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LIBRARY
Michigan Slate
University

 

 

 

This is to certify that the

dissertation entitled

Geochemistry of the Elkhorn Mountains Volcanics,
Southwestern Montana: Implications for the Early
Evolution of a Volcanic-Plutonic Complex

presented by

Carolyn Rutland

has been accepted towards fulfillment
of the requirements for

Doctoral degree in Geology

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Date 11/7/85

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GEOCHEMISTRY OF THE ELKHORN MOUNTAINS VOLCANICS,
SOUTHWESTERN MONTANA: IMPLICATIONS FOR THE EARLY
EVOLUTION OF A VOLCANIC-PLUTONIC COMPLEX

By

Carolyn Rutland

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Geological Sciences

1985

ABSTRACT
GEOCHEMISTRY OF THE ELKHORN MOUNTAINS VOLCANICS,
SOUTHWESTERN MONTANA: IMPLICATIONS FOR THE EARLY
EVOLUTION OF A VOLCANIC-PLUTONIC COMPLEX
By

Carolyn Rutland

The Cretaceous Boulder batholith and Elkhorn Mountains Volcanics,
southwestern Montana, are an example of a large volume, volcanic-plutonic
association whose level of erosion has exposed the cogenetic intrusive rocks while
preserving sizeable portions of the volcanic field. Geochemical studies of the
volcanic rocks yield information about the origin, composition, and evolution of
the magmas; such conclusions may then be compared to similar information about
the crystallized products, represented in the plutonic rocks. Taken together, the
volcanic and plutonic rocks provide an improved understanding of the mechanisms
influencing the earliest and latest stages of development of a large, complex
magmatic system.

Relatively constant Th/Ta and enrichment of Ce, Ta, Hf, Zr, Yb, and Th in a
sequence of lava flows and ash-flow sheets in the Elkhorn Mountains Volcanics are
consistent with the hypothesis that the sequence is related to evolution of a single
magmatic system, envisioned as several separately evolving magma bodies of
combined batholithic dimensions. Interpretation of chemical trends within this
sequence has shown that 1) some fractionation of olivine influenced the evolution

of one group of lava flows, 2) a small amount of fractionation of plagioclase

Carolyn Rutland

occurred in the ash-flow sheets, but none in the lava flows, and 3) no fractionation
of alkali feldspar occurred. Relationships among the lava flows and ash-flow
sheets were not dominated by either crystal-liquid fractionation or magma
mixing, but may have been influenced somewhat by a combination of these
mechanisms. The absence of zoning in the ash-flow sheets is interpreted as
indicating that eruption occurred before development of a highly silicic upper
zone in the magma chamber(s).

The Elkhorn Mountains Volcanics are similar to the main series of the
cogenetic Boulder batholith in K20, NazO, CaO, and Rb contents. However, the
plutonic phase was influenced more by feldspar fractionation than was the
volcanic phase.

From interpretation of Zr abundances and (NazO + K20)/A1203 in the
volcanic rocks, it is considered probable that the parent magma was peralkaline.

Peralkalinity of the parent magma (as an indicator of high K20 contents)
and the relatively high Rb (approximately 35-165 ppm) and Sr (approximately
300-1200 ppm) are evidence for a magmatic source in crust of intermediate
thickness above a subduction zone at intermediate depth. This is evidence in
support of earlier conclusions (Doe and others, 1968; Tilling, 1973), that the
magmas originated in the lower crust or upper mantle. The lack of a significant
Eu anomaly in the volcanic rocks, implying that the source region was not in the
stability range of feldspar, is consistent with this interpretation.

Shallow level processes are rejected as dominant processes in the evolution
of the Elkhorn Mountains Volcanics. Instead, the major chemical characteristics
of the magmas could reflect the source(s) of the liquids. Characteristics inherited
during magma generation at depth, rather than those acquired during residence at
shallow crustal levels, may be typical of similar eruptive units of intermediate

SiO2 range.

ACKNOWLEDGEMENTS

I thank my supervisor, T. A. Vogel, for his excellent guidance and support.
Tom is to be commended for his patience during my long tenure as his student and
for his tact in helping me choose a research project. He carefully considered my
requirements that the study a) be of plutonic rocks and b) involve very little field
work and then sent me off to study volcanic rocks in an almost inaccessible
wilderness area. I also thank my committee members, F. W. Cambray,
D. T. Long, and J. T. Wilband, for their interest and advice. I am indebted to John
Wilband in particular for computer software used in modeling of fractionation
processes.

