CHARACTERIZING AND IDENTIFYING THE ERUPTIVE SOURCE OF THE
SOLDIER MEADOW TUFF IN NORTHWESTERN NEVADA
Julie A. Smith
B.S., University of California, Davis, 2007
THESIS
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
GEOLOGY
at
CALIFORNIA STATE UNIVERSITY, SACRAMENTO
SUMMER
2011
CHARACTERIZING AND IDENTIFYING THE ERUPTIVE SOURCE OF THE
SOLDIER MEADOW TUFF IN NORTHWESTERN NEVADA
A Thesis
by
Julie A. Smith
Approved by:
__________________________________, Committee Chair
Dr. Brian P. Hausback
__________________________________, Second Reader
Dr. Lisa Hammersley
__________________________________, Third Reader
Dr. Christopher D. Henry
____________________________
Date
ii
Student: Julie A. Smith
I certify that this student has met the requirements for format contained in the University
format manual, and that this thesis is suitable for shelving in the Library and credit is to
be awarded for the thesis.
__________________________, Department Chair ___________________
Dr. David Evans
Date
Department of Geology
iii
Abstract
of
CHARACTERIZING AND IDENTIFYING THE ERUPTIVE SOURCE OF THE
SOLDIER MEADOW TUFF IN NORTHWESTERN NEVADA
by
Julie A. Smith
The Soldier Meadow Tuff (SMT) consists mostly of a voluminous sheet of distinctive
phenocryst-rich comendite ash-flow tuff distributed around the eastern to southern margin of the
High Rock caldera (HRC), source of the 16.3 Ma Summit Lake Tuff. The SMT is a lithologically
complex single cooling unit composed of several repetitions of planar and cross-bedded surges,
lag-breccias, and massive lithofacies. Stratigraphic repetition of these deposits suggests the SMT
erupted in several, closely-spaced pulses, each of which developed eruption columns and column
collapses. The HRC is the likely source of the SMT, which erupted approximately 16.1 Ma. The
ash-flow sheet of the SMT thins away from ring fracture vents, and pumice imbrications and
surge deposits indicate southeast flow away from the HRC. Eruption of the SMT may have
caused additional caldera collapse as indicated by aeromagnetic lows in the southeastern part of
the HRC. Collinear trends of major element-oxides, trace elements, and rare earth elements
genetically link Summit Lake Tuff, Soldier Meadow Tuff, Soldier Meadow lavas, and postSoldier Meadow pyroclastic and lava units to a common magmatic source, likely a continental
tholeitte. The magma evolved through typical crystal fractionation processes.
_______________________, Committee Chair
Dr. Brian P. Hausback
_______________________
Date
iv
ACKNOWLEDGMENTS
I extend great gratitude to my advisor, Dr. Brian Hausback, of California State University,
Sacramento, for his support and guidance since we started this project in 2009. Dr. Hausback has
demonstrated the attributes of a remarkable geologist in the field, laboratory, and classroom. He
has been a great companion in the field, and I thank him for the all the great memories and
experiences from one of the most beautiful and geologically fascinating places I have visited.
I also express thanks to Dr. Christopher Henry of the Nevada Bureau of Mines and Geology at
University of Nevada, Reno, for his insurmountable contributions to this project. Dr. Henry has
generously provided geochemical data and Ar/Ar dates for samples he collected prior to my
involvement in this project. Much of the data and interpretations of this project come from
insightful discussions with Dr. Henry. Also, I thank Richard Hilton of Sierra College for his
contributions to field work, as well as his continued support and guidance. It has been very
rewarding conducting field work with Dr. Henry and Richard Hilton and I appreciate their
dedication to this project.
Additionally, I would like to thank Robin Wham for her voluntary position as my field assistant.
Her enthusiasm and geologic curiosity are respectable traits that inspired me in the field. I thank
her for the many great memories we shared on our trips.
I extent great thanks to my thesis committee: Dr. Brian Hausback, Dr. Lisa Hammersley, and Dr.
Christopher Henry, for their time, patience, and guidance in reviewing my work.
Finally, I give immense thanks to my family and friends. I appreciate their continued love and
support that helped me to achieve my goals.
This project was mostly funded by the United States Geological Survey EDMAP cooperative
mapping program in early 2010. Funding from EDMAP covered expenses for field work, as well
as XRF and ICP-MS geochemical analyses conducted at the GeoAnalytical Laboratory at
Washington State University. I give thanks to the staff at the GeoAnalytical Laboratory for their
time and effort into analyzing my samples.
Additional funding originated from the Jack Kleinman Memorial Fund for Volcanic Research
Scholarship in 2009, awarded by the Community Foundation for Southwest Washington in
collaboration with the Cascades Volcano Observatory. Funding was also received in 2009 from
the Northern California Geological Survey Scholarship. This funding helped with field expenses
for my first field season in High Rock caldera in summer 2009.
v
TABLE OF CONTENTS
Page
Acknowledgments..................................................................................................................... v
List of Tables ........................................................................................................................ viii
List of Figures .......................................................................................................................... ix
Illustrations ................................................................................................................................ x
Chapter
1. INTRODUCTION ................................................................................................................. 1
Previous Studies and Proposed Calderas ...................................................................... 3
Geologic Setting .......................................................................................................... 7
Regional Geology ............................................................................................ 7
High Rock Caldera Complex ........................................................................... 7
Development of the Basin and Range Province .............................................. 9
2. METHODS .......................................................................................................................... 10
3. RESULTS ............................................................................................................................ 12
Geologic Mapping – Stratigraphy of Volcanic Units in the Eastern HRC ................. 12
Summit Lake Tuff .......................................................................................... 12
Rhyolite Lava................................................................................................. 12
Soldier Meadow Tuff ..................................................................................... 15
Soldier Meadow Lavas .................................................................................. 20
Bedded Sequence of Intermediate-Silicic Pyroclastic Deposits .................... 22
Rhyolite Lavas and Domes ............................................................................ 23
Basaltic Andesite to Dacite Lavas and Dikes ................................................ 24
Petrography ................................................................................................................. 25
Geochemistry .............................................................................................................. 32
XRF Analyses of Major and Trace Elements ................................................ 32
ICP-MS Analyses of Rare Earth Elements .................................................... 50
4. DISCUSSION ...................................................................................................................... 54
Mapping Interpretations .............................................................................................. 54
Eruptive Model for the Soldier Meadow Tuff ............................................................ 57
Geochemical and Petrological Interpretations ............................................................ 64
vi
5. CONCLUSIONS.................................................................................................................. 68
Appendix A. Methods ............................................................................................................ 71
Geologic Mapping................................................................................ 71
Petrographic Analysis .......................................................................... 71
Geochemical Analysis.......................................................................... 73
Ar/Ar Dating ........................................................................................ 74
Appendix B. Petrography of the High Rock Caldera Volcanic Suite .................................... 76
Appendix C. XRF Data: Major Element Oxides of the High Rock Caldera Volcanic Suite .....
....................................................................................................................... 86
Appendix D. XRF Data: Trace Elements of the High Rock Caldera Volcanic Suite ............ 91
Appendix E. ICP-MS Data of the High Rock Caldera Volcanic Suite .................................. 95
References ............................................................................................................................. 100
vii
LIST OF TABLES
Page
1.
Table 1 Radiometric Dating of the High Rock Caldera Volcanic Suite ....................... 13
2.
Table 2 Petrography of the High Rock Caldera Volcanic Suite ................................... 25
viii
LIST OF FIGURES
Page
Figure
1.
Figure 1 Location map of the HRCC and Yellowstone hot spot track ............................... 2
2.
Figure 2 Location map of the calderas of the HRCC. ......................................................... 6
3.
Figure 3 Map of geographic features of the High Rock caldera ....................................... 14
4.
Figure 4 Proximal lithofacies of the Soldier Meadow Tuff .............................................. 18
5.
Figure 5 Massive lithofacies of the Soldier Meadow Tuff ............................................... 19
6.
Figure 6 Coarse-grained Soldier Meadow Tuff ................................................................ 28
7.
Figure 7 Fine-grained Soldier Meadow Tuff .................................................................... 28
8.
Figure 8 Total alkalis vs. silica of the High Rock caldera volcanic suite ........................ 33
9.
Figure 9 Agpaicity Index (AI) vs. silica of the High Rock caldera volcanic suite ........... 34
10.
Figure 10 Harker variation diagrams of major element oxides......................................... 38
11.
Figure 11 Trace element plots of the High Rock caldera volcanic suite .......................... 44
12.
Figure 12 Spider diagram of rare earth elements .............................................................. 51
13.
Figure 13 TiO2 – Y/Nb discrimination diagram ............................................................... 53
14.
Figure 14 Statigraphic deposits of a complete eruption episode....................................... 58
15.
Figure 15 Eruptive model for the Soldier Meadow Tuff .................................................. 63
ix
ILLUSTRATIONS
Page
Plate
1.
Plate 1 Geologic Map of the Southeastern Part of the High Rock Caldera, Northwestern
Nevada ................................................................................................................... in-pocket
2.
Plate 2 Stratigraphic Section of the Soldier Meadow Tuff: Ampitheatre .............. in-pocket
3.
Plate 3 Stratigraphic Section of the Soldier Meadow Tuff: High Rock Peak ........ in-pocket
CD-ROM containing pdfs of Plates 1-3................................................................. in-pocket
x
1
Chapter 1
INTRODUCTION
The High Rock caldera complex (HRCC) in northwestern Nevada is in the oldest
part and represents the location of initial impingement of the Yellowstone hotspot plume
beneath the western edge of the North American craton around 16.5 Ma (Perkins and
Nash, 2002; Castor and Henry, 2000; Figure 1). The High Rock caldera (HRC) is one of
four probable calderas in a northeast-aligned series that make up the HRCC, and is infilled and surrounded by numerous rhyolitic tuffs, lavas, and domes. The Soldier
Meadow Tuff and Summit Lake Tuff are voluminous peralkaline to mildly peralkaline
pyroclastic-flow deposits widely distributed around the HRC, but the source of these
outflow sheets and other major ash-flow tuffs and possible source calderas has been
controversial. Korringa (Korringa and Noble, 1970; Korringa, 1972; Korringa, 1973)
interpreted that the Soldier Meadow Tuff and lava members erupted from a series of
north-trending fissures located in Soldier Meadow. Greene and Plouff (1981) suggested
the Soldier Meadow Tuff erupted from a buried caldera in the Badger Mountain area
northwest of Soldier Meadow. Smith and others (2009; 2010) suggested HRC as a likely
eruption source of Soldier Meadow Tuff and related lavas. This study provides detailed
mapping, petrographic and geochemical analyses that support HRC as the likely eruption
source of the Soldier Meadow Tuff and related lavas.
2
Figure 1. Location map of the HRCC and Yellowstone hot spot track (modified from Perkins
and Nash, 2002). The dashed line represents the path of the Yellowstone hot spot track. Dark gray
polygons represent silicic volcanic centers; medium gray is the northern Nevada rift zone; black
circles illustrate four calderas of the High Rock Caldera Complex (HRCC); HRC – High Rock
caldera, which formed 16.33 Ma (Noble et al., 2009); McD – McDermitt caldera.The HRCC
likely represents volcanism associate with the incipient Yellowstone hot spot (Perkins and Nash,
2002).
3
PREVIOUS STUDIES AND PROPOSED CALDERAS
Field studies by Noble and others (1970) and Korringa (1973) identified
numerous ash-flow sheets distributed throughout northwest Nevada. Soldier Meadow
Tuff is a distinctive marker unit and named for exposures in Soldier Meadow, east of
HRC. Noble and others (1970) and Korringa (1973) described field characteristics and
petrography of the Soldier Meadow Tuff and a group of lavas that are petrographically
and compositionally- similar to Soldier Meadow Tuff (herein referred to as Soldier
Meadow lavas (SM-lavas)). Korringa (1973) suggested the Soldier Meadow Tuff and
lava members of middle Miocene age erupted from a linear fissure-vent complex 5 miles
north of Soldier Meadow Ranch, herein referred as Korringa vent area (KVA; Figure 2).
Greene and Plouff (1981) identified several gravity lows and magnetic anomalies
in northwestern Nevada and interpreted them to be associated with calderas. An area
centered approximately 30-35 kilometers slightly northwest of Soldier Meadow in the
Badger Mountain region has distinct gravity and magnetic minimums (Greene and Plouff,
1981). Greene and Plouff (1981) interpreted this 300 km2 area as the Badger Mountain
caldera and the possible eruption source for the Soldier Meadow Tuff and
petrographically-similar rhyolite of Badger Mountain. A gravity minimum around Rock
Spring Table, northeast of the proposed Badger Mountain caldera, was interpreted to be a
smaller (approximately 80 km2) caldera. An area of approximately 250 km2 west of
Soldier Meadow displays a magnetic minimum that was interpreted to be a possible
caldera (Greene and Plouff, 1981), but gravity data was not available to further support
this interpretation.
4
The tuffs of Badger Mountain (TBM) and Alkali Flat (TAF) and the rhyolite of
Badger Mountain (TRB) are exposed along the northern margin of HRC near Badger
Mountain. These units were initially mapped as Soldier Meadow Tuff by Greene and
Plouff (1981) based on lithologic similarities. Park (1983) did detailed mapping in the
northern margin of HRC and around the Badger Mountain caldera, and distinguished
TBM, TAF, and TRB from the Soldier Meadow Tuff based on geochemical,
petrographical, and paleomagnetic data. Geochemical and petrological difference led
Park (1983) to conclude that TBM, TAF, and TRB erupted from different vents than that
of the Soldier Meadow Tuff. Additionally, TBM has reversed polarity of thermoremnant
magnetization (TRM), whereas TAF and Soldier Meadow Tuff have identical TRM
directions (Park, 1983). The stratigraphic relation of TBM, TAF, and TRB to other HRC
units remains uncertain.
Ach and Swisher (1990) named the High Rock caldera complex (HRCC) and
defined it as a northeast-aligned series of “nested” calderas that generated local, smallvolume ash-flow tuffs and lavas from ring fractures, including the Soldier Meadow Tuff.
Ach and Swisher (1990) suggested that Summit Lake Tuff, a relatively high-volume ashflow tuff in the HRCC, erupted from the proposed Badger Mountain caldera, and did not
identify the source of the Soldier Meadow Tuff and related lavas.
