.65
0.60
0.67
0.73
Zr (ppm)
138
123
127
131
176
126
157
180
Hf (ppm)
6.2
4.8
5.3
5.5
5.9
5.5
6.2
8.3
Y
Lu (ppm)
Nb (ppm)
23
22
26
29
30
22
26
32
Ta (ppm)
2.8
2.2
2.8
3.0
3.5
2.5
5.4
5.5
15.0
15.9
15.3
16.6
16.4
16.2
17.5
17.3
2.5
5.0
2.8
2.9
3.1
5.8
4.0
5.9
22.2
25.8
24.0
23.6
29.9
22.8
25.2
21.7
6.0
3.2
5.5
5.7
5.3
2.8
4.4
2.9
0.22
0.13
0.17
0.15
0.19
0.13
0.18
0.08
la /K
4.4
10.6
7.6
7.9
9.1
6.9
8.5
6.9
Cs/K
0.39
1.32
0.30
0.40
0.29
0.42
0.09
0.78
Th (ppm)
U
(ppm)
Zr/Hf
Th/U
Eu/Eu
(1)
(2)
(3)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1 for complete analyses
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
71
Q)
1000.0
Biotite Tuff/CrystalRich Rhyolite
0
100.0
10.0
1.0 1
Biotite Tuff
CrystalRich Rhyolite
0,1
IT IIITTITT11-711111
Cs Rb K Th U Sr Bo La Ce
Figure 8.
Nd
SmEuGd*Tb Y
Yb Lu Sc Zr Hf Nb To
Normalized elemental abundances of biotite tuff and crystalrich rhyolite associated with the Ash Peak
Rhyolite Peak
eruptive complex.
72
1000.0
100.0
0.1
o
0
o
o Biotite Rhyolite
Biotite Tuff/CrystalRich Rhyolite
1-
1
1 l-111111f 1 111-11
CsRb K Th U Sr BaLa Ce
Figure 9.
Nd
SmEuGd*Tb Y
Yb Lu Sc Zr Hf Nb Ta
Normalized elemental abundances of biotite rhyolite and
biotite tuff/crystal-rich rhyolite associated with the Ash Peak
Rhyolite Peak eruptive complex.
73
respect to the petrochemical patterns of biotite rhyolite and crystalpoor rhyolite (Fig. 10).
Ash Peak Glass
The Ash Peak Glass is typified by the large glassy unit that
forms the northern and western flank of Ash Peak.
However, this group
also inclides some samples of spherulitic rhyolite,
Ash Peak Glass is
an enigmatic litho-chemical group as its petrochemical pattern deviates
from those defined by the other rhyolitic volcanic rocks (Tables 11 and
16 and Fig. 11).
Stratigraphic relationships establish that the Ash
Peak Glass was erupted after the emplacement of the pumice-lithic-
crystal pyrolastic units and before the eruption of the crystal-poor
rhyolites.
Ash Peak Glass has the highest Si02 content (77.4 %) of any of
the rhyolitic volcanic rocks.
It is the most depleted in TiO2 (0.04
Ba (36 ppm), Zr (103), and has the lowest Zr/Hf ratio (18) of any
of the rhyolites.
For most elements, the petrochemical pattern of the
Ash Peak Glass is intermediate between those of biotite tuff/crystalriCh rhyolite and crystal-poor rhyolite, as illustrated in Figure 12.
Incompatible elements continue to increase to concentration levels
higher than those of biotite tuff/crystal-rich rhyolite and this trend
reaches a maximum in samples of crystal-poor rhyolite (Table 11).
The
most striking petrochemical feature of the Ash Peak Glass is the
extreme depletion of the LREE relatiVe to all other rhyolitic volcanic
rock types of the Ash Peak eruptive center (Fig. 12).
74
1000.0
100.0
1
10.0
1.0
0
0. 1
A
Biotite Tuff/CrystalRich Rhyolite
0
Biotite Rhyolite
1 I1111111-1-111-11111-1/ifil1
Cs Rb K Th U Sr Ba La Ce
Figure 10.
Nd
SmEuGdTb Y
ff
Yb Lu Sc Zr Hf Nb To
Normalized elemental abundances of biotite rhyolite,
biotite tuff/crystal-rich rhyolite, and crystal-poor rhyolite,
APEC associated with the Ash Peak
Rhyolite Peak eruptive
complex.
75
Table 16.
Chemical Analyses of Ash Peak Glass Associated with the Ash
Peak
Rhyolite Peak Eruptive Complex.
Ash Peak Glass
Sample #
AP83063
AP84139 AP84204
AP84159
AP84056
Si02
(%) (1)
77.2
77.3
77.4
77.5
TiO2
(X)
0.05
0.04
0,04
0.04
A1203 (X)
12.7
12.6
12.5
12.4
Fe203 (%) (2)
Fe0
(X)
0.46
0.47
0.48
0.46
0.63
0.65
0.63
Fe0
(%) (3)
1.03
1.05
1.05
(X)
0.04
0.04
0.03
0.03
Mg0
(X)
<0.10
<0.10
<0.10
<0.10
Ca0
(X)
0.67
0.61
0.85
Na20
(X)
3.67
4.14
3.47
3.86
K20
(X)
4.52
4.20
4.64
3.48
P205
(X)
<0.05
<0.05
<0.05
Sc (PM)
1.2
1.2
Cr (ppm)
3
-3
6
Co (ppm)
0.2
1).2
4.8
2.0
Rb (ppm)
220
250
222
239
182
Cs (ppm)
4.7
5.1
3.9
5.0
3.1
Sr (ppm)
15
19
12
5
5
Ba (ppm)
44
20
24
25
65
La (ppm)
19
19
17
18
22
Ce (ppm)
51
50
50
45
53
Nd (ppm)
20.5
21.4
21.3
19.4
25.3
Sm (ppm)
6.22
6.10
5.38
5.92
6.69
Eu (ppm)
0.13
0,14
0.13
0.15
0.19
Tb (ppm)
1.23
1.28
1.84
1.16
1.41
(ppm)
55
51
47
49
51
Yb (ppm)
5.91
5.73
5.55
5.74
6.23
Lu (ppm)
0.91
0.79
0.76
0.82
0.84
Zr (ppm)
116
107
108
104
78
Hf (ppm)
5.5
5.8
5.5
5.8
5.7
I
1.1
0.69
1.05
1.09
0.36
3.42
5.02
<0.05
1.6
1.2
7
1
3.5
Nb (ppm)
33
29
29
30
Ta (ppm)
3.8
3.3
5.2
3.6
5,1
Th (ppm)
19.1
20.7
19.7
19.6
20.5
4.5
7.6
8.5
5.0
10.3
21.0
18.6
19.6
17.9
13,8
4.2
2.7
2.3
3.9
2.0
0.06
0.07
0.08
U
(ppm)
Zr/Hf
Th/U
Eu/Eu*
0.06
0.06
30
La/K
4.2
4.5
3.7
5.1
4,4
Cs/K
1.03
1.21
0.84
1.44
0.61
(1)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1
(2)
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
(3)
for complete analyses
76
Biotite Tuff /Crystal Rich Rhyolite
o Biotite Rhyolite
Cs Rb K Th U Sr Bo La Ce
Figure 11.
Nd
SmEt.LdTb Y
Yb Lu Sc Zr Hf Nb To
Normalized elemental abundances of biotite rhyolite,
biotite tuff/crystal-rich rhyolite, and Ash Peak Glass associated
with the Ash Peak
Rhyolite Peak eruptive complex.
77
0 CrystalPoor Rhyolite
° -° Ash Peak Glass
Biotite Tuff/CrystalRich Rhyolite
0 Biotite Rhyolite
U1 T1 7 ITA111111-1111117711111
Cs Rb K Th U Sr Bo La Ce
Figure 12.
Nd
SmEuGd*Tb Y
Yb Lu Sc Zr Hf Nb To
Normalized elemental abundances of biotite rhyolite,
biotite tuff/crystal-rich rhyolite, Ash Peak Glass, and crystalpoor rhyolite associated with the Ash Peak
Rhyolite Peak
eruptive complex.
78
Crystal-Poor Rhyolite
Most bedrock exposed within the Ash Peak area consists of flows
and domes of crystal-poor rhyolite.
This litho-chemical group is
characterized by the highest abundances of incompatible elements and
the lowest abundances of compatible elements of any of the rhyolitic
volcanic rocks analyzed, as shown in Tables 11 and 17.
Although there
are differences in the total elemental abundances between the crystalpoor rocks associated with each of the eruptive centers, the overall
patterns, as illustrated in Figure 13, are the same.
Total abundances
of the REE (especially the LREE, La 76 vs. 53 ppm, Ce 172 vs. 131 ppm,
and Nd 70 vs. 47 ppm), TiO2 (0.09 vs. 0.07), and Sc (1.4 vs. 1.0 ppm)
are higher in samples from the Rhyolite Peak eruptive center compared
to those of the Ash Peak eruptive center and they are depleted in Ca0
(0.45 vs. 0.64 %).
By averaging the ten more completely analyzed
samples of crystal-poor rhyolites associated with the Ash Peak eruptive
center, one can distinguish a number of patterns.
Crystal-poor
rhyolites are high in Si02, Na20, and K20 compared to other Ash Peak
rhyolitic rocks, they have the highest abundances of Rb, Cs, REE, Zr,
Nb, Hf, Ta, Th, and U, the lowest abundances of A1203, CaO, Sc,
Sr, and
Ba, and they have the deepest Eu anomalies (Eu/Eu*= 0.04) of any of the
rhyolitic litho-chemical groups.
Continuing the tendency exhibited by
biotite rhyolite and biotite tuff/crystal-rich rhyolite, crystal-poor
rhyolite displays enrichment of incompatible elements and depletion of
compatible elements.
79
Table 17.
Chemical Analyses of Crystal-Poor Rhyolites Associated with
the Ash Peak - Rhyolite Peak Eruptive Complex.
Sample ft
AP83036
AP83050
AP83056
AP83058
AP83062
AP84084
AP84066
AP84085
Si02
(X)
76.8
75.8
76.0
76.5
77.5
76.3
76.8
77.6
TiO2
(X)
0.07
0.06
0.09
0.07
0.06
0.06
0.07
0.06
A1203 (X)
12.5
11.3
12.4
12.4
12.1
12.7
12.3
11.9
0.67
0.60
0.68
0.61
0.62
0.64
0.53
0.60
Fe203 (X)
(1)
(2)
Fe0
(X)
0.92
0.86
1.09
0.93
0.93
1.00
0.84
0.90
Fe0
(X) (3)
1.49
1.37
1.66
1.45
1.45
1.54
1.29
1.41
Mn0
(X)
0.04
0.04
0.06
0.04
0.03
0.04
0.03
0.03
Mg°
(X)
<0.10
0.23
<0.10
0.11
<0.10
<0.10
0.11
0.12
Ca0
(X)
0.54
2.71
0.34
0.43
0.27
0.13
0.38
0.31
Na20
(X)
4.13
3.80
4.70
4.14
4.16
4.29
3.78
K20
(%)
4.31
4.58
4.61
4.69
4.27
4.83
5.11
(X)
<0.05
<0.05
<0.05
0.05
<0.05
<0.05
0.09
<0.05
Sc (ppm)
0.9
0.9
1.3
0.9
1.0
0.9
0.9
Cr (ppm)
4
4
4
4
4
6
7
0.2
0.4
0.1
0.9
0.3
6.9
Rb (ppm)
293
254
278
287
271
Cs (ppm)
7.6
3.2
1.9
5.2
3.1
5
4
7
10
36
43
83
P205
Co (ppm)
Sr (ppm)
ea (ppm)
23
AP84098
AP84101
1.71
1.61
3.89
4.70
3.99
4.59
4.15
3.61
0.8
1.0
1.3
9
6
6
9.1
3.2
4.2
1.8
291
286
265
380
340
2.9
3.5
4.2
8.6
9.4
20
15
24
22
5
83
26
59
78
46
28
56
La (ppm)
53
60
65
42
43
53
62
47
44
Ce (ppm)
49
141
137
140
124
134
128
133
113
106
109
Nd (ppm)
49.5
58.6
51.4
39.1
39.7
49.7
46.3
45.4
Sm (ppm)
47.7
32.1
12.09
13.61
13.03
10.25
10.75
10.62
9.80
9.43
Eu (ppm)
12.68
10.92
0.12
0.14
0.29
0.13
0.12
0.13
0.31
0.11
0.23
Tb (ppm)
0.29
2.06
2.07
2.06
2.01
1.81
2.27
2.02
2.11
2.74
2.00
122
98
79
105
84
83
91
75
165
68
7.96
7.97
9.04
8.73
8.11
8.96
7.61
8.35
12.47
8.57
1.08
1.20
1.08
1.10
1.16
1.20
0.99
1.12
1.66
1.16
Zr (ppm)
240
208
364
213
217
252
186
214
Mf (ppm)
369
312
10.1
9.7
13.1
10.2
10.5
11.1
7.7
10.0
16.1
11.4
Y
(ppm)
YID (ppm)
Lu (ppm)
Nb (ppm)
87
58
52
56
53
43
51
41
88
Ta (ppm)
56
4.7
4.4
5.3
4.8
4.8
8.1
8.2
5.8
8.2
5.9
22.2
21.5
22.6
22.1
23.0
25.6
25.0
23.0
31.4
24.1
5.6
4.4
4.6
4.0
4.3
13.3
13.6
10.6
7.6
5.7
23.8
21.4
27.9
20.9
20.7
22.8
24.3
21.5
22.9
27.4
4.0
4.9
4.9
5.6
5.3
1.9
1.8
2.2
4.1
4.2
0.03
0.03
0.07
0.04
0.03
0.03
0.09
0.03
0.05
0.08
Th (ppm)
U
(ppm)
Zr/Hf
Th/U
Eu/Eu*
La/K
12.3
13.1
14.1
9.0
10.1
11.0
12.1
10.2
10.5
13.4
Cs/K
1.75
0.69
0.42
1.12
0.72
0.59
0.68
0,91
2.07
2.60
(1)
(2)
(3)
Major element oxides represent anhydrous recalculation
to 100X, see Appendix 1 for complete analyses
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
80
Table 17.
(continued).
sample #
AP83001
AP83002
AP83021
AP83028
AP84171
Si02
(X) (1)
76.0
75.8
78.5
75.6
76.2
TiO2
(X)
0.09
0.08
0.10
0.09
0.09
A1203 (X)
13.0
12.8
11.5
12.8
13.1
Fe203 (X) (2)
0.78
0.79
0.73
0.74
0.86
Fe0
(X)
1.05
1.17
1.03
1.17
1.03
Fe0
(X) (3)
1.73
1.85
1.66
1.79
1.79
Mn0
(X)
0.05
0.02
0.04
0.05
0.06
Mg0
(X)
<0.10
<0.10
<0,10
<0.10
0.18
Ca0
(%)
0.54
0.64
0.36
0.28
1.01
Na20
(%)
4.84
4.28
2.89
4.37
3.31
K20
(%)
3.55
4.38
4.77
4.87
4.09
P205
(%)
<0.05
0.10
0.10
<0.05
<0.05
Sc (ppm)
1.2
1.4
1.6
1.4
1.3
Cr (ppm)
3
6
5
10
3
Co (ppm)
2.6
3.1
0.3
2.0
3.3
Rb (ppm)
270
279
226
260
352
Cs (ppm)
5.1
2.4
2.8
3.2
8.5
Sr (ppm)
3
8
2
5
43
Ba (ppm)
22
36
86
101
26
La (ppm)
72
86
66
80
73
Ce (ppm)
163
183
144
199
167
Nd (ppm)
59.9
68.3
56.2
94.9
66.7
Sm (ppm)
11.18
12.97
13.07
16.80
11.68
Eu (ppm)
0.14
0.16
0.23
0.16
0.14
Tb (ppm)
2.29
2.40
1.70
2.36
2.19
(ppm)
94
112
73
77
102
Yb (ppm)
8.52
9.21
6.54
8.48
8.22
Lu (ppm)
1.13
1.17
0.91
1.27
1.11
Y
Zr (ppm)
288
297
311
295
303
Hf (ppm)
11.7
12.4
11.3
11.3
11.8
Nb (ppm)
52
58
43
55
53
Ta (ppm)
5.8
5.9
3.4
5.0
5.5
Th (ppm)
24.1
26.7
19.5
26.3
23.6
U
13.3
13.2
3.9
7.0
10.4
24.7
23.9
27.5
26.1
25.8
1.8
2.0
5.0
3.8
2.3
Eu /Eu*
0.04
0.04
0.06
0.03
0.04
La/K
20.3
19.6
13.8
16.4
17.8
Cs/K
1.43
0.54
0.58
0.65
2.07
(ppm)
Zr /Hf
Th/U
(1)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1 for complete analyses
(2)
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
(3)
Total iron as FeO
81
1000.0
100.C) 7
0
O
10.0
C
C
(1.)
cU
0
-0
C
0,1
IT TII-IlIT-ITT
Cs Rb K Th U Sr Bo La Ce
Figure 13.
Nd
SmEuGd*Tb Y
Yb Lu Sc Zr Hf Nb To
Normalized elemental abundances of crystal-poor rhyolite
from the Ash Peak and Rhyolite Peak eruptive centers.
82
Porphyritic Rhyolite
Porphyritic rhyolites were the final phase of rhyolitic volcanism
within the Ash Peak area.
With the exception of the HREE of the dome
phase, they exhibit consistent trace element abundances as illustrated
in Figure 14.
Although care was taken in the preparation of
porphyritic samples for INAA, the possible effect of contamination by
xenoliths of basaltic andesite must be considered when evaluating the
data .for this grOup of rhyolites.
Contrary to the pattern of
incompatible. element enrichment and compatible element depletion
exhibited by the earlier phases of rhyolitic volcanic rocks, the magma
chamber producing the various phases of porphyritic rhyolite appears to
have regressed.
The Si02 content, as depicted in Tables 11 and 18, is
the lowest of any of the rhyolitic litho-chemical groups and A1203,
T102, total FeO, MgO, Sc, Sr, Ba, Zr, and Hf concomitantly increase in
abundance.
Moreover, abundances of Rb, Cs, Nb, Ta, Th, and U all
decrease and the Zr/Hf and Th/U ratios increase to levels associated
with biotite rhyolite.
The REE of the porphyritic rhyolites exhibit a
modest decrease in abundance compared to the values of the crystal -poor
rhyolites of the Rhyolite Peak eruptive center (Fig. 15).
In the
broadest terms, concentrations of compatible elements of the magma have
increased whereas thOse of the incompatible elements have deCreased.
The petrogenetic implications of these changes are discussed in the
following chapter.
SUMMARY
Compared to :'average" andesites and basaltic andesites, the
andesitic volcanic rocks of Ash Peak are enriched in Ti02, Na20, K20,
83
Cs Rb K Th U Sr Bo La Ce
Nd
SmEuGd*Tb Y
Yb Lu Sc Zr Hf Nb To
Figure 14. Normalized elemental abundances of porphyritic rhyolite
from the Ash Peak
Rhyolite Peak eruptive complex.
84
Table 18.
Chemical Analyses of Porphyritic Rhyolite Associated with
the Ash Peak
Rhyolite Peak Eruptive Complex.
