UNLV Retrospective Theses & Dissertations
1-1-2000
Evolution of a small silicic system: The McCullough Pass Caldera, Clark County, Nevada
Aaron Lee Sanford University of Nevada, Las Vegas
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Repository Citation Sanford, Aaron Lee, "Evolution of a small silicic system: The McCullough Pass Caldera, Clark County, Nevada" (2000). UNLV Retrospective Theses & Dissertations. 1224. http://dx.doi.org/10.25669/4e8p-m2rk
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. EVOLUTION OF A SMALL SILICIC SYSTEM:
THE MCCULLOUGH PASS CALDERA,
CLARK COUNTY, NEVADA
by
Aaron Lee Sanford
Bachelor of Science University of Idaho 1997
A thesis submitted in partial fulfillment of the requirements for the
Master of Science Degree Department of Geoscience College of Sciences
Graduate College University of Nevada, Las Vegas December 2000
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thesis Approval IINTV The Graduate C ollege University of Nevada, Las Vegas
December 11______, 2000
The Thesis prepared by
Aaron Lee Sanford
E ntitled
Evolution of a Small Silicic System: The McCullough Pass Caldera.
Clark County. Nevada
is approved in partial fulfillment of the requirements for the degree of
Master of Science in Geoscience
Examination Committee Chair
Dean of the Graduate College
tee Member
Graduate Couege-Faculty Representative
PR/1017-B/1-00 u
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT
Evolution of a Small Silicic System: The McCullough Pass Caldera, Clark County, Nevada
by
Aaron Lee Sanford
Dr. Eugene I. Smith, Examination Committee Chair Professor of Geoscience University of Nevada, Las Vegas
The McCullough Pass Caldera is a small caldera in southern Nevada formed by the
eruption of the McCullough Pass rhyolites. The McCullough Pass rhyolites erupted as
three successive rhyolitic magma batches from 14.12 to 13.98 Ma and produced the
McCullough Pass caldera. Each magma batch was chemically zoned by sidewall
crystallization prior to erupting. The first magma batch erupted to produce the
McCullough Pass tuff and caused collapse of the Jean Lake caldera along a ring fracture
identified by the pattern of Jean Lake domes. The second magma batch produced the
Ramhead rhyolite and resulted in collapse of the Ramhead caldera. The third magma
batch produced dikes, domes and flows of the Capstone rhyolite, which produced a
topographic high. The topographic high prohibited Hidden Valley andésite, which
erupted from a cinder cone field surrounding the McCullough Pass caldera, from flowing
into the caldera. After volcanism had ended, preferential erosion of the McCullough Pass
rhyolites produced the present topographic low known as the McCullough Pass caldera.
in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
ABSTRACT...... ni
LIST OF FIGURES AND TABLES...... vi
ACKNOWLEDGEMENTS...... vii
CHAPTER 1 INTRODUCTION...... 1 Purpose of Thesis ...... 1 Regional Setting ...... 2 Previous Work ...... 2
CHAPTER 2 MCCULLOUGH RANGE...... 7 Stratigraphy ...... 7 Volcanic Centers in the McCullough Range ...... 9 Structure...... 10
CHAPTER 3 THE MCCULLOUGH PASS CALDERA...... 13 Physical Characteristics ...... 13 Intracaldera Stratigraphy ...... 16
CHAPTER 4 GEOCHRONOLOGY...... 18
CHAPTERS GEOCHEMISTRY...... 33
CHAPTER 6 PETROGRAPHY...... 44
CHAPTER 7 PETROGENESIS ...... 46 Origin of Basaltic Magma ...... 46 Origin of RhyoUtic Magmas ...... 47 Petrogenetic Model ...... 49 Magma Batch Models ...... 50
CHAPTER 8 PHYSICAL EVOLUTION OF THE MCCULLOUGH PASS CALDERA...... 69
CHAPTER 9 COMPARISON OF MPC TO OTHER SILICIC SYSTEMS...... 72 Small Süicic Systems...... 72 Large Silicic Systems...... 74
IV
Reproduced witti permission of ttie copyrigfit owner. Furtfier reproduction profiibited witfiout permission. CHAPTER 10 CONCLUSIONS...... 79
REFERENCES CITID...... 80
APPENDIX 1 ANALYTICAL TECHNIQUES...... 87
APPENDIX 2 ■’W ^ A tDATA ...... 93
APPENDIX 3 GEOCHEMISTRY DATA...... 99
APPENDIX 4 ISOTOPE DATA...... 107
APPENDIX 5 DISTRIBUTION COEFFICIENTS...... 109
VITA...... I ll
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF nOURES
Figure 1 Location Map ...... 5 Figure 2 Geologic map of the central McCullough Range ...... 6 Figure 3 Stratimphy ...... 12 Figure 4 ‘*°Ar/^ Ar inverse isochron for MPC-1 ...... 23 Figure 5 Probability diagram for MPC-1 ...... 24 Figure 6 ‘*°Ar/^^Ar inverse isochron for MPC-19 ...... 25 Figure 7 Probability diagram for MPC-19 ...... 26 Figure 8 ‘‘°Ar/^^Ar inverse isochron for MPC-42 ...... 27 Figure 9 Probability diagram for MPC-42 ...... 28 Figure 10 "*°Ar/^^Ar inverse isochron for MPC-44 ...... 29 Figure 11 Probability diagram for MPC-44 ...... 30 Figure 12 ■‘°Ar/^^Ar inverse isochron for MPC-43 ...... 31 Figure 13 Probability diagram for MPC-43 ...... 32 Figure 14 AFM diagram...... 35 Figure 15 Rock classification diagram ...... 36 Figure 16 Harker diagrams...... 37 Figure 17 La versus Rb ...... 38 Figure 18 Enrichment diagram ...... 39 Figure 19 Trace elements normalized to chondrite abundances ...... 40 Figure 20 Trace elements normalized to primitive mantle ...... 41 Figure 21 ®^Sr/“ Sr versus Si02...... 42 Figure 22 Epsilon Nd versus SiOz ...... 43 Figure 23 Pb signature of Mojave crustal province ...... 55 Figure 24 Ab-Q-Or ternary diagram ...... 56 Figure 25 Chemical and physical evolution of the McCullough Pass Caldera ...... 57 Figure 26 Fractional crystallization model for McCullough Pass Tuff ...... 63 Figure 27 Fractional crystallization model for McCullough Pass Tuff ...... 64 Figure 28 Fractional crystallization model for Ramhead Rhyolite ...... 65 Figure 29 Fractional crystallization model for Ramhead Rhyolite ...... 66 Figure 30 Fractional crystallization model for Capstone Rhyolite ...... 67 Figure 31 Fractional crystallization model for Capstone Rhyolite ...... 68
VI
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS
While completing this work, I have received assistance, financial and otherwise,
fi'om many people. This work would not have been possible without their help. 1 would first like to thank my family for their love and support It has meant a great deal. I need to thank my advisor. Dr. Eugene Smith. His open-door policy and eagerness to help has
made my task much easier. 1 would also like to thank my thesis committee, particularly
for making time to read my thesis during a busy time of year. The Graduate Student
Association (GSA) is thanked for providing two grants. The UNLV Geoscience
Department has provided scholarship money and a friendly, encouraging atmosphere and
I thank the faculty, staff and students for these valuable contributions. I would like to thank Dr. Terry Spell for providing access to the Nevada Isotope Geochronology Lab
(NIGL). NIGL provided valuable data, which greatly enhanced this work. 1 give props
to my friends, buddies, pals, cronies and partners in crime for providing many
memorable moments from my time spent in fabulous Las Vegas.
vu
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1
INTRODUCTION
Purpose of Thesis
Relatively few small calderas have been studied or even recognized. This may be
due to a general lack of interest among volcanologists or burial of small volume volcanic
systems by large-volume ash-flow sheets (Lipman, 1984). The McCullough Pass caldera
(MPC) offers an excellent opportunity to document a small silicic center and enhance
the knowledge of these systems. The purpose of this study is to create a generalized
model for the physical and chemical evolution of a small silicic system and compare this
model to models for the formation of large silicic systems. This was done by
documenting physical and chemical characteristics of the MPC and comparing them to
other small calderas to determine which characteristics are typical. Typical
characteristics of small silicic systems were then compared to the Valles and Long
Valley calderas, classic larger volume silicic systems, to determine if small and large
silicic systems are similar. This comparison is important because, in general, large
silicic systems are better understood than small silicic systems. If similarities can be
documented, knowledge of large silicic systems may be applied to studies of other small
silicic systems and may aid in mineral exploration and volcanic hazard assessment
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Regional Setting
The McCullough Pass Caldera (MPC) lies in the central McCullough Range in Clark
County, Nevada (Schmidt, 1987) (Figures land 2). The McCullough Range is in the
southern Basin and Range province on the western margin of the Colorado River
Extensional Corridor (CREC), a 70-100 km wide area of large magnitude extension
(Howard and John, 1987). The range trends northeast and extends from the Las Vegas
Valley south to the New York Mountains in California. Although the McCullough
Range was extended only 20 percent during the Cenezoic (Weber and Smith, 1987), the
River Mountains just to the north were extended 40% (Smith, 1988) and the Eldorado
Mountains to the east were extended over 100% (Anderson, 1971). The small amount of
extension in the McCullough Range has provided a unique opportunity to study an intact
caldera in a highly extended terrane.
Previous Work
The McCullough Range was not studied in detail until recently due to its lack of
mineral wealth. Hewett (1956) began mapping the Ivanpah quadrangle, including the
McCullough Range, in 1921. He mapped the McCullough Range as Precambrian
granite and gneiss and Tertiary "sediments and flows." Hewett described several
stratigraphie sections in and around the McCullough Pass caldera although he did not
recognize the caldera. In his stratigraphie sections, he was the first to describe rocks that
were later named the Eldorado Valley volcanics. Tuff of Bridge Spring, McCullough
Pass tuff and the Hidden Valley andésite. Longwell (1965) subdivided the volcanic
section in the Eldorado Range into the Patsy Mine, Golden Door and Mount Davis
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. i volcanic rocks, but mapped the northern McCullough Range as Tertiary undivided.
Bingler and Bonham (1973) mapped the northern and central McCullough Range as
Patsy Mine and Mount Davis volcanics with the McCullough Pass volcanic section
designated as Mount Davis volcanics. They did not recognize the Tuff of Bridge Spring
in the McCullough Range although they mapped it to the south in the Lucy Gray and
Highland Ranges. Work in the central McCullough Range by Kohl (1978) focused on
the relationship between the Tertiary volcanic rocks and caraotite occurrences in caliche
in the Hidden Valley area. He concluded that the source of the uranium is the Tuff of
Bridge Spring. Kohl (1978) mapped dikes, domes and flows within and around the
McCullough Pass caldera but did not recognize the caldera. He also mapped the Erie
Tuff which he correlated to the Tuff of Bridge Spring. Schmidt (1987) mapped the
central McCullough Range at a scale of 1:24,000 and first recognized and described the
McCullough Pass caldera. She suggested that the caldera formed as a result of the
eruption of a rhyolite ash-flow tuff which she named the McCullough Pass Tuff.
Schmidt mapped intracaldera units as andésite, rhyolite and caldera fill facies (ash-flow
and surge and volcaniclastic deposits) and located dikes and domes within the caldera.
Schmidt also completed major and trace element chemistry on 25 samples. Her work
showed that the McCullough Pass rhyolites are not genetically related to basalts of the
central McCullough Range but she suggested that they formed, at least in part, as the
result of fractional crystallization. Textural evidence such as skeletal olivine xenocrysts
in dacites and andésites, and clinopyroxene xenocrysts rimmed by hornblende in
rhyolite, indicated to Schmidt (1987) that magma mixing or assimilation accompanied
fractional crystallization. Bridwell (1991), while studying the Sloan volcanic section in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 the central McCullough Range, obtained an ^ÛAr/^^Ar date of 15.23 Ma from a sanidine
separate from the Tuff of Bridge Spring. This date was an important contribution to the
regional geology because the Tuff of Bridge Spring occurs over a large area in southern
Nevada and Arizona, including the McCullough, and Eldorado Mountains. Boland
(1996) mapped the volcanic rocks of the northern McCullough Range, separating them
into four informal units: Ivan Canyon andésite. Farmer Canyon andésite. Colony dacite
and the Fog Ridge andésite. Boland (1996) suggested that the Ivan Canyon andésite
correlates with the Eldorado Valley and Patsy Mine volcanic section and the Fog Ridge
andésite with the Hidden Valley and Mount Davis sections.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Frenchman Mountain
Las Vegas Nevada
Henderson Caldera
Sloan Sag
Highland Spring Range
Figure 1 : Map showing the location of the McCullough Pass Caldera (MPC). Shaded box on inset represents the location map.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 115'la 115! 09" Explanation Thv-Hidden Valley Andésite Tmp-McCollou^ Pass Rhyolite Tmpi-Intracaldera Rhyolite 3 and Associated Domes Tmpo-McCulIough Pass Tuff Tpv-Pumice Mine Vslcanic Rocks Tbs-'Iuft of Bridge Spring Tev-Hdorado Vallqr Wlcanic Rocks PC-Precambrian Basement “ Outline of McCuDugh Pass Caldera Normal Fault
0.5 km Figure 2: Geology of the central McCullough Range (modified firom Schmidt, 1986). SI, S2 and S3 are sampled sections of the McCullou^ Pass Tuff
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER!
MCCULLOUGH RANGE
Stratigraphy
In the central McCullough Range Miocene volcanic rocks unconformly overlie
Precambrian crystalline rocks (Figures 2 and 3). The Precambrian rocks, comprised of
1.7 Ga granitic paragneiss and monzogranite, form a buttress on the western margin of
the McCullough Range (Figures 2 and 3). The buttress controlled deposition of volcanic
rocks throughout Miocene time (Schmidt, 1987). No Paleozoic or Mesozoic rocks are
exposed within the McCullough Range in the immediate vicinity of the MPC.
Two conglomerate units overlie Precambrian basement and underlie the Tertiary
volcanic section (Figures 2 and 3). Both were deposited in chaimels on the flanks of the
Precambrian buttress. The first is composed of well rounded to subangular clasts of
quartzite and carbonate and the second is composed of angular to subrounded clasts of
Precambrian basement rocks (Schmidt, 1987). Work by Herrington (in progress)
suggests that the conglomerates were deposited between about 60 and 18.5 Ma.
At the base of the volcanic section in the McCullough Range are volcaniclastic rocks
and flows of andésite and dacite of the Eldorado Valley volcanic section (Schmidt,
1987) (Figures 2 and 3). The Eldorado Valley volcanic section consists of two
members. The lower member lies unconformably on Precambrian crystalline rocks and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. s is composed of flows of hornblende andésite, olivine basaltic andésite and dacite
(Schmidt, 1987). These flows are locally interbedded with overlying breccia units.
Overlying and locally interbedded with Eldorado Valley flows is poorly sorted and
matrix supported breccia containing subrounded to angular breccia clasts ranging fi'om 1
mm to 4 m in size (Schmidt, 1987). Clasts consist of flow-banded dacite, basalt,
andésite and Precambrian crystalline basement rock. The upper member of the
Eldorado Valley volcanic section (200 m thick) is composed of flows of olivine and
pyroxene basalt and andésite (Schmidt, 1987).
The Tuff of Bridge Spring is a moderately to densely welded dacite ash-flow tuff
originally described by Anderson (1971) and studied in detail by Morikawa (1993)
(Figures 2 and 3). The tuff is densely welded and vitric at its base and grades upward
into a moderately welded tuff. The Tuff of Bridge Spring contains up to 25% andésite
clasts and phenocrysts of sanidine, plagioclase, quartz and biotite. In the McCullough
Range, the Tuff of Bridge Spring is thickest (145 m) just north of the MPC. The Tuff of
Bridge Spring forms the wall of the MPC in most areas (Figure 3).