Noble and others (1970) identified and described the Summit Lake Tuff from
exposures in northern Black Rock Range, mapped approximate distributions of the tuff
(D.C. Noble’s unpublished map), and did preliminary geochemical analyses on Summit
Lake Tuff (Noble et al., 1970). Noble and others (2009) suggested that eruption of this
5
voluminous ash-flow tuff resulted in initial collapse of HRC approximately 16.33 Ma.
Detailed geologic mapping by Smith and others (2009; 2010) indicates Soldier Meadow
Tuff erupted from the eastern to southern HRC ring-fractures approximately 16.1 Ma,
followed by SM-lavas and other post-Soldier Meadow volcanic units.
6
Figure 2. Location map of the calderas of the HRCC (modified from Erwin et al., 2005). The
HRCC was defined by Ach and Swisher (1990) as a series of NE-aligned “nested” calderas.
Virgin Valley caldera formed 16.30 Ma from the eruption of Idaho Canyon Tuff (Castor and
Henry, 2000). High Rock caldera (HRC) formed 16.33 Ma from the eruption of Summit Lake
Tuff (Noble et al., 2009). The Soldier Meadow Tuff erupted approximately 16.1 Ma from the SE
ring fractures of the HRC (Smith et al., 2009; 2010). The general area of study is illustrated by
the gray rectangle and is focused near the southeastern HRC ring fractures. TBM – tuff of Badger
Mountain; TRB – rhyolite of Badger Mountain; TAF – Tuff of Alkali Flat; KVA – Korringa vent
area (Korringa, 1973).
7
GEOLOGIC SETTING
Regional Geology
The HRCC lies in Humboldt and Washoe counties in far northwestern Nevada,
near 42˚N Latitude and 119˚W Longitude, within the North American Cordilleran
complex and west of the Northern Nevada rift zone. Models from Coney and Harms
(1984) indicate that during formation of the Cordillera, crustal thickening from largescale thrust faulting resulted in an estimated total crustal thickness of 40 kilometers in
northwestern Nevada. Gravitational and various lithospheric processes made the
thickened crust structurally unstable above the mantle, and the Cordillera region was
intruded with a swarm of plutons in Late Cretaceous time (Wernicke et al., 1987).
Wernicke and others (1987) suggest that rising plutons beneath the thick, unstable crust
lead to the onset of extensional faulting within the Cordillera Complex during the Eocene
epoch, further enabling magma to rise to the surface. Northwestern Nevada experienced
strong magmatism in Middle Miocene, coincident with Nevada rift-zone volcanism to the
east (Zoback et al., 1994). During this time, volcanism in the HRCC climaxed, creating
silicic calderas and widespread ash-flow sheets, such as Soldier Meadow Tuff and
Summit Lake Tuff.
High Rock Caldera Complex
Figure 2 (modified from Erwin et al., 2005) illustrates the approximate outlines of
four silicic calderas in the HRCC, located near the Black Rock Range, Pueblo Mountains,
and Calico Mountains in Washoe and Humboldt Counties: Virgin Valley, Badger
8
Mountain, High Rock, and Cottonwood caldera, from northeast to southwest,
respectively. (Cottonwood caldera has also been referred to as Hog Ranch caldera by
Erwin and others (2005). This study uses Cottonwood caldera to avoid confusion with the
acronym for High Rock caldera). Approximate ages of the calderas of the HRCC range
from 14.5 Ma to 16.5 Ma, based on field relationships and 40Ar/39Ar dating of their
associated volcanic units (Hilton, et al., 2008). Virgin Valley caldera formed during the
eruption of Idaho Canyon Tuff approximately 16.30 ± 0.06 Ma (Castor and Henry, 2000).
Badger Mountain and High Rock calderas were interpreted from gravity and
aeromagnetic anomalies from geophysical studies conducted by Greene and Plouff
(1981). Mapping studies by Park (1983) further support the hypothesis of a caldera
structure near Badger Mountain. Cottonwood caldera has not been studied in detail.
Volcanic rocks associated with this caldera are of unknown age.
Noble and others (1970) describe the stratigraphy and depositional relationships
of the units located within the HRCC from the Black Rock Range in the south to the Pine
Forest Range in the north. The oldest outcrops are Lower Miocene olivine basalt lavas,
found in the central Black Rock Range, and Middle Oligocene pyroclastic rocks in the
southern Calico Mountains, with ages of approximately 31.3 Ma (Bonham, 1969; Noble
et al., 1970). Overlying the lower units are Upper Miocene volcanic rocks dating from
16.6 Ma to 16.1 Ma (Perkins and Nash, 2002), including the Summit Lake Tuff, Soldier
Meadow Tuff, SM-lavas, and post-Soldier Meadow volcanic units.
9
Development of the Basin and Range Province
Despite extensive Basin and Range normal faulting in northwestern Nevada,
eruptive products and their field relationships are relatively well preserved. Faults
through northwestern Nevada typically express normal motion and generally trend
slightly northwest to northeast with high dip angles of 60 to 90 (Korringa, 1973), thus
coinciding with regional Basin and Range deformational trends. Attitudes of volcanic
units within the northern Black Rock Range show gentle tilting of 5 to 10 degrees to the
northwest. Basin and Range deformation in this region began after 15 Ma, but lowtemperature thermochronology in the northern Black Rock Range indicates that the
majority of normal faulting in this region occurred after 12 Ma (Lerch et al., 2008). A
major fault that has raised the west side of the Black Rock Range above the east side of
Soldier Meadow has offset the Soldier Meadow Tuff more than 3,000 feet (Noble et al.,
1970). More than 3,000 feet of displacement of volcanic units has been recognized by
Willden (1964) in the Pine Forest Range to the northeast of HRC.
10
Chapter 2
METHODS
This study included geologic mapping, petrography, geochemical analyses, and
Ar40/Ar39 dating analyses. Refer to Appendix A for more thorough mapping,
petrographic, and geochemical methods.
Detailed geologic mapping was conducted in parts of the Soldier Meadow,
Yellow Hills East, Yellow Hills West, and High Rock Lake 7.5 minute quadrangles, and
focused on mapping the Soldier Meadow Tuff, Soldier Meadow-type lavas, and other
related volcanic rocks (Plate 1). Special attention was given to document contacts, faults,
ring fractures associated with caldera margins, eruptive vents, and other important
features useful for establishing stratigraphic, structural, and magmatic relationships.
Fifty-one standard 30-micron thin-sections were made from these hand samples at
California State University, Sacramento. Detailed petrographic descriptions of multiple
units were recorded from these thin-sections, including mineralogy, modal percentages,
colors, presence of lithic fragments, and textures from volcanic and depositional
processes.
Whole-rock and pumice samples of ash-flow tuffs, lavas, and dikes representative
of igneous activity of the HRC were analyzed by XRF (57 samples) and ICP-MS (24
samples) at Washington State University GeoAnalytical Laboratory (Appendices C, D,
and E). Radiometric ages of collected samples were calculated from measured 40Ar/39Ar of
sanidine and anorthoclase phenocrysts using an Ar-Ar total fusion method. Radiometric
11
analyses were conducted by Christopher Henry at the New Mexico Geochronology
Research Laboratory (NMGRL) (results in Table 1).
12
Chapter 3
RESULTS
GEOLOGIC MAPPING – STRATIGRAPHY OF VOLCANIC UNITS IN THE
EASTERN HRC
Summit Lake Tuff
The Summit Lake Tuff does not crop out within the mapping area, but its
stratigraphic relationship is important for interpretation of the HRC eruptive sequence.
Stratigraphic and age relationships show Summit Lake Tuff is the oldest volcanic unit of
the HRC eruptive sequence, and yields an 40Ar/39Ar date of 16.33 ± 0.03 Ma (sample
H06-18; Table 1). Summit Lake Tuff is a voluminous ash-flow tuff that is preserved over
a 100 km diameter area and is covered to the west (Noble et al., 2009). Exposures are
found to the northeast near Summit Lake, to the northwest near Massacre Lake, and
beyond McConnel Canyon more than 20 kilometers south of HRC (refer to Figure 3 for
geographic locations). Summit Lake Tuff is massive in outcrop and provides no evidence
of a cooling break. It is recognized in the field by tabular, columnar-jointed ledges that
are commonly reddish-brown to very dark brown. Pumice imbrications in exposures
north of Steven’s Camp indicate a flow direction to the northwest, outward from HRC.
Rhyolite Lava
Rhyolite lava overlies the Summit Lake Tuff along the western HRC ring
fractures near Stevens Camp. To the south of HRC, similar rhyolite lava is exposed along
13
the base of tilted fault blocks east of High Rock Lake. Outcrops of rhyolite lava are
characterized by sparse (<1% by volume) sanidine phenocrysts. Sample H08-78,
collected from rhyolite lava to the north-northeast of Stevens Camp, yields an 40Ar/39Ar
date of 16.22 ± .03 Ma (Table 1). Although this particular rhyolite lava does not crop out
in the mapping area, 40Ar/39Ar dates and stratigraphic relationships suggest this rhyolite
lava may be precursor lava to Soldier Meadow Tuff.
TABLE 1. RADIOMETRIC DATING OF THE HIGH ROCK CALDERA VOLCANIC SUITE
Map Unit
Ttp
Tra; lava
Age (Ma)
Sample
Description
14.28 ± .18
-
K-Ar of glass in ash-fall horizon, Fly Canyon (Ach et al., 1991)
14.44 ± .18
-
K-Ar of plag in ash-fall horizon, Fly Canyon (Ach et al., 1991)
15.71 ± .03
H08-119
Massive rhyolite lava on top of SMT; Little HR Canyon
15.9 ± .04
H08-134
Upper SM lava- Korringa vent area
15.93 ± .07
H08-79
Grassy Rock lava; SM-like lava
16.04 ± .03
H08-123
SM-lava at base of SMT type-locality
15.95 ± .05
H08-130
Soldier Meadow pumice; Korringa vent area
15.86 ± .04
H08-124
Pumice-rich SMT; lowest at Type-locality
16.12 ± .05
H06-112
SMT; base of cuesta; west of SM Ranch
Tra
16.22 ± .03
H08-78
Rhyolite lava NE of Stevan's Camp; W-NW ring fracture
SLT
16.33 ± .03
H06-18
(anorthoclase); Summit Lake Tuff (top); N of Summit Lake
SM-lava
SMT
Table 1. 40Ar/39Ar (K-Ar where noted) dates of select samples from the High Rock caldera
volcanic suite. All reported ages are in millions of years. A (-) symbol indicates that no sample
number was reported in the indicated studies. 40Ar/39Ar dates on H08- samples are on sanidine
phenocrysts (anorthoclase where noted) and were analyzed by Christopher Henry at the New
Mexico Geochronology Research Laboratory.
14
Figure 3. Map of geographic features of the High Rock caldera. The Soldier Meadow Tuff
type-locality was defined by Noble and others (1970). Stratigraphic sections were measured at
Ampitheatre and High Rock Peak. The geologic mapping area is defined in light gray (Plate 1).
The dark gray area is KVA – Korringa vent area (Korringa, 1973). Grassy Rock and South
Grassy Rock are ring fracture lavas that are lithologically indistinguishable from SM-lava.
TBM – Tuff of Badger Mountain; TRB – rhyolite of Badger Mountain; TAF – Tuff of Alkali
Flat.
15
Soldier Meadow Tuff
The Soldier Meadow Tuff is distinct in appearance from other volcanic units in
the mapping area. Fresh surfaces are typically light bluish gray to dark bluish gray, and
weathered surfaces are rusty reddish-brown. Vertical columnar jointing and laterallycontinuous horizontal jointing are common. Two 40Ar/39Ar dates on Soldier Meadow
Tuff provide distinctly different apparent ages: 16.12 ± 0.05 Ma (H06-12) from an
outcrop west of the Soldier Meadow Ranch, and 15.86 ± 0.04 Ma (H08-124) from the
Soldier Meadow Tuff type locality (Table 1). The difference in these ages is outside of
normal analytical uncertainty and is one factor that prompted this study. The older age
may have been a calibration problem, and there will be dating of all major units again in
future studies.
The Soldier Meadow Tuff is mostly distributed from the east to the south of the
HRC. Northernmost exposures of Soldier Meadow Tuff occur near Upper Warm Springs
Canyon and Summit Lake; easternmost exposures are in the Black Rock Range;
southernmost exposures are near McConnel Canyon and Willow Creek opal mine;
westernmost exposures are in Little High Rock Canyon (nearly due south of HRC). The
type locality of the Soldier Meadow Tuff is located west of Soldier Meadow near 41.375º
N, -119.261º, and was defined and described by Noble and others (1970). The Soldier
Meadow Tuff covers an area greater than 700 km2 with a roughly-estimated volume of 60
km3 calculated from unit distribution from mapping (Noble, D.C., unpublished) and
average thickness, varying from 150 m to 30 m. The only exposure of the base of the tuff
occurs at the bottom of the Soldier Meadow Tuff type locality, where it overlies an older
16
SM-lava (H08-123). Thick exposures of the Soldier Meadow Tuff occur near the
approximated ring fracture locus. The thickest measured exposure (herein termed the
“Ampitheatre” section) is a minimum of 157 m approximately 1 kilometer west of Franco
Reservoir and 3.5 kilometers south of the Soldier Meadow Tuff type-locality (Figure 3).
Detailed stratigraphic sections measured at the Ampitheatre (Plate 2) and near High Rock
Peak (Plate 3) illustrate the complexity of the Soldier Meadow Tuff deposits. Three
common lithofacies of the Soldier Meadow Tuff repeat stratigraphically through the tuff
and include laminated, breccia, and massive ignimbrite deposits. The lithofacies change
in thickness and appearance depending on distance from vents. No complete cooling
breaks are observed throughout the entire Soldier Meadow Tuff sequence.
Laminated deposits of the Soldier Meadow Tuff contain three sub-types of thin
layers: fine-grained planar, coarse-grained planar, and cross-bedded. Fine-grained layers
typically contain only 10-15% phenocrysts by volume with sizes typically less than 2
mm, and individual layers vary from approximately 1 to 10 cm in thickness. Coarsegrained layers contain from 15-35% phenocrysts by volume. Phenocrysts are commonly
up to 3.5 mm diameter. Coarse-grained layers are more abundant than fine-grained
layers, and range significantly in thickness from 10 cm up to 3 m. All laminated deposits
of Soldier Meadow Tuff are interpreted as surge deposits.