Intrusive Phase
Sample #
AP84161
AP84162
AP84163
Si02
(X) (1)
71.5
TiO2
(%)
0.32
A1203 (X)
14.5
Fe203 (%) (2)
1.13
AP84164
Oomal Phase
AP84168
AP84187
AP84165
AP84166
AP84167
75.0
0.24
13.4
0.69
Fe0
(X)
Fe0
(%) (3)
Mn0
(%)
Mg0
(%)
CaO
(X)
Na20
(%)
4.23
4.56
4.57
4.15
3.67
4.65
4.17
4.02
(X)
3.95
4.60
4.67
5.11
4.57
4.59
4.41
4.39
4.76
4.39
K20
P205
1.72
2.46
2.44
2.68
1.04
2.37
3.03
2.30
1.62
0.03
0.03
0.36
0.20
0.73
(%)
0.96
0.60
0.08
0.06
Sc (ppm)
4.0
3.1
3.9
3.7
Cr (ppm)
4
4
4
5
2.3
2.4
2.3
2.4
Co (ppm)
1.57
4.2
5.1
3.4
3.5
3
7
6
6
0.9
3.7
2.9
2.3
7.1
3.6
Rb (ppm)
205
224
207
209
195
170
Cs (ppm)
235
233
212
2.1
2.5
2.5
2.5
2.4
2.3
Sr (ppm)
2.5
4.2
1.8
70
62
87
75
81
154
72
Ba (ppm)
65
68
341
232
282
225
443
481
180
228
160
77.1
La (ppm)
85.4
87.1
77.8
71.8
66.9
75.6
69.8
Ce (ppm)
69.8
159
164
148
147
136
145
Nd (ppm)
150
167
166
57.6
58.5
46.1
50.5
53.6
45.0
45.1
Sm (ppm)
48.2
62.6
11.29
12.36
10.97
10.98
10.56
11.52
Eu (ppm)
10.98
9.95
14.90
0.69
0.58
0.62
0.50
1.01
1.08
0.41
0.54
0.57
1.67
1.80
1.59
1.54
1.49
1.51
1.60
1.46
3.03
59
55
58
81
73
54
81
58
145
7.10
7.55
6.78
6.55
6.30
6.56
6.89
7.11
16.05
1.06
0.93
0.96
0 86
0.95
0.97
0.91
2.26
Tb (ppm)
Y
(ppm)
Yb (ppm)
Lu (ppm)
0.95
Zr (ppm)
365
343
381
336
426
434
292
Hf (ppm)
279
299
10.9
11.4
10.4
11.3
11.7
11.6
Nb (ppm)
10.1
11.0
12.0
35
30
34
41
37
31
44
Ta (ppm)
45
37
2.8
3.2
3.3
3.3
2.4
2.4
3.1
3.4
3.2
18.5
19.3
18.3
20.1
17.0
14.1
21.8
23.3
21.2
3.1
2.3
4.1
3.3
3.4
3.0
4.8
4.3
4.0
33.5
30.1
36.6
29.7
36.4
37.4
28.9
25.4
24.9
6.0
8.4
4.5
6.1
5.0
4.7
4.5
5.4
5.3
0.19
0.15
0.18
0.14
0.30
0.30
0.12
0.17
0.11
Th (ppm)
U
(ppm)
2r/lif
Th/U
Eu/Eu.
La/K
18.6
18.7
15.2
15.7
14.6
17.1
Cs/K
15.9
14.7
17.6
0.46
0.54
0.49
0.55
0.52
0.52
0.57
0.87
0.41
(1)
(2)
(3)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1
for complete analyses
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
85
Table 18.
(continued).
Flow Phase
Sample 0
AP84025
AP84169
AP84170
AP84178
AP84185
AP84199
Si02
(X) (1)
72.3
73.1
71.1
TiO2
(%)
0.25
0.28
0.31
Al203 (X)
14.2
14.3
15.1
Fe203 (%) (2)
1.01
0.84
1.07
1.28
1.60
Fe0
(X)
1.55
Fe0
(X) (3)
2.40
MnO
(X)
0.05
2.33
2.32
2.16
1.98
2.51
0.05
0.03
Mg0
(%)
0.26
0.20
0.24
Ca0
(%)
0.69
0.66
0.66
Na20
(%)
4.62
4.70
4.21
4.32
4.41
4.40
K20
(X)
4.98
4.69
4.03
5.08
4.83
5.35
P205
(%)
0.05
0.08
0.15
4.3
Sc (ppm)
3.4
3.7
3.6
3.3
3.7
Cr (ppm)
4
4
4
5
4
3
Co (ppm)
3.4
3.1
2.8
1.9
4.7
4.7
Rb (ppm)
235
211
206
237
210
197
Cs (ppm)
2.7
5.0
2.5
2.7
2.3
2.2
Sr (ppm)
71
52
76
64
66
72
8a (ppm)
281
308
251
165
441
616
67.0
La (ppm)
80.0
74.8
72.7
78.7
87.0
Ce (ppm)
180
152
160
157
184
160
Nd (ppm)
60.7
56.9
39.7
39.8
91.7
59.4
Sm (ppm)
12.00
11.37
11.37
10.95
13.78
10.28
Eu (ppm)
0.53
0.66
0.63
0.41
0.56
1.03
Tb (ppm)
2.06
1.54
1.64
1.57
2.65
1.69
(ppm)
71.00
54.00
70.00
60.00
69.00
68.00
Yb (ppm)
7.79
6.76
6.91
6.98
7.52
6.24
Lu (ppm)
1.04
0.93
1.00
0.99
1.16
0.86
I
2r (ppm)
356
350
342
331
348
445
Hf (ppm)
12.1
10.9
11.1
10.8
11.5
13.5
Nb (ppm)
45
41
35
38
39
37
Ta (ppm)
4.2
3.1
2.8
2.7
4.0
4.3
Th (ppm)
22.6
19.8
19.9
20.6
22.5
19.0
U
10.6
4.2
3.9
4.0
5.6
8.2
29.3
32.1
30.8
30.6
30.2
33.0
2.1
4.7
5.1
5.2
4.0
2.3
Eu/Eu
0.13
0.19
0.17
0.12
0.12
0.30
la/K
16.1
15.9
18.0
15.5
18.0
12.5
Cs/K
0.53
1.07
0.62
0.53
0.48
0.40
(ppm)
Zr/Hf
Th/U
(1)
(2)
(3)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1 for complete analyses
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as Fe0
86
1000.0
CrystalPoor Rhyolite, RPEC
0
100.0
10.0
1.0
porphyritic Rhyolite
Biotite Rhyolite
0.1
Cs Rb K Th U Sr Ba La Ce
Figure 15.
Nd
SmEu3d *Tb Y
Yb Lu Sc Zr Hf Nb Ta
Normalized elemental abundances of biotite rhyolite,
crystal-poor rhyolite, RPEC, and porphyritic rhyolite associated
with the Ash Peak
Rhyolite Peak eruptive complex.
87
Sr, Ba, REE, Zr, Hf, Nb, Th, and U.
Enrichment of the alkali and
alkaline earth elements may be due in part to the metasomatic addition
of material by hydrothermal processes.
Trace element abundances of the
upper and lower andesites are virtually identical despite the
separation of the andesites by a large volume of rhyolite and possibly
considerable time.
Major and trace element contents of the rhyolitic volcanic rocks
erupted at Ash Peak are within the limits of elemental abundances
reported from petrologically similar rhyolitic volcanic systems at the
Twin Peaks, Coso, and Medicine Lake fields.
However, abundances of the
REE are higher than the median values for other systems cited in the
literature (Tables II and 13).
At least two cycles of incompatible element enrichment coupled
with compatible element depletion are recognized at Ash Peak.
In the
suite biotite rhyolite to biotite tuff/crystal-rich rhyolite to Ash
Peak Glass, compatible elements such as Mg, Ca, Ba, Sr, Eu, and Sc are
progressively depleted while incompatible elements such as Cs, Rb, Th,
U, REE, Nb, and Ta are relatively enriched.
Abundances of major and
trace elements of crystal-poor rhyolites relative to the previously
erupted rhyolites suggest that crystal fractionation was augmented by
the addition of a small amount of less evolved magma to the chamber.
Major and trace element contents of porphyritic rhyolite, the final
rhyolitic phase erupted, clearly demonstrate that the petrochemistry of
the chamber was substantially modified by less evolved magma.
88
PETROGENESIS OF THE VOLCANIC ROCKS
The principal objective of this study has been to determine the
origin and subsequent history of the Ash Peak
Rhyolite Peak eruptive
complex by utilizing a combination of field, petrographic, and chemical
techniques.
Volcanic rocks, both andesitic and rhyolitic, erupted
within the Ash Peak area were originally postulated to have resulted
from the crystal fractionation of a parent magma.
Petrochemical data
suggest that the andesitic and rhyolitic volcanic rocks were produced
by two different, but related mechanisms.
Parental basaltic magmas
constitute tbe common petrogenetic theme for the andesitic and
rhyolitic volcanic rocks of the Ash Peak area.
The upper and lOwer
andesitic volcanic rocks are proposed to be the differentiatipn product
of a parental basaltic magma primarily by crystal fractionation,
possibly with an assimilation component.
Magmas that forMed the
rhyolitic volcanic rocks are probably the result. of partial melting of
crustal rocks by the parental basaltic magma.
The generation of
andesitic or rhyolitic lavas by the basaltic magma was controlled by
changes in the local tectonic regime.
The source of the parental
basaltic Magma cannot be resolved by this study but can be constrained
to either subduction or rift related processes,
PETROGENESIS OF ANDESITIC VOLCANIC ROCKS
Magmatism of intermediate composition at Ash Peak is postulated
to be primarily the result of crystal fractionation of parental basalt.
The parental basaltic magma may have been produced by either processes
of subduction at convergent plate boundaries or extension at an active
rift.
89
Voluminous literature exists discussing the generation of magmas
at convergent plate boundaries (Ringwood, 1975; Wyllie, 1979 and 1981;
and Gill, 1981).
Gill (1981) has proposed that orogenic andesites are
produced primarily by crystal fractionation of basaltic magma generated
by partial melting of mantle peridotite under the influence of
introduced volatiles.
Studies by Christiansen and Lipman (1972),
Lipman (1980, 1981), and Damon and others (1981) suggest that changes
in the angle and (or) rate of subduction of the Pacific plate allowed
the locus of arc magmatism to migrate away from the convergent margin
toward the continental interior during the middle Tertiary.
Parental
basaltic magmas may have been formed under Ash Peak by subduction
related processes and migrated upward into the continental crust.
Studies of active rift systems suggest that regional extension
may produce basaltic magmas that may or may not undergo modification
prior to eruption (Lipman and others, 1989).
The early stages of
formation of the Rio Grande rift may have produced parental basaltic
magmas that migrated into the crust below the Ash Peak area (Perry and
others, 1987 and Lipman and others, 1989).
The elevated TiO2 and Nb
abundances of the andesitic volcanic rocks provide support that the
parental basaltic magmas formed in a rift rather than a subduction
environment.
In either case, subduction or rifting, parental basaltic
magmas were available under Ash Peak to undergo crystal fractionation
to andesitic compositions.
However, elevated trace element abundances, particularly of REE
(Tables 12 and 13), suggest that simple crystal fractionation of
a
basaltic parent, similar in composition to those erupted at Medicine
Lake, was not the comprehensive petrogenetic mechanism for the
90
formation of the andesitic volcanic rocks of Ash Peak.
Preliminary
calculations require such a basalt to undergo 75-80 percent
crystallization to achieve the trace element concentrations observed in
the basaltic andesites and andesites erupted at Ash Peak.
The
remaining liquid would possess major element abundances representative
of a highly evolved rhyolite rather than those of a basaltic andesite
or andesite as suggested by the trace and major element models of Grove
and Donnelly-Nolan (1986).
Petrogenetic models using simple crystal
fractionation of a basaltic liquid must, therefore, be modified to
comply With the observed petrochemical data.
Cycles of crystal fractionation and magma mixing of a primitive
basaltic liquid may result in magmas of intermediate composition with
elevated abundances of trace elements (O'Hara, 1977, and O'Hara and
Mathews, 1981).
Primary basaltic liquids formed by either processes of
subduction or rifting will gravitationally ascend to the crustal level
at which they are bouyantly compensated (Gill, 1973 and Cox, 1980)
Crystal fractionation of the stationary magma will enrich the
incompatible and deplete the compatible major and trace element
components of the magma.
Periodic pulses of primitive magma from below
will mix with previous batches of magma, although Huppert and Sparks
(0980) and McBirney (1980) suggest that liquids with large density
differences may not mix.
Mixing evolved and primitive magmas will
produce hybrid magmas possessing major oxide composition's intermediate
between the two end members, as determined by their mineralogy, crystal
content, and relative quantities (McBirney, 1979).
However,
.incompatible trace eleMents such as the REE, Y, Th, Zr, Ta, and fslb will
become progressively enriched in the magma through repeated cycles of
91
fractionation and mixing despite the relative uniformity of the major
oxide compositions (O'Hara, 1977).
Elevated incompatible and depleted compatible trace element
contents relative to typical rocks of intermediate composition, suggest
that the basaltic andesites and andesites erupted at Ash Peak formed in
a continuously fractionating and periodically replenished magma
chamber.
Compared to typical basaltic andesites (Tables 11, 12, and
13), the basaltic andesites of Ash Peak are enriched in incompatible
trace elements such as the REE (La 60 vs. 15 ppm, Ce 128 vs. 33 ppm, Y
48 vs. 22 ppm, and Yb 4.3 vs. 1.6 ppm), Zr (402 vs. 111 ppm), Hf (10
vs. 2.3 ppm), Nb (19 vs. 7 ppm), Th (7.1 vs. 5.3 ppm), and U (2.1 vs.
1.3 ppm) and depleted in compatible trace elements such as Sc (16 vs 31
ppm), Cr {52 vs 220 ppm), and Co (19 vs. 29 ppm).
Additionally, lower
and upper andesites, although separated in time, possess nearly
identical trace element concentrations (Fig. 6).
Thus similar magmatic
conditions must be established for the generation of the two
stratigraphically and temporally distinct types of andesite.
O'Hara (1977) has calculated that steady-state conditions may be
established by a continuously fractionating and periodically
replenished magma chamber.
Such a magma chamber will maintain uniform
abundances of major oxides but, abundances of incompatible trace
elements will increase to levels normally associated with extreme
amounts of crystal fractionation.
Additionally, periodic tapping of
the steady-state magma chamber will produce lavas of nearly identical
major and trace element compositions.
Progressive changes in the
abundances of the major oxides at Ash Peak require that the steadystate conditions evolved toward more siliceous compositions with time.
92
Thus, modification of the steady-state conditions caused the change
from early basaltic andesitic to later andesitic volcanism in the lower
andesites.
Additionally, magma chambers possessing similar steady-
state conditions may be established at different times, such as those
for the lower and upper andesites.
Parental basalt was not supplied to
the steady -state magma chamber during the period of rhyolitic volcanism
at Ash Peak, and lavas of intermediate composition were not erupted.
The early basaltic andesites of the upper andesite group were erupted,
following the cessation of rhyolitic volcanism, from a steady-state
magma chamber similar to that of the lower andesites.
Thus, it is proposed that the andesitie volcanic rocks were
formed in continuously fractionating and periodically replenished
steady-state magma chambers.
This hypothesis explains the elevated
abundances of incompatible trace elements, the depleted abundances of
compatible trace elements, and the nearly identical trace element
contents overall of the lower and upper andesites despite their
separation in time.
PETROGENESIS OF RHYOLITK VOLCANIC ROCKS
Crystal fractionation of a magma similar in composition to the
andesitic volcanic rocks was the original petrogenetic model considered
for the rhyolitic volcanic rocks erupted at Ash Peak.
Petrochemical
restrictions, predominartly the elevated abundances of the REE in the
andesitic relative to the rhyolitic volcanic rocks, argue against this
hypothesis.
The petrogenesis of the rhyolitic volcanic rocks
investigated for this study may be accounted for by models of crystal
fractionation of phenocryst assemblaaes and (,or) magma mixing of rock
93
types present within the area.
Trends of major and trace element
abundances indicate that three petrogenetic suites or lineages were
produced by the magma chamber that erupted the rhyolitic volcanic rocks
of Ash Peak.
Crystal fractionation of the phenocryst assemblage
present in the previous rock type may have been the sole process for
the creation of the volcanic suite biotite rhyolite to biotite
tuff/crystal-rich rhyolite to Ash Peak Glass.
Crystal fractionation
coupled with mixing small amounts of more primitive magma may have been
responsible for the formation of the suite biotite tuff/crystal-rich
rhyolite to crystal-poor rhyolite.
Finally, two plausible magma mixing
models were produced to simulate the formation of the porphyritic
rhyolites.
Mixing magmas similar in composition to biotite rhyolite or
alternatively, upper andesite and crystal-poor rhyolite from the
Rhyolite Peak eruptive center may have been responsible for the
formation of the porphyritic rhyolites.
Modal mineralogies were used
to represent the liquidus phases and proportions for the crystal
fractionation models.
Published partition coefficients for similar
rocks from other areas such as Twin Peaks, Utah (Nash and Crecraft,
1985) were used to calculate the crystal fractionation models.
Biotite Rhyolite Petrogenesis
The combination of basal stratigraphic position and relative
petrochemical abundances suggest that biotite rhyolite was the first
rhyolitic volcanic rock type erupted after the emplacement of the lower
andesites.
Compared to later rhyolites, the biotite rhyolites exhibit
low concentrations of incompatible elements, high concentrations of
compatible elements (especially Ba and Sr), and a relatively flat REE
94
pattern with minimal Eu anomaly.
TheSe petrochemical patterns suggest
that biotite rhyolite magmas were not substantially modified by crystal
fractionation of the parent magma.
Rhyolites erupted after biotite
rhyolite have been modelled using crystal fractionation of a biotite
rhyolite parent magma.
The formation of the biotite rhyolite magma is
more problematic.
Abundances of REE, Zr, Hf, and Nb in biotite rhyolite relative to
lower andesite, as shown in Figures 16 and 17, are inconsistent, for
the most part, with crystal fractionation models of lower andesite to
produce biotite rhyolite.
Although trends of particular elements (i.e.
Rb and Ba, Fig. 17) suggest that magmas similar in composition to the
andesitic rocks may have been parental to the rhyolites, the majority
of the petrocheMital evidence argues against this hypothesis.
The
liquidus phases of andesitic magmas (e.g. olivine, pyroxene,
plagioclase feldspar) have very low partition coefficients for REE and
other incompatible elements, (i.e. DREc <<1, see Appendix 2).
Thus,
crystallization of these minerals can change the major oxide contents
of the magMa to that of rhyolite but would increase the REE content.
The REE content of the lower andesites, and the other andesitic
volcanic rocks, is greater than that of biotite rhyolite.
Crystal
fractionation of magma similar in composition to the lower andesites
will produCe rhyolites with REE contents higher than those of the lower
andesites, and very much higher than those of biotite rhyblite.
However, rhyolites have been modelled as crystal fractionation
derivatives of andesitic magmas in other magmatic systems (e.g.
Medicine Lake volcano, California, Grove and Donnelly-Nolan,
1986).
Lower andeSite frOm Ash Peak was modelled using the liquidus phases,
95
1000.0
100.0
10.0
o Lower Andesite
* Biotite Rhyolite
0.1
11117 1-11771117 7111111171 1
CsRb K Th U Sr Ba La Ce
Figure 16.
Nd
SmEtCd *Tb Y
1
!
Yb Lu Sc Zr I-If Nb To
Normalized elemental abundances of biotite rhyolite and
lower andesite associated with the Ash Peak
Rhyolite Peak
eruptive complex.