The Pumice Mine basalt is composed of andésite flows, basalt flows and breccia
(Figures 2 and 3), and erupted from a cinder cone field in the central McCullough Range
(Smith, personal communication). In hand sample, the Pumice Mine basalt contains
olivine phenocrysts altering to iddingsite set in a fine-grained matrix. Several outcrops
show well-developed Leisegang banding.
The McCullough Pass tuff (Figures 2 and 3) is a nonwelded, vitric rhyolite tuff that
contains phenocrysts of sanidine, plagioclase, biotite and quartz, and rhyolite clasts in a
matrix of pumice and glass shards. Detailed observations of three sections of the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 McCullough Pass tuff indicate three eruptive units separated by conglomerate, sandstone
and reworked tuff. Each eruptive unit consists of one or more ash-flow deposits up to 20
m thick.
The Hidden Valley volcanic section consists of a lower section of dacite flows and
domes and an upper section of andésite flows (Schmidt, 1987) (Figures 2 and 3).
Andésite flows, up to 3 m thick, contain pyroxene and olivine phenocrysts and are
massive to vesicular. The dacite domes are up to 400 m in diameter (Schmidt, 1987).
The Hidden Valley section is 65 m thick at McCullough Pass just south of the MPC and
over 150 m thick north and east of the MPC (Figure 3) (Schmidt, 1987). Although the
Hidden Valley section crops out to the north, south and east of the MPC, it does not
occur within the MPC (Figure 3).
Volcanic Centers in the McCullough Range
In addition to the MPC, the Henderson caldera and the Sloan Sag are other volcanic
centers in the McCullough Ratige. Bridwell (1991) identified the Sloan Sag in the
Hidden Valley area (Figure 1). The Sloan Sag is a 13.5 km diameter volcanotectonic
depression that formed during eruption of the Sloan volcanic section. The section
consists of the Mount Hanna andésite, the Center Mountain dome complex and the
Mount Sutor dacite. The Mount Hanna andésite and the Center Mountain dome
complex lie above the Hidden Valley volcanic section but exposures do not overlap;
therefore, their relative ages are not known. The Mount Sutor dacite lies above the
Center Mountain dome complex and the Mount Hanna andésite. Eruption of the Mount
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 Hanna andésite, as a hot, dry aphyric lava, by lava fountaining may be responsible for
the formation of the Sloan Sag (Bridwell, 1991).
Switzer (1993) described the Henderson caldera in the northern McCullough Range
about 25 km north of the MPC (Figure 1). The caldera forms a semicircle open to the
north with a diameter of 10 km. Units related to the Henderson caldera are of late
Miocene age and include flows and domes of dacite and andésite, ash-flow and surge
deposits, basalt flows and debris flows (Switzer, 1993). Rocks of the Henderson caldera
are stratigraphically equivalent to the Sloan volcanic rocks in the central McCullough
Range (Smith and others, 1993).
Structure
Regional extension in the Basin and Range Province resulted in the tilting and
faulting of the McCullough Range. In the central McCullough Range, northeast-
striking, east-dipping normal faults cut north-striking normal faults (Anderson and
others, 1985). Schmidt (1987) recognized the two generations of faults observed by
Anderson and others (1985) but discussed them in relation to the detailed stratigraphy of
the central McCullough Range. The north-striking normal faults cut the Eldorado
Valley volcanic section and Tuff of Bridge Spring and controlled emplacement of the
rhyolite dikes within and near the MPC. Schmidt (1987) also observed that the younger
northeast-striking normal faults are responsible for up to 40° of rotation of the Hidden
Valley Volcanic section. Offset along north-striking normal faults increases southward
and several faults have as much as 90 m of displacement near the southern extent of the
volcanic section. Although faults in the northern McCullough Range generally have less
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11 than 18 m of displacement, Boland (1997) located a fault in the northern McCullough
Range with more than 400 m of offset. Generally, northeast-striking normal faults have
minimal offset (Anderson and others, 1985).
Volcanic strata dip 25 to 40 degrees to the east in the central part of the range and 15
to 25 degrees to the west in the northern part of the range (Anderson and others, 1985;
Boland, 1997) forming a synform extending through the northern margin of the MPC
and the Sloan Sag (Figure 1 ). The Henderson Caldera in the northern McCullough
Range also lies near the axis of this synform. Boland (1997) suggested a genetic
relationship between this structure and the location of volcanic centers; either volcanism
controls structural geometry or structural geometry controls the occurrence of volcanic
centers. According to Anderson and others (1985), extension within the McCullough
Range cannot alone explain the tilting of the strata; a west-dipping normal fault east of
the McCullough Range is required. Weber and Smith (1987) proposed a west-dipping
normal fault in the Eldorado Valley (the Eldorado Valley fault) that soles into a
detachment fault that projects beneath the Eldorado Mountains and the McCullough
Range. The presence of this fault was recently supported by a gravity survey
(Langenheim and Schmidt, 1996). The Eldorado Valley fault separates the highly
extended Eldorado Range from the McCullough Range. Weber and Smith (1987)
explained the minor internal deformation of the McCullough Range by projecting the
detachment fault deep beneath the range. The McCullough Range, therefore, was
transported to the west as a relatively intact block.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD O Q. C g o.
■D CD
C/) C/) Sloan Volcanics
■D8 Hidden Valley Andésite , 1 3 . 9 8 / % ) Capstone Rhyolite
CD
3. 3 " CD McCullough Pass Rhyolite ----- ■DCD Ramhead Rhyolite O Q. C a Pumice Mine Basalt 3O Pumice Mine Basalt "O o Tuff of Bridge Spring
CD Q. Ù4.ii)(:YA Jean Lake Rhyolite Eldorado Valley Volcanics \ / • ■ • A / • ■••■II , JO ■D CD Unnamed Conglomerate
Figure 3: Stratigraphy of the McCullough Range (Schmidt, 1986) and McCullough Pass Caldera. Ages are in millions of years. _ CHAPTERS
THE MCCULLOUGH PASS CALDERA
Physical Characteristics
The MPC was formed by the eruption of the McCullough Pass tuff and Ramhead
rhyolite at about 14.10 Ma (see Geochronology Section) (Plate I). It is a small caldera
(2.4 km by 1.5 km in diameter) when compared to other calderas in the Basin and Range
Province where calderas are typically larger than 6 km in diameter and 75% exceed 18
km (Walker, 1984). The caldera has an elliptical shape elongate east-west with
embayments to the northwest and southeast (Plate 1). Intracaldera units include rhyolite
and basalt flows, high-silica rhyolite ash-flow tuff surge deposits, rhyolite domes,
rhyolite and basalt dikes and volcaniclastic units.
Rhyolite domes lie in a circular pattern in the eastern half of the caldera with five
domes forming a prominent ridge just inside the eastern caldera wall. Domes also
cluster in the northwestern embayment Dikes generally trend north-south and occur
only in the central and western portions of the caldera (Plate 1 ).
The caldera wall is composed dominantly of the Tuff of Bridge Spring (Plate 1).
The Eldorado Valley basalt also makes up a small part of the northern wall. The caldera
wall is well exposed except in the embayments where the exact location of the wall is
13
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 difficult to determine. Megabreccia blocks (4 to 6 m in size) of the Tuff of Bridge
Spring lie on intracaldera units just inside the southwestern wall (Plate 1, location A).
The MPC is a topographic low and shows no evidence of resurgent doming, although
intracaldera units could obscure a resurgent dome. The caldera is surrounded by
escarpments 50-100 m higher than the highest points within the caldera. These ridges
are capped by 65-150 m of Hidden Valley basalt, which does not crop out within the
MPC (Figure 2). Two peaks, eastern and western, occur within the caldera (Plate 1).
The peaks contain the most complete sections of intracaldera stratigraphy and the names
“eastern peak” and “western peak” will be used throughout the thesis to describe these
locations.
Intracaldera Stratigraphy
Volcanic rocks of the MPC are exposed inside and outside the caldera. Intracaldera
units are herein named the Jean Lake rhyolite. Pumice Mine basalt, Ramhead rhyolite
and the Capstone rhyolite, and are discussed in stratigraphie order.
Jean Lake Rhyolite
The Jean Lake rhyolite forms the base of the section and consists of rhyolite flows
and domes, breccia and ash-flow deposits (Figure 3). The rhyolite of the domes contains
phenocrysts of sanidine, plagioclase, quartz and biotite in a glassy matrix. The domes
exhibit well-developed flow foliation and black, glassy margins. The Jean Lake domes
crop out in a circular pattern in the eastern part of the caldera (Plate 1), and six of the
domes form a ridge just inside the eastern caldera wall. Pyroclastic surge deposits ramp
onto a Jean Lake dome just inside the eastern caldera wall, and a breccia unit forms an
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15 apron at the base of the five rhyolite domes along the eastern caldera wall. The breccia
contains blocks of tuff, flow-banded rhyolite and andésite in a fine-grained matrix. In
the northeastern comer of the MPC the breccia is overlain by Pumice Mine basalt (Plate
1, location B). Rhyolite flows are exposed in the northwest and northeast comers of the
caldera. The flow in the northeast comer is 1-2 m thick and ramps onto the caldera wall
(Plate 1, location C). The flow contains sanidine and plagioclase phenocrysts and
andésite clasts set in a glassy matrix. The flow in the northwest comer has a minimum
thickness of 2 m (the bottom of the flow is not exposed at the surface). In hand sample,
the flow contains phenocrysts of sanidine and plagioclase and quartz, as well as andésite
and pumice clasts. Both flows are overlain by Pumice Mine basalt (Plate 1).
Pumice Mine Basalt
Schmidt (1987) described two sections of mafic rocks in the vicinity of the MPC; the
Pumice Mine basalt and intracaldera andésite. During field work completed for the
present study the author observed basalt flows north of the caldera, mapped as
intracaldera andésite by Schmidt (1987), lying stratigraphically above the Tuff of Bridge
Spring and interbedded with the McCullough Pass tuff (Plate 1). These field
relationships indicate that Schmidt’s intracaldera andésite is the same unit as the Pumice
Mine basalt.
Stratigraphy and crosscutting relationships indicate that Pumice Mine basalt was
erupted throughout the development of the caldera. Within the caldera. Pumice Mine
basalt lies on Jean Lake rhyolite and beneath Ramhead rhyolite and is an excellent
stratigraphie marker (Figure 3, Plate 1). In addition, two Capstone dikes, which
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 represent the last volcanic activity within the caldera, are composite and include
separate intrusions of basalt and rhyolite.
Outcrops of Pumice Mine basalt in the western part of the MFC consist of one or
more flows that are less than 5 m thick. The thickest section of Pumice Mine basalt
(150 m) forms part of the eastern peak of the MPC, and lacks evidence of flow
boundaries or agglomerates. A small vent occurs at the top of this outcrop (Plate 1,
location D). The vent is less than 10 m in length and 1-2 m wide and defined by red
vesicular basalt and numerous small bombs. Similar vents are found associated with
most flows of Pumice Mine basalt within the caldera (Plate 1).
Ramhead Rhvolite
The Ramhead rhyolite crops out throughout the caldera (Plate 1). It is composed of
a rhyolite flow, ash-flowtuff and surge deposits interbedded with volcaniclastic deposits
and Pumice Mine basalt flows. The Ramhead rhyolite lies on Jean Lake rhyolite where
Pumice Mine basalt is not present (Figure 3). The rhyolite flow contains phenocrysts of
sanidine, plagioclase and biotite in a pumiceous matrix and lies conformably on Pumice
Mine basalt in the northeast comer of the caldera (Plate 1). In the section forming the
eastern peak, Ramhead rhyolite consists of several meters of thinly bedded
volcaniclastic units overlain by a 1-2 m thick pyroclastic surge deposit with well-
developed bedding, cross bedding and bomb sags. An ash-flow tuff containing four flow
units, each 2-3 m thick, interbedded with thin (~0.1 m) deposits of air-fall tuff or thinly
bedded volcaniclastic units lies above the surge deposit. The ash-flow tuff and
volcaniclastic units pinch out to the west as they near Jean Lake rhyolite domes in the
central part of the MPC (Plate 1, location E). The western peak has a section of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17 Ramhead rhyolite similar to the eastern peak but the section of volcaniclastic units that
lies below the ash-flow tuff is thicker (20 m). Ash-flow tuffs contain phenocrysts of
sanidine, plagioclase and biotite, sparse pumice clasts and andésite xenoliths in a fine
grained matrix. Air-fall tuffs are well bedded, fine grained, aphyric and contain rare
bomb-sags. Volcaniclastic units consist of reworked ash-flow tuff, andésite clasts and
rare clasts of Precambrian crystalline basement
Capstone Rhvolite
The Capstone rhyolite consists of rhyolite domes, flows and dikes. The flows lie
conformably on the Ramhead rhyolite while the dikes and domes cut the Jean Lake
rhyolite. Pumice Mine basalt and the Ramhead rhyolite (Figure 3). Capstone rhyolite
domes contain sanidine and plagioclase phenocrysts in a glassy matrix. They form a
dome complex in the northwestern embayment and cluster in the south-central part of
the MPC (Plate 1) along the same trend as the westermnost Jean Lake domes. A
Capstone rhyolite flow, up to 50 m thick, caps the eastern and western peaks and lies
conformably on the Ramhead rhyolite. The flows are massive and contain large blocks
of devitrified pumice, up to 0.5 m in size, in a glassy matrix. Both the matrix and
pumice blocks contain large (1 cm) sanidine and plagioclase phenocrysts. Dikes occur
within the caldera and up to 200 m to the west. Along the western edge of the western
peak, an arcuate non-foliated, vitric Capstone rhyolite dike cuts the Pumice Mine basalt
(Plate 1, location F). Two Capstone dikes intruded along the same path as Pumice Mine
basalt forming composite dikes. (Plate 1, locations G and H). Evidence for magma
commingling is absent fi^om these dikes although rhyolite at its contact with basalt
commonly contains clasts of basalt.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4
GEOCHRONOLOGY
Sanidine and plagioclase phenocrysts from a Jean Lake rhyolite dome (MPC-1),
Capstone rhyolite dome (MPC-19) and from the lower (MPC-42), middle (MPC-44) and
upper (MPC-43) eruptive units of section 2 (Figure 3) of the McCullough Pass tuff were
dated using the Ar /^^Ar single crystal laser fusion method at the Nevada Isotope
Geochronology Laboratory (NIGL). Ages were calculated using three different
methods: mean, weighted mean and inverse isochron. Weighted mean ages were
calculated after omitting ages that lay outside of the 95% confidence interval (2 a) as
determined from the mean and standard deviation.
Inverse isochrons were plotted using the York (1969) regression routine. Isochron
ages are considered valid if the mean squared weighted deviation (MSWD) is below a
critical value, which is dependant on the number of data points (n) used to calculate the
isochron (Wendt and Carl, 1991). If an isochron has a MSWD above the critical value it
is considered invalid (often referred to as an “errorchron”). If the critical value is
exceeded the data point with the largest weighted residual is omitted and the isochron is
recalculated. This process is repeated until an isochron with a MSWD below the critical
value is calculated. The weighted residual is determined using the precision and
distance of a data point or “scatter” from the isochron. A data point with low precision
18
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 can have a large scatter and still have a low weighted residual, while a data point with
very high precision will have a high weighted residual unless it plots extremely close to
the isochron.