Laminated surge deposits of the Soldier Meadow Tuff are generally finesdepleted near the presumed vents and grade into fines-rich deposits in medial distances.
Pumice imbrications in near-vent surge deposits at High Rock Peak and near Franco
Reservoir indicate flow direction ranging from 170º azimuth (southeast) to 190º azimuth
17
(southwest) (Figure 4 (a)). Finer-grained surges west of Soldier Meadow Ranch exhibit
low-angle cross-bedding with flow direction towards 162º azimuth to 165º azimuth
(southeast), away from the southeastern HRC ring fractures (Figure 4 (b)). Laminated
surge deposits are interpreted to be proximal, near-vent facies that typically occur less
than 4 km from the ring fractures.
Soldier Meadow Tuff breccia deposits contain up to 80% by volume lithic clasts.
Sizes of the clasts range from a few centimeters up to 1.5 m diameter near the base of the
Soldier Meadow type-locality proximal to the eastern HRC ring fracture. Figure 4 (c)
shows breccia deposits that are stratigraphically above surge deposits at the High Rock
Peak section, close to the approximate HRC ring fracture (Figure 4 (c); Plate 2 and Plate
3). The breccias are interpreted as lag deposits produced by Soldier Meadow pyroclastic
flows, and are proximal facies that mostly occur less than 4 km from the ring fractures.
Massive Soldier Meadow Tuff deposits (Figure 5) are typically medium- to coarsegrained and characterized by 25-60% crystals by volume. Individual flow-unit bedding
planes are difficult to distinguish in these massive deposits, but laterally-continuous
horizontal jointing every 2-10 meters is common and likely coincides with flow-unit
boundaries. The massive Soldier Meadow Tuff lithofacies forms deposits over 100
meters thick in medial to distal locations (i.e. more than 4 km) from the eastern to
southern ring fractures of the HRC. Columnar jointing is common in most massive
outcrops, and eutaxitic foliation of fiamme structures indicates a strong-degree of
welding. The massive, columnar-jointed character of the tuff likely formed as a result of
ponding in topographically low-lying areas.
18
Figure 4. Proximal lithofacies of
the Soldier Meadow Tuff.
Proximal distances are generally <
4 km distance from the HRC
southeastern ring fractures.
(a) Surge beds with pumice
imbrications near Franco Reservoir
indicate flow direction of the
Soldier Meadow Tuff to the
southeast, away from the HRC ring
fractures; flow is left to right in the
photo.
(b) Surge deposits with low-angle
cross bedding west of Soldier
Meadow Ranch. Surge beds are
fine-grained consisting of less than
15% crystals by volume. Flow
direction of the Soldier Meadow
Tuff is to the southeast, away from
the HRC ring fractures; flow is left
to right in the photo.
(c)Below the dashed black line are
low-angle cross-bedded surge
deposits, overlain by Soldier
Meadow Tuff breccia. Surges and
breccias repeat throughout
stratigraphy. This photo is from the
thick section of Soldier Meadow
Tuff at High Rock Peak.
19
Figure 5. Massive lithofacies of the Soldier
Meadow Tuff. Massive of Soldier Meadow
Tuff generally crops out in distal areas (> 4
km) from the southeastern ring fractures of
the HRC. Mild to moderate columnarjointing is common.These deposits likely
formed as pyroclastic density currents
slowed away from the vent, and ponding
may have occurred in some areas. (Top)
Massive Soldier Meadow Tuff just east of
High Rock Lake. Soldier Meadow Tuff is
tabular and mildly columnar-jointed. The
exposed upper ledge is approximately 5 m in
thickness. (Bottom) Southernmost exposures
of Soldier Meadow Tuff near Willow Creek
Opal Mine. This outcrop expresses mild
columnar joints and laterally continuous
horizontal joints, probably indicating
boundaries of individual pyroclastic density
current flow units.
20
Soldier Meadow Lavas
SM-lavas are a group of lavas with minor inter-lava tuffs that have a similar
phenocryst assemblage and composition as Soldier Meadow Tuff. The SM-lava group
includes lavas of the Korringa vent area (KVA; Korringa, 1973), SM-lava at the base of
the Soldier Meadow Tuff type locality, lavas directly above the Soldier Meadow Tuff just
north of Ampitheatre, SM-lava at Grassy Rock along the western ring fracture, and SMlava south of Grassy Rock. All SM-lavas are likely HRC ring-fracture lavas. A highlysilicified lava in the HRC interior is petrographically similar to SM-lavas, consisting of
approximately 20% (by volume) phenocrysts of sanidine, quartz, and minor arfvedsonite.
However, the stratigraphic relationship of this interior lava to the SM-lavas and rhyolite
lavas of Badger Mountain (TRB) are unknown.
The SM-lavas contain abundant sanidine and quartz phenocrysts with minor
arfvedsonite and rare iron-rich clinopyroxene. In outcrop, SM-lavas are rusty reddishbrown in color on weathered surfaces and light to medium grayish-blue on fresh surfaces.
Korringa (1973) described and mapped in detail SM-lavas that crop out northwest of
Soldier Meadow Ranch. Here, a prominent linear vent area (KVA) is evident from radial
flow patterns in SM-lava defined by concentric ogives away from five or more northaligned vents. Korringa (1973) mapped SM-lava at the base of the section, with a
pumice-rich, poorly-welded tuff directly above. This tuff layer is interpreted in this study
to be Soldier Meadow Tuff (sample H08-130). Directly above the Soldier Meadow Tuff
is the upper SM-lava of this section. Noble and others (1970) reported that the tuff and
21
lower and upper lavas were emplaced very closely in time since is no evidence for a
cooling break between the tuff and lavas.
Most SM-lavas overlie the Soldier Meadow Tuff, but one underlies the tuff at its
type locality. No evidence of a cooling break at contacts between SM-lavas and the
Soldier Meadow Tuff outside of the KVA further supports that these lavas erupted
shortly after the tuff. Two outcrops of SM-lava lay directly above the Soldier Meadow
Tuff approximately 1.5 kilometers northwest of Franco Reservoir. An intrusion of Soldier
Meadow mineralogy crops out approximately 1.4 kilometers east of the Soldier Meadow
Tuff type locality, and has a general northeast trend parallel to the HRC eastern ring
fractures. This intrusion may be a feeder dike for a subsequently eroded SM-lava.
Sanidine from four samples of SM-type rocks using the 40Ar/39Ar method (Table
1). The lava that underlies Soldier Meadow Tuff at the type locality yielded 16.04 ± 0.03
Ma. A single pumice fragment from the inter-lava tuff in the SM-lava sequence from the
KVA yielded an age of 15.95 ± 0.05 Ma (sample H08-130), and the uppermost lava in
that sequence yielded 15.90 ± 0.04 Ma (sample H08-134). SM-lava at Grassy Rock
yielded an age of 15.93 ± 0.07 Ma. Thus, these ages are consistent, with all post-Soldier
Meadow Tuff lavas being indistinguishable in age from Soldier Meadow Tuff itself. The
analytical uncertainties are consistent with the post-Soldier Meadow Tuff lavas being as
much as many tens of thousands of years younger than the Soldier Meadow Tuff.
22
Bedded Sequence of Intermediate-Silicic Pyroclastic Deposits
A sequence of unwelded to slightly-welded bedded pyroclastic deposits (Ttp map
unit) directly overlies the Soldier Meadow Tuff (Plate 1) and is exposed from the eastern
to southern margins of the HRC, as well as in the southeastern HRC interior. This
pumiceous sequence ranges from andesite to rhyolite in composition and contains sparse
(1-10% by volume) phenocrysts of sanidine, plagioclase, quartz, hornblende, and biotite.
Near the ring fractures, accretionary lapilli indicate hydrovolcanic eruptions that were
subaqueous or that interacted with water-saturated material. Accidental lithics are
abundant and typically composed of aphyric rhyolite lava and Soldier Meadow Tuff.
Blocks of densely-welded Soldier Meadow Tuff measure up to 3 meters in diameter and
are commonly found in areas proximal to Ttp eruption vents along the eastern to southern
ring fractures (Smith et al., 2010). The locations of abundant Soldier Meadow blocks are
illustrated by small blue dots on the geologic map (Plate 1). In addition, Soldier Meadow
blocks are found in Ttp in High Rock Canyon (west of the southern mapping boundary).
Bomb sag structures in Ttp bedding at the base of some of the Soldier Meadow Tuff
blocks indicate the blocks were balistically-ejected and probably not transported far from
source. Slight imbrications in glassy dacitic ribbon bombs (sample JS10HR-135A) in
layers south of Buck Springs indicate transport direction to the south-southeast away
from the HRC.
In outcrops at Fly Canyon and the Fly Canyon tributary to the east, Ttp contains
layers of gray and white glassy ash-fall, dark gray to black scoria, and brown lahar
deposits with occasional mammal bones. Merriam (1910a; 1910b; 1911), Stirton (1939),
23
and Swisher (1992) described the vertebrate fauna in the fossiliferous horizon of Ttp at
Fly Canyon. According to Noble and others (1987), the fossils from Fly Canyon are
contained in bedded and reworked tuff, largely of rhyolitic composition, that overlies the
Soldier Meadow Tuff. Swisher (1992), reports that the beds directly above Soldier
Meadow Tuff at Fly Canyon date to 16.12 ± 0.025 Ma. Swisher (1992) reported K-Ar
ages of 14.48 ± 1.12 Ma, 14.28 ± 0.36 Ma, and 14.49 ± 1.14 Ma on biotite, glass, and
plagioclase, respectively, from a single sample of tuff approximately 25 feet above the
fossil-bearing beds. At the top of this sequence is a 1-2 meter-thick, brick-red, resistant
layer of foliated, fused tuff that was heated and compressed during emplacement of
overlying basaltic andesite to dacite lava (Tad map unit). This fused tuff layer acts as a
marker bed for Ttp and makes for easy identification of this unit in the field (Plate 1).
Rhyolite Lavas and Domes
Sparsely porphyritic rhyolite lava and domes (Trd map unit) locally intrude Ttp
and crop out along eastern ring fractures near Buck Springs and Chukar Gulch. Massive
rhyolite domes also crop out along southern ring fractures in High Rock Canyon. The
rhyolite domes are flow-foliated and commonly display well-developed columnar joints.
They contain less than 1% sanidine phenocrysts that are up to 3 mm in length. The
rhyolite domes are coeval with Ttp since they intrude and are also locally overlain by
pumiceous tuff of Ttp (Ach et al., 1991). It is possible the rhyolite domes may have filled
Ttp eruption vents.
24
Basaltic Andesite to Dacite Lavas and Dikes
Thick basaltic andesite to dacite lavas cap the Soldier Meadow Tuff-Ttp sequence
from the eastern to southern parts of the HRC (Tad map unit, Plate 1). The lavas erupted
from the eastern to southern HRC ring fractures and mainly flowed northwest into the
caldera interior, while some lavas flowed outward of the HRC near Chukar Gulch and
west of Warm Springs Canyon. The overall unit averages 50 to 100 meters thick, with a
maximum exposed thickness of 240 meters (Ach et al., 1991). On fresh surfaces, the
lavas are dark gray to black with a glassy groundmass. Weathered surfaces have iron
oxidation and are dark purple-brown. Outcrops are dense and massive with vesicular
zones primarily in the upper parts. Tad lavas are typically aphyric to crystal-poor with up
to 2% plagioclase phenocrysts that measure less than 3 mm diameter. Near vents, Tad
lavas contain rare lithic clasts of Soldier Meadow Tuff measuring from a few centimeters
up to 5 centimeters across.
Local eruptive centers for Tad lavas (Tac map unit, Plate 1) are characterized by
spatter and scoriaceous lava mounds that are commonly rusty red to brick red. Tac
eruptive vents commonly contain lithic clasts of Soldier Meadow Tuff that range from a
few centimeters up to 12 cm in diameter. The margins of most Soldier Meadow Tuff
lithics are partly melted from heating in Tac vents that likely ranged from 1,000º- 1,200º
C. Tac eruptive centers appear to be concentrated along the southeastern ring fractures of
the HRC. Aphyric basaltic andesite feeder dikes (Td) for Tad lavas intruded through Ttp
and Soldier Meadow Tuff. The Td dikes are steep with northeast preferential orientations
and may roughly define the eastern to southern HRC ring fractures (Smith et al., 2010).
25
PETROGRAPHY
Detailed petrographic descriptions of 51 samples from within and around the
HRC are recorded in Appendix B. Descriptions include phenocryst assemblage, modal
percentages, groundmass content, and textures. Table 2 lists a brief overview of
phenocryst assemblage and average modal percentages of the lithologies sampled.
TABLE 2. PETROGRAPHY OF THE HIGH ROCK CALDERA VOLCANIC SUITE
Unit
Trd
Phenocryst %
Phenocrysts
(by volume)
Major phenocryst size
(mm)
≤1
san
up to 3 mm
Tad lava & Td-dikes
<1
plag
< 1 mm
Ttp
1-5
san, qtz, plag
< 2 mm
SM-lava, Grassy Rock
15-20
san, qtz, arfved*
0.5 – 4 mm
SM-lava; SM-dike
10-25
san, qtz, arfved*
1.5 – 4 mm
SMT-fine
10-15
san, qtz, arfved*
0.5 – 2 mm
SMT-coarse
15-35
san, qtz, arfved*
0.5 – 3.5 mm
SMT-breccia
15-35
san, qtz, arfved*
0.5 – 3.5 mm
SLT
15-25
anorth, cpx*, amph*, bt*
0.5 – 3 mm
* Accessory phenocrysts not included in phenocryst size.
TABLE 2. Generalized petrography of the High Rock caldera volcanic suite. Phenocryst percent
and sizes were estimated and averaged from petrographically-similar thin-sections of each unit.
Abbreviations: san – sanidine; plag – plagioclase; qtz – quartz; arfved – arfvedsonite; anorth –
anorthoclase; cpx – clinopyroxene; amph – amphibole; bt – biotite.