96
400
350 -
300250
0_
ID
ain
Ilk
Arii .v
.111
200
II
a
150-
CrystalPoor
Ash Peak Glass
100-
50-
Porphyritic
Biotite Tuff/XRich
Da
Biotite Rhyolite
Andesites
I
5
-"1
-
-
10
-
--
20
15
25
30
35
100
Porphyritic
CrystalPoor
80
Ash Peak Glass
Biotite Tuff /XtI Rich
Biotite Rhyolite
Andesites
60
"F
40
vy
**.
Y
20
0
1
0
5
10
15
20
25
30
35
Th (ppm)
Figure 17.
Variation diagrams of Rb, Nb, La, and Ba versus Th for
andesitic and rhyolitic volcanic rocks associated with the Ash
Peak
Rhyolite Peak eruptive complex.
97
100
Porphyritic
CrystalPoor
Ash Peak Glass
80
y
V
60
A
40
AA::
Biotite Tuff/XtlRich
20
Biotite Rhyolite
Andesites
ti
0
0
5
10
15
20
25
30
35
1500
Porphyritic
CrystalPoor
Ash Peak Glass
Biotite Tuff/XtlRich
1000
Biotite Rhyolite
Andesites
E
1,11
0
OD
500
tA
goi
0
0
5
10
15
20
Th (ppm)
Figure 17.
(continued).
Iry
25
30
35
98
initial phenocryst proportions, and bulk partition coefficients
reported by Grove and Donnelly-Nolan (1986) using the crystal
fractionation program Magma86 supplied by Hughes Magmatics (see
Appendix 2).
Crystal fractionation of the lower andesite magma was
modelled in two steps following the procedure described for the
Medicine Lake volcano.
The first step involved crystallization of 18
percent of the magma to olivine, clinopyroxene, and plagioclase
feldspar in the weight proportions 27, 26, and 47 percent,
respectively.
To attain the major oxide contents typical of a
rhyolite, the derivative liquid must undergo an additional 20 percent
crystallization of plagioclase feldspar, amphibole, and orthopyroxene
in the weight proportions of 60, 10, and 7 percent, respectively.
In
addition, an unreasonable-amount of apatite (23 percent) was needed to
bring the REE content of the derivative rhyolite to that observed in
biotite rhyolites of Ash Peak.
Calculations using other accessory
phases that concentrate incompatible elements, such as sphene, require
similarly high proportions of accessory minerals.
Mixing a magma similar in composition to that of the lower
andesitic volcanic rocks with another that is depleted in REE, Zr, Hf,
and Nb may dilute these trace elements to levels observed in biotite
rhyolite.
The mixing of magmas more primitive than that of the lower
andesite requires that the mixed magma undergo crystal fractionation to
produce rhyolitic magma.
Because of the low distribution coefficients,
crystal fractionation will increase the REE content of the magma to
concentrations at least as high as those in the lower andesites.
If
the introduced magma is more siliceous than the andesitic magma,
crystal fractionation may not be required to form a rhyolitic magma,
99
but the REE content of the new magma would need to be lower than that
of the andesitic magma.
Petrochemical data gathered for this study
suggest that the igneous rocks erupted at Ash Peak possess high REE
concentrations relative to those of similar volcanic rocks reported in
the literature such as at the Coso volcanic field (Bacon and others,
1981), Medicine Lake volcano (Grove and Donnelly-Nolan, 1986), and Twin
Peaks volcanic field (Nash and Crecraft, 1985):
Geologic evidence to substantiate the source of the magma that
erupted as the biotite rhyolites was not discovered during the course
of this study.
Based on studies of other volcanic systems similar to
Ash Peak, it is probable that parental biotite hyolite magma was
produced by partial melting of crustal rocks by primitive magmas
created by rifting or subduction processes discussed earlier.
The
change from magmas produced by crystal fractionation of primitive
basaltic magma to magma produced by crustal melting may, in part,
explain the large compositional gap observed at Ash Peak (Fig. 5).
Fundamental to any petrogenetic model of the Ash Peak area is an
explanation for the change in volcanism from andesitic to rhyolitic and
back to andesitic.
Hypotheses developed by Walker and Richter (198)
and independently by Gans (1987) and Gans and others (1989;, propose
that changes in the local tectonic regime may control the type o
ultimately erupted or emplaced.
magma
In a pre-extensional tectonic regime,
primitive basalts may ascend slowly and undergo crystal fractionation
to andesitic compositions which accumulate in periodically tapped and
replenished magma chambers.
A change to an extensional tectonic regime
may allow the primitive basaltic magmas to rise more rapidly through
the crust without undergoing significant modification.
Such magmas
100
would then be available to act as sources of heat and volatiles for
partial melting at higher crustal levels.
Partial melting of andesitic
rocks, such as those that dominate the Mesozoic section under Ash Peak,
could yield rhyolitic magmas that might collect in magma chambers,
undergo crystal fractionation, periodic tapping, and occasional
replenishment with magma from below.
The post-extensional tectonic
regime would terminate the supply of upper level primitive magma and
re-establish the conditions that might allow for the formation of
another periodically tapped and replenished magma chamber similar to
that represented by the lower andesites.
Thus, the lower andesitic,
rhyolitic, and upper andesitic volcanic rocks of the Ash Peak area, are
unified into a common petrogenetic theme.
Changes in magmatism from
andesitic to rhyolitic and back to andesitic are explained by local
tectonic fluctuations, not major changes in overall magma genesis.
Circumstantial support for this hypothesis is provided by the
metamorphic core complex that forms the Pinaleno Mountains located
approximately 50 km west of the Ash Peak area (Fig. 1).
Coney (1980)
has suggested that metamorphic core complexes form in extensional
tectonic regimes, and that the Pinaleno Mountains are approximately
early Miocene in age.
An alternative explanation for the changes in magmatism from
andesitic to rhyolitic and back to andesitic is that proposed by Bacon
(1985) for the Coso volcanic field.
He suggests that the formation of
a magma chamber of rhyolitic composition above the ascending andesites
would block their upward movement.
Thus producing a "shadow zone" from
which more mafic magmas will not be erupted.
Crystallization of the
rhyolitic magma chamber would allow the blocked andesites to continue
101
to the surface.
Satisfactory petrogenetic models to explain the petrochemical
differences between biotite rhyolite sub-types I and II have not been
developed during the current investigation.
Abundances of Si02, Ti02,
total Ee0, incompatible trace elements Cc, Rb, and Th, And compatible
trace elements Ba and Sc in sub-type II suggest that it formed as the
result of crystal fractionation of sub-type I.
However, abundances of
REE, Zr, Hf, and Sr are not consistent with a fractionation model that
has sub-type I as the parent.
Small differences in the amount of
crystal fractionation of the parent magma and (or) amounts of partial
melting of the parent rock may account for the observed petrochemical
differences.
Biotite Tuff/Crystal-Rich Rhyolite Petrogenesis
Stratigraphic position indicates that biotite tuff was erupted
after the emplacement of biotite rhyolite and before the main phase of
pyrocla$tic cone construction.
The same criteria suggests that the
eruption of crystal-rich rhyolite was contemporaneous with the early
phase of Ryroclastic cone construction.
Similarities in the abundances
of major and trace elements of biotite tuff and crystal-rich rhyolite
(Table 11) imply that the magma chamber from which they were erupted
was petrochemically nearly identical.
Thus, the emplacement of
crystal-rich rhyolite may represent periods during which the magma
chamber was depleted in volatiles relative to those marked by the
eruption of pyroclastic material.
Petrochemically, biotite
tuff/crystal-rich rhyolites are more evolved than biotite rhyolites as
suggested by the elevated abundances of incompatible elements such as
102
Cs, Rb, Th, U, Sm, Y, Yb, Nb, and Ta in contrast to depleted abundances
of compatible trace elements such as Sr, Ba, Eu, and Sc, as depicted in
Figure 9.
Petrogenesis of the biotite tuff/crystal-rich rhyolite magma was
modelled as a crystal fractionation derivative of parental biotite
rhyolite.
Mineral phases and proportions of the liquidus assemblage
were determined from the modal mineralogy of biotite rhyolite sample
AP83067 (Table 3), and these minerals were crystallized from an average
of the two biotite rhyolite sub-types listed in Table 11.
Modelled
element partition coefficients for the constituent minerals were taken
predominantly from Nash and Crecraft (1985), with supplemental values
from Mahood and Hildreth (1983), and are tabulated in Appendix 2.
Crystal fractionation models of the eighteen elements Rb, Th, U,
Sr, Ba, REE, Sc, Hf, Nb, and Ta in one percent increments were
calculated using the computer program Magma86 (Hughes, 1987).
By means
of this program, the starting magma composition undergoes one percent
equilibrium crystallization and removal of the liquidus minerals to
form a derivative liquid.
This derivative liquid then undergoes
another one percent increment of equilibrium crystallization to form a
new derivative liquid.
Accordingly, the process proceeds in one
percent increments until the desired amount of crystallization has been
achieved.
Initial models to form biotite tuff/crystal-rich rhyolite from
the biotite rhyolite magma employed 32 one-percent crystal
fractionation increments of the liquidus mineralogy plagioclase
feldspar 49 percent, sanidine 32 percent, biotite 17 percent, and
clinopyroxene 2.2 percent.
This crystal fractionation model was
103
reasonably successful with the exception of Th which was too low in the
calculated rhyolite, and Hf and the LREE which were too high, relative
to the analytical data.
To refine further the model, trace amounts of
zircon and allanite, 0.025 percent and 0.027 percent respectively, were
added to the liquidus assemblage.
Zircon was identified in many of the
thin sections examined for this study, and allanite has been
tentatively identified as minute inclusions in larger phenocrysts of
feldspar.
Thus, the inclusion of these accessory minerals as liquidus
phases is appropriate for the crystal fractionation model.
For
comparison, the elemental abundances of the modelled liquid, the
biotite tuff/crystal-rich rhyolite average, and the biotite rhyolite
average calculated from Table 11 are illustrated in Figure 18.
Modelled abundances agree within the analytical uncertainty of the
measured petrochemical data for the biotite tuff/crystal-rich rhyolite.
Ash Peak Glass Petrogenesis
Stratigraphic succession, petrochemical evolution, and field
relationships are consistent with the proposal that the Ash Peak Glass
was the next rhyolitic phase erupted at the Ash Peak eruptive center.
Except for depletion of the LREE, Ash Peak Glass continues the trend of
incompatible element enrichment and compatible element depletion
established by biotite rhyolite and biotite tuff/crystal-rich rhyolite
(Tables 11 and 16, and Fig. 11).
The average petrochemical abundances
of biotite tuff/crystal-rich rhyolite (Table 11) were modelled as the
parental magma to the Ash Peak Glass.
Liquidus phases and proportions
were assigned on the basis of the modal mineralogy of sample AP84073
104
Crystal Fractionation Model
Biotite Tuff/CrystalRich Rhyolite
1000.0
T7
32% Fractionation of
Biotite Rhyolite
0
00.0
0
0
O
o
, 3 0
0
0
C
0
U
A
A Biotite Tuff/CrystalRich
v
v Model
C
-0 Biotite Rhyolite
0
Rb Th U Sr Bo La Ce
Nd
SmEuGd*Tb Y
Yb Lu Sc Hf Nb To
Figure 18.
Crystal fractionation model of biotite tuff/crystal-rich
rhyolite derived from biotite rhyolite.
105
(Table 3) from a crystal-rich rhyolite dome.
Partition coefficients
used for the crystal fractionation model were taken primarily from
Mahood and Hildreth (1983) and secondarily from Nash and Crecraft
(1985).
Ash Peak Glass was modelled from biotite tuff/crystal-rich
rhyolite by calculating 20 one-percent increments of the liquidus
mineralogy consisting of sanidine 77 percent, plagioclase feldspar 15
percent, and biotite 8 percent.
Depletion of the LREE was accomplished
by modelling 0.12 percent allanite as part of the liquidus assemblage.
The crystal fractionation model together with the petrochemical
abundances of Ash Peak Glass and biotite tuff/crystal-rich rhyolite are
presented in Figure 19.
Modelled abundances of the eighteen elements
investigated are within the analytical uncertainty of the measured
petrochemical data for Ash Peak Glass.
Discussion
Abundances of major and trace elements for the suite biotite
rhyolite to biotite tuff/crystal-rich rhyolite to Ash Peak Glass follow
progressive trends of incompatible element enrichment and compatible
element depletion.
Consistent with these trends, biotite tuff/crystal-
rich rhyolite and Ash Peak Glass can be modelled as successive magma
batches formed by crystal fractionation of a parent magma similar in
composition to the average of the biotite rhyolites.
Crystal-Poor Rhyolite Petrogenesis
Stratigraphic relationships at both eruptive centers indicate
that the crystal-poor rhyolites were emplaced following the cessation
of pyroclastic activity and the eruption of the Ash Peak Glass.
The
106
Crystal Fractionation Mod&
Ash Peak Glass
000.0
20% Fractionation of
Biotite Tuff/CrystalRich
oao
0
o
0
0
.0
1
0
3
Rb Th
U Sr Bc La Ce
Nd
SmEuCd*Tb Y
Yb Lu Sc Hf Nb Ta
Figure 19.
Crystal fractionation model of Ash Peak Glass derived from
biotite tuff/crystal-rich rhyolite.
107
absence of petrochemical data for the pyroclastic rocks precludes
petrogenetic modelling of the crystal-poor rhyolites associated with
the Rhyolite Peak eruptive center.
Graphs of abundances of major
oxides for the rhyolitic volcanic rocks (Fig. 4) demonstrate that the
formation of the crystal-poor rhyolites was not a continuation of the
simple crystal fractionation process responsible for the previous
rhyolitic rocks.
Elemental abundances of rhyolitic volcanic rock samples plotted
in Figure 20, exhibit progressive enrichment of highly incompatible
elements such as Th, Rb, and Nb.
Thus, changes in the petrochemistry
of the magma chamber may be illustrated by graphing incompatible and
compatible elemental abundances against one of these elements.
was selected because of its analytical
Thorium
accuracy and precision using
INAA techniques and the wide range in Th contents of the Ash Peak
rhyolitic volcanic rocks.
The behavior of the highly incompatible
elements provides permissive support to the hypothesis that the
rhyolitic volcanic rocks are, at least in part, petrogenetically
related by crystal fractionation processes outlined by Hanson (1978,
1980).
The petrochemical behavior of the REE, due to differences in
ionic radii and charge, is depicted in Figure 21 for the rhyolitic
volcanic rocks of Ash Peak.
For the suite biotite rhyolite to biotite
tuff/crystal-rich rhyolite to Ash Peak Glass, La and Eu display
compatible element behavior, Sm maintains a nearly constant abundance,
and Yb displays incompatible element behavior.
Abundances of REE of
crystal-poor rhyolites do not exhibit a continuation of this trend, as
they behave as incompatible elements.
Further, the graphs of the REE,
especially La, Sm, and La/Yb, are suggestive of the plausible
108
100
Porphyritic
80-
CrystalPoor
Ash Peak Glass
Biotite Tuff/CrystalRich
60-
Biotite Rhyolite
yy v
,
*
40 -
II%
A.
20-
0
5
10
15
20
25
30
35
20
25
30
35
10
Porphyritic
CrystalPoor
Ash Peak Glass
8
Biotite Tuff/CrystalRich
Biotite Rhyolite
A-I
2
0
0
10
15
Th (ppm)
Figure 20.
Variation diagrams of Ta, Nb, Nb/Ta, and Rb versus Th for
rhyolitic volcanic rocks associated with the Ash Peak
Rhyolite
Peak eruptive complex.
109
20
Porphyritic
CrystalPoor
Ash Peak Glass
*y
A
10
as
.*
0
Biotite Tuff/CrystalRich
Biotite Rhyolite
`
1.-..-1.1-.1'
5
10
20
15
25
30
35
30
35
500
Porphyritic
400-
CrystalPoor
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
i
v
vi
v
..m
At
4)"
411,.
m
100-
0
5
10
'I'
15
20
Th (pp m)
Figure 20.
(continued).
25
110
100
80-
*,
V
60 -
Q_
40-
Porphyritic
AA.
CrystalPoor
Ash Peak Gloss
20-
0
Biotite Tuff/CrystalRich
Biotite Rhyolite
5
10
15
20
25
30
35
30
35
20
Porphyritic
16-
CrystalPoor
V
Ash Peak Glass
Biotite Tuff/CrystalRich
a_
12
E
v
Biotite Rhyolite
8
Cl)
A
(DIM
A
AA
ti
in
4-
5
10
15
20
25
Th (ppm)
Figure 21.
Variation diagrams of REE La, Sm, Eu, and Yb versus Th for
rhyolitic volcanic rocks associated with the Ash Peak
Rhyolite
Peak eruptive complex.
111
Porphyritic
CrystalPoor
0.9 -
..$
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
E
0.5
LJ
0.3 -
1,
0.'
15
VV
20
V
25
Porphyritic
CrystalPoor
2-
VT
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
30
35
'
y
: ot
6-
toe%
41
.3 -
.
0
0
5
10
.
.
15
20
Th (ppm)
Figure 21.
(continued).
25
30
35
112
hypothesis that the biotite tuff/crystal-rich rhyolite magmas followed
two petrogenetic paths.
One path consisted of continued crystal
fractionation toward Ash Peak Glass and the other of fractionation and
mixing toward crystal-poor rhyolite.
Abundances of Zr and Hf,
presented in Figure 22, also support the trace element trends exhibited
by the REE.
In the suite biotite rhyolite to biotite tuff/crystal-rich
rhyolite to Ash Peak Glass, Zr behaves compatibly and is progressively
depleted.
The abundances of Hf remain fairly constant to slightly
depleted.
In the crystal-poor rhyolites, Zr and Hf behave incompatibly
and are enriched as the Th content increases.
As with the REE,
patterns of Zr and Hf abundances tentatively suggest that biotite
tuff/crystal-rich rhyolite followed two petrogenetic paths which
resulted in the Ash Peak Glass and the crystal-poor rhyolites.
Trends
of compatible element abundances of Ba, Sc, and the ratio Yb/Sc are
depicted in Figure 23.
Contrary to the incompatible elements, the
depletion to constant abundance of Ba and Sc suggests that crystal-poor
rhyolites are a continuation of the crystal fractionation trend that
produced the previous rhyolitic volcanic rocks.
The ratio Yb/Sc
further illustrates that crystal-poor rhyolites are probably a
continuation of the same crystal fractionation trend.
However,
abundances of major oxides of the rhyolitic volcanic rocks discussed in
the rock description chapter (Fig. 4) and the behavior of some of the
incompatible elements (i.e. REE, Zr, and Hf) do not support the
hypothesis that crystal-poor rhyolites formed by the continued
fractionation of Ash Peak Glass.
Abundances of Si02, Ti02, A1203,
total FeO, MgO, Na20, and K20 clearly demonstrate that the incompatible
elements were depleted (or diluted) and the compatible elements were
113
500
Porphyritic
Crystal Poor
400-
300-
,
200AAA%
00 -
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
15
o
25
20
30
35
20
Porphyritic
CrystalPoor
6-
2-
4v,
11' v
8
.41
A
A
4-
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
-
15
20
25
30
35
Th (ppm)
Figure 22.
Variation diagrams of Zr, Hf, and Zr/Hf versus Th for
rhyolitic volcanic rocks associated with the Ash Peak
Rhyolite
Peak eruptive complex.
114
40
30 -
s
*
0
A
A
A
A
20 -
Porphyritic
CrystalPoor
10-
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
-
-
10
15
20
1
25
Th (ppm)
Figure 22.
(continued).