Mean, weighted mean and inverse isochron ages are presented but inverse isochron
ages were used in this study. Inverse isochron ages are preferred because they calculate
the *°Ai/^Ax ratio, which is needed to analyze for excess argon. Average and weighted
mean calculations use apparent ages which assume an atmospheric ‘*°Ar/^^Ar ratio. The
presence of excess argon is particularly important for these samples since some of the
dated crystals were plagioclase. Plagioclase has low potassium content; therefore, any
excess argon will have a larger affect on the sample age. Analytical procedures are
presented in Appendix 1 and raw geochronological data are presented in Appendix 2.
All errors are given at lo.
MPC-1, a Jean Lake rhyolite dome, yielded ages from 14.06 to 14.16 Ma with an
average of 14.10 + 0.04 Ma (n=10), a weighted mean age of 14.10 + 0.04 Ma (n=10) and
an inverse isochron age of 14.11 ± 0.12 Ma (n=10) (Table 1) (Figure 4). An '*°Ar/^^Ar
ratio o f306 + 44, indistinguishable from the atmospheric ratio of295.5, was calculated
from the inverse isochron indicating that the crystals did not contain excess argon.
Individual crystal ages indicate that no xenocrysts were dated (Figure 5).
MPC-19, a Capstone rhyolite dome, yielded ages from 13.85 to 14.02 Ma with an
average age of 13.93 + 0.04 Ma (n=10), a weighted mean age of 13.94 + 0.038 Ma
(n=10) and an inverse isochron age of 13.98 +_0.1 Ma (n=10) (Table 1) (Figure 6 ). An
“°Ar/^^Ar ratio o f299 + 22, indistinguishable from the atmospheric ratio o f295.5, was
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 given by the inverse isochron indicating that the crystals did not contain excess argon.
Individual crystal ages indicate that no xenocrysts were dated (Figure 7).
MPC-42, the lowermost sample from a section of the McCullough Pass tuff, yielded
three crystal populations with individual ages from 13.97 Ma to 1.5 Ga. The juvenile
(youngest) population yielded an average age of 14.11 + 0.06 Ma (n=7), a weighted
mean age of 14.11 + 0.02 Ma (n=7) and an inverse isochron age of 14.11 ± 0.06 Ma
(n= 6 ) after omitting one outlier (Table 1) (Figure 8 ). An “*°Ar/^®Ar ratio of283 + 13,
indistinguishable from the atmospheric ratio o f295.5, was calculated from the inverse
isochron indicating that the crystals did not contain excess argon. A probability diagram
shows that in addition to the juvenile population described above, two xenocrystic
populations were dated (Table 2) (Figure 9). A population of three xenocrystic crystals
gave a mean age of 14.77 + 0.26 Ma and an older population of six xenocrystic crystals
gave a mean age of 1.24 + 0.16 Ga (Table 2).
MPC-44, collected from the middle of a section of the McCullough Pass tuff
(between MPC-42 and MPC-43), yielded three crystal populations with individual ages
from 14.08 Ma to 1.5 Ga. The juvenile population yielded an average age of 14.25 + 0.1
Ma (n=9), a weighted mean age of 14.18 + 0.02 Ma (n= 6 ) and an inverse isochron age of
14.12 + 0.13 Ma (n=9) (Table 1) (Figure 10). An ‘‘“Ar/^^Ar ratio of 281 + 12,
indistinguishable from the atmospheric ratio o f295.5, was given by the inverse isochron
indicating that the crystals did not contain excess argon. A probability diagram shows
that in addition to the juvenile population described above, two xenocrystic populations
were dated (Table 2) (Figure 11). A population of six xenocrystic crystals gave a mean
age of 15.12 + 0.12 Ma and an inverse isochron age of 15.13 + 0.1 Ma (n= 6 ) and a
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 second population of six xenocrystic crystals gave a mean age of 1.32 + 0.38 Ga (Table
2).
MPC-43, the uppermost sample collected from a section of the McCullough Pass
tuff, yielded an average age of 14.15 + 0.25 Ma (n=16), a weighted mean age of 14.09 +
0.03 Ma (n=16) and an inverse isochron age of 14.09 + 0.09 Ma (n=12) after omitting
four outliers (Table 1) (Figure 12). An '‘°Ar/^^Ar ratio o f261 + 24, indistinguishable
from the atmospheric ratio o f295.5, was given by the inverse isochron indicating that
the crystals did not contain excess argon. Individual crystal ages indicate that no
xenocrysts were dated (Table 2) (Figure 13).
Inverse isochron ages from all samples are within 1 sigma, indicating they are
statistically the same age (Table 1); however, the Capstone rhyolite dome (MPC-19)
with an age of 13.98 +_0.1 Ma does apper to be younger than the other four samples.
This relationship is in agreement with the intracaldera stratigraphy where the Capstone
rhyolite is the uppermost unit.
The single i rystal laser fusion technique allowed individual xenocryst ages to be
obtained and showed two separate populations for the lower (MPC-42) and middle
(MPC-44) eruptive units of the McCullough Pass tuff, but no xenocrystic populations for
the upper unit (MPC-43). The ages of the xenocrystic populations (Table 2) are
consistent with ages of Precambrian crystalline basement and the Tuff of Bridge Spring
(Bridwell, 1991).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22 Table l-'*°Ar/^®Ar single crystal laser fusion ages of five samples from the McCullough Pass volcanic section (All ages and errors are in Ma).
Inverse Average Weighted Sample 1 a 1 a Isochron 1 CT Age Mean Age Age
MPC-1 14.10 0.04 14.10 0.04 14.11 0 . 1 2
MPC-19 13.93 0.04 13.94 0.04 13.98 0 . 1
MPC-42 14.11 0.06 14.11 0 . 0 2 14.11 0.06
MPC-43 14.15 0.25 14.09 0.03 14.09 0.09
MPC-44 14.25 0 . 1 14.18 0 . 0 2 14.12 0.13
Table 2-‘*°Ar/^^Ar single crystal laser fusion ages of two populations of xenocrysts from the McCullough Pass Tuff. Miocene ages and errors are Ma. Proterozoic ages and errors are Ga.
Inverse Sample Population Average Age 1 a Isochron 1 CT Age Miocene Xenocrysts 14.77 0.26 NA NA (Ma) MPC-42 Proterozoic Xenocrysts 1.24 0.16 NA NA (Ga) Miocene Xenocrysts 15.12 0 . 1 2 15.13 0 . 1 (Ma) MPC-44 Proterozoic Xenocrysts 1.32 0.38 NA NA (Ga)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23
0.004
MPC-1 Age=14.Il +0.12 Ma 0.003 ^Ary^Ar=306±44 MSWD=0.1 (n=10)
'Ar/°Ar
0.002
0.001
0 .0001— 0.00 0.04 0.08 0.12 0.16 0.20 ''A r r A r Figure 4; Inverse isochron for MPC-1. Arrow represents atmospheric “Ar/"Ar.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24
es
(U 00 < u eu 2
Xi p
«N 3AUBp'y[
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25
0.004
MPC-19 Age=I3.98 ±0.10 Ma 0.003 "°ArrAr=299±22 MSWD=0.1 (n=10)
Ar/^ Ar
0.002
0.001
0. 0001— 0.00 0.04 0.08 0.12 0.16 0.20 ''A r r A r
Figure 6 ; Inverse isochron for MPC-19. Arrow represents atmospheric ^Ar/^Ar.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26
O
o vd
o v i TO
2 (U OÛ < o
u c. 2 à £ S CO o es cn = 6
S i S 3 •8
o 2 es S) A^^iqgqojj 3Aub [3>i
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27
0.004
MPC-42 Age=14.11 ±0.06 Ma 0.003 ^Ar/“Ar=283 ± 13 MSWD=2.3 (n=6)
0.002
0.001
0.000 0.00 0.04 0.08 0.12 0.16 0.20 39
Figure 8 : Inverse isochron for MPC-42. Arrow represents atmospheric “Ar/“Ar.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28
O fN
o O o<
o S i oc
o
t— Xl[(iqEqojj 3AqB[3^
(U Où o < \o es TT ù Cl. 2
CO 2 T3
J3 ce -8 c\ o 2 3 m SO yQ^iqsqojj 3AqB|3^
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29
0.004
MPC-44 Age=14.12 ± 0.13 Ma 0.003 ^Ar/^"Ar=281 ± 12 MSWD=0.4 (n=9)
0.002
0.001
.00 0.04 0.08 0.12 0.16 0.20 39
Figure 10: Inverse isochron for MPC-44. Arrow represents atmospheric “Ar/"Ar.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. "OCD O Q. C g Q.
"O CD
C/) C/)
8 ci'
X i Proterozoic Xenocrysts 3 3 " CD è u "OCD O Q. I C a g 3O "O O
CD Q. 0,0 0.4 0,8 1,2 1,6 Age (Ga)
"O CD / w 13,0 14,0 15,0 16,0 17,0 18.0 19.0 20.0 C/) C/) Age (Ma) Figure 11: Probability diagram for MPC-44, u; O 31
0.004
MPC-43 Age=14.09 ± 0.09 Ma 0.003 ^Ar/'"Ar=261±24 MSWD=1.7 (n=12)
0.002
0.001
.00 0.08 0.12 0.160.04 0.20
Figure 12: Inverse isochron for MPC-43. Arrow represents atmospheric “Ar/”Ar.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. es
&
Ô a. 2
E I en Xi CO ■§
fN i)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTERS
GEOCHEMISTRY
Rocks of the MFC are calc-alkaline and range in composition from basalt to rhyolite
(Figures 14 and 15). MgO, FezOs, AI 2 O3 , CaO, Ti0 2 , P2 O5 and MnO decrease with
increasing Si 0 2 while K 2O and NajO increase slightly (Figure 16). Major elements
define mafic and felsic sample groups with two hybrid samples lying between the groups
(Figure 16). The mafic group consists of the Pumice Mine basalt and a basalt dike that
cuts the Capstone rhyolite. The hybrid samples are Pumice Mine basaltic trachyandesite
and trachyandesite. Rhyolites of the Jean Lake rhyolite, Ramhead rhyolite. Capstone
rhyolite and McCullough Pass tuff (collectively referred to as the McCullough Pass
rhyolites) comprise the felsic group. Major and trace element data are presented in
Appendix 3.
McCullough Pass rhyolites fall into two groups on plots of LREE (Figure 17). The
gap between the two groups is most pronounced for La; therefore, the groups are
referred to as either high La or low La. Three sampled sections (Figure 2) of
McCullough Pass tuff each increase in La (and other LREE) upsection. Each section is
composed of three eruptive units, consisting of one or more ash-flow tuffs, separated by
conglomerate, sandstone or reworked tuff. One sample was collected from each
eruptive unit of each section. Samples from the lower eruptive unit of sections one and
33
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34 two occur in the low La group while samples from the upper eruptive unit lie in the high
La group. Samples from all three eruptive units of section three lie in the low La group
but still increase in La upsection. Ramhead rhyolite occurs in high and low La groups.
Capstone rhyolite domes are low La while flows are high La.
Sections two and three of the McCullough Pass tuff show a crossover between Nd
and Sm on a plot of “early/late” emichment (Figure 18) such that older flows are
depleted in LREE but enriched in HREE. Section one does not show a crossover
indicating that the older flow is depleted in all REE relative to the youngest flow (Figure
19).
On chondrite normalized diagrams basalts and andésites have a shallow negative
slope (Figure 19). The McCullough Pass rhyolites show a negative slope from La to Sm,
a small Eu anomaly and a nearly flat trend for the HREE. On primitive mantle
normalized diagrams basalts, andésites and rhyolites show depletion in Nb, P and Ti.
Rhyolites show depletion of Ba and Sr as well (Figure 20).
Two samples of Pumice Mine basalt (MPC-3 and MPC-4) and one Jean Lake
rhyolite dome (MPC-1) were analyzed for Sr, Nd and Pb isotopes (Appendix 4). Figure
2 1 shows that SiOz correlates to progressive enrichment of^^Sr/^Sr, which ranges from
0.706852 in MPC-4 to 0.710184 in MPC-1. Figure 22 shows how SiO? correlates to a
progressive decrease of Si® from -3.86 in MPC-4 to -8.75 in MPC-1. Pb isotope values
for MPC-1 and MPC4 are 39.178 to 39.031 for “ W '^ P b , 15.56 to 15.596 for
207pb/204pb, 18 091 to 18.628 for ^°®Pb/^°^b, respectively (Appendix 4).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35
F-FeO*
Tholeiitic
A A
Calc-alkaline A
A-Na20+K20 M-MgO
Figure 14: AFM diagram of rocks of the McCullough Pass volcanic section. Diagram is after Irvine and Baragar (1971).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36
161 n 11111111 n 1 !j 1111 I I11 1111 II I ! 11
14- Phonofie
1 2 -
I 1 0 Rhyolite Fdcfite Thdivhole I» •S
6 -
4 Aidesïe
2
■1 . 1 J I I I I I I I I I I I I I I I I I I I I I I I
Figure 15: Classification of rocks of the McCullough Pass volcanic section. Diagram is after LeBas and others (1986).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37
5 en 4 J: § 1-- S 3 A ^ A < 0 . 2 - ^ 2 A ()/- A 1 A f 1 1 0.( 0 A 1.6 . r < l .]
H O i- 1
A 0 /
1 1 . o.c -2 ^ ▲A 8 à A (n § 6 # (U 8 ^ A \ ■ tu Z 2 >
1 A ± 0 1 1 1 A A aV A A 8 A
A 9 ^ AA “ A
^ 4 S. A 2 2 0 : % 0 1 1 / i u ■ 40 50 60 70 80 40 50 60 70 80 Si02 Si02 Figure 16: Marker variation diagrams.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38
j McCullough Pass Tufi I Section 1 ▲ 6 0 - I Section 2 ^ I Section 3 g I Jean Lake Rhyolite 9
20 0 100 200 300 Rb
Figure 17: Plot of Rb versus La showing the high and low La groups. Arrows point upsection for each section of McCullough Pass Tuff.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. "OCD O Q. C g Q.
"O CD
C/) Explanation 3o" O Section 1 ■ Section 2 "O8 ■ Section 3
CD c « o 3. 3 " CD I "OCD O Q. 1 C Oa 3 "O O
CD Q. 0 "O Rb Sr Y Zr Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu CD Element C/) C/) Figure 18: Early/Late enriclunent plot for three sections of the McCullough Pass Tuff, Diagram after Hildreth (1981),
n 40
1000 iPumice Mine Basalt # |McCullough Pass Rhyolite A
100
La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu
Figure 19; Chondrite normalized spider diagram after McDonough and Sun (1988).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. J î
100(k-T—I— I— I— i— i— I— I— r—I— r Pumice Mine Basalt # McCullough Pass Rliyolite A. lOCr-'
i a> %
Si
I I I I I » I I I I 1 1— I— I— i— I—K_l— I— I— L .0001 CsRbBaTli UNb KLaCePbPrSr P NdZiSnEuJDy YYbLu
Figure 20: Primitive mantle normalized spider diagram after McDonough and Sun (1988).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42
0.711
0.710 MPC-1
0.709
87 ,
MPC-3 0.708
0.707 MPC-4
0.706 SiO.
Figure 21: Plot showing *^Sr/“Sr enrichment with increasing SiO^.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43
MPC-4
MPC-3
MPC-1 -10 SiO,
Figure 22: Plot showing enrichment of epsilon Nd with increasing SiO^.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6
PETROGRAPHY
Thirty thin sections from intracaldera units and the McCullough Pass tuff were
examined to determine phenocryst assemblages and textures. Phenocryst abundances
presented below are estimates.
Jean Lake domes have a vitric matrix with phenocrysts (25 to 40%) of sanidine (46
to 60%), plagioclase (25 to 46%), biotite (5 to 15%), quartz (<5%), hornblende (1 to
2%) and less than 1% titanite and apatite. Jean Lake rhyolite block-and ash-flow units
have a vitric matrix containing phenocrysts (8%), of sanidine (55%), plagioclase (38%),
quartz (5%) and biotite (2%). Pumice clasts (5 to 35%) range from 1 to 10 cm in
length. The block-and ash-flow units also contain lithic clasts (5 to 10 percent) of
andésite and rhyolite and rare clasts (< 1%) of Precambrian crystalline basement rock.