I acknowledge the important support of W. R. Greenwood and R. I. Tilling,
both of the U.S. Geological Survey. This project could not have been completed
without their interest and encouragement. Neither was ever too busy to discuss
the problems and progress of the project, or to offer insight into any of its
aspects. Bill Greenwood arranged for material support of the project by the
Office of Mineral Resources, as a follow-up of work on the Elkhorn Wilderness
Study Area. The preliminary draft of this dissertation was greatly improved by
careful and critical reading by Bob Tilling. I am grateful to them both for their
contributions.

1 also benefited from conversations with P. Lipman (U.S.G.S.) and especially
with R. L. Smith (U.S.G.S.).

I was ably and enthusiastically assisted in the field by M. Adelman, whose
questions forced me to consider field relationships that I might otherwise have
overlooked and whose cooking abilities were the best in camp. T. A. Vogel and

L. W. Younker (Lawrence Livermore National Laboratory) also participated in

ii

many helpful discussions in the field. H. W. Smedes (now retired from the U.S.
Dept. of Energy) taught me a great deal about the field aspects of welded ash-
flow tuffs and about the Elkhorn Mountains Volcanics in particular. C. J. Schmidt
(Western Michigan University) unselfishly contributed time to my field work from
his own graduate students and projects, and C. B. Schmidt provided vital
inspiration. The field work was supported primarily by the U.S. Geological
Survey, Office of Mineral Resources, as a follow-up of work on the Elkhorn
Wilderness Study Area. Other sources of support were the Society for Sigma Xi,
Chevron grants-in-aid of field-founded graduate research to Michigan State
University, Department of Geological Sciences, and T. A. Vogel. L. J. Suttner,
Director, generously allowed use of the Indiana University Geologic Field Station
facilities.

I thank the Department of Geological Sciences, Michigan State University,
for supporting my graduate studies there. I also thank W. R. Greenwood for
arranging for most of the chemical analyses to be done by the U.S.G.S., Office of
Mineral Resources as a follow-up of work on the Elkhorn Wilderness Study Area.
The remaining analyses were done by Lawrence Livermore National Laboratory,
for which I thank L. W. Younker. Work was performed under the auspices of the
U.S. Department of Energy under Contract W-7405-Eng-48.

Part of this research was carried out while I was living in Montana (198‘!)
and in Texas (1985), and I am grateful to the Montana Bureau of Mines and
Geology for drafting assistance and to the Texas A 6: M University, Department
of Computer Science for use of their computer facilities. G. Lunsky and
especially 3. Gell (Michigan State University) were helpful in sample preparation.

Finally, I gratefully acknowledge the unflagging support of my parents, Ann
and Walter Rutland, and especially of my husband, Chris Schmidt. Chris must be
given credit for always insisting that somebody ought to study that pile of
volcanics out in southwestern Montana.

iii

TABLE OF CONTENTS

LIST OF FIGURES I O O O O O O O O O O O O O O O O O O O O

LlST OF TA BLES O O O O O O O O O O O O 0 O O O O

INTRODUCHON O O 0 O O O O O O O O O O O O O O O O

REGIONAL GEOLOGIC SETTING . . . . . . . . . . .
GEOLOGYOF THESTUDY AREA . . . . . . . . . . .
Field Characteristics . . . . . . . . . . . . . .

Lower Memmr O O O O O O O O O O O O O O
MiddleMember..............

Summary of Thin Section Descriptions . . . . . . . .

SAMPLING AND ANALYTICAL METHODS . . . . . . .

CHEMICAL COMPOSITIONS OF THE ELKHORN
MOUNTAINS VOLCANICS . . . . . . . . . . . .

MajorElements...................

LavaFlows................

Ash-flowTuffs...............

Trace Elements 0 O O O O O O O O O ...... O O 0 O

Ash-fallAnalyses. . . . . . . . . . . . . . . .
DISCUSSION 0 O O O I O O I O O O O O O O O O 0

Chemical Trends within the Volcanic Sequence -
ASingle Magmatic System . . . . . . . . .