30
35
115
800
Porphyritic
CrystalPoor
600-
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
CL
400-
200AA
v
10
w
-
v
15
20
25
30
35
Porphyritic
CrystalPoor
Ash Peak Gloss
Biotite Tuff/CrystalRich
V
Biotite Rhyolite
AA
IN
vy
Ir ;
0
10
-
15
20
25
30
35
Th (pp m)
Figure 23.
Variation diagrams of Ba, Sc, and Yb/Sc versus Th for
rhyolitic volcanic rocks associated with the Ash Peak
Rhyolite
Peak eruptive complex.
116
Porphyritic
CrystalPoor
Ash Peak Glass
Biotite Tuff/CrystalRich
Biotite Rhyolite
vv
t
NM
AA.
a,
0
1
0
-
5
I
10
-
"
-
15
1
1
I
20
25
30
Th (ppm)
Figure 23.
(continued).
35
117
enriched in the magma chamber that gave rise to the crystal-poor
rhyolites relative to that of Ash Peak Glass.
Modelling of crystal-poor rhyolite petrogenesis employed the
identical starting composition, biotite tuff/crystal-rich rhyolite
(Table 11), and liquidus proportions, sample AP84073 (Table 3), as that
of the Ash Peak Glass.
Relative abundances of major and trace elements
discussed above suggest that the magma chamber had received an addition
of more primitive magma following the eruption of the Ash Peak Glass.
The composition of the primitive magma is not known, but it had the
effect of reestablishing the petrochemical
abundances of the magma
chamber to approximately that of biotite tuff/crystal-rich rhyolite.
The model consists of 43 one-percent incremental fractionation events
of the minerals sanidine (77%), plagioclase feldspar (15 %), biotite (8
%), and allanite (0.007 %).
Partition coefficients were taken
primarily from Mahood and Hildreth (1983) and secondarily from Nash and
Crecraft (1985) and these are listed in Appendix 2.
The crystal
fractionation model together with the petrochemical abundances of
crystal-poor rhyolite and biotite tuff/crystal-rich rhyolite are
presented in Figure 24.
Modelled abundances of crystal-poor rhyolites
are within the analytical uncertainty of the measured petrochemical
data.
The addition of 15 weight percent "typical" basalt listed in
Table 13 was necessary to reconcile the major oxide abundances to those
of the analyzed samples.
Porphyritic Rhyolite Petrogenesis
Porphyritic rhyolites were the final phase of rhyolitic activity
associated with the Ash Peak
Rhyolite Peak eruptive complex.
Lava
118
Crystal Fractionation Model
1000.0
CrystalPoor Rhyolite, Ash Peak Eruptive Center
43% Fractionation of
0
Biotite Tuff/CrystalRich
00.0
C__)
a
0
10.0
0
a
1.0 1
A CrystalPoor, APEC
v Model
C
0
_O
0-0 Biotite Tuff/CrystalRich
0.1
Rb Th U Sr Ba La Ce
Nd
Srn EuCd*Tb Y
Yb Lu Sc Hf Nb Ta
Figure 24.
Crystal fractionation model of crystal-poor rhyolite, APEC
derived from biotite tuff/crystal-rich rhyolite.
119
flows of porphyritic rhyolite overlie crystal-poor rhyolites associated
with the Rhyolite Peak eruptive center and these in turn are overlain
by upper andesitic volcanic rocks.
Domal and intrusive phases of
porphyritic rhyolites were intruded into the crystal-poor rhyolites.
Crystal fractionation models do not adequately account for the trace
element abundances observed in the porphyritic rhyolites.
Models
involving mixing of magmas compositionally similar to the rocks erupted
within the study area provide reasonably close duplication of the
analytical data for the porphyritic rhyolites.
Hypotheses of crystal fractionation that use compositions of any
of the rhyolitic volcanic rocks as initial magmas are not consistent
with the major and trace element contents of the porphyritic rhyolites.
Abundances of compatible trace elements such as Sr, Ba, and Eu in
porphyritic rhyolites are higher than those of all
except biotite rhyolite (Table 11).
other rhyolites
The contents of Sc, Ti02, A1203,
and Fe0 in the porphyritic rhyolites are higher than those in all other
rhyolitic volcanic rocks, whereas the Si02 content is lower (Table 11).
In addition, abundances of the incompatible trace elements, except La
and Zr, of crystal-poor rhyolites associated with the RPEC (Rhyolite
Peak eruptive center) are higher than those of the porphyritic
rhyolites, as illustrated in Figure 15.
Thus, crystal fractionation
models of the porphyritic rhyolites that involve parental magmas of
crystal-poor rhyolite composition are not consistent with reasonable
liquidus mineralogies and partition coefficients.
Review of the
element versus element graphs and spider diagrams discussed previously
(Figs. 4, 15, 20, 21, 22, and 23) suggests that the addition of magma,
more primitive than crystal-poor rhyolite, to the chamber was essential
120
to the petrogenetic formation of the porphyritic rhyolites.
The petrogenetic models for the porphyritic rhyolites consist of
mixing magmas and crystals compositionally similar to rocks identified
within the Ash Peak area.
For one model, crystal-poor rhyolites
associated with the Rhyolite Peak eruptive center were combined with
biotite rhyolite and the phenocryst assemblage of the porphyritic
rhyolites.
The proportions and types of minerals used for the
phenocryst assemblage were taken from the description of the intrusive
phase by Richter and others (1983), which consists of 20 percent alkali
feldspar and 5 percent clinopyroxene.
The phenocryst assemblage was
modified by modes determined during this study and "fine tuning" of the
mixing model with respect to the accessory minerals.
Trace element
contents of the crystals were determined by 20 percent fractional
crystallization of a biotite tuff/crystal-rich rhyolite parent magma.
Partition coefficients were taken from Mahood and Hildreth (1983) and
are listed in Appendix 2.
A magma of biotite tuff/crystal-rich
rhyolite composition was selected because it was modelled as parental
to the crystal-poor rhyolites and the Ash Peak Glass.
The amount of
crystallization was based on the overall phenocryst content of the
porphyritic rhyolites (i.e. 20 %).
The mixing model for the
porphyritic rhyolites dealt with the same eighteen elements as the
crystal fractionation models for the previous rhyolitic volcanic rocks,
and the mixing calculations were performed using Magma86.
Elemental
abundances of the porphyritic rhyolites were obtained by mixing the
analytical abundances of the crystal-poor rhyolites from the RPEC,
biotite rhyolite, and the calculated abundances of the crystals from
the biotite tuff/crystal-rich rhyolites, in the proportions 50, 30, and
121
20 percent respectively.
The elemental abundances of the modelled
magma and the petrochemical data of the porphyritic rhyolite are
presented in Figure 25.
This graph demonstrates the close agreement
between the two sets of values.
The presence of andesitic xenoliths that exhibit textures
suggestive of an included liquid phase raises the possibility of their
likely importance in any mixing model.
Although analytical data for
these xenoliths was not obtained for the present study, upper andesite
was modelled as a possible component in the formation of the
porphyritic rhyolites.
This model consists of mixing magmas similar in
composition to the crystal-poor rhyolite of the Rhyolite Peak eruptive
center, upper andesite, and the same crystal assemblage described above
in the proportions 64, 16, and 20 percent, respectively.
The results
of the model are presented in Figure 26, and compare favorably to the
analytical data gathered for the porphyritic rhyolites.
Major and
trace element data for these inclusions, to be gathered in future
studies should provide a more comprehensive view of their role in the
formation of the porphyritic rhyolites and the Ash Peak
Rhyolite Peak
eruptive complex.
SUMMARY
Trace element contents and chondrite-normalized patterns of the
lower and upper andesites are essentially identical, which indicate
that they shared a common petrogenesis or similar petrogenetic process.
Elevated abundances of REE and other incompatible trace elements in the
andesitic volcanic rocks suggest that they formed in continuously
122
Mixing Model
Porphyritic Rnyolites
000.0
50 % CrystalPoor, RPEC
30 % Biotite Rhyolite
20 % Crystals
100.0
'0.0
-0
0
1.0
v Porphyritic Rhyolite
6 Model
v
U
U
11-11111-111111-1-1-111111111
SmEuCd*Tb Y
Rb Oh U Sr Ba La Ce
0
Nd
Yb Lu Sc Hf Nb Ta
1000.0
0
50 % CrystalPoor, RPEC
30 % Biotite Rhyolite
0
/0 -0
1 00 0
20 % Crystals
8
----0
0-0
0
--,,,
-,
0
0.0
o;0
0
0
*--- 0
O
f-LL:
0
0 CrystalPoor, RPEC
O
o
-1:3
Biotite Rhyolite
Crystals
111
Rb
U Sr Ba La Ce
Nd
SniEuGd*Tb Y
Yb Lu Sc Hf Nb Ta
Figure 25.
Magma mixing model for porphyritic rhyolite involving the
combination of biotite rhyolite, crystal-poor rhyolite, RPEC, and
hypothetical crystals.
123
Mixing Model
Porphyritic Rhyolites
1000.0
50 % CrystalPoor, RPEC
0
.30 % Biotite Rhyolite
20 % Crystals
1 00.0
0u
lao,
0
0
v
1.0
Porphyritic Rhyolite
Model
0
O
--o
0.1
0
0
o CrystalPoor, RPEC
o Biotite Rhyolite
0
O Crystals
Rb fh U Sr Bo La Ce
Figure 25.
(continued).
Nd
SmEuGd*Tb Y
Yb Lu Sc Hf Nb Ta
124
Mixing Model
Porphyritic Rhyolites
1000.0
64.5 % CrystalPoor, RPEC
15.5 % Upper Andesite
20.0 % Crystals
100.0
0
10.0
Mixing Model
1.0 -;
o
0.1
0 Porphyritic Rhyolite
0
11111111T1I11
1 111111-7-111
Sm EuGd *Tb Y
Rb Th U Sr Ba La Ce
Nd
Yb Lu Sc Hf Nb Ta
1000.0
64.5 % CrystalPoor, RPEC
15.5 % Upper Andesite
20.0 % Crystals
100.0
v
v
0
10.0-
1.0 -
Crystal Poor, RPEC
6 --6 Upper Andesite
o
0.1
IIIII11111111 11111111-111
Rb Th U Sr Ba Lo Ce
Figure 26.
o Crystals
Nd
Sm EuGd*Tb Y
Yb Lu Sc Hf Nb Ta
Magma mixing model for porphyritic rhyolite involving the
combination of upper andesite, crystal-poor rhyolite, RPEC, and
hypothetical crystals.
125
Mixing Model
Porphyritic Rhyolites
1000.0
""
64.5 % CrystalPoor, RPEC
15.5 % Upper Andesite
713
20.0 % Crystals
0 V/A
1 00.0
II
10.0
0
7
Crystal Poor. RPEC
1.0
a Upper Andesite
0 o Crystals
o
0.1
111111 -I-
Rb Th U Sr Bo La Ce
Figure 26.
o Mixing Model
Porphyritic Rhyolite
o
(continued).
7
U T T T T T I
SmEuGd*Tb Y
Nd
1
1
7
1
1
II II
Yb Lu Sc Hf Nb To
126
fractionating, periodically replenished magma chambers.
The lower
abundances of the REE in the five rhyolitic volcanic rock types,
compared to those of the andesitic volcanic rocks, suggest that they
did not form by crystal fractionation of magmas similar in composition
to the andesitic volcanic rocks.
Petrogenesis of the biotite tuff/crystal-rich rhyolites
is
modelled as 32 percent fractional crystallization of a parental magma
of biotite rhyolite composition.
Ash Peak Glass is formed by an
additional 20 percent fractional crystallization of the derivative
magma of biotite tuff/crystal-rich rhyolite composition.
Crystal-poor
rhyolites are modelled using a combination of processes that include 43
percent fractional crystallization of biotite tuff/crystal-rich
rhyolite and mixing with 15 percent primitive basaltic magma.
Crystal fractionation models for the porphyritic rhyolites are
not supported by the abundances of major and trace elements.
Instead,
their petrogenetic model consists of mixing magmas of crystal-poor and
biotite rhyolite composition with hypothetical minerals, crystallized
from a parental magma of biotite tuff/crystal-rich rhyolite, in the
proportions 50, 30, and 20 percent, respectively.
The presence in the
porphyritic rhyolites of andesitic xenoliths that resemble crystallized
globules of liquid and xenocrysts that exhibit anatectic textures
suggest an alternative mixing model.
Assuming that the xenoliths
represent magma similar in composition to upper andesite, the mixing of
crystal-poor rhyolite (64 %) and upper andesite (16 %) magmas with
hypothetical crystals (20 %) produces a hybrid magma similar in
composition to the porphyritic rhyolites.
Further, the xenoliths and
xenocrysts suggest that partial melting and magma mixing processes were
127
at least partly responsible for the formation of the rhyolitic magmas
from which evolved the Ash Peak
Rhyolite Peak eruptive complex.
Variations in the local tectonic regime provided the petrogenetic
framework within which the andesitic and rhyolitic volcanic rocks of
the Ash Peak area were formed.
Parental basaltic magmas formed by
processes of either subduction at a convergent plate margin or
extension at an active rift will ascend into the continental crust.
The ascent of the parental basaltic magmas would be impeded in a preextensional stress field and accelerated in an extensional stress
field.
A slow ascent would allow heat loss and crystal fractionation
which would likely result in magma modification to intermediate
compositions.
The lower andesites of the Ash Peak area possibly
represent the eruptive products of such a process.
A more rapid ascent
inhibits heat loss and allows the basalts to act as sources of heat and
volatiles for the partial melting of crustal rocks.
The parental
rhyolitic magma (biotite rhyolite) may have formed by such a process.
In the post-extensional tectonic environment, the retarded ascent of
the parental basaltic magmas would reestablish magmatism of
intermediate composition, which is preserved as the upper andesites.
128
GEOCHEMISTRY AND GENESIS OF THE MINERAL DEPOSITS
Although located in the central portion of "porphyry copper
country", the mineralization of the Ash Peak District is not directly
related to tectonic and magmatic events of Laramide age.
Mineralization at Ash Peak consists of gold-silver-carbonate-silica and
carbonate-manganese oxide veins similar in style to other districts of
the Southwest hosting Tertiary epithermal vein mineralization such as
at the Steeple Rock and Mogollon districts of New Mexico.
The earliest record of mining activity in the Ash Peak district
was by Grant (1918, as cited in Lines, 1940) who reported that the
Goldfield Consolidated Mines Company was applying for patents on five
lode claims and two millsites located at the present Ash Peak mine.
Development at that time consisted of three shafts, the Commerce (152
m, 500 ft), the Harding (33.5 m, 110 ft), and the Shamrock (244 m, 800
ft) and included roughly 1880 m (6167 ft) of drifts and raises.
Production figures for the Goldfield Consolidated Mines Company are not
available, but silver was the metal extracted (Lines, 1940).
Veta Mines, Inc. acquired the property in 1936 and began mining
operations in 1937 (Lines, 1940).
The Shamrock shaft was extended to
300 m (975 ft) and stoping was begun on the 500, 600, and 700 levels.
The 500 level of the Commerce shaft was connected to the 350 level of
the Shamrock to provide haulage, ventilation, and emergency evacuation.
For the year 1938, metals production from the Ash Peak district totaled
1,752 oz gold, 527,706 oz silver, 9,389 lbs copper, and 26,247 lbs lead
(Lines, 1940).
Detailed mining records for the 1940's and 1950's are not
available, but production and development of the mine was probably
129
sporadic until the patented claims were acquired by Mr. Paul Turney of
Tucson, Arizona.
He has granted leases to various companies for mining
and development of the Shamrock and Commerce shafts.
At the time of
the present study, the Morenci Branch of the Phelps Dodge Corporation
held a lease on the property.
Mining operations consisted of
extraction and transportation of vein silica to the Morenci copper
smelter for use as a flux.
The costs of mining and transporting the
silica were reclaimed by recovery of the contained silver as a byproduct.
Phelps Dodge Corporation has since relinquished their lease
and the Ash Peak mine is now operated by Arizona Flux Mines,
Incorporated of Tucson.
Several geologic studies have been conducted by leasees on the
mineral deposits, prospects, and mineralized zones of the Ash Peak
district.
The few available studies viewed by the author were
considered proprietary by the Phelps Dodge Corporation and thus cannot
be included in this report.
GOLD-SILVER-CARBONATE-SILICA VEINS
Gold-silver-carbonate-silica epithermal veins are present at the
Ash Peak mine which is located approximately 1 km northeast of Ash Peak
(Fig. 2).
Within the mine area, the veins are hosted in lower andesite
(basaltic andesite phase) but locally extend upward into a thin veneer
of pyroclastic(?) and epiclastic deposits that are exposed at or near
the present surface.
The Ash Peak veins are spatially and temporally associated with a
1.5 km section of the Ash Peak fault system (Fig. 2).
Right lateral
strike-slip movement dominates the fault system which is approximately
130
10 km in length.
Slickenside orientations in the mine area vary from
subvertical to subhorizontal, indicating a dip-slip component as well
as the predominate strike-slip movement.
Dreier (1984) reported that
the general trend of the fault system is N70°W to N80°W with segments
that trend east-west and segments that trend N40°W which contain the
mineralization.
Mapping by Richter and others (1983) and during the
present study demonstrate that the entire fault system, including the
mineralized section, trends N60°W.
Within the mine area, mineralization is encountered in three
veins that are nearly vertical and en echelon downward to the
southwest.
They are the Ash Peak, Hanging Wall, and Footwall
veins.
Total widths of individual veins pinch and swell from 1 m to over 10 m
with an average of 3
m.
Vein widths generally increase with depth.
The Ash Peak vein is exposed at the surface; the Hanging Wall vein is
encountered between 10 and 20 m depth; and the Footwall vein is
encountered below 60 m.
Propylitic, argillic, advanced argillic, and silicic alteration
types have been identified in association with the Ash Peak veins.
Recognizable hydrothermal alteration types are restricted to close
proximity to the fault system.
As mentioned previously, regionally the
lower andesites appear altered and (or) weathered, probably by
processes that predate and are unrelated to the formation of the Ash
Peak veins.
Propylitic alteration consists of replacement of mafic minerals
by chlorite and iron oxides.
The subordinate development of clays
after plagioclase feldspar has been noted in some thin sections.
Chlorite and calcite also fill vesicles, form veinlets up to 1 cm
131
thick, and develop rosettes within the groundmass.
Propylitic
alteration extends up to ten meters from the silicified portions of the
veins.
Argillic alteration consists of moderate to strong transformation
of the phenocrysts and groundmass to clay minerals.
Within the
argillic zone, chlorite and calcite are present and locally dominate.
Argillic alteration is gradational with propylitic but is restricted to
within a few meters of the silicic veins.
Advanced argillic alteration consists of near complete conversion
of original rock-forming minerals to clay minerals and iron oxides.
The zone of advanced argillic alteration is restricted to what the
miners and the scant literature have referred to as the "diabase".
The
"diabase" is located between the Ash Peak and Hanging Wall veins, and
is a septum of lower andesite that has undergone advanced argillic
alteration.
Within the "diabase", little remains of the original lower
andesite mineralogy or texture.
The "diabase" is very incompetent and
0.5 to 1 m of ore was left in place by earlier mining operations to
provide stability within the stopes.
Silicic alteration consists primarily of chalcedonic silica with
minor amounts of quartz, manganocalcite, and calcite.
Chalcedony is
generally light in color with white, yellow, grey, and light brown
varieties most abundant.
In addition, patches, pods, and stringers of
dark brown to black chalcedony occur scattered throughout the vein.
Manganocalcite forms light brown to nearly black coarsely crystalline
masses.