26
Petrographic observations of Summit Lake Tuff samples H08-136, H08-137,
H09-12A and H09-13 (refer to Appendix B) are similar to previously reported
descriptions by Noble and others (1970), Korringa (1973), and Park (1983). Summit Lake
Tuff includes 15-25% phenocrysts by volume, mainly anorthoclase (3 mm) that
commonly display grid-iron twinning. Minor phenocrysts of clinopyroxene, amphibole
and biotite are usually less than 1 mm diameter and are most common in the groundmass.
Light-colored volcanic lithics of intermediate composition are less than 1% abundance
and range from 1-5 cm in diameter (Park, 1983). The groundmass is commonly glassy
with some minor devitrification in some samples. Moderate welding is indicated by
elongated pumice clasts (fiamme) from compaction and retained heat in the ash-flow tuff.
Soldier Meadow Tuff, SM-lavas, and Soldier Meadow feeder dike (JS10HR-73)
have identical phenocryst assemblages of euhedral sanidine and quartz with minor
arfvedsonite (sodic amphibole) (Korringa, 1973; Smith et al., 2009; Smith et al., 2010).
Sanidine phenocrysts commonly display blue chatoyancy (adularescence) in hand
sample, and have Carlsbad twins in thin-section. A significant percentage of quartz grains
are embayed along the crystal margins. In Soldier Meadow Tuff samples, both sanidine
and quartz phenocrysts are commonly fragmented, likely due to vesiculation and
decompression during highly explosive eruptions (Best and Christiansen, 1997).
Phenocryst sizes and percentages vary drastically in each of the Soldier Meadow
Tuff lithofacies. Coarse-grained layers and breccia clasts of the Soldier Meadow Tuff
contain 15-35% phenocrysts that range up to 3.5 mm in size (Figure 6). Fine-grained
Soldier Meadow Tuff layers usually contain only 10-15% phenocrysts with a range from
27
0.5 – 2 mm diameter (Figure 7). Most of the Soldier Meadow Tuff sequence is highlywelded to moderately-welded, as indicated by significantly compacted pumice fragments
(fiamme). Unwelded samples JS10HR-93, JS10HR-85, and JS09HR-35, collected from
the uppermost parts of the Soldier Meadow Tuff, display vapor-phase arfvedsonite in
relatively equant pumice clasts.
28
FIGURE 6. Coarse-grained Soldier Meadow Tuff. Contains phenocrysts of sanidine, quartz, and
arfvedsonite. Groundmass contains glass (welded), sanidine, quartz, Fe-Ti oxides, and occasional
vapor-phase arfvedsonite. Maximum phenocryst size is 3.5 mm. Field of view is 4.5 mm width at
4x magnification. (a) Plane-polarized light. (b) Crossed-nichols.
FIGURE 7. Fine-grained Soldier Meadow Tuff. Contains phenocrysts of sanidine, quartz, and
arfvedsonite. Groundmass contains glass (welded), sanidine, quartz, Fe-Ti oxides, and occasional
vapor-phase arfvedsonite. Maximum phenocryst size is 2.5 mm. Field of view is 4.5 mm width at
4x magnification. (a) Plane-polarized light. (b) Cross-nichols.
29
In most samples, the groundmass of Soldier Meadow Tuff and SM-lavas is
partially devitrified and contains minor amounts of euhedral to subhedral phenocrysts of
clinopyroxene, arfvedsonite, and Fe-Ti oxides. Sanidine and quartz microlites are also
common in the groundmass. Well-developed acicular vapor-phase crystals of
clinopyroxene and arfvedsonite are attributed to the peralkalinity of the Soldier Meadow
units. These typically peralkaline minerals of the Soldier Meadow Tuff and lavas
distinguish them from the tuffs of Badger Mountain and Alkali Flat, which contain biotite
phenocrysts, a typically metaluminous phase (Park, 1983).
The Ttp sequence of bedded pyroclastics overlies the Soldier Meadow Tuff and
significantly varies in petrography depending on stratigraphic position and geographic
location (i.e. distance from vent). Sample JS10HR-87 was collected near the base of Ttp,
approximately 1 meter above the top of Soldier Meadow Tuff in the northern Chukar
Gulch area west of upper Warm Springs Canyon. This sample is glassy, very pumicerich, and contains approximately 10% phenocrysts of sanidine and quartz. Felsic lithics
up to 3 mm in diameter in thin-section contain microlites of sanidine and quartz, although
the modal percentages of each are somewhat difficult to determine due to the small sizes.
It is likely these lithic fragments are accidental clasts of Soldier Meadow Tuff.
Accretionary lapilli are present in the bedded layers close to the presumed ring-fracture
vents.
Just south of Buck Springs, Ttp is palagonitized. Palagonitization likely
developed from hydrothermal alteration in the porous and permeable unwelded sequence.
In the Ttp fused tuff marker bed in the uppermost parts of the sequence, sample JS10HR-
30
34 has a highly-welded glassy matrix with plagioclase microlites. This tuff layer was
fused by heating and compression from overlying Tad lava.
Rhyolite lavas and domes (Trd) that locally intrude Ttp contain sparse
phenocrysts of sanidine that measure up to 2.5 mm in length. Rare phenocrysts of
amphibole (arfvedsonite?) with brown to green pleochroism may indicate a peralkaline
composition. The glassy groundmass contains microlites of sanidine (?) and possibly
some quartz, although it is difficult to determine due to the small size. Sparsely
porphyritic rhyolite lava (sample H08-119) overlies Soldier Meadow Tuff in Little High
Rock Canyon. It is unclear whether this rhyolite lava is related to Trd lavas and domes,
but it is indistinguishable from other Trd occurrences mapped in this study. In thinsection, both contain sparse (<1% by volume) sanidine phenocrysts in a glassy, microlitic
groundmass.
Basaltic andesite to dacite lavas and basaltic andesite dikes (Tad and Td,
respectively) are petrographically indistinguishable. Sample JS10HR-39 is a sample from
a dike (Td map unit) that intruded through Soldier Meadow Tuff at the top of the
“Ampitheatre” stratigraphic section approximately 1 km west of Franco Reservoir.
Sparse phenocrysts of plagioclase are less than 2 mm long. The groundmasss is dark gray
to black in hand sample and is glassy, nearly opaque in thin section. The groundmass has
a microlitic texture with abundant feldspar (likely plagioclase) microlites. Tad lava is also
sparsely porphyritic and contains plagioclase phenocrysts less than 2 mm diameter. Rare
phenocrysts of clinopyroxene exist in hand specimen and in the groundmass. As in Td
31
dikes, the glassy groundmass of Tad lavas has a microlitic texture with plagioclase
microlites set in a nearly opaque groundmass, as observed in thin-section.
32
GEOCHEMISTRY
XRF Analyses of Major and Trace Elements
XRF analyses of major element oxide and trace element concentrations were
conducted on 57 whole rock and pumice samples of the High Rock caldera volcanic
suite. (Refer to Appendix C for major element oxide data; Refer to Appendix D for trace
element data). A total alkalis versus silica (TAS) diagram (Figure 8), defined by Le Bas
and others (1986) and modified by Le Maitre (1989), illustrates the range in compositions
of the HRC volcanic suite, which is marginally alkalic, except for the Soldier Meadowrelated eruptive products, which are subalkaline. Summit Lake Tuff is an alkali-rich, lowSiO2 rhyolite, while the Soldier Meadow Tuff and SM-lavas are subalkaline high-SiO2
rhyolites. Post-Soldier Meadow units of bedded pyroclastics (Ttp) and aphyric dikes and
lavas (Td and Tad, respectively) are basaltic trachy-andesite to trachydacite.
The Agpaicity Index (AI) was defined by Shand (1947) to characterize the
alkalinity of volcanic rocks. Agpaitic coefficients of High Rock samples were calculated
by dividing the sum of molecular (Na2O + K2O) by molecular Al2O3. Samples with
Agpaitic coefficients (AI) greater than one are as peralkaline, and values less than one are
metaluminous or peraluminous. All samples of Soldier Meadow Tuff and related tuffs
and lavas are peralkaline (Figure 9). Summit Lake Tuff is metaluminous with the
exception of the most silicic samples, which are slightly peralkaline (Figure 9).
33
Figure 8. Total alkalis vs. silica of the High Rock caldera volcanic suite. Fields were defined by
Le Bas and others (1986) and modified by Le Maitre (1989). The alkaline-subalkaline dividing
line (in red) was defined by Irvine and Baragar (1971). Tad-lava – basaltic trachy-andesite to
trachydacite; Td-dike – basaltic trachy-andesite; Ttp – trachydacite; Rhyolite group: SM-lava, GR
(Grassy Rock); SM- lava, South GR; SM-lava; SM-dike; SMT (Soldier Meadow Tuff); Tr
rhyolite lava; SLT (Summit Lake Tuff); TBM (Tuff of Badger Mountain); TAF (Tuff of Alkali
Flat).
34
Figure 9. Agpaicity Index (AI) vs. silica of the High Rock caldera volcanic suite. AI is based on
the definition by Shand (1947) to describe alkalinity of volcanic rocks. Agpaitic coefficients of
High Rock samples were calculated by dividing molecular (Na2O + K2O) by molecular Al2O3.
Peralkaline rocks have AI coefficients > 1, whereas metaluminous and peraluminous rocks have
AI coefficients < 1. Peralkaline: lava in HRC interior; SM-lava at Grassy Rock (GR); SM-lava at
South GR; SM-lava; SM-intrusion (feeder dike); SMT (Soldier Meadow Tuff); Tr rhyolite lava.
Metaluminous: Tad-lava; Td-dike; Ttp; SLT (Summit Lake Tuff; northern samples mildly
peralkaline).
35
Summit Lake Tuff was sampled at five sections, three north and two south of the
HRC. The tuff is a high-SiO2 trachydacite to low-SiO2 rhyolite, with a range from 69% to
72% SiO2. Harker variation diagrams (Figure 10) show decreases in TiO2, Al2O3, FeO*,
Na2O, CaO, and MgO and with increasing SiO2. Figure 10 (a) (inset) is a simplified
variation diagram with only select samples from stratigraphic sections of the Summit
Lake Tuff and Soldier Meadow Tuff. In the Summit Lake section south of Little High
Rock Canyon, a basal sample (H09-12A) has intermediate SiO2 (70.3%) and lower TiO2,
whereas an upper sample (H09-13) has 68.9% SiO2 and higher TiO2 (Figure 10 (a) inset).
Trace element concentrations of the Summit Lake Tuff also vary with
stratigraphic position. Compositions vary only slightly at three sections (north of Summit
Lake, near Massacre Lake, and in the southeastmost occurrence) but more distinctly at
the sections north of Stevens Camp and south of Little High Rock Canyon. For example,
four samples, including a basal vitrophyre (H09-35) and three single pumice samples
(H09-36, H09-37, H09-38) that represent most of the thickness of the tuff north of
Stevens Camp, have nearly indistinguishable compositions. The three single pumice
samples are slightly peralkaline. A single pumice (H09-39) from the top of the tuff in the
same area has distinctly lower SiO2, K2O, Zr, Nb, Zn, and Y and higher TiO2, FeO, CaO,
Sr, and Ba and is distinctly not peralkaline (refer to Appendices C and D for major
element oxide and trace element data, respectively).
Compositions of Summit Lake Tuff also vary geographically relative to the
caldera (Figure 10). Most samples north of the caldera, except the upper pumice sample
north of Stevens Camp, are more differentiated and have higher SiO2. Samples south of
36
the caldera, except the basal sample south of Little High Rock Canyon, are less
differentiated, with lower SiO2.
Indistinguishable geochemical signatures of Tr-rhyolite lava (one sample that
stratigraphically underlies the Soldier Meadow Tuff east of High Rock Lake, and one
sample collected from northeast of Stevens Camp) to the Soldier Meadow Tuff may
indicate that Tr-rhyolite lava might be a related, locally-extruded, precursor lava.
37
Figure 10. Harker variation diagrams of major element oxides. Major element oxides have been
normalized to 100 %. Major element oxides (a) TiO2, (b) Al2O3, (c) FeO*, (d) Na2O, (e)CaO, and
(f) MgO are plotted against silica. Relatively collinear trends genetically connect the Summit
Lake Tuff, Soldier Meadow group, and post-Soldier Meadow units and suggest the magma
evolved from typical fractionation processes. Samples of the Summit Lake Tuff consistently
show geographic zonation in which northern samples are more evolved than southern samples. (a)
Inset: this is a simplified inset showing select basal and upper samples from stratigraphic sections
of the Summit Lake Tuff (SLT) and Soldier Meadow Tuff (SMT). Summit Lake Tuff and Soldier
Meadow Tuff show vertical zonation in which basal samples are more evolved (i.e. higher silica
content and lower major element oxides) and erupted earlier than their stratigraphic upper
counterparts. The Summit Lake Tuff section is from a canyon south of Little High Rock Canyon;
the Soldier Meadow Tuff section is from upper Warm Springs Canyon. GR – Grassy Rock.
38
Figure 10. (Figure caption on page 37)
39
Figure 10 (cont). (Figure caption on page 37)
40
Figure 10 (cont). (Figure caption on page 37)
41
The Soldier Meadow Tuff is a high-SiO2 rhyolite with a range from 76% to 80%
SiO2. It is important to note that 77.4% is generally considered the maximum silica
content for an igneous rock (Hildreth, 1981, p. 10154). Silica contents greater than this
value are likely due to post-depositional alteration processes, such as silicification.
Nevertheless, the more immobile elements, such as TiO2, Zr, and the REEs, can be used
to evaluate geochemical characteristics and evolution even in somewhat altered rocks.
Soldier Meadow Tuff was sampled at five sections along the eastern to southern
margins of the caldera. The Soldier Meadow Tuff, similar to the Summit Lake Tuff,
shows notable variations in major element oxide and trace element concentrations
through stratigraphy. Total alkali (Na2O + K2O) content ranges from 8.17% to 9.22%
(Figure 8), with four of the five sections having higher total alkali concentrations near the
base of the tuff compared to poorly-welded samples collected in the top parts of the tuff.