Evidence of Processes Affecting the Distinct Groups
of Lava Flows and Ash-flow Sheets . . . . . .
TheAsh-flowSheets . . . . . . . . . . . .
TheLavaFlows..............
Processes Common to Both the Ash-flow Sheets

andtheLavaFlows . . . . . . . . . .

Possible Relationships Between the Lava Flows
and the Ash-flow Sheets . . . . . . . . . .

The Evolution of the Entire Magmatic System within
the Time Frame Represented in the Study Area
Interpretation of Caldera Cycle Remnants. . .

iv

vi

viii

15
15
l7
18

24

29
35
37
39
#5
1+9

49

52
52
54
59
62

66
66

TABLE OF CONTENTS (continued)

Comparison of the Elkhorn Mountains Volcanics
to the Similar-Sized Timber Mountain and
Associated Calderas Complex. . . . . . . .

Comparison of the Elkhorn Mountains Volcanics
to the Boulder Batholith . . . . . . . . . . . .

Comparison of Volcanic and Plutonic Processes . . . . .
CONCLUSIONS 0 O O O O O O O O I O O O O O O O O O 0
APPENDIX: CHEMICAL ANALYSES OF ASH-FALL SAMPLES

REFERENCES

69

71
83
86
9O
91

Figure 1.

Figure 2.

Figure 3.
Figure 14.
Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.
Figure 12.

Figure 13.

Figure 14.

Figure 15.
Figure 16.

Figure 17.

LIST OF FIGURES

The tectonic setting of the Elkhorn
Mountains Volcanics-Boulder batholith
region 0 O O O O O O O O O O O I O O O O O O 0

Map of remnants of the Elkhorn Mountains
Volcanics in the Elkhorn Mountains . . . . . . .

Geologic sketch map of the study area . . . . . .
Volcanic stratigraphy of the study area . . . . .

Plot of SiO against FeO*/MgO of all
available Efi<horn Mountains Volcanics analyses . .

Major-element variation diagrams of the
lavaflows..................

Major-element variation diagrams of the
aSh-flOW Sheets 0 o o o o o o o o o o o o o o o

Plots of Ba, Ce, Co, Cr, Cs, Hf, Ni, Rb, Sc,
Sr,Ta,aneragainstTh. . . . . . . . . . . .

Plots of K20, NaZO, and CaO against Th . . . . .

Chondrite-normalized rare earth element profiles
for the Elkhorn Mountains Volcanics . . . . . . .

PlotofCeagainsth‘. . . . . . . . . . . . . . .

Plot of Ta against Th for the Elkhorn Mountains
Volcanics compared with some other eruptive units

Results of modeling: Predicted and actual values

of Ni, Cr, SC, and Co in lava groups V and VIII. . . .

Plots of Eu/Eu* against Th and Sr for The
Elkhorn Mountain Volcanics . . . . . . . . . .

SiOZ ranges of some selected eruptive units . . .
Plots of SiOz, MgO, and Fe as Fe203 against Th. .

Ratio-ratio plots to evaluate role of magma mixing

vi

10
11
ll!

34
36
33

40
M

#7
51
53
55
58
61

64
65

Figure 18.
Figure 19.

Figure 20.
Figure 21.
Figure 22.
Figure 23.

Figure 24.

Figure 25.

LIST OF FIGURES (continued)

PlotofThagainsttime . . . . . . . . . . .

Map of the Timber Mountain and related

calderascomplex..................

Highly schematic sequence of pre-erosion Elkhorn

Mountain Volcanics cross-sections based on Timber

Mountain and related calderas complex analogy . . .

KO-NagO-Cao diagram of the two plutonic main

3 d sodi series fields and the Elkhorn

MountainsVolcanics. . . . . . . . . . . . . . . . .

Plots of Rb and Sr against 510 in the Elkhorn
Mountains Volcanics and the p utonic main

andsodicseriesfields................

Abundances of Y, Cu, Zr, and Ba in the Elkhorn Mountain
Volcanics and the plutons of the southern part of the
BOUIder bathouth O O O O O O O O O O O O O O O O O O

Chondrite-normalized rare earth element profiles
fortheBoulderbatholith. . . . . . . . . . . . . . .

Plot of Eu/Eu* against Th for the Elkhorn
Mountains Volcanics and the Boulder batholith

vii

67

7O

72

7‘}

76

77

81

82

Table 1.
Table 2.
Table 3.
Table 14.