Calcite forms snow white, finely to coarsely crystalline
masses and coatings and pale yellow dogtooth spar crystals.
Quartz
forms coarsely crystalline seams terminated by amethyst crystals and
132
clear drusy coatings in chalcedonic cavities.
The silicic vein
material exhibits textures of open-space filling punctuated by multiple
stages of brecciation which in turn is followed by recementation.
Breccia fragments of lower andesite are enclosed in silica and are
either silicified or completely altered to clay minerals.
Adjacent to
the footwall of the Ash Peak vein is a zone of brecciated lower
andesite partially cemented by iron oxides.
Textural evidence gathered from samples of the Ash Peak vein
suggests the following paragenetic sequence.
Original brecciation of
the lower andesite was followed by flooding of the interstices by
chalcedony.
Fault movement was at least partially contemporaneous with
silica deposition that resulted in chalcedony enclosing early formed
chalcedonic fragments of breccia.
The paucity of lower andesite
fragments within the silica vein suggests that most of this rock unit
underwent complete silicification to chalcedony.
Amethyst crystallized
after the majority of the chalcedony was deposited, but prior to the
termination of silica infusion and brecciation.
deposited toward the end of hydrothermal
brecciation had ceased.
Manganocalcite was
activity but before
Textures of calcite suggest that it has
undergone little brecciation and thus is late in the paragenetic
sequence.
Quartz crystals appear to have formed as a result of
recrystallization of chalcedony after the termination of hydrothermal
activity.
Samples of lower andesite from within the mine were analyzed for
major and trace elements to quantitatively assess the migration of
chemical constituents during hydrothermal
alteration.
Analyses of
samples collected from the "diabase" and along declines that approach
133
the silicic veins from the south and northeast are presented in Table
19.
The migration of elements in response to the hydrothermal fluid
is depicted in Figure 27.
Abundances of Si02 and A1203 remain
basically unchanged in the samples from the various alteration facies.
Equivocal data suggests that Fe0 and CO may have been mobilized by the
hydrothermal fluid.
The abundances of TiO2 and P205 are depleted
during propylitic alteration and progressively enriched during argillic
and advanced argillic alteration.
Relative to concentrations in the
lower andesites, all facies of alteration in the samples analyzed are
strongly enriched in MgO.
The alkali elements exhibit differing
behavior; Na20 is progressively depleted as the intensity of the
alteration increases.
Abundances of K20 are enriched in samples of
propylitic and argillic alteration, but are depleted in the sample
exhibiting advanced argillic alteration.
Trends of the abundances of the major oxides are consistent with
the mineralogical changes observed for the various alteration
assemblages.
The increase in K20 content of the propylitic and
argillic assemblages may be the result of the formation of smectitic
clays that preferentially absorbed K as interlayer cations.
Conversion
of the smectites to kaolinitic clays during advanced argillic
alteration presumably eliminated the cation sites and thus facilitated
the removal of K.
The increase in Mg0 content of the alteration
assemblages is probably the result of the formation of chlorite.
The
destruction of the plagioclase feldspars should deplete the rocks in
Na20 and CaO.
Removal of Na20 is obvious, whereas the removal of Ca0
by alteration of the plagioclase feldspars may have been negated by the
134
Table 19. Chemical Analyses of Altered Andesites Associated with the
Ash Peak mine.
Alteration
Advanced
Type
Propylitic
Argillic
Argillic
Average
Lower
Basaltic
Sample 0
AP84220
AP84225
Average
AP83068
AP84215
AP84212
Average
AP84209
Andesite
Si02
(%) (1)
56.2
n.a.
56.2
54.3
n.a.
n.a.
54.3
56.0
54.5
TiO2
(%)
1.17
0.81
0.99
2.54
3.07
3.09
2.90
3.88
1.58
A1203 (X)
16.8
19.0
17.9
11.6
16.0
15.7
14.4
22.2
17.9
Fe203 (%) (2)
3.76
n.a.
3.76
6.29
n.a.
n.a.
6.29
4.41
4.82
Fe0
(%)
3.41
n.a.
3.41
6.48
n.a.
n.a.
6.48
2.12
4.76
Fe0
(%) (3)
6.83
5.56
6.19
12.12
10.21
10.87
11.07
4.45
8.36
Mn0
(%)
0.13
0.10
0.11
0.08
0.17
0.09
0.11
0.02
0.09
Ago
(%)
5.08
8.53
6.81
8.46
9.57
9.26
9.10
4.50
1.01
Ca0
(%)
5.58
3.70
4.64
1.60
3.06
1.39
2.02
4.01
5.18
Na20
(%)
2.80
3.60
3.20
0.68
1,18
0.19
0.68
0.16
4.55
K20
(%)
4.64
4.74
4.69
6.95
6.71
9.03
7.56
1.33
3.38
P205
(%)
0.44
n.a.
0.44
1.03
n.a.
n.a.
1.03
1.33
0.79
Sc (ppm)
14.1
13.5
13.8
21.3
21.6
20.6
21.1
23.4
15.7
Cr (ppm)
182
260
221.1
22
21
14
19
17
52
Co (ppm)
22.3
20.6
21.5
34.8
35.0
33.4
34.4
27.7
19.3
V
n.a.
(ppm)
n.a.
171
171
134
321
137
197
360
Ni (ppm)
n.a.
53
53
58
49
95
67
78
n.a.
Zn (ppm)
n.a.
83
83
164
197
248
203
236
n.a.
As (ppm)
n.a.
2.1
2.1
7,7
n.a.
7.7
7.7
1.8
2.0
Sb (ppm)
n.a.
0.9
0.9
1.6
2.3
2.5
2.1
0.7
0.6
Se (ppm)
n.a.
1.2
1.2
1.8
1.4
4.2
2.4
3.3
n.a.
Rb (ppm)
228
264
246
667
484
540
497
84
77
Cs (ppm)
8.9
1.9
5.4
5,6
5.1
4.2
5.0
29.4
0.9
Sr (ppm)
391
222
306
1187
786
1604
1192
209
582
Ba (ppm)
984
810
897
1540
1349
1372
1420
421
1098
La (ppm)
54.0
47.5
50.7
41,1
37.8
49.6
42.8
40.3
60.2
Ce (ppm)
114.0
91.7
102.8
90.3
80.3
81.8
84.1
87.8
128.0
Nd (ppm)
47.4
39.4
43.4
48.5
37.2
35.3
40.3
48.8
60.7
Sm (ppm)
8.79
8.03
8.41
9.89
8.90
9.03
9.27
9.75
12.02
Eu (ppm)
1.92
1.74
1.83
2.96
2.89
2.72
2.86
2.88
3.33
Tb (ppm)
1.17
1.02
1.10
1.34
1.28
1.40
1.34
1.17
1.53
Dy (ppm)
n.a.
6.06
6.06
7.79
6.48
7.13
7.13
8.39
n.a.
(ppm)
42
17
30
16
33
17
22
52
48
Yb (ppm)
2.77
3.09
2.93
3.29
2.68
3.53
3.16
3.33
4.31
Lu (ppm)
0.40
0.30
0.35
0.41
0,32
0.43
0.39
0.55
0.29
Zr (ppm)
333
313
323
307
361
459
375
356
402
Nf (ppm)
7.4
6.9
7.2
7,3
6.9
7.0
7.1
8.3
10.0
Y
Nb (ppm)
14
9
11.5
13
17
8
12.7
23
19
Ta (ppm)
0.9
0.7
0.8
1,4
1.3
1.2
1.3
1.5
1.7
Th (ppm)
11.1
9.3
10.2
2.7
2.5
2.7
2.6
3.0
7.1
1.7
3.5
2.6
1.6
1.0
1.6
1.4
2.7
2.1
Zr/Hf
45.00
45.07
45.04
42.24
51.91
65.45
53.20
42.94
40.1
Th/U
6.57
2.63
4.60
1.72
2.43
1.70
1.95
1.09
3.5
Eu/Eu
0.71
0.71
0.71
0.96
1.03
0.93
0.97
0.99
0.91
La/K
14.02
12.07
13.04
7.13
6.78
6.62
6.84
36.41
17.8
Cs/K
2.32
0.48
1.40
0.98
0.91
0.56
0.82
26.53
0.3
U
(ppm)
(1)
Major element oxides represent anhydrous recalculation to 100%, see Appendix 1 for complete analyses
(2)
Iron (ferrous-ferric ratio) recalculated using the method of LeMaitre (1976a)
Total iron as FeO
(3)
n.a. = not analyzed
135
Andesites from the Ash Peak Nine
V
Si02
TiO2
A1203
Fe0
Mn0
Mg0
Ca0 Na20 K20 P205
o
Propylitic
Argillic
1 0.0 1
Advanced Argillic
1.0
0.1
Y171111"
Sc Cr Co As Sb Rb Cs Sr Ba La Ce NdSmEu Tb Y Yb Lu Zr Hf Nb To Th U
Figure 27.
Normalized elemental abundances of altered andesites
associated with the Ash Peak
Rhyolite Peak eruptive complex.
136
Mine
Peak
Ash
the
from
Andesites
100.0
00
7
10.0
oC
Argillic
UC
Propylitic
-
-o
o
(1.)
Argillic
Andesite
Lower
0
Average
v
o
Advanced
v
0 C
I
To
Yb
Lu
Sc
Zr
Hf
1 Nb
lllll
Y
Dy
SmEuGd*Tb
Nd
Ce
La
Ba
Cs
Rb
K
Th
U
Sr
I
TTTTTTTTT
1.0
(continued).
27.
Figure
137
deposition of calcite.
Titania should have been leached during the
destruction of the pyroxenes, but stabilized by the formation of
insoluble iron-titanium oxides.
The mobility of the trace elements observed at Ash Peak is
typical of that described for hydrothermal alteration in other systems
(Humphris and Thompson, 1978a, 1978b, Henderson, 1984).
As proposed by
Pearce (1983), the elements of high ionic potential (Z/r) are
relatively immobile and those of low ionic potential
mobile in the hydrothermal environment.
are relatively
Thus, abundances of the REE,
Sc, Co, Zr, Hf, Nb, and Ta remain approximately similar to those of the
average lower andesite where the samples have been subjected to the
conditions of propylitic, argillic, or advanced argillic alteration.
The alkali trace elements are very mobile, following the behavior of
Na20 and K20.
For example, Rb behaves similarly to K20 as it is
progressively enriched in samples subjected to propylitic and argillic
alteration and depleted in those affected by advanced argillic
alteration.
Antithetic to Na20, Cs is continuously enriched as the
grade or intensity of alteration increases.
The alkaline earth
elements, Sr and Ba, are depleted during propylitic alteration,
enriched during argillic alteration, and depleted during advanced
argillic alteration.
The enrichment of Lu and the scatter of Nb may be
related to analytical uncertainties rather than to geochemical effects.
The anomalous behavior of Y was verified by analyses of Dy in selected
samples.
Although Y behaves chemically like a REE (Dy) in the magmatic
environment, in the hydrothermal environment it does not.
This
behavior may presumably account for the depletion of Y relative to the
other REE in several of the samples of altered andesite.
Investigation
138
of this anomalous chemical behavior will be one aspect of future
research at Ash Peak.
Precious metal mineralization is spatially and temporally
associated with the silicified portions of the veins.
Silver is the
most important metallic commodity within the district, and total
production has been estimated to be at least 97,000 kg or 2.6 million
oz. (Richter and Lawrence, 1983).
Silver mineralization has been
variously described as argentite (Ag2S) or aurorite ((Mn,Ag,Ca)Mn307
3H20).
In either case, the minerals are very finely crystalline and
dispersed throughout the silica gangue.
Results of the present study
indicate that silver is also contained in both manganocalcite and
calcite.
Historically, production has come mainly from the Ash Peak
vein, although recent data indicate that the grade of silver in the
Hanging Wall vein increases with depth.
CARBONATE-MANGANESE OXIDE VEINS
Carbonate-manganese oxide mineralization is present at two
locations within the Ash Peak area.
The Thurston-Hardy (Godfrey) mines
are located north of Ash Peak (Fig. 2).
The Crow, A-1, Paradise, SPW,
and Black Beauty mines and prospects are located east of Ash Peak and
are collectively referred to as the Rattlesnake Pit deposits in this
report.
Neither group of mines is presently in production and the
development to date consists of shallow trenches and shafts.
Epithermal veins of carbonate-manganese oxide at the Thurston-
Hardy deposits are hosted in lower andesite.
The veins are associated
with two predominantly right-lateral strike slip fault zones trending
N55°W and N70°W, respectively.
The veins, approximately 40 m in
139
length, are nearly vertical and 1 to 4 m wide.
investigation of the hydrothermal
Preliminary
alteration associated with the
Thurston-Hardy mines identified a weak propylitic alteration (minor
chlorite) halo up to 5 m from the vein.
Weak argillic alteration is
developed adjacent to the veins, but advanced argillic and silicic
assemblages were not observed.
Vein minerals at the Thurston-Hardy deposits consist of manganese
oxides, manganocalcite, and calcite.
Mineral deposition occurred as
open-space fillings which coat breccia fragments of lower andesite and
the walls of the fault zone.
Manganese oxides and manganocalcite
exhibit multiple episodes of brecciation followed by recementation.
Textural paragenetic evidence suggests that deposition of the manganese
oxides predates the deposition of most of the dark manganocalcite.
White calcite generally lacks brecciation textures and is interpreted
to have been deposited toward the end of both hydrothermal activity and
fault movement.
Carbonate-manganese oxide epithermal veins of the Rattlesnake Pit
deposits are hosted in rhyolite similar in composition to subtype II
biotite rhyolite.
The veins are associated with north-south to
northwest-southeast trending fracture zones of undetermined relative
motion.
There is considerable diversity between the various prospects,
but generally the veins are 0.5 to 2 m wide and 7 to 10 m in length
Vein minerals at the Rattlesnake Pit deposits consist of
manganese oxides and calcite.
Mineral deposition occurred as open-
space fillings that coat breccia fragments of rhyolite and as finegrained flooding of rhyolite fragments.
The fracture zones exhibit
multiple episodes of brecciation and recementation by manganese oxides
140
and (or) calcite.
Manganese oxides of the Rattlesnake Pit deposits,
as
previously noted for the Thurston-Hardy deposits, formed before the
deposition of calcite.
A total of 1,704 metric tons of 39-45 percent manganese were
produced from the Thurston-Hardy deposits (Richter and Lawrence, 1983).
A combined total of 116.5 metric tons averaging 25 percent manganese
were produced from the Rattlesnake Pit deposits, predominantly from the
Crow mine.
The manganese oxides of both areas have been identified as
psilomelane and pyrolusite by early workers (as cited in, Richter and
Lawrence, 1983).
Preliminary X-ray diffraction studies performed
during this study indicate that the manganese minerals are complex
mixtures of pyrolusite and other manganese oxides and hydroxides of
various oxidation states.
GEOCHEMISTRY
To investigate the interaction between the epithermal
mineralization and the magmatism of the Ash Peak area, trace element
determinations of ore and gangue minerals were performed.
Chalcedony,
amethyst, manganocalcite, and calcite from the Ash Peak vein, and
manganese oxide, manganocalcite, and calcite from the Thurston-Hardy
and Rattlesnake Pit deposits were analyzed.
The trace element data
were acquired using INAA techniques and were complemented by data from
commercial laboratories.
Abundances of the trace elements in
chalcedony and amethyst were too low to yield meaningful information
using these analytical techniques.
The trace element characterization of ore and gangue minerals is
141
a relatively unique approach to the study of mineral deposits.
Most
investigations of mineral deposits have been concerned with the trace
element contents of the host rocks rather than the ore minerals
themselves.
The difficulty in obtaining trace element data for the ore
and gangue minerals has contributed to this deficiency.
This study and
others (e.g. Graf, 1977) indicate that INAA is, at least in some cases,
amenable to the determination of trace element contents of ore and
gangue minerals as well as the host rocks.
Recent studies (Ayuso,
1987) have shown that the electron microprobe is also ideally
applicable to the determination of trace element contents of individual
ore and gangue minerals.
However, the relatively high cost, the large
amount of time involved, and the small size of the sample are obvious
drawbacks of this technique.
The elemental data for the carbonates from the Ash Peak vein and
Thurston-Hardy deposits are presented in Table 20.
These data are
plotted as Figure 28 for the Ash Peak vein and as Figure 29 for the
Thurston-Hardy deposit.
The manganocalcites from the Ash Peak vein
display relatively flat REE patterns and modest Eu enrichments.
The
single calcite analysis from the Ash Peak vein displays LREE enrichment
and a relatively strong enrichment of Eu.
Both manganocalcite and
calcite from the Thurston-Hardy deposits exhibit slight enrichment of
the LREE, no Eu enrichment, and variable amounts of Ba enrichment and
Ce depletion.
Abundances of As and Sb are higher and Ba is lower in
the Ash Peak vein carbonates relative to those of the Thurston-Hardy
deposits.
Although the absolute abundances of the trace elements for
the carbonates are highly variable, the overall patterns of the trace
142
Table 20. Chemical Analyses of Carbonates from the Ash Peak and
Thurston-Hardy Epithermal Vein Systems.
Ash Peak Vein
Thurston-Hardy
Type (1)
Mn
Mn
Mn
Ca
Mn
Mn
Mn
Mn
Ca
Ca
Sample #
00C1
0003
Average
ODC2
ODC4
ODC5
ODC8
Average
ODC6
ODC7
Fe0
Ca
Average
(%)
0.47
6.38
3.42
0.27
0.09
0.57
0.10
0.25
0.27
0.01
0.14
Na20 (%)
0.01
0.01
0.01
0.01
0.02
0.05
0.02
0.03
0.01
0.001
0.01
K20
0.06
n.d.
0.06
0.01
0.12
0.34
0.11
0.19
0.06
0.01
0.03
Sc (ppm)
n.d.
52.0
52.0
0.4
0.3
1.3
0.3
0.6
0.3
0.1
0.2
Cr (ppm)
3
5
4
I
1
3
1
2
1
0.3
0.7
Co (ppm)
1.9
5.4
3.6
2.5
2.5
3.4
0.5
2.1
0.6
0.1
0.3
Zn (ppm)
162
150
156
11
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
As (ppm)
71.0
116
93.5
2.9
7.0
13.5
0.9
7.1
6.8
0.3
3.6
Sb (ppm)
23.1
7.5
15.3
0.8
1.4
1.2
0.3
1.0
0.2
n.d.
A9 (ppm)
105
26.5
65.8
10.5
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
Cs (MO
0.2
0.2
0.2
0.1
0.1
0.8
0.2
0.4
0.1
0.01
0.07
Sr (ppm)
234
278
256
423
376
291
79
249
100
49
74
8a (ppm)
100
181
140
37
1476
1277
170
974
216
27
122
La (ppm)
0.3
3.8
2.0
3.2
2.7
4.1
1.4
2.7
1.0
1.4
1.2
Ce (ppm)
1.0
7.2
4.1
4.2
1.1
6.0
1.4
2.8
1.3
0.2
0.7
(%)
0.2
Nd (ppm)
2.5
13..4
7.9
3.5
2.5
6.4
2.2
3.7
1.6
1.8
1.7
Sm (ppm)
0.12
1.08
0.60
0.21
0.46
0.79
0.29
0.51
0.19
0.69
0.44
Eu (ppm)
0.07
0.59
0.33
0.17
0.11
0.26
0.07
0.14
0.06
0.08
0.07
Tb (ppm)
0.04
0.27
0.16
0.04
0.03
0.12
0.04
0.06
0.03
0.01
0.02
Yb (ppm)
0.04
1.79
0.92
0.12
0.08
0.29
0.11
0.16
0.10
0.03
0.07
0.01
0.26
0.13
0.01
0.01
0.03
0.02
0.02
0.01
0.01
0.01
Hf (ppm)
0.1
0.1
0.1
0.04
0.04
0.4
0.2
0.2
0.1
0.01
0.05
'la (ppm)
n.d.