Pumice Mine basalt flows are fine to medium grained with phenocrysts (10 to 30%)
of olivine (15 to 90%), plagioclase (5 to 60%) and clinopyroxene (5 to 25%). Basalt
dikes have a fine-grained matrix of plagioclase with olivine and oxide minerals.
Subhedral olivine altering to iddingsite is the only phenocryst (10%) phase.
Ash-flow tuffs of the Ramhead rhyolite contain phenocrysts (25 to 40%) of sanidine
(45 to 65%), plagioclase (25 to 45%), biotite (3 to 5%), hornblende (1 to 3%) and quartz
(2 to 5%) set in a groundmass of glass shards and pumice. Accessory phases include
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 titanite (< 1%), apatite (< 1%) and zircon (< 1%). Ash-flow tuffs also contain small
andésite clasts up to 0.5 cm and pumice clasts up to 1 cm in size. Ramhead rhyolite
flows contain phenocrysts (40%) of plagioclase (60%), sanidine (20%), quartz (15%),
biotite (2%), iron oxides (2%) and hornblende (1%) in a flow banded glassy matrix.
Capstone rhyolite domes are vitric and contain phenocrysts (25 to 30%) of sanidine
(45 to 55%), plagioclase (25 to 40%), quartz (10 to 12%), biotite (2 to 8%) and
hornblende (1%). Accessory phases include titanite (< 1%), apatite (< 1%) and zircon
(< 1%). Capstone rhyolite block and ash deposits are vitric. The matrix and pumice
blocks contain phenocrysts (30 to 35%) of sanidine (35 to 55%) up to 1 cm, plagioclase
(30 to 45%), biotite (8 to 15%), quartz (2 to 5%), hornblende (1 to 5%) and opaque
minerals (1 to 2%). Accessory phases include titanite (1%) and apatite (< 1%).
Capstone rhyolite dikes are vitric with phenocrysts (15%) of sanidine (93 to 95%),
plagioclase (3 to 5%) and minor amounts of quartz (1%) and biotite (1%).
The McCullough Pass tuff is a nonwelded, high-silica rhyolite. The matrix is vitric
and composed of glass shards and ash. Phenocrysts include (10 to 25%) sanidine (30 to
65%), plagioclase (25 to 30%), quartz (10 to 30%), biotite (1 to 3%), clinopyroxene (1 to
5%) and apatite (<1%). Pumice clasts (10 to 15%) are vitric and up to 1 cm in size.
Basalt xenoliths (1 to 10%) contain clinopyroxene, plagioclase and olivine altered to
iddingsite. Granitoid xenoliths (0 to 10%) consist dominantly of quartz and plagioclase.
The clinopyroxene crystals may be xenociysts plucked from the basalt xenoliths.
Xenocrysts of microcline (1 to 2%) occur only in the lower and middle part of the
McCullough Pass tuff.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER?
PETROGENESIS
Origin of Basaltic Magma
Major and trace element and isotopic data suggest the Pumice Mine basalt was
derived by partial melting of lithospheric mantle. Recent studies have shown that basalt
with >8 wt % MgO generally retains trace element and isotopic compositions of
primitive mantle (Kempton and others, 1991; Fitton and others, 1991). Sample MPC-4
has an MgO concentration of 7.7 wt % and is therefore somewhat evolved. Feuerbach
and others (1998); however, reasoned that lavas with 6 to 8 wt% MgO, although
differentiated may still retain primitive mantle traits because concentrations of
incompatible trace elements, such as Mb, REE and Sr, in assimilants (continental crust)
are lower or nearly equal to concentrations in mantle basalts. A very large amount of
assimilation, therefore, would be required to significantly change the composition of the
magma. I conclude that the trace-element concentrations of the Pumice Mine basalt
reflect their mantle source.
The isotopic character of the lithospheric mantle beneath the McCullough Range is
unknown; however, Kempton and others (1991) suggested that Pb isotopic compositions
of lithospheric mantle are mimicked by those of the overlying Proterozoic crust. If we
accept this assumption, then describing the composition of the Proterozoic basement in
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47 the McCullough Range may help in characterizing underlying lithospheric mantle. The
McCullough Range lies in the Mojave crustal province that extends from Death Valley
to the western edge of the Colorado Plateau. Rocks of the province are defined by Snd
values less than -16.0, crystallization ages of 1.65 to 1.80 Ga, Nd model ages of 2.0 to
2.3 Ga (Bennett and De Paolo, 1987), high -°^Pb/^°^Pb and “ ®Pb/^°^b, and low
^°’Pb/^°^Pb (Wooden and Miller, 1990). Jean Lake rhyolite (MPC-1) and Pumice Mine
basalt (MPC-3,4) have Pb isotopic signatures similar to the Mojave crustal province
(Figure 23). Assuming that Proterozoic crust and lithospheric mantle share isotopic
characteristics, it can be argued that MPC-4 (Pumice Mine basalt) is (or retains traits
similar to) a partial melt of lithospheric mantle spinel peridotite. Spinel peridotite is
suggested rather than garnet peridotite due to the shallow to moderate negative slope of
REE for the Pumice Mine basalt on chondrite normalized spider diagrams. Garnet
peridotite would exhibit a steeper slope because HREE are highly compatible in garnet.
Compared to MPC-4, MPC-3 has a higher concentration of SiO; and lower Sr, Ni, Cr
(Appendix 3), and enrichment of ®’Sr/®^Sr and End (Figures 22 and 23), suggesting MPC-
3 represents crustally contaminated MPC-4 type magma.
Origin of Rhyolitic Magmas
I evaluate two models for the evolution of McCullough Pass rhyolitic magmas;
fractional crystallization from basaltic magma and partial melting of continental crust.
Fractional crystallization is a differentiation process that is capable of producing rhyolite
from a basaltic parent, resulting in a continuous compositional trend from parent to
daughter magma. The McCullough Pass rhyolite and Pumice Mine basalt are closely
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48 related spatially and temporally, suggesting they may be oogenetic. Harker diagrams,
however, show compositional gaps between rhyolite and basalt suggesting differences
between the two rock types which cannot be explained by fractional crystallization
(Appendix 4, Figures 21 and 22). No isotopic difference should be observed between a
parent and daughter magma if crystal fractionation is the only differentiation
mechanism.
Partial melting of crust initiated by the injection of basaltic magma is another model
for the production of rhyolites. In this model, mantle-derived basalt provides the heat
required to melt silicic crust (Patchett, 1980; Hildreth, 1981). Although, only present in
small erupted volumes (approximately 0.5 km^), the Pumice Mine basalt may represent
the mafic magma that caused partial melting of the crust to produce the McCullough
Pass rhyolite. Large volumes of basalt at the surface are not required, since in thick
continental crust, voluminous felsic magmatism can take place without a significant
amount of basalt reaching the shallow crust. Basaltic magma cools quickly and solidifies
as it loses heat to the crust (Hildreth, 1981). In addition, basalt is often not capable of
penetrating larger felsic systems due to the low density “cap” of rhyolite (Hildreth,
1981). Examples of felsic volcanoes not associated with large volumes of basalt are the
Valles, Long Valley and Yellowstone calderas, which lack significant volumes of basalt
within the caldera but are partially surrounded by an apron of basalt flows (Hildreth,
1981). Hildreth further argues that in a small-scale magmatic system and/or extending
crust (conditions similar to those of the MPC) some basalt may reach the surface. This
is the case at the MPC where Pumice Mine basalt occurs around and inside the caldera.
Although no isotopic age has been determined for the Pumice Mine basalt, its
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49 stratigraphie position inside and outside of the caldera provides ample evidence that its
eruption was coeval with the emplacement of the McCullough Pass rhyolites (Plate 1,
Figures 2 and 3). Therefore, it is plausible that the Pumice Mine basalt could represent
the heat source that resulted in the partial melting of the crust.
Petrogenetic Model
The following petrogenetic model is based on a complete set of chemical data for a
small number of samples and therefore, while plausible, cannot be fully documented.
The model assumes that basaltic magma, represented by MPC-4 (Pumice Mine basalt),
rose into and melted crust to produce the McCullough Pass rhyolite magma. The
potential crustal source rocks can be inferred from work by Anderson and others (1985),
who mapped the Proterozoic crystalline basement rocks in the central McCullough
Range, 5 km south of the McCullough Pass caldera. Two rock types dominate this
region; granitic paragneiss and monzogranite. Experimental studies indicate that the
melting of granitic rock with normative quartz, orthoclase and sodic plagioclase will
produce felsic magma similar in composition to the McCullough Pass rhyolite. When
melting occurs in the system Ab-Q-Or (Tuttle and Bowen, 1964), the first melts form in
the thermal minimum or at the ternary eutectic if plagioclase contains normative An.
McCullough Pass rhyolites lie within or very near the ternary eutectic of the system Ab-
Q-Or at water pressures of 1000 to 500 kg/cm^ (Figure 24). Melts will continue to be of
the eutectic composition until one crystal phase is exhausted, at which point the melt
composition will change and eventually, with complete melting, will have the same
composition as the original rock. McCullough Pass rhyolites cluster about the ternary
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 eutectic, suggesting the amount of melting was relatively low (Figure 24); otherwise
melt composition would deviate from the ternary eutectic. Furthermore, when partial
melting occurs, compatible elements remain in the crystalline residue and the melt is
depleted in these elements. Ba and Sr are compatible in the source rock and exhibit
troughs on primitive mantle normalized spider diagrams, which suggests small degrees
of melting. These elements would be released to the liquid with large degrees of
melting and no troughs would be observed on spider diagrams. As discussed above. End
values of Proterozoic granitoids in the Mojave cmstal province are less than -16 but the
End value of MPC-1 (Jean Lake rhyolite) is -8.7. The model proposed above, therefore,
is problematic if Proterozoic granitic paragneiss and monzogranite are alone invoked as
the source rocks of the McCullough Pass rhyolites. Work by Anderson and others
(1985) may provide an explanation. In addition to the Proterozoic granitoids,
Proterozoic amphibolite and meta-ultramafic rocks were identified in the central
McCullough Range. These rock types may have mid-ocean ridge basalt (MORE) or
ocean island basalt (OIB) values of End (+2 to +10) and, if melted along with granitoid
crustal rock, may explain the higher End values of the McCullough Pass rhyolites. This
model is difficult to evaluate because isotopic analyses are not available the Proterozoic
basement rock.
Magma Batch Models
Magma batch models have been used to explain geochemical trends observed in
rhyolites erupted from large calderas such as Valles and Timber Mountain (Spell and
Kyle, 1989; Stix and Gorton, 1993; Cambray and others, 1995). The magma batch
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51 model presented here is based on a limited data set and should be interpreted with
caution. The model assumes an open system where active recharge and fractional
crystallization occur before eruption. I also assume that magma volume lost to eruption
is roughly balanced by periodic recharge of melts derived from the underlying crustal
source.
Step 1-McCullough Pass tuff-Jean Lake Rhyolite
The first step of the model is intrusion of magma into the MPC chamber followed by
sidewall crystallization (Figure 25a) (McBiraey, 1980) to produce a chemically zoned
chamber. Sidewall crystallization involves fractionation along the walls of a magma
chamber resulting in a buoyant liquid phase. This buoyant evolved liquid rises along the
walls of the chamber and collects under the roof. Because sidewall crystallization is a
form of fractional crystallization (FC), equations designed to model FC are valid in this
case. MPC-3 8 (McCullough Pass tuff) was chosen as the parent because it was collected
from the upper eruptive unit and is the least evolved with respect to LREE. MPC-39
(McCullough Pass tuff) was chosen as the daughter because it was collected from the
lower eruptive unit and is the most evolved with respect to LREE. Fractional
crystallization models require about 30 percent fractionation to reproduce the observed
trace element trends of the McCullough Pass tuff (Figures 26 and 27). Fractionated
phases used in the model occur as phenocrysts in the McCullough Pass tuff with the
exception of allanite. Allanite was not observed in thin section; however, allanite is a
very rare accessory mineral in rhyolite and could be present in the McCullough Pass tuff
and not be observed. Supporting this observation are studies by Huppert and Sparks
(1984) and Sparks and others (1984) who suggested that observed phenocryst
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 assemblages do not necessarily represent phases that crystallized along chamber walls to
produce compositional diversity. Additional justification for the inclusion of allanite in
fractional crystallization models comes from the REE “crossover” (Figure 18) exhibited
by sections two and three of the McCullough Pass tuff. The same characteristic has
been observed in the Bishop tuff (Hildreth, 1981) and the Bandelier tuff (Stix and
Gorton, 1993) and has been attributed to sidewall fractional crystallization in which
zircon and allanite control the bulk distribution coefficients of REE (Michael, 1983;
Cameron, 1984; Stix and Gorton; 1993).
Magma from the zoned chamber produced during step 1 erupted to produce the
McCullough Pass tuff (Figure 25b), initially drawing highly evolved (LREE poor)
magma from the top of the chamber. As eruptions continued, progressively deeper
levels of the chamber were tapped, resulting in emplacement of less chemically evolved
ash flows. Jean Lake domes plot in the high La group, suggesting that they represent the
high La residual magma left after eruption of the McCullough Pass tuff.
Chemical differences between sections one, two and three of the McCullough Pass
tuff could be produced by sector emplacement as suggested by Hildreth and Mahood
(1983). Sector emplacement occurs when topography or eruption mechanics cause ash
flows to be emplaced locally rather than as a sheet. Sections one and two contain rocks
of both the low and high La groups and may be a more complete representation of the
magma chamber from which they erupted. Section three consists only of low La rhyolite
and may represent only the first, highly evolved, magmas erupted from the chamber.
Late flows, which would be less evolved and La rich, apparently were not emplaced in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53 the vicinity of section three, due to sector emplacement, or were emplaced and quickly
eroded.
Step 2 Ramhead Rhvolite
After eruption of the McCullough Pass tuff, the chamber was recharged and again
became chemically zoned prior to erupting the Ramhead rhyolite (Figure 25c).
Chemical zonation was produced by the same mechanism as in step 1, although the
fractionating assemblage is apparently slightly different (Figures 28 and 29). Note that
Ramhead rhyolite cannot be derived from the same magma batch that empted to
produce the McCullough Pass tuff and Jean Lake domes (produced in step 1 of the
model). Ramhead rhyolite flows are high and low La and younger than the Jean Lake
domes, which are high La and are thought to represent tapping of lower levels of the
magma chamber after low La magma had been erupted from the upper part of the
magma chamber. Therefore, a second period of zonation is necessary to produce
evolved (low La) magma.
Step 3-Capstone Rhvolite
Following eruption of the Ramhead rhyolite, the magma chamber was recharged and
again became chemically zoned. Chemical zonation probably occurred in the same way
as described in steps 1 and 2 of the model, although the fractionating assemblage is
apparently slightly different (Figures 30 and 31). Initially, low La (upper) levels of the
chamber were tapped as Capstone rhyolite domes were extruded (Figure 25d). Capstone
flows are high La and may represent lower parts of the magma chamber tapped
following extrusion of the domes (Figure 25d). The relative ages of Capstone domes
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54 and flows cannot be determined firom field relations. Extrusion of Capstone rhyolite
flows marked the end of volcanism related to the McCullough Pass caldera.
Convincing evidence for magma chamber recharge is difficult to obtain when
intruding magma is similar in composition to magma previously in the magma chamber.