Table 5.
Table 6.

Table 7.

LIST OF TABLES

Estimates of analytical accuracy. . . . .
Chemical analyses of lava flows . . . . .
Chemical analyses Of ash-flow sheets . . .

Partition coefficients used in modeling of
crystal fractionation . . . . . . . . . .

Trends in actual and modeled Mg* . . . .

Eu/Eu* in the Boulder batholith and the
Elkhorn Mountains Volcanics. . . . . . .

Z? “d (N320 + K20/M203) o o o o o o 0

viii

27
30
32

57
6O

79
81}

INTRODUCTION

The Cretaceous Boulder batholith and Elkhorn Mountains Volcanics in
southwestern Montana are an excellent example of a large volume, volcanic-
plutonic association whose level of erosion has exposed the cogenetic intrusive
rocks while preserving sizeable portions of the volcanic field. In such a plutonic-
volcanic association, the plutonic rocks yield information primarily about the
crystallized products and the compositions of the magmas must be inferred. In
contrast, the volcanic rocks, which consist primarily of lava flows and ash-flow
sheets in the Elkhorn Mountains Volcanics, yield information about the
composition of the magmas.

It is well accepted that both ash-flow sheets and calderas are products of
violent, rapid evacuations of the upper portions of shallow-level magma chambers
(Williams, 1941; Smith, 1960). In this context, a sequence of ash flows preserves
quenched magma as ash-flow tuffs from the uppermost part(s) of a magmatic
system and hence a partial record of its evolution in time. Lava flows from the
same system complement the record of the ash-flow sheets by furnishing samples
of magma from other portions of the magmatic system. However, lava flows
generally represent much smaller volumes of magma than do ash-flow sheets.

The sources of the ash flows and lava flows of the Elkhorn Mountains
Volcanics may have comprised several overlapping calderas and vents. Although
each eruptive episode may have tapped its own localized magma chamber, all
were probably related to the larger, complex magmatic system, whose final
crystallization products were the plutons of the Boulder batholith. The volcanic

products of an individual volcanic center or vent would thus largely be determined

by the evolutionary processes operating there, but could also be influenced by
events common to the other parts of the overall magmatic system.

In general, each eruptive cycle would begin with a pre-caldera stage
dominated by eruptions of lavas and pyroclastic materials, followed by a climactic
caldera-forming event with attendant voluminous ash flows. The post-caldera
stage would again be a period of less energetic and low-volume eruption of lavas
and pyroclastic materials (Christiansen, 1979).

Because the Elkhorn Mountains Volcanics are only the remnants of a huge,
complex volcanic field, no single caldera cycle is likely to be preserved
completely. However, the intercalated ash-flow sheets and lava flows do record
the overall Chemical evolution of the larger magmatic system; individual caldera
cycles may possibly be identified within these ash-flow sheets and lava flows.

The chemical variation of each successive ash-flow sheet and lava flow
reflects the long-term evolution of the system. Possible compositional zoning
within the ash-flow sheets is especially significant. The recognition that
compositional zoning within ash-flow sheets probably reflects pre-eruption
gradients in the magma body (Smith and Bailey, 1966; Lipman and others, 1966)
has provided a fundamental basis for many detailed studies of ash flow sheets (Cf.
Hildreth, 1979; Mahood, 1981; Bacon and others, 1981; Whitney and Stormer, 1985;
Leat and others, 1981!).

Recent modeling of ash flow eruption dynamics has demonstrated that an
ash-flow sheet is not a simple inversion of the pre-eruptive trends (Spera, 1981;
Blake, 1981). Nevertheless, the overall Chemical pattern in the magma body is
generally preserved in the ash-flow sheets (Lipman and others, 1966).

Studies of zoned ash-flow sheets in turn have led to the formulation of

models for the evolution of their source chambers (Smith, 1979; Hildreth, 1981).

The nature of the causes of such differentiation is still much debated (Hildreth,
1981; Michael, 1983; Cameron, 1981:; Baker and McBirney, 1985).