0.2
0.2
0.02
0.04
0.1
0.03
0.06
n.d.
n.d.
n.d.
0.01
0.07
0.04
0.01
0.02
0.2
0.1
0.1
0.04
0.01
0.02
0.8
n.d.
0.8
0.1
n.d.
0.1
0.3
0.2
n.d.
n.d.
n.d.
Lu (ppm)
Th (ppm)
U
(1)
(ppm)
Mn denotes manganocalcite and Ca denotes calcite.
n.a. = not analyzed
n.d. = not detected
143
100.0 ;
10.01
o Manganocalcite
A Manganocalcite
Calcite
1.0E 2
Cs K Th U Sr Bo La Ce
Nd
SmEuGd*Tb
Yb Lu Sc Hf To
100.0 ,
o---0 Manganocalcite
10.0
A---A Manganocalcite
v
v Calcite
1.0
0.1
1.0E-2 1
LOE 3,
LOE 4
1-1111-1711-1-11i111-111III1- I
Fe Sc Cr Co Zn As Sb Na K Cs Sr Bo La Ce NdSm Eu Tb Yb Lu Hf To Th U
Figure 28. Normalized elemental abundances of carbonates
associated
with the Ash Peak vein.
144
o ---0 Manganocakfte
100.0
L---6 Manganocalcite
v---v Manganocalcite
0---0 Calcite
o
Calcite
10.0
1 .0
0.1
1.0E 2
Cs K Th U Sr Ba La Ce
Nd
SmEuGd*Tb
llllllll
Yb Lu Sc Hf Ta
100.0
0---0 Manganocalcite
Manganocalcite
v---v Manganocalcite
1.0E-3
1.0E-4
IIITITITATIITIFIFIIIIIII
Fe Sc Cr Co Zn As Sb No K Cs Sr Ba La Ce NdSm Eu Tb Yb Lu Hf Ta Th U
Figure 29. Normalized elemental abundances of carbonates associated
with the Thurston-Hardy epithermal vein deposits.
145
elements are consistent within each deposit type.
The differences in
the elemental abundances and patterns between the deposit types is
probably related to their relative paragenetic position in the
hydrothermal system.
Abundances of the major and trace elements for the manganese
oxides from the Thurston-Hardy and Rattlesnake Pit deposits are
presented in Table 21.
These data are plotted as Figure 30 and include
normalizations by non-volatile Cl chondrite and average biotite
rhyolite.
Manganese oxides from the two mineralized areas exhibit
essentially the same elemental abundances and patterns and possess very
high abundances of Sr, Ba, As, and Sb.
Samples from the Rattlesnake
Pit deposits are enriched in As and Sb and are depleted in Sr and Ba
relative to those of the Thurston-Hardy deposits.
The depletion of Ce
noted in the carbonates is more pronounced in the manganese oxides,
especially those samples from the Thurston-Hardy deposits.
Abundances
of REE, except Ce, are nearly identical to those of average biotite
rhyolite.
The foregoing petrogenetic hypothesis of epithermal
mineralization of the Ash Peak district was developed from a comparison
of the elemental abundances and patterns of the carbonates and
manganese oxides to those of the volcanic rocks.
INTERACTION OF MAGMATISM AND MINERALIZATION
A common petrogenesis for the Ash Peak, Thurston-Hardy, and
Rattlesnake Pit deposits is suggested by a comparison of the elemental
contents of the ore and gangue minerals to those of the associated
igneous rocks.
Based on these comparisons, the principal
source of the
146
Table 21. Chemical Analyses of Manganese Oxides Associated with the
Thurston-Hardy and Rattlesnake Pit Epithermal Vein Deposits.
Thurston-Hardy
Sample #
Fe0
ODM6
00115
Rattlesnake Pits
Average
00112
00111
00113
00114
Average
(%)
0.03
0.04
n.d.
0.04
0.03
6.50
0.37
2.30
Na20 (X)
0.19
0.20
0.17
0.18
0.31
0.69
0.73
0.58
9.7
Sc (ppm)
1.6
1.4
3.8
2.2
10.8
12.1
6.3
Cr (ppm)
3
6
6
5
12
10
n.d.
Co (ppm)
80.3
92.6
167
113
22.6
30.1
13.0
21.9
101
82
n.d.
n.d.
n.d.
n.d.
n.d.
151
1459
1762
n.d.
1611
Ni (ppm)
Zn (ppm)
57
151
89
n.d.
11
As (ppm)
382
332
943
552
2932
3220
1451
2534
Sb (ppm)
26.8
25.5
19.5
23.9
331
1852
591
925
Ag (ppm)
0.5
n.a.
0.5
0.5
0.5
0.5
0.5
0.5
Cs (ppm)
0.3
0.4
0.4
0.3
0.9
2.3
0.9
1.4
Sr (ppm)
11461
13309
6562
10444
9463
8335
4665
7488
Ba (ppm)
87130
86063
95803
89665
74229
68278
35963
59490
La (ppm)
72.6
53.4
56.2
60.7
37.4
31.7
19.8
29.6
Ce (ppm)
4.9
7.4
18.3
10.2
28.6
22.9
26.7
26.1
Nd (ppm)
32.4
27.7
29.7
29.9
28.0
67.4
29.9
41.7
Sm (ppm)
4.91
4.44
4.53
4.63
3.72
5.44
2.49
3.88
Eu (ppm)
2.48
2.43
2.09
2.33
0.98
2.08
0.75
1.27
Tb (ppm)
0.48
0.44
0.50
0.47
0.43
0.79
0.56
0.59
Yb (ppm)
0.54
0.58
0.97
0.70
2.33
2.46
1.64
2.14
1.11 (ppm)
0.11
0.08
0.12
0.11
0.39
0.38
0.26
0.34
Zr (ppm)
131
n.d.
n.d.
131
n.d.
157
n.d.
157
Ta (ppm)
0.1
0.02
n.d.
0.06
0.2
0.7
0.3
0.4
Th (ppm)
0.2
0.5
0.6
0.4
0.7
0.4
2.0
1.0
U
2.1
1.9
6.0
3.3
6.2
17.2
n.d.
11.7
(ppm)
n.a. = not analyzed
n.d. = not detected
147
1111IITIIIIIIIII1
Cs K Th U Sr Bo La Ce
Nd
SmEuGd *Tb
Yb Lu Sc Hf Ta
0-0 RS Pit
loom
°' RS Pit
v RS Pit
0-0 TH
0-0 TH
100.0
TH
10.0
1.0
0.1
1.0E-2
1.0E 3
I
I
I
1
I
I
I
Fe Sc Cr Co Zn As Sb No K Cs Sr Ba La Ce NdSm Eu Tb Yb Lu Hf To Th U
Figure 30.
Normalized elemental abundances of manganese oxides
associated with the Thurston-Hardy and Rattlesnake Pit epithermal
vein deposits.
148
1.0E4
1000.0
100.0 -
10.0
1.0
0.1
/11-1-1-1-1
Cs K Th U Sr Ba La Ce
Figure 30.
(continued).
Nd
TT
SmEuGd*Tb
I
Yb Lu Sc Hf Ta
149
hydrothermal fluids that produced the mineralization of the Ash Peak
district was the magma chamber that erupted the biotite rhyolites.
The
strong enrichments of Sr, Ba, As, Sb, Mn, Au, and Ag in the ore and
gangue minerals provide additional evidence for the magmatic origin of
the hydrothermal fluid.
The similar REE patterns of the carbonate and manganese oxide
minerals of the Ash Peak, Thurston-Hardy, and Rattlesnake Pit deposits,
depicted in Figure 31, are suggestive of a common genesis of the
hydrothermal fluids that formed these deposits.
However, several
sources for the hydrothermal fluids that developed the epithermal
mineralization within the Ash Peak area are available.
These include
the formation of an aqueous phase in the magma chamber that erupted the
rhyolites and the leaching of the lower andesites by a circulating,
meteoric dominated, hydrothermal fluid.
Because the ore and gangue minerals have relatively high
concentrations of Eu, Sr, and Ba, the strong depletion of these
elements in the rhyolitic litho-chemical
groups, excluding biotite
rhyolite, indicates that the magmatic sources of these rocks were not
the source of the hydrothermal fluid.
In addition, the lack of hydrous
mineral phases in these rocks suggests that the separation of an
aqueous phase was unlikely.
A circulating fluid dominated by meteoric water could acquire the
required REE pattern by leaching the lower andesites.
Calculations
based on the manganese present in the Thurston-Hardy deposit are
permissive that leaching the lower andesites might provide the observed
content of metals.
The necessary volumes of lower andesite may be
calculated assuming that the manganese extracted from the Thurston-
150
Average Rock or Mineral Type
Biotite Rhyo
1.0E4
a
o
Lower Bas And
* RS Pit MnOx
0-0
loom
TH MnOx
1 00.0
10.0
1.0
0.1
10E 2
I
1
1
I
1
I
Cs Rb K Th U Sr Bo La Ce
Figure 31.
Nd
SrnEuGd*Tb Y
Yb Lu Sc Zr Hf Nb To
Normalized elemental abundances of average rock and mineral
types from the Ash Peak, Thurston-Hardy, and Rattlesnake Pit
epithermal vein deposits.
151
Hardy deposits represents 10 percent of the total manganese of the
deposits, a density of 2.5 g/cm3 for the lower andesites, and the
abundances of Mn, Sr, Ba, As, and Sb of the lower andesites determined
during this study.
To account for the observed content of Mn, Sr, Ba,
As, and Sb in the Thurston-Hardy deposits, approximately 5 x 106 m3 of
lower andesite must be totally leached of these metals.
Data from the
altered andesites suggest that approximately 50 percent of the total
elemental content may be leached under the most extreme conditions
(advanced argillic) of hydrothermal alteration.
This constraint
indicates that approximately 107 m3 or a cube 215 m on a side must be
leached to account for the observed concentrations of Mn, Sr, Ba, As,
and Sb in the Thurston-Hardy deposits.
Although the volume of andesite
necessary to obtain these metals appears reasonable, it must be
considered a lower limit.
The quantity of metal deposited in the veins
may greatly exceed the estimate used in the calculations.
In addition,
because the veins are exposed at the surface, unknown and possibly
large amounts of vein material may have been removed by erosion.
Likewise, the downward extent of the veins is not known.
The enrichment and depletion patterns of the altered andesites do
not support the hypothesis that the Mn, Sr, Ba, As, and Sb deposited in
the veins was leached from the lower andesites.
The mobility of these
elements in the different alteration zones is summarized in Table 22.
Although Sr, Ba, and Mn are leached under advanced argillic conditions,
As and Sb are unaffected by the hydrothermal fluid.
Under conditions
of propylitic alteration, Sr and Ba are depleted and Mn, As, and Sb are
enriched or unaffected.
However, with argillic alteration, all five
elements are enriched in the lower andesites.
The immobility of the
152
Table 22.
Element Mobility in Response to Differing Hydrothermal
Conditions, as Represented by the Average Elemental Abundance of
the Alteration Type Divided by the Elemental Abundance of Average
Lower Andesite.
Alteration
Type
Advanced
Propylitic
Argillic
Argillic
Sr
0.53
2.05
0.36
Ba
0.82
1.29
0.38
Mn
1.22
1.22
0.22
As
1.05
3.85
0.90
Sb
1.50
3.50
1.17
153
REE under conditions of propylitic, argillic, and advanced argillic
alteration suggests that the lower andesites were not the source of REE
in the hydrothermal fluid (Fig. 27).
In addition, the progressive
depletion of Ce in the upper levels of the hydrothermal system
demonstrate the increased influence of oxygenated meteoric waters.
Thus, if leaching of the lower andesites by circulating meteoric waters
was the source of the REE and other metals, the Ce anomaly should be
present at all levels of the system.
In addition, the abundances of Au
and Ag in the lower andesites were below the analytical detection
limits (10 ppb Au), suggesting that extremely large volumes of lower
andesite would need to be leached to account for the observed
quantities of these metals in the epithermal deposits.
Formation of the epithermal veins is believed to have resulted
from the separation of an aqueous phase from the magma that erupted the
biotite rhyolites.
The presence of biotite and hornblende in the
biotite rhyolites suggest that the magma was water-saturated and could
evolve an aqueous phase.
In the magmatic environment, incompatible
elements such as As, Sb, Au, and Ag will be strongly partitioned into
an aqueous phase.
Although the REE may be preferentially partitioned
into the magma, the aqueous phase will contain the same REE pattern as
that of the magma.
The REE patterns of the carbonates and the
manganese oxides approximate those of the biotite rhyolites and suggest
a common origin.
As discussed above, the anomalous behavior of Ce is
postulated to be the result of mixing oxygenated meteoric waters with
the aqueous phase, derived from the biotite rhyolite magma, at some
point during its migration from the source to the site of deposition.
Very likely, an appreciable fraction of Ce+3 in the aqueous phase was
154
oxidized to the highly insoluble Ce(OH)4, which precipitated out of the
solution before adsorption of the REE+3 into the carbonate and
manganese oxide minerals.
The weak Ce anomaly in the carbonates of the
Ash Peak vein, relative to those associated with the manganese oxides,
indicates that a smaller meteoric component was present in the
hydrothermal fluid.
This weak Ce anomaly suggests that the Ash Peak
vein formed at a greater depth than the manganese oxide deposits.
Thus, there is the possibility that gold-silver-carbonate-silica
mineralization may exist beneath the Thurston-Hardy and Rattlesnake Pit
deposits.
Comparisons of the geochemical data gathered for the ore and
gangue minerals relative to the igneous rocks of the Ash Peak area
provide evidence for the genesis of the epithermal deposits.
Hypotheses involving the hydrothermal leaching of the lower andesites
or separation of an aqueous phase from the rhyolitic magma chambers,
other than biotite rhyolite, are inconsistent with the observed data.
However, the geochemical data do support the hypothesis that the
epithermal veins formed from a hydrothermal fluid that was originally
an aqueous phase that separated from the biotite rhyolite magma.
The
hydrothermal fluid was progressively modified by oxygenated, probably
meteoric, waters as it attained higher levels in the hydrothermal
system.
155
SUMMARY AND CONCLUSIONS
Petrogenetic relationships between the andesites and rhyolites of
the Ash Peak area have been suggested by petrochemical investigations
of the volcanic rocks.
Changes in the local tectonic regime and its
affect on ascending parental basaltic magmas are proposed as the
unifying petrogenetic theme.
In addition, hypotheses relating the
formation of the mineral deposits and the magma that produced the
biotite rhyolites have been proposed.
Andesitic magmatism both preceded and followed the eruption of
the low- and high-silica rhyolites of the Ash Peak area.
Formation of
the andesites is proposed to be the result of crystal fractionation,
with or without assimilation, of parental basaltic magmas as they
ascended relatively slowly through the crust in pre- and postextensional regimes.
The basaltic magmas may have been produced either
by processes related to subduction at a convergent margin or extension
associated with active rifting.
The lack of TiO2 and Nb depletion in
the andesites suggests that the basalts were associated with the origin
of the Rio Grande rift.
However, the enrichment of Ba relative to La
in the andesites suggests in contrast that the basalts were associated
with subduction related processes.
High abundances of some trace
elements, particularly the REE, in the andesites of Ash Peak relative
to those andesites elsewhere as reported in the literature, suggest
that they evolved in a continuously fractionating and periodically
replenished magma chamber.
Elevated abundances of the trace elements, particularly the REE,
of the lower andesites provide evidence that they did not undergo
differentiation to produce the rhyolites.
The rhyolitic rocks of the
156
Ash Peak area are postulated to have originated as derivatives of
a
parental rhyolitic magma that formed by partial melting of the
continental crust.
The Precambrian Pinal schist or the Mesozoic
intermediate volcanic rocks that underlie Ash Peak would have been
suitable source rocks for the formation of a parental rhyolitic liquid.
In a locally extensional regime, parental basaltic magmas may have
ascended through the crust more rapidly.
With minor heat loss as
compared to a slow ascent, these basaltic magmas would not likely
undergo crystal fractionation to more intermediate compositions.
At
the crustal level where they were bouyantly compensated, they may have
acted as sources of heat and volatiles for the partial melting of the
continental crust.
Stratigraphic position and the comparatively
primitive petrochemical abundances of the biotite rhyolites suggest
that a compositionally similar magma was parental to the rhyolites of
the Ash Peak area.
Crystal fractionation models using biotite rhyolite
as the starting composition and modal minerals and proportions for the
liquidus assemblage were calculated to simulate the analytical
abundances of 18 elements in the rhyolitic rocks.
Two petrogenetic
lineages have been proposed based on the observed petrochemical
abundances.
The suite biotite rhyolite to biotite tuff/crystal-rich
rhyolite to Ash Peak Glass was modelled using crystal fractionation.
The addition of a more primitive magma to the rhyolitic magma chamber
is suggested by the abundances of the major oxides, particularly Si02,
in the crystal-poor rhyolites.
Following the addition of the primitive
magma, a composition similar to biotite tuff/crystal-rich rhyolite is
proposed for the rhyolitic magma chamber which then underwent crystal
fractionation to crystal-poor rhyolite.
Crystal fractionation models
157
for the formation of the porphyritic rhyolites were inadequate to
explain the observed petrochemical
abundances.
Magma mixing was used
to model the petrogenesis of the porphyritic rhyolites.
Two models
were developed based on the petrochemistry and stratigraphic position
of the last erupted rhyolite, the proposed parental role of biotite
rhyolite, and the presence of xenoliths of intermediate composition in
the porphyritic rhyolites.
These consist of mixing magmas similar in
composition to either biotite rhyolite or upper andesite with crystalpoor rhyolite, RPEC (Rhyolite Peak eruptive center) and hypothetical
crystals representing the phenocryst assemblage of the porphyritic
rhyolites in the proportions 20, 60, and 20 percent, respectively.
Rather than proposing two petrogenetic mechanisms, by changing
the tectonic regime to produce the andesites and rhyolites from the
parental basaltic magmas, a unifying petrogenetic model has been
developed.
Moreover, this mechanism serves to explain the nearly
identical petrochemical abundances of the lower and upper andesites
despite their separation by the rhyolite sequence and possibly a
considerable length of time.
The similar trace element patterns of the ore and gangue minerals
associated with the gold-silver-carbonate-silica and carbonatemanganese oxide mineral deposits suggest a common origin for the fluids
from which they crystallized.
The pattern of trace elements in these
minerals is nearly identical to that of the biotite rhyolites.
The
presence of hydrous minerals in the biotite rhyolite indicate
conditions of water saturation for the magma and the possibility of the
separation of an aqueous phase as the source for the hydrothermal
fluids.
Trace elements that are enriched in the epithermal veins such
158
as As, Sb, Au, and Ag would preferentially partition into an aqueous
phase.
Likewise, the REE pattern of the magma would be inherited by
the aqueous phase, but not necessarily the magmatic abundances.
Because Ce(OH)
4
is more insoluble than the REE+3, the depletion of Ce
in the carbonate-manganese oxide deposits relative to the gold-silvercarbonate-silica deposits indicates the progressive influence of
oxygenated meteoric waters on the hydrothermal fluid.
Thus, the
increase in the negative Ce anomaly between the two deposit types may
also indicate relative position in the hydrothermal system.