Recharge is invoked in this model because of an overall lack of chemical evolution firom
the earliest to latest eruptions. Sidewall crystallization operating in a magma chamber
with no recharge would result in magmas of the earlier eruptions being less evolved than
those of later eruptions. Although the Jean Lake (representing early magmas), Ramhead
and Capstone rhyolite (representing later eruptions) show evidence of intra-unit
geochemical evolution, they have similar trace element abundances and trends (high and
low La groups) to one another. These geochemical observations are inconsistent with the
trends expected to result from sidewall crystallization alone.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55
39.5 MPC-3 MPC-
38.5 Mojave
208/204
37.5
36.5 17.0 17.5 18.0 18.5 19.0 206/204
Figure 23: Plot of “*Pb/^b versus ”^b/^“Pb showing the Mojave crustal province field (Feuerbach and others, 1998) and three samples from the McCulloughPass volcanic section. NHRL is the northern hemisphere reference line.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56
0 3 > P - -
Figure 24: Ternary diagram o f the system NaAlSijO.-KAlSijO*- SiOi-H^O, showing the effect of water-vapor pressure on the position of the ternary eutectic. Solid line represents the ternary eutectic. Dashed lines indicate water-vapor pressure. Figure is after Tuttle and Bowen (1964').
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57
Low La
High La
Convectmg Magma f
f \
Intrusion of McCullough Pass Rhyolite Magma From Source
Figure 25a: Generalized diagram of the McCullough Pass magma chamber, before emption of the McCullough Pass Tuff, depicting intrusion of magma from a crustal source, sidewall crystallization (x) and buoyant rise of evolved magma (arrows).
Reproduced witti permission of ttie copyrigfit owner. Furtfier reproduction profiibited witfiout permission. 58
Jean Lake Rhyolite
f Convectmg Magma f f
Intrusion o f McCullough Pass Rhyolite Magma From Source Figure 25b: Generalized diagram of the Jean Lake caldera and magma chamber, after eruption of the McCullough Pass Tuff and extrusion of the Jean Lake rhyolite. Low and high La magma erupted to form the McCullough Pass Tuff. Only high La m:^ma remained in the chamber to be extnaded and form the Jean Lake Rhyolite.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ramhead Rhyolite
îiTevri
f Convectmg Magma f f
Intrusion of McCullough Pass Rhyolite Magma From Source Figure 25c: Generalized diagram of the MPC and magma chamber, after extrusion of the Ramhead rhyolite. The chamber was rechai^ed and chemically zoned by sidewall crystallization (x) and buoyant rise of evolved magma (arrows) prior to eruption. Magma ftom the low and high La magma layers was tapped in the eruption.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Capstone Rhyolite
1 Convectmg Magma f T
Intrusion of McCullough Pass Rhyolite Magma From Source Figure 25d: Generalized diagram of the MPC and magma chamber, after eruption of the Capstone rhyolite. The chamber was recharged and chemically zonedby sidewall crystallization (x) and buoyant rise of evolved magma (arrows) prior to extrusion. High La magma extruded as rhyolite domes followed by extrusion of low La magma as rhyolite flows. Extrusion of Capstone rhyolite formed a topographic high and marked the end of volcanism associated with the McCullough Pass magma chamber
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hidden Valley Andésite
i
Figure 25e: Generalized diagram of the MPC following extrusion of the Hidden Valley Andésite (Thv).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. O-
U
Figure 25f: Generalized diagram of the MPC after erosion formed the present day topographic low.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63
60 McCullough Pass Tuff Section 1 A
Section 2 ^
50 Section 3 I
\ Ao% FC La A I 40 ^ 2 0 % FC ♦ ■ ♦
30 30% FC
20 0.2 0.3 0.4 0.5 0.6 Lu
Figure 26: Fractional crystallization model for the McCullough Pass tuff. Fractionation vector was produced from the following assemblage: plagioclase (35 %), sanidine (35 %), quartz (20 %), biotite (10 %), apatite (0.1 %), allanite (0.1 %) and zircon (0.12 %). Arrows represent 10 percent fractionation. Bulk Distribution coefficients are in Appendix 5.
Reproduced witti permission of ttie copyrigfit owner. Furtfier reproduction profiibited witfiout permission. 64
800 McCullough Pass Tuff Section 1 A
Section 2
600 Section 3
Sr 400
200
200 400 600 800 Ba
Figure 27; Fractional crystallization model for the McCullough Pass Tuff. Fractionation vector was produced from the following assemblage: plagioclase (20 %). sanidine (45 %), quartz (25 %), biotite (10 %), apatite (0.1 %), allanite (0.1 %) and zircon (0.1 %). Arrows represent 10 percent fractionation. Bulk Distribution coefficients are in Appendix 5.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65
10% FC
La 40 20% FC
0.3 0.4 0.5 0.6 Lu
Figure 28: Fractional crystallization model for the Ramhead rhyolite. Fractionation vector was produced from the following assemblage: plagioclase (35 %), sanidine (35 %), quartz (20 %), biotite (10 %), apatite (0.1 %), allanite (0.1 %) and zircon (0.18 %). Arrows represent 10 percent fractionation. Bulk Distribution coefficients are in Appendix 5.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66
800
600
Sr 400
200
0 200 400 600 800 Ba
Figure 29: Fractional crystallization model for the Ramhead rhyolite. Fractionation vector was produced from the following assemblage: plagioclase (15 %), sanidine (50 %), quartz (25 %), biotite (10 %), apatite (0.1 %), allanite (0.1 %) and zircon (0.1 %). Arrows represent 10 percent fractionation. Bulk Distribution coefiicients are in Appendix 5.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67
70 Capstone Rhyolite Flow s -^>-
Domes 60
10% E C
50
La 20% EC
40
30% EC
30
20L- 0.2 0.3 0.4 0.5 0.6 Lu
Figure 30: Fractional crystallization model for the Capstone rhyolite. Symbols are as in Figure 7. Fractionation vector was produced from the following fractionating assemblage: plagioclase (35 %), sanidine (35 %), quartz (20 %), biotite (10 %), apatite (0.1 %), allanite (0.1 %) and zircon (0.21 %). Arrows represent 10 percent fractionation. Bulk Distribution coefiicients are in Appendix 5.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68
800 : Capstone Rhyolite Flow s -O -
Domes 600
Sr 400
200
200 400 600 800 Ba Figure 31: Fractional crystallization model for the Capstone rhyolite. Symbols are as in Figure 7. Fractionation vector was produced from the following fractionating assemblage: plagioclase (20 %), sanidine (45 %), quartz (25 %), biotite (10 %), apatite (0.1 %), allanite (0.1 %) and zircon (0.1 %). Arrows represent 10 percent fractionation. Bulk Distribution coefficients are in Appendix 5.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTERS
PHYSICAL EVOLimON OF THE MCCULLOUGH PASS CALDERA
The MPC is the erosional remnant of two calderas: the Jean Lake and the Ramhead
calderas. The Jean Lake-Ramhead caldera complex was formed by eruption of ash-flow
tuffs and was subsequently filled by rhyolite and basalt flows, dikes and rhyolite domes.
When volcanism ended, preferential erosion of intracaldera rocks formed the present
topographic low known as the McCullough Pass caldera.
Ash flows of the McCullough Pass tuff erupted at 14.10 Ma (Table 1) and resulted in
collapse along a ring fracture to produce the Jean Lake caldera (Figure 25b). The ring
fracture is outlined by Jean Lake domes which extruded at 14.1 Ma and form an arcuate
pattern in the eastern part of the MPC (Plate 1, Figiue 25b).
Xenocryst ages determined by the ‘’°Ar/^^Ar single crystal laser fusion technique can
aid in interpretation of the eruption that produced the McCullough Pass tuff (Table 2).
The presence of xenocrystic crystals only in the lower and middle eruptive units of the
McCullough Pass tuff may indicate the xenocrysts were scoured from the paleosurface
by ash flows rather than being incorporated in the vent. If the source of the xenocrysts
was the vent, xenocrysts should be found in ash-flows throughout the McCullough Pass
Tuff section. An explosive eruption would erode vent material and entrain xenocrysts
and xenoliths. The absence of xenocrysts in the upper eruptive unit of the McCullough
69
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 Pass tuff suggests that the eruption was less explosive and a Plinian eruption column
may have never been produced. Instead, a less explosive eruption may have “boiled”
out of the vent and over the caldera walls, similar to the 1877 eruption of Cotopaxi in
Ecuador, which was described by eyewitnesses as “a pan of rice boiling over” (Cas and
Wright, 1987). This type of eruption may have entrained Precambrian and Tuff of
Bridge Spring xenocrysts as it flowed over the paleosurface. Ash flows of the lower and
middle eruptive units may have covered most exposures of Precambrian basement rocks
and the Tuff of Bridge Spring so that as ash flows of the upper eruptive unit moved
across the surface few xenocrysts were incorporated.
Eruption of several ash-flows produced the Ramhead caldera (Figure 25c). Caldera
collapse occurred along northern and western margins of the present MPC and along the
western margin of the Jean Lake caldera. Unlike the Jean Lake caldera, the extent of the
Ramhead caldera is not well defined by rhyolite domes. The Ramhead caldera is
therefore defined as that part of the MPC not occupied by the Jean Lake caldera.
Furthermore, Ramhead Rhyolite preferentially crops out within the Ramhead Caldera.
An alternative interpretation is that the Ramhead caldera is an embayment of the Jean
Lake caldera similar to the Toledo embayment of the Valles Caldera in New Mexico
(Self and others, 1988). However, because of its large size and the outcrop pattern of
Ramhead Rhyolite, the caldera model is preferred.
Capstone rhyolite domes and dikes extruded after caldera collapse. The pre-existing
Jean Lake Caldera ring fracture may have controlled the location of Capstone domes
(Figure 25d and Plate 1). Capstone rhyolite flows, domes, and dikes filled the caldera
complex, creating a topographic high (Figure 25d).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 71 The MPC is surrounded by 65-150 m thick flows of the Hidden Valley andésite,
which crop out 50-100 m higher than the caldera. The Hidden Valley andésite erupted
from a cinder cone field surrounding the caldera complex between about 14.1 and 13.07
Ma (Figure 25e). The mafic flows surrounded the caldera complex but were prohibited
from flowing into it because the caldera was a topographic high at this time. Following
extrusion of the Hidden Valley andésite, preferential erosion of the Capstone rhyolite
and other intracaldera units produced the MPC, an erosional remnant of the Jean Lake
and Ramhead calderas (Plate 1, Figure 25f).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 9
COMPARISON OF MPC TO OTHER SILICIC SYSTEMS
Small Silicic Systems
The Trans-Pecos volcanic field of Texas in the southern Basin and Range Province
contains several small calderas (Henry and Price, 1984; Henry and others, 1989; and
Henry and others, 1997). A detailed petrogenetic study has been completed only on the
Solitario (Spell and Henry, 1998), therefore, comparisons to other Trans-Pecos calderas
are limited to physical characteristics.
Phvsical Characteristics
The Van Horn Mountains caldera is roughly elliptical in shape with a maximum
diameter of 4 km. Volcanism began with the eruption of flow-banded rhyolite,
followed by eruption of air-fall and ash-flow tuffs and deposition of collapse breccias.
Trachyte, basalt and rhyolite flows then extruded before eruption of a second ash-flow
tuff, which ponded within the caldera. Volcanism occurred over a relatively short
period of time at about 37 Ma based on K-Ar dates obtained from alkali feldspar (Hemy
and Price, 1984).
The 35.4 Ma Solitario, also located in the Trans-Pecos area, is a combination
laccolith-caldera that formed during three distinct magmatic pulses at 36.0,35.4 and
35.0 Ma (Henry and others, 1997). Emplacement of a large laccolith resulted in the
72
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73 formation of a dome with a diameter of 16 km. The caldera formed during the second
magmatic pulse at 35.4 Ma. The caldera collapsed along a ring fracture associated with
eruption of an ash-flow tuff extrusion of trachytic lava and emplacement of radial dikes,
rhyolite breccia and megabreccia (Henry and others, 1997). The caldera is elliptical
with dimensions of 6 x 2 km.
Four calderas formed from 40 to 44 Ma (Henry and others, 1989) and comprise the
Christmas Mountains caldera complex in the Christmas Mountains of the Big Bend
region. Initially, emplacement of a silicic magma body formed the elliptical Christmas
Mountains dome (8x5 km). This event was followed by eruption of an ash-flow tuff
and collapse to form the First caldera, with a diameter of 1-1.5 km. A second ash-flow
tuff and lava doming produced the Second caldera, with a diameter of 1.5 km. The main
Western and Eastern calderas, both with a diameter of 1.5 km, were then formed by
separate eruptions of ash-flow tuffs. Extrusion of quartz trachyte porphyry stocks and
lava domes along a ring fracture of the Western caldera marked the end of volcanism
related to this system.
Calderas in the Trans-Pecos and the MPC have similar physical characteristics. All
of these calderas formed by the eruption of an ash-flow tuff. The Van Hom Mountains
caldera, Christmas Mountains caldera complex and the MPC were formed by collapse
related to multiple eruptions. The Christmas Mountains caldera complex and the MPC
both contain rhyolite domes related to ring fractures. None of the calderas exhibit
resurgence and all the calderas contain megabreccia deposits. No detailed geochemical
studies of the rocks related to the Trans-Pecos calderas exists, therefore, comparison of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 74 the petrogenetic model for the MPC to these other small silicic systems is not possible at
this time.
Chemical Characteristics
The Solitario and MPC have chemical similarities and differences. Both systems
produced dominantly high silica rhyolites thought to be the result of crystal
fractionation. ‘*°Ar/^ W ages indicate volcanism related to the Solitario caldera
formation lasted from 35.49 to 35.38 Ma, a period of less than 100 ka (Spell and Henry,
1998). The MPC formed over a similar length of time, 140 ka. The MPC formed from
three separate magma batches. The Solitario system also produced three magmatic
pulses, which may represent separate magma batches, however; the caldera was formed
during eruption of only the second pulse of magmatism, which may represent a single
magma batch. REE behavior was different in the two systems. LREE and HREE
behaved incompatibly in the Solitario magma chamber (Spell and Henry, 1998). LREE
behaved compatibility and HREE behaved incompatibly in the McCullough Pass magma
chamber resulting in a crossover on a plot o f early/late enrichment (Figure 19).
Large Silicic Systems
The Valles and Long Valley calderas and associated volcanic sections have been the
focus of numerous scientific studies designed to provide information as to the physical
and chemical evolution of large silicic systems. Comparisons between the McCullough
Pass, Valles and Long Valley calderas are summarized in Table 3. Detailed discussions
of the Valles caldera can be found in Smith and Bailey, (1966); Gardner and others
(1986); Self and others (1988); Spell and Kyle (1989); Spell and others (1990); Stix and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 75 Gorton (1993); Wolff and others (1999). Detailed discussions of the Long Valley
caldera can be found in Bailey and others (1976); Hildreth (1979); Hildreth (1981);
Hildreth and Mahood, 1986; Michael (1983); Cameron (1984); Metz and Mahood
(1985); Halliday and others (1989).
Phvsical Characteristics
An important similarity between these systems is that the MFC, Valles and Long
Valley calderas formed in areas of active extension. The MFC lies in the southern Basin
and Range Province, the Valles caldera lies along the Rio Grande rift and the Long
Valley caldera lies along the Hilton Creek extensional fault system near the western
margin of the Basin and Range Province. Another similarity is all systems were
preceded and/or accompanied by basaltic to andesitic volcanism. These observations
suggest mafic magmatism and extension promote the development of both large and
small rhyolitic systems. Eruption and emplacement of ash-flow and ash-fall tuffs
accompanied collapse of each caldera (Table 3).