Samples JS10HR-33, JS10HR-93, and H08-133 were collected in unwelded,
uppermost parts of the Soldier Meadow Tuff. These three samples are consistently higher
in concentrations of major element oxides, most notably in Al2O3, CaO, and MgO, and
are slightly off-trend from the rest of the Soldier Meadow group. These uppermost
samples have relatively low pre-normalized total major element oxide values (90.22%,
91.47%, and 96.92%, respectively), which may reflect post-depositional alteration or
hydration of glass. Outliers JS10HR-93 and JS10HR-33 are not included in Figures 9 and
10 due to their low major-element oxide totals (refer to Appendix B for major element
oxide data). Despite a somewhat low major-element total for H08-133, this sample plots
42
along the same trend as the Soldier Meadow group in Figures 9 and 10, with the
exception of higher MgO concentretation as indicated in Figure 10 (f).
Figure 10 (a) (inset) is a simplified inset variation diagram with only select
samples from stratigraphic sections of the Summit Lake Tuff and Soldier Meadow Tuff.
Figure 10 (a) (inset) illustrates the variations in TiO2 concentrations in Soldier Meadow
Tuff samples collected from the base (H09-20) and top (JS09HR-35) of the Soldier
Meadow Tuff section in upper Warm Springs Canyon. Additionally, samples collected
near the base of all sections consistently have higher concentrations of incompatible trace
elements Nb, La, Ce, and Zr (refer to Appendices C and D). Sample H09-20 is a basal
vitrophyre sample from the Soldier Meadow Tuff section in Upper Warm Springs
Canyon, and has higher concentrations of Ce and La than its upper counterpart, sample
H09-23 (Figure 11 (a) and (b)).
There are three Soldier Meadow Tuff outliers in Zr and Nb concentrations (for
simplification, these samples were not plotted in Figure 11; refer to Appendix D for trace
element data). BH08HR-49 and JS09HR-35 are unwelded uppermost Soldier Meadow
Tuff whole-rock samples collected near Upper Warm Springs Canyon; H08-130 is a
single, frothy pumice from the KVA. Since whole rock XRF analyses were conducted on
samples BH08HR-49 and JS09HR-35, it is possible there may have been some lithic
inclusions that affected analyses of trace element concentrations. It is difficult to decipher
from the available data what alteration or contamination processes could have affected Zr
and Nb concentrations of these samples.
43
Stratigraphic variations of major oxide and trace element concentrations suggest
the Soldier Meadow Tuff erupted from a vertically-zoned magma chamber in which
more-evolved compositions were concentrated in the upper parts of the chamber and
therefore erupted earlier in the sequence. Both the Summit Lake Tuff and the Soldier
Meadow Tuff display similar stratigraphic chemical variation. Unlike the Summit Lake
Tuff, the Soldier Meadow Tuff shows no noticeable geographic variations in composition
across its eastern to southern exposures.
44
Figure 11. Trace element plots of the High Rock caldera volcanic suite. In (a) and (b), the
constant ratio slopes of Ce vs. La and Nb vs. La, respectively, genetically link the Summit Lake
Tuff (SLT), Soldier Meadow units, and post-Soldier Meadow units to a common magma source.
Soldier Meadow Tuff (SMT) shows vertical zonation in the Upper Warm Springs Canyon
section, in which the base (H09-20) is more evolved than the upper counterpart (H09-23). SMlava outliers indicated with labels may have experienced contamination or post-depositional
alteration processes. In (c) and (d), Summit Lake Tuff, Soldier Meadow units, and post-Soldier
Meadow units follow a collinear trend and are genetically connected. Summit Lake Tuff shows
geographic zonation in which northern samples are more evolved than southern samples. SM-lava
outliers are indicated with labels and may have experienced contamination or post-depositional
alteration processes. GR – Grassy Rock.
45
Figure 11 (cont). (Figure caption on page 44)
46
Figure 11 (cont). (Figure caption on page 44)
47
Directly overlying the Soldier Meadow Tuff are SM-lavas, with the exception of
SM-lava at the base of the Soldier Meadow Tuff type locality. SM-lavas are
petrographically very similar to the Soldier Meadow Tuff and have major-element
geochemical signatures identical to Soldier Meadow Tuff (Figure 10). SM-lava sample
H08-129, collected from the lower SM-lava at KVA, is an outlier in both Ce vs. La and
Nb vs. La ratios (Figure 11 (a) and (b)). However, H08-129 plots well with the Soldier
Meadow group in both Zr and Nb vs. SiO2 ratios (Figure 11 (c) and (d)). Sample H08134, collected from the upper SM-lava at KVA, is an outlier in Zr and Nb vs. SiO2 ratios
(Figure 11 (c) and (d)), but plots well with the Soldier Meadow group in both Ce vs. La
and Nb vs. La ratios (Figure 11 (a) and (b)). Sample H08-123 of SM-lava from the
bottom of the Soldier Meadow Tuff type locality is significantly higher in Nb
concentrations compared to the rest of the Soldier Meadow group. This SM-lava sample,
however, plots similarily with the Soldier Meadow group in other trace element
concentrations. The SM-lava south of Grassy Rock is notably in higher in Ce, Nb, and Zr
concentrations plots outside of the pattern of each of the trace elements plotted in Figure
11. In this study, with the exception of the SM-lava at Grassy Rock, each sample of SMlava is an outlier in one or more trace element concentrations. More analyses need to be
conducted to explain the implications of the SM-lava outliers, but simple fractionation
processes alone cannot account for these variations. It is possible that contamination,
possibly from incorporation of lithic inclusions, or possibly post-depositional alteration
processes, affected analyses of trace element concentrations in these SM-lava samples.
However, overall patterns of SM-lava petrography and geochemical analyses suggest that
48
SM-lavas are related to the rest of the Soldier Meadow group, and likely represent HRC
ring fracture lavas.
Post-Soldier Meadow units, including dacitic pyroclastic deposits (Ttp), basaltic
andesite dikes (Td) and basaltic andesite to low-silica-dacite lavas (Tad), are not as
evolved as the Soldier Meadow Tuff. A lesser degree of fractionation is indicated by the
lower silica content, higher concentrations of the compatible oxides TiO2, Al2O3, FeO*,
CaO, and MgO (Figure 10), and lower concentrations of the incompatible elements Ce,
Nb, La, and Zr (Figure 11).
Analyses of the tuffs of Badger Mountain (TBM) and Alkali Flat (TAF) in Figure
11(a) show values in Ce vs. La that are collinear with Summit Lake Tuff and the Soldier
Meadow Tuff. However, the trends of the tuffs of Badger Mountain and Alkali Flat in
Figure 11 (b), (c), and (d) are not collinear with other High Rock units. Incompatible
element concentrations and ratios from the tuffs of Badger Mountain and Alkali Flat
generally differ somewhat from other High Rock units, suggesting that there is either no
genetic link or a more complex relationship to the magmas of the Summit Lake Tuff and
Soldier Meadow Tuff.
Trace elements plots are useful for evaluating genetic relationships between
different rock types. The concentration of an incompatible element in the liquid fraction
of a magma increases as crystallization proceeds, and the ratio of two or more equallyincompatible elements should remain constant as crystallization proceeds. If the ratio plot
of two incompatible elements is linear, then all samples are likely related to a single
magma system. If the ratio of incompatible elements for the samples lie along parallel but
49
not collinear trends, this could indicate crystallization of rocks from magmas that had
different ratios of the incompatible elements.
Trace element plots were used to interpret genetic relationships of the Summit
Lake Tuff, Soldier Meadow Tuff, SM-lavas, post-Soldier Meadow units, and the tuffs of
Badger Mountain and Alkali Flat (Figure 11 (a-d)). Incompatible-element pairs Ce vs. La
and Nb vs. La were used to compare ratio trends between these units in Figures 11 (a)
and (b), respectively. These trace elements are fairly immobile and not prone to drastic
changes in concentrations during alteration. In Figures 11 (a) and (b), the Summit Lake
Tuff, Soldier Meadow Tuff, and post-Soldier Meadow groups plot on a collinear trend.
Incompatible elements Zr and Nb (Figure 11 (c) and (d), respectively) increase linearly as
the degree of magma fractionation increases. For Summit Lake Tuff samples, basal
samples of northern exposures have higher Zr and Nb concentrations, and upper samples
from southern exposures have the lowest Zr and Nb concentrations. The vertical and
geographic variations in Zr and Nb concentrations further suggest the Summit Lake Tuff
erupted from a zoned magma chamber, with earlier eruptions occurring in the north.
With the exception of the previously discussed Soldier Meadow Tuff and SMlava outliers, ratio-slopes for Summit Lake Tuff, Soldier Meadow Tuff, SM-lavas, Ttpbedded pyroclastics and younger dikes and lavas follow curvilinear trends on major
element oxides and trace element plots.The curvilinear trend of the Summit Lake Tuff,
Soldier Meadow Tuff, SM-lavas, and post-Soldier Meadow units suggests a genetic
relationship in which of all these magmatic units evolved from a common parental
magma probably more mafic than the Td analyses.
50
ICP-MS Analyses of Rare Earth Elements
ICP-MS analyses were conducted on 24 High Rock samples for Rare Earth
Element (REE) abundances. For units with multiple analyses, average REE
concentrations were calculated. These REE concentrations were normalized to chrondrite
abundances from Sun and McDonough (1989) and plotted on a spider diagram (Figure
12). Summit Lake Tuff shows a mildly negative Eu anomaly, also documented by Noble
and others (1979). This Eu anomaly is best explained by the fractionation of
anorthoclase as Eu readily substitutes for Ca in feldspars, causing a difficiency of Eu
within the remaining melt. Figure 12 illustrates a strong negative Eu anomaly for Soldier
Meadow Tuff and the Soldier Meadow intrusion (sample JS10HR-73, a feeder-dike for
the Soldier Meadow lavas). Post-Soldier Meadow unit Td-dike does not exhibit this Eu
anomaly. The relatively constant negative slope from light REE to heavy REE in all
samples indicates that the suite of REE, except Eu, had a similar spectrum of
incompatibilities in all the HRC magmas, as well as the tuffs of Badger Mountain and
Alkali Flat. The tuffs of Badger Mountain and Alkali Flat have even stronger negative
Eu-anomalies than the Soldier Meadow Tuff.
Additionally, it is important to note the strong negative Eu anomaly of a lava
found in the HRC interior (sample BH10HR-23) that is petrographically-similar to SMlava. The REE geochemical signature may suggest a genetic relationship of this lava in
the caldera interior to the tuffs of Badger Mountain or Alkali Flat. The lava and tuffs are
similar in many trace-element characteristics, but the lava has distinctly higher Ta/Yb and
Th/Yb. Additional analyses are required to further characterize this lava.
51
Figure 12. Spider diagram of rare earth elements. REEs are normalized to chrondrite abundances
(Sun and McDonough, 1989). The negative slope from light REEs to heavy REEs reflects crystal
fractionation processes. SLT shows an Eu depletion from crystallization of anorthoclase. Further
fractionation resulted in strong Eu depletion in the magmas for SMT, SM-dike, and Tr rhyo lava.
Lava in the HRC interior has a significant Eu anomaly and is likely related to TBM and/or TAF.
Abbrev: Td-dike – basaltic-andesite dike; Ttp – bedded pyroclastics; Lava, HRC interior –
identifified lava in the High Rock caldera interior; SMT – Soldier Meadow Tuff; Tr rhyo. lava –
Tr rhyolite lava; SLT – Summit Lake Tuff; TBM – tuff of Badger Mountain; TAF – tuff of Alkali
Flat.
52
Determining the geochemical signature of the parental magma source for the High
Rock volcanic suite is difficult because most rocks are highly evolved. A discrimation
plot of TiO2 versus Y/Nb for basalts can be used to determine the magma series of the
source, with some indication of continental or MORB tectonic settings (adapted from
Floyd and Winchester, 1975 and Rollinson, 1993). The discrimination plot is only valid
for rocks with low degrees of fractionation, i.e., basalt to andesite compositions.
Therefore, data for Td dikes are plotted on the TiO2 – Y/Nb discrimination diagram
(Figure 13) to characterize the likely magma origin of the High Rock caldera volcanic
suite. The geochemical signatures of these feeder dikes suggest a magma source most
related to continental tholeiite, which have higher SiO2, K2O, and light rare earth element
concentrations than do oceanic tholeittes (Campbell, 1985).
53
Figure 13. TiO2 – Y/Nb discrimination diagram. Three fields are plotted and overlap in some
parts: Alk. (alkali basalt, including continental and ocean-island alkali basalt), Cont. thol.
(continental tholeiite), and MORB (Mid-Ocean Ridge Basalt) (adapted from Floyd and
Winchester, 1975 and Rollinson, 1993). This discrimination diagram is used to determine the
magma series of the source, with some indication of continental or MORB tectonic settings. The
discrimination plot is only valid for rocks with low degrees of fractionation, i.e., basalt to andesite
compositions. Td-dikes are the least evolved of the High Rock caldera volcanic suite and reflect a
magma source most related to continental tholeitte.
54
Chapter 4
DISCUSSION
MAPPING INTERPRETATIONS
Noble and others (2009) state that the eruption of Summit Lake Tuff induced
initial collapse of the HRC at approximately 16.33 Ma. Data used to support this
statement included distribution of the Summit Lake Tuff to the northwest, north, east, and
south of the HRC, as well as paleocurrent indicators showing flow direction outward
from the HRC.
Eruption of the Soldier Meadow Tuff occurred at least 200,000 years and possibly
as much as 400,000 years after the caldera-forming eruption of the Summit Lake Tuff.
Korringa and Noble (1970) and Korringa (1972; 1973) interpreted that the Soldier
Meadow Tuff erupted from a linear fissure vent area marked by and underlying the SMlavas in the KVA, but did not indicate any relationship to a caldera structure. Field
observations in this study further characterize the Soldier Meadow units and define an
eruptive relationship to the HRC. Field data, including unit thicknesses, paleocurrent
indicators, clast sizes, lithic inclusions and other deposit characteristics were used to infer
the eruptive source of the Soldier Meadow Tuff. The thickest exposures of the Soldier
Meadow Tuff occur along the eastern to southern ring fractures of the HRC; however,
thickness alone cannot determine an eruptive source due to topographic influences during
deposition of ash-flow sheets. Schmincke and Swanson (1967), Mimura and MacLeod
(1978), and Kamata and Mimura (1983) report that pumice imbrications can be useful for
55
determining the flow direction of pyroclastic flow deposits. Kamata and Mimura (1983)
successfully used data from pumice imbrications to determine flow direction and locate
the source of the Handa pyroclastic flow in Japan. In this study, pumice imbrications and
surge beds in the Soldier Meadow Tuff indicate a flow direction towards the southeast,
away from the southeastern HRC ring fractures (refer to Plate 1).