The gold-
silver-carbonate-silica veins at Ash Peak may have formed at a deeper
level in the same hydrothermal system that produced the carbonate-
manganese oxide veins of the Thurston-Hardy and Rattlesnake Pit
deposits.
Assuming the validity of this hypothesis, gold-silver-
carbonate-silica mineralization may exist at depth below the ThurstonHardy and Rattlesnake Pit deposits.
159
REFERENCES
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Anderson, C.A., 1966, Areal geology of the Southwest: in Titley, S.R.
and Hicks, C.L., eds., Geology of the Porphyry Copper Deposits,
Southwestern North America, Tucson, University of Arizona Press,
p. 3-16.
Anderson, C.A. and Silver, L.T., 1976, Yavapai Series-A Greenstone Belt:
Arizona Geological Society Digest X, p. 13-26.
Armstrong, A.K. and Mamet, B.L., 1978, The Mississippian system of
southwestern New Mexico and southeastern Arizona:
in Callender,
J.F., Wilt, J.C., and Clemons, R.E., eds., Land of Cochise
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APPENDICES
168
Appendix 1.
Energy Dispersive X-Ray Fluorescence
Data for Andesites and Rhyolites Associated with
the Ash Peak
Rhyolite Peak Eruptive Complex,
Southeastern Arizona
169
Table A.
Energy Dispersive X-Ray Flourescence Analyses
K20
Ca0
TiO2
(X)
(X)
(X)
0.02
0.02
0.002
Detection
Limits:
Fe0(T)
(X)
Rb
Sr
7
Zr
Nb
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
(ppm)
(ppm)
0.02
Ba
La
Ce
Lower Andesite
basaltic andesite
Sample #
K20
Ca0
TiO2
(X)
(X)
(X)
Fe0(T)
(X)
Rb
Sr
7
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
Zr
Nb
Ba
La
Ce
83065
2.90
5.61
1.779
10.5
63
698
42
363
15
1054
83040A
76
3.84
153
6.25
1.619
9.4
87
752
54
413
15
1166
76
155
84193
2.93
3.88
1.474
9.4
56
414
50
441
28
1083
83040B
65
134
3.32
6.08
1.518
8.6
76
684
47
383
11
1090
78
142
84111-1
3.11
3.95
1.487
8.6
62
446
48
397
26
1054
84111
64
124
3.08
3.72
1.302
8.1
65
451
50
398
18
1055
66
138
n.a.
7.6
103
463
45
413
21
1104
68
129
84137
n.a.
n.a.
Average
3.20
4.92
1.530
8.9
73
558
48
401
19
Std Dev
1087
70
0.35
139
1.19
0.16
0.96
17
145
4
25
6
40
6
12
6
6
7
7
7
7
7
7
7
7
7
Fe0(T)
Rb
T
Zr
(X)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
N=
6
andesite
Sample A
K20
Ca0
1102
(X)
(X)
(X)
Sr
Nb
Ba
La
Ce
(ppm)
84114
3.56
3.55
1.337
6.9
75
435
57
389
25
84113
1194
65
3.56
130
3.73
1.157
6.0
79
395
56
396
26
1148
57
117
84180
2.72
4.39
0.785
4.8
90
487
28
351
6
84181
1187
72
3.23
122
4.75
0.861
4.6
91
481
30
346
14
1067
67
123
3.85
1.893
4.4
93
501
41
353
14
1246
73
130
0.801
4.2
90
496
34
354
8
1068
65
n.a.
5.2
27
302
20
154
11
n.a.
n.a.
n.a.
84194
6.86
84192
3.25
4.77
84001
n.a.
n.a.
127
Average
3.86
4.17
1.139
5.2
78
442
38
335
15
Std Dev
1152
67
1.50
125
0.53
0.430
1.0
23
73
14
82
8
72
6
5
7
7
7
7
7
6
6
6
N=
6
6
n.a. = not analyzed
n.d. = not detected
6
7
170
Table A. (continued)
K20
Ca0
TiO2
(%)
(%)
(K)
0.02
0.02
0.002
0.02
K20
Ca0
TiO2
Fe0(T)
(14)
(X)
(X)
Detection
Limits:
Fe0(T)
(K)
Rb
Sr
T
(PPm)
(PPm)
(PPm)
(ppm)
(ppm)
(PPm)
(PPm)
(Wm)
6
6
5
4
3
6
6
7
(p(m)
(ppm)
(PPm)
(ppm)
(ppm)
(PPm)
(PPm)
(ppm)
Nb
Zr
Ba
La
Ce
Upper Andesite
basaltic andesite
Sample #
(X)
Rb
Sr
Nb
Zr
Y
Ba
Ce
La
84183
2.73
4.87
1.880
11.1
61
487
50
359
24
941
57
123
84182
2.91
4.64
1.568
10.0
63
545
91
419
30
1114
77
139
84006
2.28
7.28
1.570
9.1
51
742
34
278
25
817
47
91
84007
2.37
7.14
1.430
9.0
54
728
38
272
19
838
46
89
84008
2.50
7.12
1.138
8.5
56
732
33
258
19
838
45
91
84014
2.24
8.04
1.429
7.3
35
608
26
221
6
890
42
79
Average
2.51
6.52
1.503
9.2
53
640
45
301
21
906
52
102
Std Dev
0.27
1.41
0.243
1.3
10
110
24
73
8
111
13
23
N.
6
6
6
6
6
6
6
6
6
6
6
6
K20
Ca0
TiO2
Fe0(T)
Rb
(X)
(X)
(X)
(X)
(PPm)
(PPm)
(PPm)
(ppm)
(Pim)
(PPm)
(PPm)
(PPm)
121
Andesite Dikes
Sample #
Sr
Y
Nb
Zr
Ba
Ce
La
84017
n.a.
n.a.
n.a.
8.9
73
593
52
365
24
1180
74
84041
n.a.
n. a.
n. a.
8.6
62
599
21
245
10
796
39
78
84058
n.a.
n.a.
n.a.
8.2
50
650
26
230
20
790
42
71
Average
8.6
62
614
33
280
18
922
52
90
Std Dev
0.4
12
31
17
74
7
223
19
27
3
3
3
3
3
3
3
3
3
n.a. = not analyzed
n.d. = not detected
171
Table A.
(continued)
Detection
Limits:
K20
Ca0
TiO2
(X)
(Z)
(X)
0.02
0.02
0.002
0.02
Fe0(T)
Fe0(T)
(X)
Rb
r
Sr
Nb
Zr
La
Ba
Ce
(pps)
(ppm)
(PFe)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
Nine Andesite
K20
Ca0
TiO2
Sample A
(X)
(Z)
(X)
83068
8.90
1.62
2.370
84211
2.48
5.06
1.882
84212
6.59
1.81
2.326
12.1
495
84215
7.45
4.19
2.542
13.2
432
84217
3.80
13.30
2.216
12.1
119
84220
4.40
5.29
1.108
7.5
228
84209
n.a.
n.a.
n.a.
4.3
Rb
Sr
Nb
Zr
Y
Ba
La
Ce
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
13.3
493
1723
16
299
13
1630
51
94
14.5
101
251
31
272
17
698
53
105
1779
17
291
8
1344
52
103
1537
33
302
17
1062
53
103
370
33
267
17
678
49
93
391
42
333
14
815
70
126
75
209
52
356
23
n.a.
n.a.
n.a.
(X)
Average
5.60
5.21
2.074
11.0
278
894
32
303
16
1038
55
104
Std Dev
2.44
4.27
0.522
3.7
190
741
13
32
5
385
8
12
N=
6
6
6
7
7
7
7
7
6
6
6
n.a. = not analyzed
n.d. = not detected
7
172
Table B.
Energy Dispersive X-Ray Flourescence Analyses
Detection
Limits:
K20
Ca0
TiO2
(X)
(X)
(X)
0.02
0.02
0.002
Fe0(T)
(X)
Rb
Sr
Nb
Zr
Y
Ba
La
Ce
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
0.02
Biotite Rhyolite
Sub-type I
K20
Ca0
TiO2
(X)
(X)
(X)
L83039
5.85
0.91
0.196
83067
4.32
0.97
0.199
84012
n.a.
n.a.
84022
n.a.
84031
n.a.
84094
84095
Sample #
Fe0(T)
(X)
Rb
Sr
Zr
Y
Nb
Ba
La
Ce
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(Km)
(ppm)
1.3
168
63
32
209
17
739
58
109
1.2
124
72
26
213
19
813
57
n.a.
1.5
174
101
34
237
21
830
n.d.
n.a.
n.a.
1.4
186
94
33
248
23
789
52
98
n.a.
n.a.
1.4
168
102
31
238
20
808
58
107
n.a.
n.a.
n.a.
1.3
183
150
36
226
17
789
56
107
n.a.
n.a.
n.a.
1.3
133
106
29
210
14
720
52
94
Average
5.09
0.94
0.198
1.3
162
98
32
226
19
784
56
104
Std Dev
1.08
0.04
0.002
0.1
24
28
3
16
3
40
3
6
N=
2
2
2
7
7
7
7
7
7
7
6
109
n.d.
Sub-type It
K20
Ca0
TiO2
Sample #
(X)
(X)
(X)
84023
n.a.
n.a.
n.a.
84024
n.a.
n.a.
n.a.
84200
3.93
1.68
84201
6.29
Average
Std Bev
N=
Fe0(T)
Rb
Y
Zr
Nb
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
0.8
212
95
27
126
20
468
45
89
0.8
146
91
27
124
22
480
49
81
0.113
0.8
184
100
24
118
15
434
48
77
0.68
0.108
0.8
186
60
31
126
20
465
47
91
5.11
1.18
0.111
0.8
182
87
27
124
19
462
47
85
1.67
0.71
0.004
0.0
27
18
3
4
3
20
2
7
2
2
2
4
4
4
4
4
4
4
4
4
n.a. = not analyzed
n.d. = not detected
(%)
Sr
Ba
La
Ce
173
Table C.
Energy Dispersive X-Ray Flourescence Analyses
Detection
Limits:
K20
Ca0
TiO2
(X)
(X)
(X)
0.02
0.02
0.002
Fe0(T)
(X)
Rb
Nb
Y
Zr
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppn)
(ppm)
6
6
5
4
3
6
6
7
(ppm)
(ppm)
(ppm)
(ppm)
0.02
Sr
Ba
La
Ce
Biotite Tuff/Crystal-Rich Rhyolite
K20
Ca0
TiO2
(X)
(X)
(X)
83031
4.58
0.45
0.083
0.8
83032
4.39
0.54
0.089
1.0
83033
4.64
0.79
0.082
83038
7.45
0.37
0.090
83044
8.48
0.36
83045
5.27
Sample #
Fe0(T)
Rb
(ppm)
Sr
Zr
Y
Nb
Ba
La
Ce
(ppm)
(ppm)
168
7
24
124
26
95
49
85
185
16
34
127
26
138
44
89
1.0
206
21
31
150
32
185
48
97
0.9
273
15
34
138
23
119
46
80
0.102
0.9
262
15
32
143
27
147
46
94
0.52
0.107
1.0
195
18
52
176
30
145
60
123
(X)
(ppm)
83046
4.96
0.68
0.090
0.8
167
25
42
137
25
176
52
96
83048
5.02
0,68
0.101
1.0
168
23
36
142
21
148
59
110
83052
4.48
0.62
0.079
0.9
204
8
42
131
29
83
55
80
84018
n.a.
n.a.
n.a.
1.1
197
36
41
179
29
158
66
130
84020
n.a.
n.a.
n.a.
1.0
227
16
41
154
23
115
54
107
84021
n.a.
n.a.
n.a.
0.9
237
53
37
159
24
181
59
103
84033
n.a.
n.a.
n.a.
0.9
115
76
32
135
21
152
48
102
84047
n.a.
n.a.
n.a.
0.9
242
20
35
131
35
71
44
85
84048
n.a.
n.a.
n.a.
1.0
189
23
32
141
33
156
58
116
84073
4.85
1.13
0.083
1.1
172
35
39
126
22
121
45
92
84076
3.56
0.52
0.079
0.9
200
24
32
123
22
60
45
83
84104
n.a.
n.a.
n.a.
1.1
233
17
53
189
31
48
65
118
76
84122
n.a.
n.a.
n.a.
0.9
195
21
26
115
19
116
42
84123
6.33
0.24
0.090
0.7
178
13
32
118
21
109
42
81
84124
5.58
0.47
0.151
1.0
174
n.d.
43
157
26
88
62
113
84129
5.08
0.19
0.087
1.2
234
65
193
43
66
84131
n.a.
n.a.
n.a.
1.1
194
n.d.
50
180
32
n.a.
n.a.
n.a.
84151
4.48
0.38
0.081
1.0
203
22
53
183
26
62
63
114
Average
5.28
0.53
0.093
1.0
201
23
39
148
27
119
53
100
Std Dev
1.27
0.23
0.018
0.1
35
16
10
24
6
42
8
17
N=
16
16
16
24
24
22
24
24
24
23
23
23
n.a. = not analyzed
n.d. = not detected
10
63
129
174
Table D.
Detection
Limits:
Energy Dispersive X-Ray Flourescence Analyses
K20
Ca0
TiO2
(X)
(X)
(X)
0.02
0.002
0.02
Fe0(T)
(X)
Rb
r
Sr
Nb
Zr
Ba
La
Ce
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
0.02
Ash Peak Glass
Sample 0
K20
Ca0
TiO2
(X)
(X)
(X)
Fe0(T)
Rb
Y
Zr
Nb
(X)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
Sr
Ba
La
Ce
83063
4.31
0.64
0.044
0.7
220
15
55
116,
33
27
21
56
83064
3.67
0.67
0.040
0.8
237
12
55
107
32
27
25
64
n.a.
n.a.
n.a.
0.8
313
6
47
115
41
24
24
55
84050
n.a.
n.a.
n.a.
0.8
261
13
43
121
38
22
25
58
84051
n.a.
n.a.
n.a.
0.8
219
9
41
107
40
44
25
55
84053
n.a.
n.a.
n.a.
0.8
278
11
37
118
42
30
21
49
n.a.
n.a.
n.a.
0.8
240
6
51
117
37
32
23
61
43
84049
84055
84057
n.a.
n.a.
n.a.
0.8
218
23
46
97
33
n.d.
25
84059
n.a.
n.a.
n.a.
0.8
262
18
47
123
43
46
35
57
84069
5.43
0.46
0.051
0.8
233
10
58
102
35
29
21
64
84096
3.86
0.84
0.042
0.7
229
n.d.
58
107
27
25
24
58
84139
n.a.
n.a.
n.a.
0.8
250
19
51
107
29
20
29
56
n.a.
n.a.
n.a.
0.8
220
12
50
110
28
26
23
84140
56
n.a.
n.a.
n.a.
0.8
211
46
34
106
25
28
64
n.a.
n.e.
63
84141
n.a.
0.8
212
35
37
101
27
50
23
43
5.03
0.45
0.067
0.8
204
13
55
103
32
37
24
61
63
84139-1
84142
84145
4.50
0.59
0.059
0.8
287
n.d.
49
92
24
26
23
84156
4.82
0.38
0.047
0.8
215
7
52
104
27
32
25
55
0.66
0.048
0.8
242
20
50
103
26
27
27
55
84158
4.06
84159
2.90
0.94
0.043
0.8
239
n.d.
49
104
30
25
23
62
84202
4.66
0.58
0.044
0.8
226
12
49
112
29
21
23
54
3.71
0.69
0.037
0.7
199
13
35
105
28
24
17
41
4.35
0.80
0.037
0.8
222
12
47
108
29
24
24
50
4.75
0.65
0.041
0.8
259
14
43
105
29
22
24
57
56
84203
84204
84205
Average
4.31
0.64
0.046
0.8
237
16
47
108
32
31
24
Std Dev
0.67
0.16
0.009
0.0
28
10
7
7
6
11
3
7
13
13
24
24
21
24
24
24
23
24
24
N.
13
n.a. = not analyzed
n.d. = not detected
175
Table E.
Detection
Limits:
Energy Dispersive X-Ray Flourescence Analyses
K20
Ca0
TiO2
(X)
(X)
(X)
0.02
0.02
0.002
Fe0(T)
(X)
Sr
Rb
Nb
Zr
T
Ce
La
8a
(ppm)
(ppm)
(ppm)
(ppm)
(PPm)
(PPm)
(PPm)
(PPm)
6
6
5
4
3
6
6
7
(ppm)
(PPm)
(PPm)
0.02
Crystal-Poor Rhyolite (Ash Peak Eruptive Center)
K20
Ca0
TiO2
Sample #
(X)
(X)
(X)
83037
6.57
0.57
0.059
1.1
309
83041
5.23
0.69
0.081
1.0
232
n.d.
83042
4.71
0.31
0.075
1.0
325
n.d.
83043
4.49
1.00
0.077
1.2
314
83049
3.78
0.72
0.075
1.2
287
83053
4.47
0.52
0.085
1.5
264
83054
4.50
0.24
0.090
1.4
267
83056
4.58
0.34
0.090
1.3
83058
4.62
0.42
0.072
83060
4.39
0.35
83061
4.31
83062
Fe0(T)
(%)
Rb
(ppm)
Sr
Zr
Y
Nb
La
Ba
Ce
(PPm)
(ppm)
(PPm)
(Wm)
6
62
227
44
38
38
86
61
228
40
19
52
116
59
278
70
38
29
111
90
260
61
44
58
122
n.d.
84
231
45
21
66
138
n.d.
76
348
55
32
73
140
9
33
356
61
54
46
118
278
7
79
364
52
30
65
140
1.0
287
10
105
213
56
62
54
129
0.064
1.3
270
n.d.
71
221
43
36
75
163
0.74
0.057
1.1
266
10
83
206
48
38
76
141
4.22
0.27
0.063
1.1
271
20
84
217
53
42
55
134
84002
n.a.
n.a.
n.a.
0.9
182
57
60
271
49
n.a.
n.a.
n.a.
84003
n.a.
n.a.
n.a.
0.9
179
21
52
253
48
n.a.
n.a.
n.a.
84004
n.a.
n.a.
n.a.
0.7
173
20
58
186
42
n.a.
n.a.
n.a.
84005
n.a.
n.a.
n.a.
0.7
148
19
54
184
37
n.a.
n.a.
n.a.
84009-1
n.a.
n.a.
n.a.
1.2
297
27
81
242
51
47
74
154
84010
4.39
0.55
0.069
1.2
302
8
99
247
48
16
71
142
84013
n.a.
n.a.
n.a.
1.1
218
15
65
187
37
60
59
109
84015
n.a.
n.a.
n.a.
1.1
235
16
64
193
35
109
44
105
84019
n.a.
n.a.
n.a.
1.2
353
10
78
293
56
26
56
117
84029
n.a.
n.a.
n.a.
1.2
332
9
80
281
55
32
55
119
84030
n.a.
n.a.
n.a.
1.2
326
n.d.
77
281
54
31
60
123
84032
n.a.
n.a.
n.a.
1.2
364
8
84
351
63
20
47
110
84035
n.a.
n.a.
n.a.
1.0
365
n.d.
65
270
69
23
48
114
84038
n.a.
n.a.
n.a.
1.2
328
23
61
295
68
68
54
123
84042
n.a.
n.a.
n.a.
1.2
249
25
73
239
55
16
61
133
84043
n.a.
n.a.
n.a.
1.2
510
32
71
254
62
15
75
142
84044
n.a.
n.a.
n.a.
1.0
280
11
63
202
53
25
64
135
84045
n.a.
n.a.
n.a.