Single and multiple collapse appear to be common. Valles, McCullough Pass, Van
Horn Mountains and the Christmas Mountains calderas exhibit multiple collapse while
Long Valley caldera formed from a single collapse. Ring fractures accommodate
collapse in large and small systems (Table 3). The Valles, Long Valley and McCullough
Pass calderas collapsed along ring fractures. Following caldera collapse, rhyolite domes
were extruded along ring fractures of all three calderas. This is an important observation
because small calderas have been referred to as “funnels” (Aramaki, 1984; Lipman,
1997), indicating a simple geometry. A “fimnel” cored out by an explosive eruption
would lack a coherent floor and a ring fracture would not be
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76
Table 3-Comparison of McCullough Pass, Valles and Long Valley calderas.
Characteristic Small Systems Large System
McCullough Pass Valles Long Valley
Physical Active extension Yes Yes Yes Previous volcanic Basaltic-andesitic Basaltic-andesitic Basaltic-andesitic activity
Eruption style Effusive Peleean Peleean
McCullough Pass Outflow sheet Bandelier tuff Bishop tuff tuff Collapse Multiple Multiple Single Ring fracture Yes Yes Yes Resurgent dome No Yes Yes Chemical High-silica High-silica High-silica Dominant rock type rhyolite rhyolite rhyolite
Multiple magma batches 3 5 2 Zoned magma chamber Yes Yes Yes Fractional Fractional Fractional Mechanism of zonation crystallization crystallization crystallization REE crossover Yes Yes Yes Geochronology
Period of activity 13.98-14.12 Ma 1.8 Ma to Recent 2.1 Mato Recent
Repose time 45 ka 270-370 ka 300 ka Note: References for caldera systems are presented in the text.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77
possible. A funnel geometry may exist in some small systems but the geometry of the
MPC and other small calderas suggests that more complex geometries, similar to large
systems, are common.
An important difference between the large and small systems is the presence or
absence of resurgent domes. The Valles and Long Valley calderas have well-developed
resurgent domes while the MFC and small calderas of the Trans-Pecos lack resurgent
domes. It should be noted, however; that all systems have produced post-collapse
rhyolitic volcanic rocks.
Chemical Characteristics
The McCullough Pass, Bandelier and Bishop tuffs are generally high-silica rhyolites
(Table 3) although rhyolitic compositions are also present. All are chemically zoned and
are thought to represent the inverted chemical stratigraphy of the magma chamber. In
addition, the zonation in all three chambers was produced by fractional crystallization.
Hildreth (1981) suggested Soret diffusion coupled with thermogravitational convection
as the process that produced the chemical zonation of the Bishop tuff but subsequent
studies (Lesher and Walker, 1983; Walker and DeLong, 1982; Cameron, 1984; Michael,
1983) have shown that Soret diffusion would not produce the observed trends and that
fractional crystallization was more likely responsible for chemical variation. The
McCullough Pass, Bandelier and Bishop tuffs exhibit similar “early/late” enrichment
trends for REE (Figure 18), which suggests the same mechanism forms chemical
zonation in large and small silicic systems. This mechanism is thought to be sidewall
crystallization where zircon and allanite control distribution coefficients of REE.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78 An important similarity between small and large silicic systems is the eruption of
multiple magma batches. The present study suggests three magma batches erupted from
14.10 to 13.98 Ma, to produce the McCullough Pass rhyolites. Work by Stix and Gorton
(1993) and Spell and Kyle (1989) suggests that the Bandelier tuff and subsequent Valles
rhyolite formation are the products of up to five separate magma batches erupted from
1.78 Ma to Recent. Rhyolites have erupted from the Long Valley magma chamber from
2.1 Ma to Recent. The Bishop tuff and late eruptive units of Glass Mountain are thought
to represent a single zoned chamber but early eruptive units of Glass Mountain have a
different chemical signature and may represent a separate magma batch (Metz and
Mahood, 1985; Halliday and others, 1989). The presence of multiple magma batches is
another indication that small systems can have a very complex evolutionary history
similar to large silicic systems.
Repose times listed in Table (3) indicate that magma erupted from the McCullough
Pass magma chamber at 45 ka intervals while the much larger Valles and Long Valley
chambers erupted chemically zoned magma every 270 to 370 ka. The 45 ka repose time
is based on the age difference between the McCullough Pass tuff and Capstone rhyolite
(140 ka) and the number of eruptions (3). The point should again be made that all ages
obtained from the McCullough Pass rhyolites overlap at 1 sigma; therefore, the repose
time is less than but not more than 45 ka. These observation support work by de Silva
and Wolff (1995) indicating that eruptions occur when a magma chamber accumulates a
critical volume of evolved magma ( - 1 0 percent) at the top of the chamber by sidewall
crystallization. This process takes 10^ to 10'* years in small magma chambers (1 to 100
km^) and 1 0 ^ to 1 0 ^ years in large magma chambers (greater than 1 0 0 0 km^).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 10
CONCLUSIONS
The McCullough Pass caldera exhibits many characteristics of typical small calderas
and these characteristics are similar to characteristics of large calderas, indicating that
small and large silicic systems follow similar evolutionary paths. Small and large silicic
systems can have complex physical characteristics. These include multiple periods of
eruption and caldera collapse and collapse along ring fractures. Small and large silicic
systems can have complex chemical characteristics. These include several periods of
recharge from an underlying crustal source and chemical zonation due to sidewall
crystallization. Repose times from the McCullough Pass, Valles and Long Valley
calderas indicate recharge and formation of a critical volume of chemically zoned
magma can occur in 1 0 "* years in small chambers and 1 0 ^ years in large chambers
indicating that chunber processes are similar but operate more rapidly in small
chambers.
79
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES CITED
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX 1
ANALYTICAL TECHNIQUES
87
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ANALYTICAL TECHNIQUES
Five samples were collected for “°Ar/^^Ar single crystal laser fusion analysis. Three
samples were collected from section 2 (Figure 2) of the McCullough Pass Tuff and one
each was collected from the Jean Lake rhyolite and the Capstone rhyolite. Samples
were placed in re-sealable plastic bags for storage. Whole rock samples were crushed to
pea size with a Braun "chipmunk." Samples were then ground to approximately 1 mm
(the average size of sanidine crystals) using a disk grinder. The samples were then
sieved and individual crystals were picked out of the proper size fractions using a
binocular microscope.
Samples were wrapped in A1 foil and stacked inside a 6 mm diameter pyrex tube.
Individual packets averaged 3 mm thick and neutron fluence monitors (ANU 92-176,
Fish Canyon Tuff sanidine) were placed every 5-10 mm along the tube. Synthetic K-
glass and optical grade CaF; were included in the irradiation packages to monitor
neutron induced argon interferences from K and Ca. Loaded tubes were packed in an A1
container for irradiation. Samples were irradiated for 3 hours in the D3 position on the
core edge (fuel rods on three sides, moderator on the fourth side) of the IMW TRIGA
type reactor at the Nuclear Science Center at Texas A&M University. Irradiations are
performed in a dry tube device, shielded against thermal neutrons by a 5 mm thick jacket
of B 4C powder, which rotates about its axis at a rate of 0.7 revolutions per minute to
88
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 89 mitigate horizontal flux gradients. Correction factors for interfering neutron reactions
on K and Ca were determined by repeated analysis of K-glass and Cap 2 fragments.
Measured (‘’°Ar/^®Ar)K values and Ca correction factors [(^^Ar/^Ar)Ca = 2.88 (± 0.1) x
10"* and (^^Ar/^^Ar)Ca = 6.85 (± 0.05) x 10"*] are listed in Appendix 1. J factors were
determined by fusion of 4-5 individual crystals of neutron fluence monitors, which gave
reproducibilities of 0.05% to 0.27% at each standard position. Variation in neutron flux
along the 80mm length of the irradiation tubes was <4%. An error in J of 0.5% was
used in age calculations. No significant neutron flux gradients were present within
individual packets of crystals as indicated by the excellent reproducibility of the single
crystal flux monitor fusions.
Irradiated crystals together with Cap 2 and K-glass fragments were placed in a Cu
sample tray in a high vacuum extraction line and were fused using a 20 W CO 2 laser at
the Nevada Isotope Geochronology Lab (NIGL). During laser fusion samples were
viewed by a video camera system and positioned with a motorized sample stage.
Reactive gases were removed by a single MAP and two GP-50 SAES getters prior to
being admitted to a MAP 215-50 mass spectrometer by expansion. The relative volumes
of the extraction line and mass spectrometer allow 80% of the gas to be admitted to the
mass spectrometer for laser fusion analyses and 76% for furnace heating analyses. Peak
intensities were measured using a Balzers electron multiplier by peak hopping through 7
cycles; initial peak heights were determined by linear regression to the time of gas
admission. Mass spectrometer discrimination and sensitivity was monitored by repeated
analysis of atmospheric argon aliquots from an on-line pipette system. Measured
‘*°Ar/^^Ar ratios were 289.79 ± 0.35% during this work, thus a discrimination correction
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 90 of 1.01963 (4 AMU) was applied to measured isotope ratios. The sensitivity of the mass
spectrometer was ~2 x 10'*^ mol mV* with the multiplier operated at a gain of 100 over
the Faraday. Line blanks averaged 3.04 x 10'*® mol for mass 40 and 1.28 x 10'*® mol for
mass 36 for laser fusion analyses. Discrimination, sensitivity, and blanks were relatively
constant over the period of data collection. Computer automated operation of the
sample stage, laser, extraction line and mass spectrometer as well as final data reduction
and age calculations were done using Labview software written by B. Idleman (Lehigh
University). An age of 27.9 Ma (Steven et al., 1967; Cebula et al., 1986) was used for
the Fish Canyon Tuff sanidine flux monitor in calculating ages for samples.
Forty-three samples were collected from the McCullough Pass volcanic section in
the central McCullough Range including samples from units within and outside of the
caldera. Unaltered samples were placed in re-sealable plastic bags for storage. Whole
rock samples of basalt and andésite flows and dacite and rhyolite domes were crushed to
pea size with a Braun "chipmunk." Samples were then crushed to powder with a Bico
"shatterbox." Pumice from the McCullough Pass tuff Ramhead rhyolite and Jean Lake
rhyolite were crushed with a Fritsch pulverisette agate mortar and pestle. Pumice was
collected from ash-flow tuff whenever possible to prevent contamination from xenoliths
and xenocrysts. Because the amount of pumice was not sufficient to adequately fill the
Bico "shatterbox”, all pumice samples were crushed with an agate mortar and pestle.
Loss on ignition percentages were determined for all samples. One to four grams of
sample were weighed and placed in a ceramic crucible in a preheated furnace for two
hours at 110°C. Samples were then reweighed and L.O.I. percent was calculated by use
of the following formula.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 91 L.O.L % = [(wt. lost by sample)/(wt. o f "wet" sample)] x 100
Samples were placed in a fiiraace at 950°C for two hours. The furnace was turned off
and allowed to cool to 300°C. Crucible and powders were removed and placed in a
dessicator and allowed to cool to room temperature at which time L.O.I. was again
calculated using the formula above.
Fused glass disks were made for major and trace element analyses. Sample powders
(1.7 g) were thoroughly mixed with 8.5 g of lithium tetraborate and 0.274 g of
ammonium nitrate. The mixture was then poured into an Au/Pt crucible and heated for
30 minutes at 1100°C at which time the powder mixture was liquid. Immediately upon
removal from the oven the sample was poured into an Au/Pt mold and allowed to cool to
form a disk. Disks were stored in a dessicator until analyzed. All sample disks were
analyzed for major and seven trace elements (Rb, Sr, Y, Ni, Cr, Nb and Zr) using a
Rigaku 3030 XRF spectrometer. L.O.L and major element weight percentages were
entered into a matrix correction spreadsheet and totals were calculated according to the
methods ofNorrish and Chappel (1967). Geochemical standards used for these analyses
are presented in Appendix 1.
Approximately 2-4 g of powder from 27 samples were sealed in pyrex vials and sent
to Washington State University to be analyzed for rare-earth elements and Ba, Rb, Y,
Nb, Cs, Hf, Ta, Pb, Th, U, Sr, and Zr. Analyses were done with a Sciex Elan model 250
ICP-MS. Sample preparation, instrument operating conditions and calibration were
done following Crock and Lichte (1982) and Knaack et al. (1994).