Detailed descriptions of pyroclastic deposits are important for determining
distance from source and flow dynamics of ash-flow tuffs. Models of eruptive dynamics
of ash-flow sheets (Sparks et al., 1973; Sheridan, 1979; Cas and Wright, 1987; Branney
and Kokelaar, 2002) are applied to the Soldier Meadow Tuff depositional facies to
interpret flow dynamics and location of the most-likely eruptive source. In general,
maximum size of lithics decreases from proximal to distal sites (Branney and Kokelaar,
2002) as the ash-flow sheet loses momentum and carrying capacity. Aguirre-Díaz and
Labarthe-Hernández (2003) record lag-breccias in voluminous ignimbrites in the Sierra
Madre Occidental that occur in proximal distances to fissure vents. Almost all Soldier
Meadow breccia deposits occur in proximal locations to the HRC ring fractures and are
not found in distal areas. This suggests the mapped ring fractures of the HRC are the
most likely vent sources of the Soldier Meadow Tuff.
The SM-lavas erupted soon after the Soldier Meadow Tuff, approximately 15.9
Ma, as indicated by their indistinguishable ages and the lack of a complete cooling break
at the contact documented by Noble and others (1970). The SM-lavas are concentrated
along the southeastern ring fractures and in the area surrounding Soldier Meadow.
Korringa (1973) mapped at least five north-south aligned eruptive vents for SM-lavas
56
north of Soldier Meadow Ranch and east of the HRC topographic rim. Howard (2010)
describes four concentric zones of subsidence structures that develop during caldera
collapse, with zone 1 located on the caldera floor and zone 4 along the outermost ring
fractures. The eruption of Soldier Meadow Tuff may have caused further subsidence of
the HRC beyond that formed in the earlier Summit Lake Tuff eruption (Smith et al.,
2010). The Soldier Meadow Tuff eruption may have been confined to vents along the
southern to eastern margin and likely widened the zone 4 ring-fractures along the
southeastern margin of the HRC. As a result, SM-lavas at the base of the type-locality
and south of the type-locality erupted near, or just outboard, of primary ring fractures of
the HRC (Plate 1). The Soldier Meadow dike also intruded along the HRC ring fractures.
SM-lavas of the KVA (Korringa, 1973) in Soldier Meadow likely erupted along
peripheral ring fractures of the HRC just east (outboard) of the inferred HRC ring
fractures (Plate 1). SM-lava at Grassy Rock erupted along western HRC ring fractures,
perhaps synchronously and from the same magma source as the SM-lavas of the
southeastern ring fractures. This signifies that the magma source for the Soldier Meadow
Tuff and SM-lavas was located under at least the southern half of the HRC.
Explosive eruptions continued after extrusion of Soldier Meadow Tuff and lavas,
resulting in a thick sequence of unwelded ash-fall and hydrovolcanic deposits (Ttp).
Large accidental blocks up to 3 m in diameter of densely-welded Soldier Meadow Tuff
were ballistically emplaced, as indicated by bomb-sag structures in Ttp pyroclastic beds.
Soldier Meadow Tuff accidental blocks are mostly located proximal to the HRC ring
fractures (refer to Plate 1). The locations and dense welding of the blocks suggest thick
57
accumulation of Soldier Meadow Tuff along the eastern to southern part of the HRC
(Smith et al., 2010).
Effusive volcanism occurred along the southeastern ring fractures after
pyroclastic eruptions ceased. Northeast-striking basaltic andesite dikes (Td) intruded into
Ttp along what is now the topographic rim of the southeastern margin of the HRC.
Eruptive vents for Tad lavas and rhyolite domes (Trd) are also aligned in a northeastern
fashion, likely along preexisting HRC ring fractures. Distribution patterns and thick
accumulation of Soldier Meadow Tuff, SM-lava, Ttp bedded pyroclastics and late
effusive volcanic units, as well as a strong magnetic minimum (Greene and Plouff, 1981),
large densely-welded accidental blocks of Soldier Meadow Tuff in Ttp, and the
preferential northeast orientation of feeder dikes and eruptive vents suggest a large, longlived magma source was concentrated beneath the eastern to southern ring fractures of
the HRC.
Eruptive Model for the Soldier Meadow Tuff
Ash-flow deposits are commonly formed by one to many flow units emplaced
sequentially, generally from a “pulsing” effect of large-scale eruptions. Thick sequences
of flow units can create a massive ignimbrite many tens of meters to hundreds of meters
in thickness (Druitt, 1998). Sparks and others (1973) described three distinct types of
pyroclastic deposits that constitute a related eruptive sequence: Layer 1 - sandwave,
massive, or planar surge deposits (Wohletz and Sheridan, 1979), Layers 2a and 2b - the
main pyroclastic flow, and Layer 3 - fine ash deposits. Sheridan (1979) added a basal
layer of Plinian fall deposits beneath the surge deposits of Layer 1 (Figure 14).
58
Figure 14. Stratigraphic deposits of a complete eruption episode (from Sheridan (1979); modified
from Sparks and others (1973)). Layer 0: inversely-graded Plinian fall deposit; Layer 1: surge
deposit of sandwave (shown), massive, or planar facies (Wohletz and Sheridan, 1979); Layer 2a:
basal layer of the pyroclastic flow that may show inverse grading; Layer 2b: main part of the
pyroclastic flow that has double-grading. Lithic inclusions (L) are concentrated near the base of
the pyroclastic flow unit, and pumice fragments (P) are concentrated near the top. FP – fumarolic
pipes that may be present throughout the pyroclastic flow unit. A lava flow may cap the sequence.
59
The fine, capping ash-cloud deposits derived from overriding co-ignimbrite
clouds are rarely preserved. Therefore, surges and the main pyroclastic flow are
commonly found as facies of a single pyroclastic flow unit. Co-ignimbrite lag breccias
are an additional, proximal facies intimately associated with surge deposits (Cas and
Wright, 1987). The three most common pyroclastic deposits in the Soldier Meadow Tuff
are surges, breccias, and massive ignimbrite lithofacies.
Commonly, surge deposits are well-stratified and have planar to low-angle cross
beds and dune formations (Sparks et al., 1973) that are generated by traction-dominated
flow (McPhie et al., 1993). Fines-depleted surge facies predominantly occur in proximal
(near vent) locations formed by a high degree of agitation-fluidisation near the vent.
Surge deposits are therefore closely associated with co-ignimbrite breccias (Cas and
Wright, 1987). The Soldier Meadow Tuff surge deposits likely formed near the eastern to
southern HRC ring fracture vents from turbulent, relatively high-velocity flow during
initial stages of multiple column collapses.
As an eruption column collapses, air is ingested into the flow head, causing the
flow to become fluidized. Upward flow of gases in the fluidized flow removes fine
particles, causing the breccia facies to be fines-depleted (Branney and Kokelaar, 2002).
In general, maximum sizes of lithics in the breccia facies decreases from proximal to
distal sites (Branney and Kokelaar, 2002). This is in part due to a decrease in flow
velocity away from the source, thus decreasing the maximum clast-carrying capabilities
of the pyroclastic current. Larger blocks deposit near vent and in higher concentrations,
thus breccias are commonly a proximal-vent facies. Blocks can be generated from
60
erosion or collapse of the eruptive vent or conduit, avalanches into the pyroclastic
deposit, or erosion of the substrate (Branney and Kokelaar, 2002).
The majority of Soldier Meadow Tuff breccias deposits occur in proximal
distances, or less than 4 km, from the southeastern HRC ring fractures. Breccia deposits
vary from 50-80% concentration of Soldier Meadow Tuff clasts. Breccia clasts are
commonly crystal rich, and sometimes glassy. Most Soldier Meadow Tuff breccia
deposits directly overlie surge deposits, which corresponds to a typical expected eruptive
sequence of a single eruption pulse described by Sheridan (1979).
Measured stratigraphic sections near High Rock Peak and the Ampitheatre expose
Soldier Meadow Tuff surge and breccia deposits that repeat multiple times in stratigraphy
(Plates 2 and 3). The repetition of breccia lithofacies in stratigraphic sections suggest the
Soldier Meadow Tuff erupted in multiple pulses; many of these produced lag breccias.
The lack of cooling breaks in the Soldier Meadow Tuff section suggests the pulses
created multiple column-collapse events that were closely spaced in time.
Lag breccias within the Soldier Meadow Tuff are composed of densely welded
clasts Soldier Meadow Tuff. This indicates that the Soldier Meadow Tuff eruption was
sufficiently long-lived for tuff to deposit, lithify, and then be re-fragmented and entrained
into subsequent pulses of the ongoing Soldier Meadow Tuff eruption. No cooling break
has been recognized within the Soldier Meadow Tuff, so the eruption was likely
continuous, with any breaks in the explosive eruption very short-lived.
Massive ignimbrite deposits are characterized by poorly-sorted non-stratified
deposits (Branney and Kokelaar, 2002). These massive deposits lack internal
61
stratification and individual beds may be hard to distinguish due to a lack of visible
bedding planes. The massive character of this type of deposit indicates minimal shearing
between particles during deposition (Branney and Kokelaar, 2002). The main body of the
pyroclastic flow, represented by Layer 2b (see Figure 14), is commonly interpreted to be
deposited en masse (Sparks, 1976), typically as velocity and the amount of shearing
between grains decrease. To meet these conditions, massive ignimbrite deposits generally
form in medial to distal areas from the vent, or where topography reduces velocity and
allows ponding of the ignimbrite. Ponding of pyroclastic density currents in low areas can
result in a strongly-welded mass with individual flow units that can exceed 100 m in
thickness. This is documented in Lower and Upper Bandelier Tuffs that ponded in
paleocanyons (Cas and Wright, 1987). Columnar jointing is common in massive
ignimbrite deposits that have cooled very slowly.
Massive Soldier Meadow Tuff mostly crops out in distal locations, more than 4
km from the southeastern ring fractures. Applying the above models to the Soldier
Meadow Tuff suggests that the massive lithofacies formed in relatively low-velocity
conditions, where minimal shearing between grains resulted in negligible development of
bedding planes. The highly-welded and columnar-jointed characteristics of the massive
deposits may suggest localized ponding of the Soldier Meadow Tuff in topographic lows
or perhaps low velocity flow over relatively flat ground. It is difficult to determine from
the massive deposits alone whether the Soldier Meadow Tuff erupted in multiple pulses,
but the strongly-layered, repetitive surge, breccia, and pyroclastic-flow deposits at the
High Rock Peak and Ampitheatre measured sections (Plates 2 and 3) demonstrate that the
62
Soldier Meadow Tuff did indeed flow in hundreds or thousands of individual pulses that
were closely-spaced in time.
An eruptive model for the Soldier Meadow Tuff and post-Soldier Meadow
volcanism is illustrated in Figure 15. During collapse of the eruption columns, turbulent,
high-velocity flows deposited lag breccias and surge beds near vent. In general, laminated
lithofacies formed in proximal locations (less than 4 km) from the southeastern HRC ring
fractures. As the pyroclastic density current moved further from the vent, velocity slowed
and the massive lithofacies of Soldier Meadow Tuff deposited in distal locations (greater
than 4 km) from the vent. Subsequent and multiple column collapses occurred closely in
time and resulted in repetition of the lithofacies in stratigraphy. Later effusive volcanism
resulted in the extrusion of Soldier Meadow lava along the southeastern HRC ring
fractures.
Post-Soldier Meadow volcanism included pyroclastic eruptions of andesitic to
rhyolitic fall and hydrovolcanic units (Ttp). Explosive eruptions, some phreatomagmatic,
ripped up and incorporated accidental blocks of Soldier Meadow Tuff measuring up to 3
m in diameter. The blocks are mostly found near the eastern to southern HRC ring
fractures, and suggest that the eruptive vents for Ttp were along the caldera ring fractures.
Late feeder-dikes that intruded along the eastern to southern ring fractures
resulted in the extrusion of basaltic andesite to dacite lava (Tad). The emplacement of
Tad lava, perhaps at roughly 1000º C, fused the upper layers of Ttp and created a welded,
resistant marker bed. Fragments of Soldier Meadow Tuff included in the mafic to
intermediate magmas were partially melted.
63
Figure 15. Eruptive model for the Soldier Meadow Tuff. Laminated Soldier Meadow
Tuff deposits and breccias were deposited in proximal (< 4 km) from the vent. Massive deposits
were deposited in distal locations (> 4 km) from the vent. Explosive post-Soldier Meadow
eruptions (Ttp) incorporated accidental blocks of the tuff in proximal locations from the vent.
Feeder dikes (Td) intruded along the southeastern ring fractures. Effusive volcanism produced
capping basaltic andesite to dacite lavas (Tad).
64
GEOCHEMICAL & PETROLOGICAL INTERPRETATIONS
It is common for caldera-forming ash-flow sheets to show compositional
gradients between early-erupted and later-erupted products (Smith, 1979). The range in
major and trace elements concentrations and degree of peralkalinity of Summit Lake Tuff
samples and the stratigraphic and geographic chemical variations indicate the 16.33 Ma
tuff erupted from a chemically zoned magma chamber. Crystallization and fractionation
of anorthoclase in the Summit Lake Tuff magma chamber likely drove the magma toward
peralkalinity and lower Eu content. Initial eruptions were from the upper, most evolved
and mildly peralkaline part of the chamber and spread mostly to the north. Later
eruptions from the less evolved, metaluminous part of the chamber spread mostly to the
south.
Significant magma differentiation within remnant magma after the Summit Lake
Tuff eruption, or from post-Summit Lake Tuff injection of new magma, produced the
silica-rich Soldier Meadow Tuff. Linear trends of incompatible elements and REE trends
suggest the Soldier Meadow Tuff and related lavas are genetically linked to the Summit
Lake Tuff and post-Soldier Meadow units of the HRC volcanic suite. Further
differentiation of the magma chamber continued after the caldera-forming eruption of the
Summit Lake Tuff to proudce a highly-evolved melt with significant Eu-depletion that
later erupted the Soldier Meadow Tuff.