0.8
173
22
31
125
32
117
41
88
84046
n.a.
n.a.
n.a.
0.9
208
25
31
150
34
112
53
97
n.a, = not analyzed
n.d. = not detected
9
176
Table E.
Detection
Limits:
(continued)
K20
Ca0
TiO2
(X)
(X)
(%)
0.02
0.02
0.002
Fe0(T)
(X)
Rb
Sr
Nb
Zr
Y
Ba
La
Ce
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
0.02
Crystal-Poor Rhyolite (Ash Peak Eruptive Center)
Sample N
K20
Ca0
TiO2
(X)
(X)
(X)
Fe0(T)
(X)
Rb
(ppm)
Sr
(ppm)
Nb
Y
Zr
8a
La
(PPm)
(ppm)
(PPNI)
(PPm)
(ppm)
Ce
(ppm)
84054
n.a.
n.a.
n.a.
1.0
330
10
51
211
56
51
84060
23
4.56
76
0.24
0.055
1.1
284
n.d.
85
236
43
15
48
131
102
84061
n.a.
n.a.
n.a.
1.1
396
8
65
284
63
33
84062
52
3.62
0.58
0.062
1.3
430
9
114
369
78
13
50
n.a.
0.8
280
17
64
106
33
n.a.
84065
n.a.
n.a.
84066
5.08
0.38
0.068
1.0
286
24
91
186
51
84068
4.75
0.46
0.077
1.2
308
17
53
296
54
0.35
0.087
1.3
315
32
91
319
62
0.56
0.072
1.2
313
83
72
315
0.087
1.4
311
26
78
84070
4.83
84071
3.68
102
n.a.
n.a.
78
62
133
113
33
116
43
67
131
49
30
60
140
377
45
26
69
142
84077
3.75
0.49
84080
4.44
0.70
0.059
1.0
251
27
77
203
40
20
84081
61
4.65
115
0.95
0.067
1.1
292
20
89
266
43
39
50
106
n.e.
n.e.
1.1
270
32
77
217
43
42
67
138
84083
n.a.
84084
n.a.
n.a.
n.a.
1.2
291
15
83
252
43
84084-1
47
72
n.a.
156
n.a.
n.a.
1.2
294
14
69
237
46
84085
38
n.a.
62
132
n.a.
n.a.
1.1
265
22
75
214
41
36
55
124
84086
n.a.
n.a.
n.a.
1.0
258
54
69
205
84087
36
32
53
n.a.
106
n.a.
n.a.
1.1
311
20
76
225
84088
40
23
62
n.a.
138
n.a.
n.a.
0.9
237
27
69
189
36
84089
59
54
n.a.
107
n.a.
n.a.
1.0
262
17
71
213
39
84090
39
3.35
65
127
0.063
1.2
301
n.d.
116
245
48
37
69
136
21
67
188
36
n.a.
n.a.
n.a.
125
0.51
84091
n.a.
n.a.
n.a.
1.0
221
84092
n.a.
n.a.
n.a.
1.4
279
21
72
354
45
84093
58
4.08
69
0.61
0.059
1.0
230
n.d.
70
189
43
38
2.84
53
113
0.48
0.041
1.2
421
29
145
370
79
15
43
103
84097
84098
3.71
0.71
0.051
1.2
380
n.d.
165
369
84099
88
28
4.53
53
122
0.45
0.091
1.1
316
n.d.
108
299
66
46
54
128
n.a.
1.2
327
77
79
313
48
97
61
124
84100
n.a.
n.a.
84101
3.87
0.74
0.088
1.2
340
83
68
312
56
84103
56
4.30
53
0.50
122
0.071
1.0
216
8
80
184
37
92
63
111
0.083
1.0
209
8
60
180
30
77
74
96
84105
4.79
0.44
n.a. = not analyzed
n.d. = not detected
177
Table E.
Detection
Limits:
(continued)
K20
Ca0
TiO2
(%)
(X)
(%)
0.02
0.02
0.002
FeO(T)
(%)
r
Zr
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
Rb
0.02
Sr
Nb
Ce
La
8a
Crystal-Poor Rhyolite (Ash Peak Eruptive Center)
K20
Ca0
TiO2
Sample Si
(%)
(X)
(X)
84106
3.48
0.88
0.065
84107
4.58
0.44
0.072
84108
4.70
0.19
84109
3.81
84110
Fe0(T)
(%)
Rb
Sr
Nb
Zr
T
Ce
La
8a
(ppm)
(ppm)
(ppm)
1.0
301
85
80
184
32
102
49
115
1.2
267
11
85
236
48
71
79
153
0.079
1.2
295
n.d.
80
275
44
30
31
100
0.65
0.055
1.2
309
18
85
260
53
124
41
104
3.90
0.45
0.056
1.2
273
6
127
268
48
34
71
110
84112
2.56
0.97
0.070
1.1
263
81
52
172
29
34
47
119
84116
5.12
0.41
0.076
1.1
211
8
52
169
27
56
48
116
84118
n.a.
n.a.
n.a.
1.1
221
14
58
186
37
n.a.
n.a.
n.a.
84119
3.81
0.42
0.123
1.2
197
26
88
188
30
86
58
130
84120
5.21
0.39
0.096
1.1
199
7
48
189
40
79
53
121
84121
4.82
0.49
0.065
0.9
192
10
72
178
34
96
47
102
84125
4.15
0.41
0.062
1.0
218
n.d.
91
183
35
76
67
126
84126
4.73
0.68
0.075
1.0
188
18
53
172
35
57
51
124
84127
5.24
0.47
0.077
0.9
220
13
64
188
36
74
49
120
84130
3.72
1.08
0.067
1.1
314
7
69
175
31
38
60
117
84132
n.a.
n.a.
n.a.
1.1
234
17
58
195
33
40
45
111
84134
2.70
0.38
0.065
0.8
115
6
52
108
17
65
36
76
84135
n.a.
n.a.
n.a.
0.9
202
16
64
186
32
82
51
108
84136
n.a.
n.a.
n.a.
1.0
277
21
66
187
27
29
51
108
84143
3.82
0.88
0.071
1.0
245
n.d.
53
169
32
34
48
113
84147
3.97
0.81
0.093
1.0
265
87
61
179
29
35
50
107
84148
4.18
0.46
0.068
1.0
189
n.d.
44
172
33
54
56
132
84149
4.38
0.28
0.077
0.8
187
16
48
147
23
31
47
91
84150
3.79
0.82
0.088
1.0
358
29
64
186
29
36
55
118
84154
4.48
0.42
0.092
0.9
210
8
54
191
33
68
49
110
84155
4.60
1.71
0.057
1.0
227
11
70
200
43
38
59
119
84208
4.28
0.42
0.070
1.1
289
10
71
235
46
18
72
147
Average
4.29
0.57
0.073
1.1
273
23
73
233
45
49
56
120
Std Dev
0.68
0.26
0.014
0.2
65
20
21
63
13
27
12
17
N.
53
53
53
89
89
73
89
89
89
82
82
82
n.a. = not analyzed
n.d. = not detected
178
Table F.
Energy Dispersive X -Ray Flourescence Analyses
Detection
Limits:
E20
Ca0
TiO2
(X)
(X)
(X)
0.02
0.02
0.002
Fe0(7)
(X)
Rb
Sr
Zr
Y
Nb
Ba
Ce
La
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
(PPm)
(PPm)
(PPm)
0.02
Crystal-Poor Rhyolite (Rhyolite Peak Eruptive Center)
K20
Ca0
ri02
Sample N
(X)
(X)
(X)
83001
3.35
0.51
0.086
83002
4.32
0.63
0.078
83003
4.18
0.55
83004
4.40
83005
83006
Fe0(r)
Rb
Sr
Zr
Y
(PPm)
(Pfam)
1.3
270
1.3
279
0.088
1.1
0.82
0.092
4.48
0.16
2.65
0.62
83009
4.76
83013
(X)
Nb
Ba
La
Ce
(PPm)
(PPm)
(PPm)
n.d.
94
288
52
22
89
186
8
112
297
58
36
119
202
247
n.d.
99
266
54
23
76
177
1.2
251
11
106
284
53
50
83
189
0.097
1.3
258
n.d.
89
311
60
31
95
184
0.098
1.3
324
13
104
297
48
51
102
208
0.25
0.109
1.5
239
n.d.
108
379
47
36
95
214
4.74
0.08
0.112
1.8
247
n.d.
98
543
51
19
94
208
83016
3.93
0.33
0.112
1.6
229
n.d.
101
522
55
13
93
193
83020
n.a.
n.a.
n.a.
1.4
224
29
95
333
42
n.a.
n.a.
n.a.
83021
4.69
0.35
0.097
1.2
226
n.d.
73
311
43
44
75
146
83022
5.23
0.23
0.116
1.7
220
6
79
521
50
31
132
215
83024
5.16
0.15
0.140
1.9
232
17
76
542
55
40
99
189
83025
4.37
0.16
0.116
1.5
221
n.d.
136
533
82
19
88
193
83026
5.15
0.57
0.094
1.3
295
17
115
316
67
39
88
227
83027
n.a.
n.a.
n.a.
1.4
273
20
116
314
62
101
102
204
83028
n.a.
n.a.
n.a.
1.3
260
n.d.
77
295
55
n.a.
n.a.
n.a.
83029
4.81
0.21
0.105
1.2
215
n.d.
73
312
42
30
59
165
84171
3.80
0.94
0.080
1.3
352
43
102
303
53
26
87
186
84174
5.35
0.49
0.124
1.9
300
11
74
527
41
14
94
194
84175
4.32
0.80
0.101
1.3
313
76
78
291
41
89
98
181
84177
4.39
0.18
0.071
1.0
248
17
83
247
38
41
67
115
84189
4.49
0.26
0.015
1.3
250
8
74
293
43
32
87
183
84191
4.65
0.29
0.112
1.5
243
n.d.
75
292
46
23
76
173
Average
4.44
0.41
0.097
1.4
259
21
93
359
52
37
91
188
Std Dev
0.64
0.25
0.025
0.2
36
19
17
104
10
22
16
24
21
21
24
24
13
24
24
24
22
22
22
N.
21
n.a. = not analyzed
n.d. = not detected
179
Table G.
Detection
Limits:
Energy Dispersive X-Ray Flourescence Analyses
K20
Ca0
TiO2
00
(14)
(X)
0.02
0.02
0.002
Fe0(T)
(X)
Rb
r
Zr
Nb
(ppm)
(ppm)
(Pew)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
6
6
5
4
3
6
6
7
(PPrn)
(PPm)
(PPm)
(ppm)
n.a.
0.02
Sr
Ba
La
Ce
Porphyritic Rhyolite
Sample S
K20
Ca0
TiO2
(X)
(%)
(X)
Fe0(T)
(X)
Rb
(Pflo)
Sr
Zr
Y
(PP0)
(ppm)
(ppm)
Nb
Ba
La
Ce
84025
n.a.
n.a.
n.a.
2.1
235
71
71
356
45
n.a.
n.a.
84026
n.a.
n.a.
n.a.
2.8
172
198
51
463
33
999
109
203
n.a.
n.a.
n.a.
2.2
221
92
55
373
37
427
99
202
84028
n.a.
n.a.
n.a.
2.3
253
70
67
303
45
155
104
205
84161
5.03
0.99
0.293
2.3
205
70
59
365
35
313
113
191
5.16
0.83
0.265
2.0
224
62
55
343
30
257
95
197
5.05
0.72
0.313
2.4
207
87
58
381
34
403
102
183
4.41
0.96
0.232
2.1
209
75
81
336
41
240
98
188
0.265
2.2
235
72
81
292
44
128
89
183
84027
84162
84163
84164
84165
4.73
0.90
84166
4.74
0.60
0.241
1.8
233
65
58
279
45
127
81
84167
172
4.58
0.59
0.271
0.8
212
68
n.d.
299
37
126
104
202
5.19
0.58
0.358
1.5
195
81
73
426
37
511
83
167
4.37
0.78
0.224
2.2
211
52
54
350
41
243
95
184
182
84168
84169
84170
5.30
0.66
0.292
2.3
206
76
70
342
35
247
99
84172
4.34
0.91
0.244
1.9
212
57
70
270
36
148
100
188
56
57
223
33
239
83
172
n.a.
84173
3.61
1.05
0.197
1.8
183
84178
n.a.
n.a.
n.a.
2.1
237
64
60
331
38
n.a.
n.a.
84179
4.17
1.20
0.238
2.0
248
66
60
325
34
215
100
205
239
63
68
346
36
243
103
209
84184
4.25
0.96
0.193
2.1
84185
4.82
0.66
0.279
2.2
210
66
69
348
39
217
100
84186
198
3,85
0.93
0.207
2.1
233
61
90
299
42
136
95
84187
202
5.13
1.75
0.440
2 8
170
154
54
434
31
645
99
84188
170
4.26
0.84
0.216
1.7
221
61
47
296
43
259
97
187
84190
4.92
1.25
0.283
2.1
206
95
66
319
36
266
106
203
84195
4.53
0.73
0.237
2.0
225
74
81
379
40
302
117
210
84197
6.91
0.79
0.235
2.1
282
59
54
250
35
163
84198
82
162
5.02
0.75
0.247
2.0
245
71
72
306
38
164
96
187
0.305
1.8
197
72
68
445
37
527
90
177
192
84199
5.25
0.65
Average
4.68
0.86
0.245
2.0
225
68
66
319
38
220
Std Dev
98
0.69
0.18
0.030
0.3
21
10
11
40
4
76
9
13
23
23
23
28
28
28
27
28
28
26
26
26
N=
n.a. = not analyzed
n.d. = not detected
180
Appendix 2.
Normalization Factors, Partition
Coefficients, and Formulae for Crystal
Fractionation Models.
181
Table H. Non-volatile Cl chondrite values used for normalization
Values listed are parts per million (ppm)
Rb
Th
U
Sr
Ba
La
3.04
0.0339
0.0107
10.4
3.00
0.312
Ce
Nd
Sm
Eu
Tb
Y
0.813
0.603
0.197
0.074
0.047
2.06
Yb
Lu
Sc
Hf
Nb
Ta
0.210
0.0323
7.7
0.157
0.33
0.0205
Modified from Anders, E. and Ebihara, M., 1982, Solar system abundances
of the elements:
Geochimica et Cosmochimica Acta, v. 46, p. 2363-2380.
182
Table I.
Partition coefficients
Modelling Biotite Tuff/Crystal-Rich Rhyolite from Biotite Rhyolite
Sanidine
Plagioclase
Biotite
Clinopyroxene
Allanite
Zircon
32.3 %
49 %
16.5 %
2.15 %
0.027 %
0.025 %
Rb
0.9
0.065
2.1
0
0
0
Th
0.02
0.05
0.72
0.094
548
91
U
0.04
0.05
0.3
0
13
294
Sr
5.75
7.8
0.6
0
0
0
Ba
8.5
3.3
5.6
0
0
0
La
0.1
0.34
1.28
1.7
2594
7.2
Ce
0.04
0.24
1.21
2.0
2278
10
Nd
0.0375
0.19
1.08
3.5
1533
4.6
Sm
0.02
0.09
1.0
3.6
753
11.1
Eu
3.3
4.1
0.59
3.2
91
9.0
Tb
0.01
0.09
0.87
4.6
140
37
Y
0.04
0.04
1.0
4.5
119
95
Yb
0.04
0.1
0.7
3.5
37
499
Lu
0.04
0.125
0.6
5.0
33
635
Sc
0.01
0.06
5.0
44
53
59.4
Hf
0.045
0.29
0.84
0.97
28
3742
Nb
0.005
0.02
2.0
0
0
0
Ta
0.001
0.02
1.2
0.09
1.0
39.8
Modelling Ash Peak Glass from Biotite Tuff/Crystal-Rich Rhyolite
Sanidine
Plagioclase
Biotite
Alanite
76.8 %
15.1 %
7.98 %
0.12 %
Rb
0.29
0.052
2.2
0
Th
0.259
0.08
2.3
551
U
0
4.3
0.27
0
Sr
0
4.3
0.27
0
Ba
8.1
3.3
5.5
0
La
0.126
0.35
1.28
2833
Ce
0.029
0.21
1.21
2033
Nd
0.106
0.19
1.8
1800
Sm
0.013
0.09
0.99
863
Eu
5.3
3.77
0.62
91
Tb
0.023
0.09
0.66
235
Y
0.003
0.04
0.5
119
Yb
0.003
0.058
0.32
23.9
Lu
0.013
0.06
0.39
36
Sc
0.065
0.06
20
63.1
Hf
0.015
0.29
0.65
31
Nb
0.0045
0.02
1.35
0
Ta
0.01
0.02
1.35
5.1
Partition coefficients taken from Nash, W.P. and Crecraft,
trace elements in silicic magmas:
H.R., 1985, Partition coefficients for
Geochimica et Cosmochimica Acta, v. 49, p. 2309-2322, and
Mahood, G.A. and Hildreth, W., 1983, Large partition coefficients for trace elements in highsilica rhyolites:
Geochimica et Cosmochimica Acta, v. 47, p. 11-30.
183
Table I.
(continued)
Modelling Crystal-Poor Rhyolite, APEC from Biotite Tuff/Crystal-Rich
Rhyolite
Sanidine
Plagioclase
Biotite
Alanite
76.8 %
15.1 %
8.093 %
0.007 %
Rb
0.29
0.052
2.2
0
Th
0.259
0.08
2.3
551
U
0.039
0.05
0.13
13
Sr
0
4.3
0.27
0
Ba
2.12
3.3
5.5
0
La
0.126
0.35
1.28
2833
Ce
0.029
0.21
1.21
2033
Nd
0.106
0.19
1.8
1800
Sm
0.013
0.09
0.99
863
Eu
1.8
3.77
0.62
91
Tb
0.023
0.09
0.66
235
Y
0.003
0.04
0.5
119
Yb
0.003
0.058
0.32
23.9
Lu
0.013
0.06
0.39
36
Sc
0.065
0.06
20
63.1
Hf
0.015
0.29
0.65
31
Nb
0.0045
0.02
1.35
0
Ta
0.01
0.02
1.35
5.1
Partition coefficients taken from Mahood, G.A. and Hildreth, W., 1983, Large partition
coefficients for trace elements in high-silica rhyolites:
Geochimica et Cosmochimica Acta, v. 47,
p. 11-30, and Nash, W.P. and Crecraft, H.R., 1985, Partition coefficients for trace elements in
silicic magmas:
Geochimica et Cosmochimica Acta, v. 49, p. 2309-2322
184
Table J.
Analytical uncertainties associated with the data.
Element or Oxide
SiO
2
TiO2
Al
2
0
3
Fe 0
Feb 3
Fe0 (total)
Mn0
MgO
Ca0
Na90
K b
2
Sc
Uncertainty
5%
5%
5%
5%
5%
5%
5%
5%
5%
3%
15%
Zn
3%
10%
5%
12%
15%
As
Sb
5%
5%
Rb
Cs
Sr
Ba
10%
5%
12%
10%
La
Ce
Nd
Sm
Eu
3%
7%
12%
5%
5%
10%
5%
5%
5%
Cr
Co
Ni
Y
Tb
Yb
Lu
Zr
Hf
Nb
Ta
Th
U
15%
5%
10%
5%
5%
7%
185
Table K.
Recalculation of Fe0 and Fe 2 0 3 using the method of Le Maitre
(1976).
Fe0 and Fe 2 0 3 are recalculated according to the following
relationship:
Fe0/(Fe0 + Fe203) = 0.93
0.0042 Si02
0.022 (Na20 + K20)