Four samples (2 g) sealed in pyrex vials were analyzed for Sr, Nd and Pb isotopes
at the University of Kansas Isotope Geochemistry Laboratory. Samples were analyzed
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92 by thermal ionization mass spectrometry (TIMS) using a VG sector 54 mass
spectrometer. The technique is described in Feuerbach et al. (1993).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX 2
^ W W d a ta
93
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 94
•d cn cn cn m cn m cn cn cn cn c/3 s o o o o o o o o o o o
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L. c " < G\ u n m CN o M T f 'OO' oo II o KK ON un v d
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95
0 0 CN 0 0 p'- *— T 3 c n c n ON CN mCN U) o o oo o oo oo oc o o CQ c n CN VO c n u n NO c n c n c n CN O n p ON Os O n 0 0 ON ON ON a s O n Ü m cn c n CO c n c n c n c n c n c n c n CO ^ < II 1) t'' < o n* 00 o r-* 0 0 c n o g s O' o u n 0 0 c n CN o CO "O m o CN NO CN c n 0 0 c n CN CN CN CN CN CN CN CN CN CN CN a s NO no' NO NO* NO NO* NO* NO NO NO o< o Tf II É ■g p NO OO + c n O n soOO O c n o c n u n 0 0 CN O' O' O n mo cn o o' II < c n O •— o c n NO O s o 9 0 o K ON o u n O n 0 0 O' cn t TT O n O n un ON n - ON O n O n O' O' (L> § Os CN u n u n O s NO u n O' . CN u n u n NO ON 0 0 c n c n a s o 0 0 4 - §; NO o T f u n cn u n vd ( d un u n CN u n u n NO CN o (M c ^ 0 0 0 0 c n CN u n r>- 4 - I CN CN o o CN c n O n ON NO — + O 0 0 o o f S § sO S OOO 0 0 c n c n c n un O ' O ' 0 0 CN u n 5" m c n CN c n & O' a u n 'Tt 2 CO VO § c o VO CN CN O'O' < n VO < m p ! r - O' m VO O' u n 00 i n VO VO VO o T f o o § m — d d o I 1 w n O n T)- u n Ê o o CN O u n r r c n < c n 00 c n u n NO O OO 00o Ï u n c n r r u n CN CN c n TT c n 1 c n o Oo oo o Oo oo '-3 3 oo •g CN NO NO O n VO c n o o n - "C S i OO c n NO ON u n CN CN o c n o I o o oCN O o o > o o CN o od d d d d £ c un cn 8 C C/3 f CN VO vO VO VO VO VO VO VO VO vO O_ g + o zr X3I Sb’O § o OOo OOo Oo o o oOo OO o o oCO s VO VO VO VO VO VO VO VO VO Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 96 c n -O O s 0 0 o o 0 0 ON ON o o 0 0 o o c n o O s u n o ON ON VO en O o o o o o o o o o un 0 0 o O s ON c n o o u n CN O o o o o o o o o o un v d K Os' v d o o u n O I or^vovovoov — oo'S'inooo'S'vOf^Ov 00 Ov o 00 Tp — OOP- «n Tpcn^'S'Tp^^^vo- O'. -. ®. 'O. «'i -. o. CM— fNp^cn d —00 m d /^^cnd o ^00 TT TP I «— ■ ■ .—■ — 1—i j ' ' ' a fsj rn m CN < II II II II VO un r r CN e n o cd U ed o u n SO o u n O 0 0 un Tf ON 0 0 o o 0 0 Tf e n e n e n VO CN VO c n 2 S 2 m TT 0 0 VO o r - 0 0 ON TT C^ C^ CN u n un T f CN un m CN c n s 2 c n CO o r n e n CN CN CN CN c n CN CN CN o CN o CN CN o "O CN CN CN CN CN CN CNCN CN CN ON vO CN "g CN CN CN e n c n CN 2 C/3 2 "d 2F Tf o CL c/3 'S' + •d • r i g c ui + 3 c CN n 00 ^ 00 CN CN 00 0 0 un O s o o CN un u n 0 0 c n CN o O n 0 0 P CN p c n O en + o p 8 S O vo VO Os' CN CN v d p - CN e n TP u n eng vri - o — 3 00 VO P-. c n l en 00 Ov t^ ov CN en en un o SU un en 1 o o o en — o O o o 8 o M Tf d d d d o d > d d d d d d d d d E 6^ 00 C un Ï5 en 00 vovovovovovovovovovovovovovovovo I cd o + 1 00 o OOOOOOOOOOOOOO I oooooooooooooo g en u n d vOvOvovOvOvOvOVOvOvOvOvOvOvO VO88 VO e n II h s T t 1 si eng ^ CN en m VO a> o> Tp«nvoP~ooo\2Z22Z t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 97 cn c n u n un u n 0 0 cn cn o cn O s CN m c n c n un •o CN a i CN CN CN o o o o O o o o o o o o o o o o o o o O O o es oosOsmt'-somoor-'SOCM'fOsP-OTp'pooMOvunOv g »— OOsr—CMOOOsOO'/ncN — vO'-'OsOsCslO'-'*— o a < - o 00 < r- os o 00 o CN VO u n o o CN O s os p os o o CO O s u n TT u n u n un VO un c n 00 c n o un u n o 00 un o * o u-T u n u n un c n u n vO 0 0 o o u n O s u C^ CN vO VO + CN CN CN un c n CN CN CN un CN c n c n o CN CN CN c n c n C ? v d v d s d s d s d s d v d s d c n < v d s d s d v d v d v d d v d v d v d v d v d E O i TP o 00 g CO O s 00 o —• CN 00 — O CN OS OS OS cn CN ^ CN CN un o o o O p-' — d o sd OS c n p- -T o c n Ov o c n u n VO TT o s VO 0 0 c n OS OS pv: d o s' SO OS O s 0 0 m VO 0 0 P- CN 0 0 un o 0 0 Tp u n c n O s CN CN s o O s un o 0 0 un c n u n 0 0 OS VO US OS CN u s U-1 u n t T c n u n VO o s CN P- v d s d K d v d o d (C TP r s | CN O s u n vO Tf- o d CN P-' OS CN CN v d CN s f Ov u n O s r - 0 0 VO VO vO Ov 0 0 OO •o 0 0 ? 3 un VO US t - - ■N" C-- CN s o SO OS u n 0 0 Ov c n c n o u s TP CN Ov c n o p TT c n p US 3 " VO 0 0 CN TP c n o s s d c n u n CN o o 0 0 c n s d CN d d & CN v d o c n CN O s 2 2 o d p"' 0 0 o s o o VO Mp 0 0 VO u s c n 0 0 Ov VO O s CN TP c n o s P - O s u s 0 0 US CN u s US c n c n VO u s u s OS CN 0 0 CN —— u s 0 0 — c n — TP vO d I d d d d d —' d d d d d d 0 0 VO c n vO c n CN OS 0 0 CN CN 0 0 OS CN OS o s — c n o o C' Ov OS c n CN c n O CN p c n Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD O Q. C g Q. ■D CD MPC-44 C/) C/) J = 0.0003518 +/- 0.5%; 4 amu discrimination = 1.01561+/- 0.59%, 40/39K = 0.01175 +/- 5.4%. 36/37Ca = 0.00028768 +/- 3.8%,39/37Ca = 0.0006856 +/- 0.7% Crystal T (C) t (min) 36Ar 37Ar 38Ar 39Ar 40Ar %40Ar* % 39Ar rlsd 40Ar*/39ArK Age (Ma) Is.d. 1 1600 6 0.04 0.15 0.37 27.00 650.29 99.0 5,0 23,7804 15.03 0.08 8 2 1600 6 0.04 0.12 0.19 13.62 336.63 97.5 2,5 23,8280 15.06 0.08 g 3 1600 6 0.79 0.07 0.15 0.67 4569.74 95.3 0,1 6528,2001 2152.08 6,93 4 1600 6 0.10 0.62 0.07 3.94 120.28 80,3 0,7 23.7327 15.00 O il 5 1600 6 0.05 0.13 0.32 24,01 583.36 98.6 4.5 23.8598 15.08 0.09 6 1600 6 0.07 0.22 0.86 66.06 1619,18 99.1 12.4 24.3206 15.37 0.08 CD 7 1600 6 1.64 0.18 1.95 127.09 3320.18 85,9 23.8 22.4937 14.22 0.08 8 1600 6 0.16 1.28 0.15 9.60 259.31 84,8 1.8 22,5572 14.26 0.08 3 3 " 9 1600 6 0.07 1.05 0.12 8.21 203.59 93,7 1.5 22.7478 14.38 0.08 CD 10 1600 6 0.26 0.11 0.75 54.09 1285.07 94,4 10.1 22.4302 14.18 0.08 ■DCD 11 1600 6 0.104 0.512 0.058 3.25 103.24 74.3 0,6 22.6525 14.32 0.15 O Q. 12 1600 6 0.14 0.105 0.595 44.39 1033.17 96,4 8.3 22,4334 14,18 0.08 C a 13 1600 6 0.109 0.65 0.075 4.00 120.02 76.8 0.7 22,2730 14.08 0.10 3O 14 1600 6 0.109 0.216 0.655 48.64 1191.88 97.8 9.1 23,9758 15.15 0.08 "O 15 1600 6 0.422 0.141 1.149 81.38 1939.69 94.0 15.2 22,4302 14.18 0.08 O 16 1600 6 0.221 0.676 0.107 4.25 156.38 63.8 0.8 22,8272 14.43 0.09 17 1600 6 0.16 0.056 0.044 1.241 2617,55 98.4 0,2 2077,8255 989.87 7.69 CD Q. 18 1600 6 0.016 0.057 0.04 3,347 9495,84 100.0 0,6 2842,8140 1250.58 8.14 19 1600 6 0.325 0.05 0.104 3.203 6577.71 0,6 2029.7821 972.17 846 20 1600 6 0.024 0.043 0.016 1.211 4505,11 100.0 0,2 3746.9974 1516.85 8.52 ■D 21 1600 6 0.031 0.05 0.025 1.595 4434.59 100.0 0,3 2796.4799 1235.82 CD 7,52 22 1600 6 1.42 0.084 0.327 3.88 10171 96.0 0,7 2530.1033 1148,52 7.38 C/) C/) note: isotope beams in mV rlsd = released, error in age includes 0.5% J error. Mean +/- s.d. = 431,13 646,02 ail errors 1 sigma Mean +/- s.d. 14Ma= 14.25 0.10 (Not corrected for decay) Mean +/- s,d, 15Ma= 15.12 0.12 Mean +/- s.d. = 1323.70 378,40 VO 00 APPENDIX 3 GEOCHEMISTRY DATA 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 100 Oxides (wt%) MPC-1 MPC-2 MPC-3 MPC^ MPC-5 MPC-6 S1Ü2 74.47 58.87 60.31 59.839Ô 73.51 77.(35“ TiO: 0 . 2 0 0.71 0.92 0.91283 0.26 0.13 AI2O3 14.21 18.11 16.68 16.55 14.12 1 1 . 6 8 Fe2 0 3 1 . 1 2 6 . 2 0 6.07 6.02268 1.31 0.38 MgO 0.33 2 . 0 2 3.60 3.57194 0.24 0.23 CaO 0.79 4.29 4.96 4.92134 1 . 1 2 0.57 Na?0 3.46 3.46 3.40 3.3735 3.71 2.98 K, 6 5.15 4.18 3.16 3.13537 4.73 5.51 0.00 P2 O5 0.02 0.53 0.38 0.37704 0.04 MnO 0.06 0.14 0.12 0.11906 0.04 0.01 LOI 0.92 0.63 2.82 1.67 0.93 0.64 Totals 100.73 99.14 102.42 100.49 100.00 99.15 Trace elements (ppm) Rb 174.90 118.06 86.37 21.56 168.59 213.00 Sr 172.14 1517.80 765.38 684.33 188.59 67.80 Y 9.28 40.19 58.74 29.62 57.35 0.00 Zr 184.16 417.36 322.16 177.48 199.53 106.18 Nb 11.31 4.06 15.23 9.58 22.72 20.04 Cr _ — 54.38 214.11 —— Ni w — — 108.43 —— La 55.43 — 68.87 — 56.45 — Ce 91.51 — 121.33 — 94.18 — Pr 9.06 — 13.22 — 9.22 — Nd 29.98 — 49.48 — 31.70 — Sm 5.13 — 9.10 — 6.84 — Eu 0.83 — 2.24 — 1.20 — Gd 3.52 — 6.66 — 8.48 — Tb 0.59 — 0.96 — 1.54 — Dy 3.50 — 5.29 — 9.15 — Ho 0.71 — 1.01 — 1.77 — Er 1.98 — 2.66 — 4.63 — Tm 0.32 — 0.38 — 0.66 — Yb 2.07 — 2.39 — 3.94 — Lu 0.33 — 0.38 — 0.58 — Ba 638.00 — 1169.00 — 584.00 — Th 22.47 — 13.71 — 23.19 — Hf 4.86 — 6.90 — 5.30 — Ta 2.12 — 1.24 — 1.98 — U 3.32 — 2.43 — 4.69 — Pb 31.74 — 17.82 — 29.97 — Cs 1.80 —— 1.47 — 2.60 — Sc 2.60 — 14.40 — 3.30 — W 135.30 — 38.60 — 103.50 — Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 101 Oxides (wt%) MPC-7 MPC- 8 MPC-9 MPC-10 MPC-11 MPC-12 S1Ü2 4 8 . 8 8 50.Ô5 74.39 73.17 75.44 74.39 TiOz 1 . 2 0 1.38 0.13 0.16 0.18 0.14 AI2O3 15.95 16.38 10.82 12.76 12.31 11.17 FezOs 10.14 8.96 0 . 6 8 1.15 0.92 0.80 MgO 9.27 5.47 0.69 1.08 0.42 0.23 CaO 9.22 9.52 1 . 1 2 3.18 1.27 1 . 2 2 NaiO 2.73 3.37 2.54 0.67 2.75 3.48 K2 6 0.74 1.84 4.59 3.01 4.74 4.66 0 . 0 2 P2O5 0.29 0.95 0 . 0 0 0 . 0 1 0 . 0 2 MnO 0.15 0.14 0.07 0.06 0.07 0.07 LOI 1.45 2.73 6.71 13.00 5.44 1 . 2 0 Totals 1 0 0 . 0 1 100.79 101.74 108.25 103.56 97.38 Trace elements (ppm) Rb 10.83 41.82 181.57 163.74 158.63 158.84 Sr 636.35 1773.20 81.24 792.56 634.90 126.07 Y 33.50 47.20 43.42 0.76 15.75 16.40 Zr 159.91 418.47 123.51 164.00 190.46 154.23 Nb 4.57 40.60 15.50 4.72 12.82 16.31 Cr 349.83 50.04 —— — — Ni 176.29 — ——— — La 20.47 94.61 40.94 38.61 — — Ce 42.57 181.92 72.22 70.01 — — Pr 5.34 22.99 7.09 8.08 — — Nd 23.40 94.77 23.47 28.36 — — Sm 5.48 18.76 4.55 6.43 — — Eu 1.73 4.72 0.60 0.77 — — Gd 4.98 12.91 3.38 4.82 — — Tb 0.80 1.64 0.60 0.78 — — Dy 4.68 8 . 0 1 3.61 4.27 — — Ho 0.94 1.39 0.74 0.79 — — Er 2.49 3.30 2.13 2.08 — — Tm 0.35 0.44 0.34 0.29 — — Yb 2 . 1 1 2.55 2.25 1.75 — — Lu 0.33 0.39 0.35 0.26 — — Ba 390.00 1941.00 159.00 79.00 — — Th 1.79 10.55 21.57 26.62 — — Hf 3.12 7.45 4.64 4.43 — — Ta 0.64 0 . 8 6 1.52 1 . 6 6 — — U 0.37 2.48 3.66 2.09 — — Pb 3.11 18.24 29.91 5.29 —— Cs 0.37 2.91 2.31 2.92 — — Sc 31.20 23.50 3.10 7.00 — — W 75.80 48.30 2 . 2 0 1 . 2 0 — — Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 102 Oxides (wt%) MPC-13 MPC-I4 MPC-15 MPC-16 MPC-17 MPC-18 SiOz 70.40 73.78 75.49 75.63 56.50 71.36 TiOz 0.24 0.23 0.17 0.19 1 . 0 2 0.19 AlzOj 13.43 11.53 1 2 . 6 6 13.32 15.73 13.00 FezOs 1.31 1.23 0.83 0.76 6.63 1.08 MgO 0.41 0.37 0.24 0.23 4.60 0.59 CaO 2.03 2 . 2 0 0.94 0.64 6.51 0.82 Na,0 3.27 2.93 3.68 3.48 3.20 3.07 KzO 4.77 4.83 4.84 5.01 3.01 5.27 P1O5 0.05 0.04 0 . 0 1 0 . 0 1 0.48 0 . 0 1 MnO 0.08 0.07 0.06 0 . 0 2 0.07 0.07 LOI 4.01 5.12 1.05 0.69 47.99 4.31 Totals 1 0 0 . 0 1 102.33 99.97 99.99 145.74 99.78 Trace elements (ppm) Rb 161.01 178.39 180.76 183.88 84.39 183.86 Sr 195.32 186.53 80.36 103.36 988.34 131.39 Y 19.22 9.92 9.85 23.97 32.41 10.77 Zr 206.88 180.06 119.86 150.48 333.66 150.36 Nb 20.80 18.75 20.23 20.54 9.78 19.45 Cr — 0.51 — — 132.55 — Ni —— — — 57.05 — La — 66.27 39.65 — — 53.91 Ce — 99.37 70.23 — — 92.65 Pr — 10.51 5.95 — — 8.74 Nd — 34.49 18.35 — — 28.61 Sm — 5.94 3.01 —— 4.87 Eu — 0 . 