The Soldier Meadow Tuff erupted as a peralkaline, high-silica rhyolite along the
southeastern ring fractures approximately 16.12 Ma. Early eruptions of the Soldier
Meadow Tuff were of highly-differentiated magma that contained relatively high silica
65
content and low concentrations of TiO2, Al2O3, FeO, MgO, CaO, and P2O5. These earlier
erupted products, deposited as basal exposures, also have higher concentrations of
incompatible elements compared to upper stratigraphic counterparts. Stratified
geochemical variation reflects a typical eruptive sequence in which the cooler, lowest
density and most-evolved components were concentrated in the uppermost parts of the
magma chamber and erupted first, followed by later eruptions of slightly lesser-evolved
magma as lower parts of the chamber were tapped.
SM-lavas were largely emplaced shortly after the uppermost parts of the Soldier
Meadow Tuff (i.e. after BH09HR-49; JS10HR-93; JS10HR-33). However, SM-lava is
also found below the Soldier Meadow Tuff at the type-locality. Geochemical data shows
that SM-lavas (and Soldier Meadow dike) have a similar chemical variation as the
Soldier Meadow Tuff.
Later, post-Soldier Meadow eruptions along the eastern to southern ring fractures
produced andesitic to rhyolitic bedded pyroclastics (Ttp) and basaltic-andesite to dacite
dikes (Td) and lavas (Tad). Linear trends of major element oxides versus silica
concentrations are consistent with the more silicic rocks of the HRC volcanic suite
forming by fractional crystallization from a lesser-evolved parental magma. The parental
magma was likely a continental tholeiite as suggested by the TiO2 – Y/Nb ratio of Td dikes
and Tad lavas, the most mafic rocks of the HRC volcanic suite.
Ratios of incompatible elements should remain relatively constant through the
course of fractional crystallization and may vary only slightly during batch melting
(Rollinson, 1993). Variation in ratios of incompatible elements could indicate
66
heterogeneous source magmas, possibly arising from contamination or magma mixing
(Bougault et al., 1980). Incompatible element ratios of most rocks of the HRC volcanic
suite are relatively constant, suggesting a genetic relationship and possible common
magmatic source of the Summit Lake Tuff, Soldier Meadow Tuff, Ttp-bedded
pyroclastics and younger Td-dikes and Tad-lavas. This constant ratio slope implies the
parental magma was relatively homogeneous and did not undergo significant mixing or
contamination processes during evolution of the magmas erupted in the HRC volcanic
suite. Summit Lake Tuff, Soldier Meadow Tuff, Ttp bedded pyroclastics, Td dikes and
Tad lavas likely evolved from a common parental magma via crystal fractionation
processes (with the exception of Soldier Meadow Tuff and SM-lava geochemical outliers
that require post-depositional alteration, contamination of analyses from incorporated
lithics, or additional petrogenic processes other than simple fractionation).
The magma chamber that erupted the Summit Lake Tuff may have been only
partly evacuated during the caldera-forming eruption 16.33 Ma. Smith and Shaw (1973;
1975) and Smith (1979) state that a pyroclastic eruption is likely to extrude a maximum
of one-tenth the magma chamber volume. Residual magma in the HRC chamber evolved
over a 200,000-year to 400,000-year period to produce the Soldier Meadow Tuff and
SM-lavas that erupted along HRC ring fractures approximately 15.9 Ma to 16.12 Ma.
New magma likely injected into the base of the magma chamber between 16.33 Ma and
16.12 Ma, providing the heat necessary for continued differentiation. However, the new
magma may not have materially mixed with the HRC chamber. Post-Soldier Meadow
units, including Ttp, Tad lavas, and Td dikes, likely erupted from less-evolved magmas
67
lower in the chamber that were closer in composition to the parent from which the
Summit Lake Tuff and Soldier Meadow Tuff magmas evolved.
68
Chapter 5
CONCLUSIONS
The HRC formed from the eruption of Summit Lake Tuff approximately 16.33
Ma (Noble et al., 2009). This study suggests the Soldier Meadow Tuff and related lavas
erupted along the eastern to southern ring fractures of the HRC approximately 16.12 Ma.
Field observations, petrographic data and geochemical analyses were used to determine
the HRC as the likely eruptive source of the Soldier Meadow Tuff and SM-lavas, and to
characterize the complex eruptive dynamics and depositional facies of the Soldier
Meadow Tuff.
XRF and ICP-MS geochemical analyses were conducted on the Summit Lake
Tuff, Soldier Meadow Tuff, SM-lavas, and an overlying sequence of andesitic to rhyolitic
bedded-pyroclastics and basaltic andesite to dacite dikes and lavas. Major oxide
concentrations show geographic and stratigraphic compositional zoning in the Summit
Lake Tuff, and stratigraphic compositional zoning in the Soldier Meadow Tuff. Overall
collinear trends of major-oxides and trace elements vs. silica concentrations, and REE
patterns relate the Summit Lake Tuff, Soldier Meadow Tuff, SM-lavas, and post-Soldier
Meadow units to a common magmatic source, likely continental tholeitte, that evolved
predominantly by typical crystal fractionation processes.
Using transport and depositional models of common ash-flow lithofacies (Sparks
et al., 1973; Sheridan, 1979; Cas and Wright, 1987; McPhie, et al., 1993; Druitt, 1998;
Branney and Kokelaar, 2002), a model of the eruptive sequence of the Soldier Meadow
Tuff and post-Soldier Meadow units was illustrated in Figure 15. In the eruptive model,
69
Soldier Meadow Tuff deposits are complex and include fine-grained and coarse-grained
thinly-laminated surge layers, breccia, and massive deposits that are distributed along and
outboard-of the eastern to southern ring fractures of the HRC. Column collapse resulted
in the deposition of low-angle cross-bedded surges as well as deposition of auto-breccia
lags in proximal (<4 km) distances from eruption vents. The massive lithofacies
deposited in distal (>4 km) locations where velocity slowed and topography allowed
ponding or decreased flow velocity. These deposits repeat in stratigraphy, implying the
Soldier Meadow Tuff erupted in sequential, closely-spaced pulses from multiple column
collapses.
Paleocurrent indicators and a widespread distribution suggest that the eruption of
Summit Lake Tuff at 16.33 Ma resulted in the initial collapse of the HRC. Paleocurrent
indicators and distribution of surge, breccia, and massive lithofacies of the Soldier
Meadow Tuff indicate that the Soldier Meadow Tuff and lavas erupted along the
southeastern ring fractures of the HRC approximately 16.1 Ma. Further subsidence likely
occurred in the southeastern part of the HRC as the Soldier Meadow Tuff erupted. PostSoldier Meadow volcanism endured along the southeastern ring fractures in the form of
andesitic to rhyolitic bedded-pyroclastic deposits, rhyolite domes, northeast-aligned
basaltic andesite dikes and eruptive centers of basaltic andesite to dacite lavas.
70
APPENDICES
Appendix A. Methods
Appendix B. Petrography of the High Rock Caldera Volcanic Suite
Appendix C. XRF Data: Major Element Oxides of the High Rock Caldera Volcanic Suite
Appendix D. XRF Data: Trace Elements of the High Rock Caldera Volcanic Suite
Appendix E: ICP-MS Data of the High Rock Caldera Volcanic Suite
71
APPENDIX A
Methods
Geologic Mapping
Previous geologic maps (Korringa, 1973; Ach, 1988a; Ach, 1988b; Ach et al.,
1991) covered portions of the Soldier Meadow, Yellow Hills East, Yellow Hills West,
and High Rock Lake 7.5’ quadrangles, respectively. Detailed geologic mapping for this
project covers portions of these quadrangles and has detailed mapping in areas not
previously mapped.
Field objectives included mapping the Soldier Meadow Tuff, Soldier Meadow
lava, other related volcanics, contacts and faults, ring fractures associated with caldera
margins, eruptive vents, and other important features useful for establishing geospatial
relationships. Soldier Meadow Tuff crops out in the linear fissure complex (Korringa,
1973) as well as along the eastern to southern margin of HRC. Therefore, detailed
mapping was focused near the eastern to southern ring fractures. Although the ring
fractures of the HRC are buried, the ring fractures were approximated based on unit
distributions, locations of eruptive vents and preferentially-oriented feeder dikes for postSMT lavas.
Petrographic Analysis
Multiple hand samples of each mapable unit were collected. Collection sites were
recorded using a handheld GPS. Petrographic descriptions of both hand specimens and
72
outcrops mapped in the field include mineralogy, modal percentages, colors, and textures
from volcanic and depositional processes.
Fifty-one standard 30-micron thin-sections were made from collected samples at
California State University, Sacramento. Detailed petrographic descriptions of mapped
units were recorded from these thin-sections and included phenocryst assemblages,
estimated modal percentages of phases, micro-textures of crystals and groundmass,
presence of lithic fragments, and other important petrographic features (refer to Appendix
B).
A thin-section staining method was used for accurate approximations of modal
percentages of quartz and sanidine phenocrysts, which are particularly difficult to
distinguish from one another in thin-section. Staining procedures were based on revised
methods from Rosenblum (1956) and Houghton (1980). Rock surfaces of uncovered thinsections were etched using 50% hydrofluoric acid. Although most staining methods
require approximately 30 to 45 seconds for etching, times may vary significantly
depending on the strength of the hydrofluoric acid, which changes over time. To obtain a
proper etch, thin-sections in this project were etched over acid for approximately 3
minutes. Etched thin-sections were then dipped in a 60-percent saturated solution of
sodium cobaltinitrite in distilled water. Again, staining times vary depending on the
reactivity of the sodium cobaltinitrite solution. Although procedures suggested staining
times of 30 to 45 seconds, successful staining was achieved when thin-sections were
dipped in the staining solution for 3 minutes. The resulting effect is that quartz remains
unstained, while potassium feldspar retains a light yellow stain. Thin-sections were
73
covered with a glass slip using standard covering procedures. The stain was easily seen in
plane polarized light using a petrographic microscope.
Geochemical Analysis
Samples of ash-flow tuffs, lavas, and dikes representative of igneous activity of
the HRC were analyzed by XRF and ICP-MS at Washington State University
GeoAnalytical Laboratory using a ThermoARL Advant'XP+ sequential X-ray
fluorescence spectrometer. The analyzed samples consisted of 47 whole rocks and 10
single pumices (57 samples total) from the High Rock caldera volcanic suite.
Each hand sample was cleaned of weathered surfaces, lichen, dirt and soil, and
other factors that may alter the analytical results. Fresh pieces were broken off, and chips
of the hand sample were hand-picked. Approximately 30 grams were required for proper
analysis of each sample. At the WSU GeoAnalytical Laboratory, XRF analyses for major
and trace elements were conducted using a single low dilution Li-tetraborate fusion
method (Johnson et al., 1999). Resulting data of major element oxides were reproduced
in a spreadsheet, plotted on calibration curves, and normalized to total 100 %; Precision
of the data using this fusion method is usually ± 2 ppm (Johnson et al., 1999).
For ICP-MS analyses, the GeoAnalytical Laboratory uses an HP (now Agilent)
4500+ ICP-MS quadrupole mass spectrometer with an inductively coupled argon plasma
ion source. This mass spectrometer can measure metals and trace elements in liquids
from the low part-per-billion (ng/mL) range to tens of parts-per-million (ug/mL) (Knaack
et al., 1994). In this routine, 27 elements were analyzed, including 13 trace elements and
14 naturally occurring rare earth elements (REEs) from La through Lu as well as Ba, Rb,
74
Y, Nb, Cs, Hf, Ta, Pb, Th, U, Sr and Zr. Zr is measured as a check for complete
dissolution of the sample during the extraction process (Knaack et al., 1994).
Personnel at the GeoAnalytical Laboratory prepared samples for analyses based
on a hybrid technique of Crock and Lichte (1982) and laboratory experimentation.
Samples were ground in an iron bowl, and then two grams of the resulting powder were
mixed with two grams of lithium tetraborate flux. The powder was placed in a furnace
and fused. The sample preparation used fusion and dilution procedures to ensure
complete dissolution of zircons and others phases.
Data are imported into a spreadsheet, and oxide- and drift corrections are then
applied. Resulting drift-corrected concentrations have a precision typically better than 5%
relative standard deviation (RSD) for the REEs and 10% for the remaining trace elements
(http://www.sees.wsu.edu/Geolab/note/icpms.html).
Ar/Ar Dating
Radiometric ages of collected samples were calculated from measured 40Ar/39Ar
of sanidine phenocrysts. Sanidine phenocrysts are ideal for radiometric analysis based on
the high potassium content, homogeneity of the mineral, and the relative abundance of
this phase in these samples. Sample preparation and radiometric analyses were conducted
by Christopher Henry at the New Mexico Geochronology Research Laboratory
(NMGRL). Individual sanidine or anorthoclase grains were totally fused with a CO2
laser, and the released gas analyzed with a Mass Analyzer Products Limited (MAP) 21550 noble gas mass spectrometer.
75
Before analysis, the rocks were crushed and sieved. The sanidine crystals were
separated from other phases using a heavy liquid technique. Residual groundmass was
removed by ultrasonically cleaning the grains in 4% hydrofluoric acid solution for 5
minutes (Nomade, 2005). This purified aliquot was used in the irradiation process at
NMGRL, where samples were irradiated in a nuclear reactor core. Ar-Ar total fusion is
most useful on sanidine crystals due to the high potassium content, and is performed
using a laser and measuring the ratios of 40Ar/39Ar. Analytical data were plotted on
probability distribution diagrams, or ideograms. Most samples were highly radiogenic so
isochron plots were not used. The reported ages are weighted means of the analyzed
grains (http://geoinfo.nmt.edu/labs/argon/home.html).
76
APPENDIX B
Petrography of the High Rock Caldera Volcanic Suite
77
78
79
80
81
82
83
84
85
86
APPENDIX C
Major Element Oxides of the High Rock Caldera Volcanic Suite
87
88
89
90
91
APPENDIX D
XRF Data: Trace Elements of the High Rock Caldera Volcanic Suite
92
93
94
95
APPENDIX E
ICP-MS Data of the High Rock Caldera Volcanic Suite
96
97
98
99
100
REFERENCES
Ach, J.A., 1988a, Geologic Map of the Yellow Hills West Quadrangle, Washoe County, Nevada:
U.S. Geological Survey, Miscellaneous Field Studies Map MF-2028.