8 8 0.45 —— 0.77 Gd — 4.22 2.24 —— 3.45 Tb —— 0 . 6 8 0.38 — — 0.56 Dy — 4.05 2.34 —— 3.25 Ho — 0.83 0.52 —— 0.67 Er — 2.33 1 . 6 8 —— 1.95 Tm 0.37 0.29 —— 0.31 Yb — 2.44 2 . 0 2 —— 2.03 Lu — 0.39 0.35 — — 0.33 Ba — 404.00 191.00 — — 503.00 Th — 24.95 25.71 —— 23.35 Hf — 5.09 4.01 — — 4.77 Ta — 1 . 6 6 2.38 ~ — 1.87 U — 4.13 3.84 —— 4.04 Pb — 29.14 29.11 —— 32.56 Cs — 3.34 3.27 —— 2.79 Sc — 3.10 2.70 — — 3.20 W — 2.30 188.50 — — 88.90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 103 Oxides (wt%) MPC-19 MPC-20 MPC-21 MPC-22 MPC-23 MPC-24 S1Ü2 76.03 48.81 73.44 72.72 51.34 51.20 TiOz 0.14 1 . 1 2 0.13 0 . 1 2 1.28 1.35 AI2 O3 12.89 15.44 1 2 . 2 1 12.54 17.38 16.58 FezOz 0.74 8.41 0.72 0.62 7.80 9.08 MgO 0.23 9.95 0.28 0.23 5.46 6.04 CaO 0.80 9.60 2.47 0.91 9.25 10.06 NazO 3.53 2 . 6 6 3.64 3.21 3.08 2.85 K ,0 4.69 1.75 4.81 4.78 1.79 1.28 P2 O5 0.03 0.62 0.03 0 . 0 0 0.41 0.47 MnO 0 . 1 2 0.15 0.06 0.08 0 . 1 2 0.13 LOI 0.77 1.47 2.07 4.79 3.10 2.38 Totals 99.98 99.98 99.87 99.99 1 0 1 . 0 1 101.42 Trace elements (ppm) Rb 240.24 22.40 175.12 169.57 41.52 27.81 Sr 51.60 1285.40 97.22 128.73 882.70 8 6 8 . 2 1 Y 0 . 0 0 39.74 5.23 12.96 27.10 31.88 Zr 96.64 279.14 116.39 109.03 233.75 2 1 2 . 1 0 Nb 22.92 4.06 18.11 2 2 . 8 6 10.03 7.04 Cr — 418.15 —— 127.40 213.69 Ni — 208.53 —— 66.58 86.71 La 33.72 70.19 41.65 —— — Ce 55.80 130.98 65.83 ——— Pr 4.82 15.69 5.94 - — — Nd 13.89 63.43 17.77 ——— Sm 2.17 12.49 2.74 — — — Eu 0.27 3.27 0.40 — — - Gd 1.53 9.07 1.98 —— — Tb 0.27 1 . 2 1 0.34 —— - Dy 1.76 6 . 1 0 2.08 —— — Ho 0.41 1 . 1 1 0.47 —— — Er 1.40 2.78 1.46 —— — Tm 0.26 0.38 0.25 —— — Yb 1.97 2.24 1.83 —— — Lu 0.36 0.35 0.31 —— — Ba 124.00 1257.00 178.00 — — — Th 33.29 1 0 . 2 2 24.43 —— — Hf 3.87 4.68 3.68 —— — Ta 2.74 0.99 1 . 8 8 —— — U 7.62 2.59 4.28 — — — Pb 33.56 15.93 31.37 — —— Cs 4.70 1.14 3.17 —— — Sc 3.50 32.00 2.70 —— — W 183.10 74.60 97.70 ——— Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 104 Oxides (wt%) MPC-25 MPC-26 MPC-27 MPC-29 MFC-30 MPC-31 SiOz 74.24 73.64 71.78 70.79 74.66 71.50 TiOz 0.25 0.24 0.25 0.43 0.25 0.31 AlzOz 13.16 13.04 11.79 1 2 . 8 6 13.31 13.13 FezOj 1.35 1.29 1.62 3.04 1.34 1.58 MgO 0.30 0.37 1.56 0.83 0.29 1 . 0 1 CaO 1 . 0 1 1.92 2.79 2.17 0 . 8 6 2.49 NazO 3.49 3.27 1.73 2.33 3.54 1.67 KzO 4.54 4.72 4.19 4.24 4.44 4.37 0.08 P2 O5 0.03 0.04 0.04 0.06 0.03 MnO 0.07 0.07 0.04 0.29 0.07 0.04 LOI 3.64 4.22 10.87 8.61 3.27 10.94 Totals 102.08 102.82 106.66 105.65 102.06 107.12 Trace elements (ppm) Rb 174.79 158.79 164.50 160.57 165.43 169.25 Sr 166.27 185.32 168.90 220.62 178.52 301.74 Y 14.58 20.03 5.52 52.01 15.13 28.69 Zr 192.30 195.64 201.67 284.67 193.67 257.87 Nb 21.14 26.13 12.44 31.90 18.99 13.05 Cr —— — - — — Ni —— — — — — La 59.56 58.04 — — 55.51 — Ce 98.64 95.67 — - 93.80 — Pr 9.68 9.33 —— 9.12 — Nd 31.90 30.86 — — 30.18 — Sm 5.52 5.25 — — 5.25 — Eu 0 . 8 8 0.89 — — 0.84 — Gd 4.03 3.80 — — 3.79 — Tb 0.65 0.62 —- 0.61 — Dy 3.85 3.62 — — 3.69 — Ho 0.78 0.75 — — 0.77 — Er 2.27 2.17 — — 2 . 2 0 — Tm 0.36 0.34 — — 0.36 — Yb 2.43 2.31 — — 2.39 — Lu 0.39 0.37 —— 0.39 — Ba 525.00 570.00 —— 528.00 — Th 24.06 22.42 — — 23.91 — Hf 5.62 5.24 — — 5.45 — Ta 3.08 2 . 0 0 — — 2.34 — U 4.16 4.03 — — 4.09 — Pb 32.61 30.05 — — 31.21 — Cs 3.33 3.15 — — 3.12 — Sc 3.30 3.00 — — 2.80 — W 318.70 121.80 — — 164.50 — Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 105 Oxides (wt%) MPC-32 MPC-33 MPC-34 MPC-36 MPC-37 MPC-38 S1Ü2 75.97 76.20 75.03 72.15 70.54 74.87 TiOz 0 . 1 2 0 . 1 2 0.13 0.14 0.15 0.17 AIzOz 13.21 13.06 13.65 11.72 7.89 1 2 . 0 1 FezOz 0.67 0.44 0.65 0.76 0.72 0.92 MgO 0.23 0.23 0.23 0.25 0.39 0.29 CaO 0.73 0.85 0.70 0.61 1.49 0.81 NazO 3.66 3.28 3.67 3.41 4.20 3.24 KzO 4.93 4.73 4.88 5.76 4.35 5.15 0 . 0 0 0 . 0 1 0 . 0 1 P2 O5 0 . 0 0 0 . 0 1 0 . 0 0 MnO 0.08 0.05 0.07 0.07 0.06 0.07 LOI 1.07 1.46 1 . 0 1 4.63 9.74 4.40 Totals 100.67 100.43 1 0 0 . 0 1 99.50 99.54 101.94 Trace elements (ppm) Rb 164.82 165.37 159.85 164.17 116.19 158.21 Sr 50.27 53.23 57.79 84.34 295.80 150.09 Y 15.78 13.16 17.69 5.56 5.05 14.71 Zr 108.32 117.06 105.41 108.34 110.93 160.47 Nb 26.17 26.36 23.94 17.36 12.57 17.94 Cr — — — — — — Ni —— — — — — La — 38.58 — 41.21 52.12 55.14 Ce — 34.92 — 71.03 89.49 92.28 Pr — 5.25 — 6.84 8.48 8.89 Nd — 15.68 — 21.62 27.44 29.29 Sm — 3.03 3.69 4.64 5.05 Eu — 0.62 —— 0.49 0.72 0.73 Gd — 2.80 — 2.65 3.27 3.53 Tb — 0.53 — 0.46 0.53 0.57 Dy — 3.37 — 2.87 3.17 3.45 Ho — 0.72 — 0.60 0.65 0.70 Er — 2 . 0 2 —— 1.81 1.91 2 . 0 1 Tm — 0.32 — 0.30 0.30 0.33 Yb — 2.07 — 2 . 1 0 2 . 0 0 2.16 Lu — 0.33 — 0.34 0.32 0.35 Ba — 1 1 2 . 0 0 — 105.00 488.00 508.00 Th — 19.24 — 25.03 22.85 24.09 Hf — 4.42 — 4.01 4.48 4.99 Ta — 2.09 — 1.62 1.44 2.29 U 3.20 — 4.51 3.87 4.34 Pb — 29.31 — 35.85 30.12 33.46 Cs — 1.69 — 3.58 2.99 2 . 8 6 Sc — 3.00 — 2.70 2.80 2.80 W — 126.70 — 2.50 2 . 0 0 168.00 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 106 Oxides (wt%) MPC-39 MPC-40 MPC-41 MPC-42 MPC-43 MPC-44 SiO, 74.99 75.33 76.33 75.86 72.50 73.83 T iO z 0 . 1 1 0.14 0 . 1 2 0 . 1 2 0.18 0.15 A lzO z 12.60 11.26 10.15 12.91 12.31 9.71 FezOj 0 . 6 6 0.71 0.60 0.64 0.99 0.93 MgO 0.61 0.23 0.23 0.23 0.27 0.42 CaO 0.70 0.50 0.35 0.48 0.58 1.06 NazO 2.39 3.60 3.25 3.42 4.05 3.79 K,0 5.43 5.33 4.53 5.02 5.27 4.30 P2O5 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 2 MnO 0.09 0.07 0.07 0.08 0.08 0.06 LOI 5.44 4.65 4.51 3.91 3.24 9.14 Totals 103.02 101.82 100.14 102.67 99.48 103.41 Trace elements (ppm) Rb 182.13 165.83 156.90 170.41 157.62 154.10 Sr 40.58 62.67 33.54 36.48 71.24 124.95 Y 14.45 10.92 9.59 19.36 1 0 . 0 1 8 . 2 2 Zr 113.18 109.95 102.42 108.23 118.95 133.57 Nb 22.04 26.18 24.12 20.91 19.96 18.04 Cr ————— 12.46 Ni — — —— — — — La 29.38 36.16 41.06 34.51 51.35 35.85 Ce 59.06 63.62 68.16 63.80 85.52 6 6 . 8 6 Pr 5.90 6.15 6.38 6.72 8.27 6.48 Nd 20.63 19.73 19.94 23.14 26.39 22.14 Sm 4.71 3.65 3.72 5.15 4.40 4.42 Eu 0.59 0.47 0.42 0.67 0.61 0.64 Gd 3.73 2 . 6 6 2.75 4.14 3.10 3.38 Tb 0.65 0.45 0.49 0.70 0.53 0.56 Dy 4.05 2.79 3.12 4.34 3.18 3.35 Ho 0.82 0.60 0.67 0.87 0 . 6 8 0 . 6 8 Er 2.31 1.79 2.04 2.45 1.94 1.87 Tm 0.36 0.30 0.34 0.38 0.32 0.30 Yb 2.30 2.06 2.27 2.44 2.17 1.92 Lu 0.36 0.34 0.36 0.38 0.35 0.31 Ba 133.00 143.00 8 6 . 0 0 131.00 287.00 295.00 Th 20.82 24.48 24.79 2 0 . 1 1 25.39 20.92 Hf 4.40 4.13 4.37 4.19 4.64 4.36 Ta 1.58 3.39 2.78 1.48 1.59 1.36 U 3.44 4.53 4.68 3.74 4.53 5.48 Pb 32.53 30.87 33.77 32.99 30.24 27.46 Cs 2.35 3.07 3.26 2.51 3.13 2.38 Sc 3.20 2.90 2 . 2 0 3.10 2.90 3.60 W 2.30 412.60 271.10 2.50 2.30 1.70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX 4 ISOTOPE DATA 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 108 Sample MPC-1 MPC-3 MPC-4 ^^Sr/^Sr 0.71018 0.70822 0.70685 Epsilon Nd -8.75 -7.58 -3.86 206pb/204pb 18.091 18.283 18.628 207pb/204pb 15.56 15.599 15.596 208pb/204pb 39.178 39.262 39.013 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX 5 DISTRIBUTION COEFFICIENTS 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD O Q. C g Q. ■O CD Element Quartz Sanidine Plagioclase Biotite Amphibole Zircon Allanite Apatite Magnetite WC/) Sc 0.00 0.03 0,17 15.50 3.20 68.70 49.40 0.00 8.90 3o" Rb 0.00 0.31 0.024 5.30 0.25 0.00 0.00 0.00 0.00 O Sr 0.00 4.00 24.00 0.53 0.57 0.00 100.00 2.00 0.15 Y 0.00 0.10 0.07 1.00 0.89 60.00 100.00 40.00 1.00 ■D8 Zr 0.00 0.05 0.36 180 1.10 6400.00 2.00 0.10 8.00 Cs 0.00 0.008 0.05 2.40 na 0.00 0.00 0.00 0.00 Ba 0.00 10.00 3.50 3.70 0.31 0.00 0.00 2.00 0.00 La 0.00 0.067 0.26 3.55 0.27 16.90 2362.00 20.00 26.00 Ce 0.00 0.027 0.14 3.49 0.34 16.80 2063.00 35.00 22.90 Sm 0.00 0.0025 0.021 1.76 0.91 14.40 756.00 63.00 12.49 3. 3 " Eu 0.00 5.10 4.90 0.87 na 16.00 122.00 30.00 2.80 CD Tb 0.00 0.013 0.005 1.20 1.40 37.00 235.00 20.00 7.50 CD ■D Yb 0.00 O 0.0025 0.15 0.69 0.97 527.00 24.50 25.00 1.00 Q. C Lu 0.00 0.003 0.12 0.80 0.89 642.00 22,00 25.00 0.91 a O Hf 0.00 0.02 0.096 0.68 0.50 3742.00 9.80 0.10 1.80 3 ■D Ta 0.00 0.0007 0.13 1.30 1.30 47.50 1.90 0.00 1.13 O Th 0.00 0.003 0.002 1.74 0.30 76.80 420.00 2.00 13.10 U 0.00 0.002 0.15 CD 0.19 na 340.50 14.00 0.00 0.21 Q. Mahood and Hildreth (1983); Michael (1983); Nash and Crecraft (1985); Stix and Gorton (1990); Bradshaw (1991), ■D CD C/) C/) Ill VITA Graduate College University of Nevada, Las Vegas Aaron Lee Sanford Home Address: P.O. Box 245 Surrey, ND 58785 Degrees: Bachelor of Science, Geology, 1997 University of Idaho Publications: Sanford, A.L., Smith, E.I., and Spell, T.L., 1999, The McCullough Pass Caldera, southern Nevada: Geometry of a relatively undeformed volcanic center in the highly extended Northern Colorado River Extensional Corridor: Geological Society of America Abstracts with Programs, v. 31, no. 7, p. 262. Omdorff, R.L., Downing, R.F., Jones, T.A., Ogan, B.D., Rotert, J.W., Sanford, A.L., Williams, N.D., and Willse, K.R., 1999, Building a GIS-based flood model for the Las Vegas basin in an applied spatial modeling course at the University of Nevada, Las Vegas: Geological Society of America Abstracts with Programs, v. 31, no. 7, p. 271 Thesis Title: Evolution of a Small Silicic System: The McCullough Pass Caldera, Clark Coimty, Nevada Thesis Examination Committee: Chairperson, Dr. Eugene Smith, Ph.D. Committee Member, Dr. Michael Wells, Ph.D. Committee Member, Dr. Terry Spell, Ph.D. Graduate Faculty Representative, Dr. Chih-Hsiang Ho, Ph D. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NOTE TO USERS Oversize maps and charts are microfilmed in sections in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, WITH SMALL OVERLAPS This reproduction is the best copy available. u m t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Plate 1 : Geologic Ma 115' Iff G ' y ., r - 47' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vlap of the McCullough Pq 1 5 o y § Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ullough Pass Caldera Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46' Qal-Quatémary Alluvium Tjl-Jean Lake Rhyolite Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 11 « n EXPLANATION Strike-and dip symbol, 40 equals dip in degrees Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. n dip m degrees Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 46: . Qal-Quatemary Alluvium : Tjl-Jean Lake Rlwolite Tliv-Hidden Valley Volcanics I Tmpo-McCullough Pass Tuff I Tcs-Capstone Rhyolite Tbs-Tuft* of Bridge Spring Trh-Ramhead Rhyolite Tev-Eldorado Valley Volcanics Tpv-Pumice Mine Basalt I PC-Precambrian Basement Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 y j I 7 . EXPLANATION 40 ean Lake Rlivolite Strike and dip symbol. 40 equals dip in degrees Fault-Bar is on downdroppcd block o-McCulIough Pass Tuff Contact Liift' of Bridge Spring “ Caldera wall Rhyolite Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ni ils dip in degrees )Iock Scale 1:12.000 0 Kilometers • I 0 Meters 1000 0 Miles Contour interval-40 feet Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.