Ach, J.A., 1988b, Geologic Map of the Yellow Hills East Quadrangle, Washoe and Humboldt
Counties, Nevada: U.S. Geological Survey, Miscellaneous Field Studies Map MF-2029.
Ach, J.A., and Swisher, C.C., 1990, The High Rock Caldera Complex: Nested “failed” calderas in
northwestern Nevada: Transactions of the North American Geophysical Union, EOS, v. 71,
no. 43, p. 1614.
Ach, J.A., Bateson, J.T., Turrin, B.D., Keith, W.J., Noble, D.C., and Swisher, C.C., 1991,
Geologic map of the High Rock Lake Quadrangle, Washoe and Humboldt Counties, Nevada:
U.S. Geological Survey, Miscellaneous Field Studies Map, MF-2157.
Aguirre-Díaz, G.J., and Labarthe-Hernández, G., 2003, Fissure ignimbrites: Fissure-source origin
for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with Basin
and Range faulting: Geology, v. 31, p. 773-776.
Best, M.G., and Christiansen, E.H.,1997, Origin of broken phenocrysts in ash-flow tuffs:
Geological Society of America Bulletin, v. 109, p. 63-73.
Bonham, H.F., 1969, Geology and mineral deposits of Washoe and Storey Counties, Nevada,
with a section on industrial minerals by K.G. Papke: Nevada Bureau of Mines Bulletin, v. 70,
140 p.
Bougault, H., Joron, J.L., and Treuil, M., 1980, The primordial chrondritic nature and largescale
heterogeneities in the mantle: evidence from high and low partition coefficient elements in
oceanic basalts: Philosophical Transactions of the Royal Society of London, Series A, v. 297,
p. 203-213.
Branney, M.J., and Kokelaar, P., 2002, Pyroclastic density currents and the sedimentation of
ignimbrites: Geology Society Memoir no. 27, 143 p.
Campbell, I.H., 1985, The difference between oceanic and continental tholeittes: a fluid dynamic
explanation: Contributions to Mineralogy and Petrology, v. 91, p. 37-43.
Cas, R.A.F., and Wright, J.V., 1987, Volcanic successions: modern & ancient: Chapman and
Hall, London, 528 p.
Castor, S.B., and Henry, C.D., 2000, Geology, geochemistry, and origin of volcanic rock-hosted
uranium deposits in northwestern Nevada and southeastern Oregon, USA: Ore Geology
Reviews, v. 16, p. 1-40.
101
Coney, P.J., and Harms, T.A., 1984, Cordilleran metamorphic core complexes: Cenozoic
extensional relics of Mesozoic compressions: Geology, v. 12, p. 550-554.
Crock, J.G., Lichte, F.E., 1982, Determination of rare earth elements in geologic materials by
inductively couple argon plasma/atomic emission spectrometry, Analytical Chemistry, v. 54,
p. 1329-1332.
Druitt, T.H., 1998, Pyroclastic density currents in Gilbert, J.S., and Sparks, R.S. (Eds.), The
Physics of explosive volcanic eruptions: Geological Society Special Publication, No. 145, p.
145-182.
Erwin, D.M., H.E. Schorn, R.C. Smith, L.M. Levy, C.I. Millar, R.D., Westfall, J.C. King, and
V.S. Moran, 2005, Nevada's buried treasure: the Lund Petrified Forest: Botanical Society of
America meeting, Austin, Texas, abstract no. 271.
Floyd, P.A. and Winchester, J.A., 1975, Magma-type and tectonic setting discrimination using
immobile elements: Earth and Planetary Science Letters, v. 27, pp. 211-218.
Gabriel, A., Cox, E.P., 1929, A staining method for the quantitative determination of certain rock
minerals, American Mineralogist, v. 14, p. 290-292.
Greene, R.C., and Plouff, D., 1981, Location of a caldera source for the Soldier Meadow Tuff,
northwestern Nevada, indicated by gravity and aeromagnetic data: Summary: Geological
Society of America Bulletin, Part I, v. 92, p. 4-6, Doc. No. S10102.
Hildreth, W., 1981, Gradients in silicic magma chambers' implications for lithospheric
magmatism: Journal of Geophysical Research, v. 86, no. B11, p. 10153-10192.
Hilton, R.P., Hausback, B.P., Bromm, G.E., Schorn, H.E., 2008, Disruption of a mid-Miocene
ecosystem associated with incipient “Yellowstone Hotspot” volcanism in northwest Nevada:
Geological Society of America Abstracts with Programs, v. 40, no. 1, p. 39.
Houghton, H.F., 1980, Refined techniques for staining plagioclase and alkali feldspars in thin
section, Journal of Sedimentary Petrology, v. 50, p. 629-631.
Howard, K.A., 2010, Caldera collapse: Perspectives from comparing Galápagos volcanoes,
nuclear-test sinks, sandbox models, and volcanoes on Mars: GSA Today, v. 20, no. 10, p. 410.
Irvine, T.N. and Baragar, W.R.A., 1971, A guide to the chemical classification of the
common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, p. 523-548.
Johnson, D.M., Hooper, P.R., Conrey, R.M., 1999, XRF Analysis of rocks and minerals for major
and trace elements on a singe low dilution Li-tetraborate fused bead, Advances in XRF
Analysis, v. 41, p. 843-867.
102
Kamata, H., and Mimura, K., 1983, Flow direction inferred from imbrication in the Handa
pyroclastic flow deposit in Japan: Bulletin of Volcanology, v. 46-3, p. 277-282.
Knaack, C., Cornelius, S.B., Hooper, P.R., 1994, Trace element analyses of rocks and minerals by
ICP-MS, Washington State University GeoAnalytical Laboratory, Technical Notes
<http://www.sees.wsu.edu/Geolab/note/icpms.html>.
Korringa, M.K., 1972, Vent area of the Soldier Meadow Tuff, an ash-flow sheet in northwestern
Nevada: Unpub. Ph.D. thesis, Stanford University, 105 p.
Korringa, M.K., 1973, Linear vent area of the Soldier Meadow Tuff, and ash-flow sheet in
Northwestern Nevada: Geological Society of America Bulletin, v.84, p. 3849-3866.
Korringa, M.K., and Noble, D.C., 1970, Ash-flow eruption from a linear vent area without
caldera collapse: Geological Society of America Abstracts with Programs (Cordillera
Section), v. 2, pt. 2, p. 108-109.
Le Bas, M.J., Le Maitre, R.W., Streckeisen, A., Zanettin, B., and IUGS Subcommission on the
Systematics of Igneous Rocks, 1986, A Chemical classification of volcanic vocks
based on the Total Alkali-Silica Diagram: Journal of Petrology, v. 27, p. 745-750.
Le Maitre, R.W., 1989, A Classification of Igneous Rocks and Glossary of Terms:
Recommendations of the International Union of Geological Sciences Subcommission
on the Systematics of Igneous Rocks. Blackwell, Oxford, 193 pp.
Lerch, D.W., Miller, E., McWilliams, M., and Colgan, J., 2008, Tectonic and magmatic evolution
of the northwestern Basin and Range and its transition to unextended volcanic plateaus: Black
Rock Range, Nevada: Geological Society of America Bulletin, v. 120, p. 300-311.
McDougall, I., and Harrison, T.M., 1999, Geochronology and thermochronology by the 40Ar/39Ar
method: New York, Oxford University Press, xii, 269 p.
McPhie, J., Doyle, M., Allen, R., 1993, Volcanic textures: A Guide to the interpretation of
textures in volcanic rocks: 1993, CODES Key Centre, pp. 197.
Merriam, J.C., 1910a, Tertiary Mammal Beds of Virgin Valley and Thousand Creek in
northwestern Nevada, Part I, Geologic History: University of California Publications,
Bulletin of the Department of Geology, v. 6, no. 2, p. 21-53.
Merriam, J.C., 1910b, Tertiary Mammal Beds of Virgin Valley and Thousand Creek in
Northwestern Nevada, Part II, Vertebrate Faunas: University of California Publications,
Bulletin of the Department of Geology, v. 6, no. 11, p. 199-304.
Merriam, J.C., 1911, Tertiary mammal beds of Virgin Valley and Thousand Creek in
northwestern Nevada, Pt. II, Vertebrate faunas of California Publications, Bulletin of the
Department of Geology, v. 6, p. 199-304.
103
Mimura, K. and MacLeod, N.S., 1978, Source directions of pumice and ash deposits near Bend,
Oregon: Geological Society of America Abstracts with Programs (Cordilleran Section), v.10,
p. 137.
“New Mexico Geochronology Research Laboratory.” Copyright 2007-2008 New Mexico Bureau
of Geology & Mineral Resources. <http://geoinfo.nmt.edu/labs/argon/home.html>.
Noble, D.C., Henry, C.D., Park, Steven L., Smith, J.A., Hausback, B.P., and Hilton, R.P., 2009,
Geologic Framework and Evolution of the High Rock Canyon Volcanic Center,
Northwestern Nevada: An Early Caldera-Focused System of the Yellowstone Hotspot Track:
Geological Society of America Abstracts with Programs, v. 41, no. 7, p. 57.
Noble, D.C., McKee, E.H., Smith, J.G., and Korringa, M.K., 1970, Stratigraphy and
geochronology of Miocene volcanic rocks in northwestern Nevada: U.S. Geological Survey
Professional Paper 700D, p. D23–D32.
Noble, D.C., Plouff, D., Bergquist, J.L., Neumann, T.R., and Close, T.J. 1987, Mineral resources
of the High Rock Lake wilderness study area, Washoe and Humboldt Counties, Nevada. U.S.
Geological Survey Bulletin 1707A, p. 1-14.
Noble, D.C., Rigot, W.L., and Bowman, H.R., 1979, Rare-earth-element content of some highly
differentiated ash-flow tuffs and lavas: Geological Society of America Special Paper 180, pp.
77-85.
Nomade S., Renne, P.R., Vogel, N., Deino, A.L., Sharp, W.D., Becker, T.A., Jaouni, A.R.,
Mundil, R., 2005, Alder Creek sanidine (ACs-2): A Quaternary 40Ar/39Ar dating standard tied
to the Cobb Mountain geomagnetic event, Chemical Geology, v. 218, p. 315-338.
Park, S.L., 1983, Paleomagnetic stratigraphy, geochemistry and source areas of Miocene ash-flow
tuffs and lavas of the badger mountain area, northwestern Nevada: Thesis submitted for
Master of Sciences in Geology, University of Nevada, Reno.
Perkins, M.E., and Nash, B.P., 2002, Explosive silicic volcanism of the Yellowstone hotspot: The
ash fall tuff record: Geological Society of America Bulletin, v. 114, no. 3, p. 367-381.
Rollinson, H., 1993, Using Geochemical Data: Evaluation, Presentation, Interpretation. UK:
Longman Group UK Limited, 352 p.
Rosenblum, S., 1956, Improved techniques for staining potash feldspars, American Mineralogist,
v. 41, p. 662-664.
Schminke, H.U. and Swanson, D.A., 1967, Laminar viscous flowage structures in ash-flow tuffs
from Gran Canaria, Canary Islands: Journal of Geology, v. 75, p. 641-664.
Shand, S.J., 1947. The Eruptive Rocks, 3rd edition. New York: John Wiley, 444 pp.
104
Sheridan, M.F., 1979, Emplacement of pyroclastic flows: A review in Chapin, C.E., Elston, W.E.,
1979, Ash-flow tuffs: Geological Society of America Special Paper 180, p. 125-136.
Smith, J.A., Hausback, B.P., Henry, C.D., Noble, D.C., and Hilton, R.P., 2009, The Soldier
Meadow Tuff of the High Rock Caldera, Northwestern Nevada: Geological Society of
America Abstracts with Programs, v. 41, no. 7, p. 140.
Smith, J.A., Hausback, B.P., Henry, C.D., Noble, D.C., 2010, The Soldier Meadow Tuff:
Eruptive and depositional processes and relationship to the High Rock Caldera, NW Nevada:
American Geophysical Union, Fall Meeting 2010, abstract no. V13A-2338.
Smith, R.L., 1979, Ash-flow magmatism: Geological Society of America Special Paper 180, p. 527.
Smith, R.L., and Shaw, H.R., 1973, Volcanic rocks as geologic guides to geothermal exploration
and evolution: EOS [American Geophysical Union Transactions], v. 54, p. 1213.
Smith, R.L., and Shaw, H.R., 1975, Igneous-related geothermal systems, in White, D.E., and
Williams, D.L., eds., Assessment of geothermal resources of the United States – 1975: U.S.
Geological Survey Circular 726, p. 58-83.
Sparks, R.S.J., Self, S., and Walker, G.P.L., 1973, Products of ignimbrite eruptions, Geology, v.
1, p. 115-118.
Stirton, R.A., 1939, The Nevada Miocene and Pliocene mammalian faunas as faunal units: Sixth
Pacific Science Proceedings, p. 627-638.
Sun, S. and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts:
implications for mantle composition and processes in Saunders, A.D. and Norry, M.J.,
editors, Magmatism in the ocean basins: Geological Society of London Special Publication,
v. 42, p. 313–345.
“Trace Element Analyses of Rocks and Minerals by ICP-MS.” Washington State University
GeoAnalytical Laboratory Technical Notes. Updated 2011. <http://www.sees.wsu.edu/
Geolab/note/icpms.html>.
Wernicke, B.P., England, P.C., Sonder, L.J., and Christiansen, R.L., 1987, Extension in the Basin
and Range Province and East Pacific Margin: Tectonomagmatic evolution of Cenozoic
extension in the North American Cordillera: Geological Society of London Special
Publication, v. 28, p. 203-221.
Willden, R., 1964, Geology and mineral deposits of Humboldt County, Nevada: American
Association of Petroleum Geologists Bulletin, v. 42, no. 10, p. 2378-2398.
Wohletz, K.H., and Sheridan, M.F., 1979, A model of pyroclastic surge: Geological Society of
America Special Paper 180, p. 177-194.
105
Zoback, M.L., McKee, E.H., Blakely, R.J., and Thompson, G.A., 1994, The northern Nevada rift:
regional tectono-magmatic relations and middle Miocene stress direction: Geological Society
of America Bulletin, v. 106, no. 3, p. 371-382.