The development of the cogenetic plutonic rocks may be broadly traced
within the same caldera cycles which produced the volcanic materials. The model
proposed by Steven and Lipman (1966) for the San Juan volcanic field, Colorado,
can likewise be applied to the Elkhorn Mountains Volcanics-Boulder batholith
association (Lipman, 1979, 19811). Thus, each cycle of volcanic activity reflects
the upward movement of magma, climaxing with the failure of the roof of each
particular Chamber or cupola of the larger magmatic system and ash flow
eruption, and waning as the remaining material crystallized. The chemistry of the
plutons of the Boulder batholith offers evidence of the evolutionary processes
operating only during the final cooling and crystallization stages. Comparison of
the chemical trends of the batholith with those in the ash-flow sheets and lavas
contrasts crystallized products (that is, the batholith) and magma (that is, the
volcanic rocks). This comparison may contribute to an improved understanding of
the mechanisms controlling the earliest and latest development of a large,
complex magmatic system.

Accordingly, this investigation of the Elkhorn Mountains Volcanics-Boulder
batholith association has the following objectives.

1) To identify the major processes that led to the Chemical diversity of
the Elkhorn Mountains Volcanics, based on interpretation of the major
and trace element variations within the volcanics, and

2) To compare the evolutionary processes inferred from the volcanic rocks

with those recorded in the plutonic rocks.

REGIONAL GEOLOGIC SETTING

The Elkhorn Mountain Volcanics-Boulder batholith region lies east of the
Idaho batholith within the Helena salient of the frontal Cordilleran fold and thrust
belt (Figure l). The region is dominated by Sevier-style thrusting on the north.
However, the southernmost exposures of the volcanics and southern satellite
plutons of the batholith are folded and faulted in structures related to Laramide-
style deformation of the Rocky Mountain foreland (Schmidt and Hendrix, 1981).
These two structural styles developed coevally from Late Cretaceous to late
Paleocene time (about 80 to 60 Ma). Eruption of the Elkhorn Mountains Volcanics
took place from about 81 to 76 Ma ago, overlapping emplacement of the Boulder
batholith from approximately 78 to 68 Ma ago(Tilling and others, 1968).

For much of the past thirty years the Boulder batholith was the subject of
research by members of the U.S. Geological Survey (e.g., Knopf, 1957, 1963;
Klepper and others, 1957, I971; Klepper, Ruppel and others, 1971; Klepper and
Smedes, 1959; Becraft and others, 1963; Ruppel, 1961, 1963; Smedes, 1962, 1966,
1967; Smedes and others, 1962; Doe and others, 1968; Tilling, 1973, 1974; Tilling
and others, 1968; Robinson and others, 1968). A part of the Elkhorn Mountains
Volcanics-Boulder batholith complex lies within the proposed Elkhorn Wilderness
Study Area, whose economic potential was evaluated jointly by the U.S.
Geological Survey and the U.S. Bureau of Mines (1978). A summary follows of the
regional igneous relationships as described in these references.

The Elkhorn Mountains Volcanics were intruded by the Boulder batholith and
make up its margins and remaining roof (Figure 1). The volcanics discontinuously
cover much of the region from the Deer Lodge valley to the Townsend valley and

from the Jefferson River north toward Helena (Klepper and others, 1957). They

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are the remnants of an extensive volcanic field that may originally have covered

up to 25,000 km2

and have been at least 3 km and perhaps as much as 4.7 km thick
(Klepper and Smedes, 1959; Smedes, 1966). If these estimates are correct, the ash
flows in the Elkhorn Mountains Volcanics may have comprised one of the earth's
largest known ash flow fields (Smith, 1960a).

Klepper and others (1957) divided the volcanics into three informal members
based on their work in the southern Elkhorn Mountains. The lower member
consists predominantly of lava flows, autobrecciated lavas, and volcanic
sediments, but also contains some thin ash flows; its thickness is as great as
1300 m locally. The middle member, as much as 2290 m thick in places, consists
of many layers of ash flows and thin layers of ash-fall sediments. The upper
member is composed of water-laid volcanic sediments totalling 630 m. The same
lithostratigraphic units of varying thickness have been recognized throughout the
region (Klepper, Ruppel and others, 1971; Klepper and others, 1957; Freeman and
others, 1958; Nelson, 1963; Smedes, 1966; Becraft and others, 1963; Ruppel, 1961,
1963); however, no correlations of chronological boundaries have been established.
There is no formal stratigraphy for the Elkhorn Mountains Volcanics, and usage of
the informal division is retained for convenience in referring to the relevant
portions of the volcanic pile.

The Boulder batholith is a composite intrusion comprised of at least fifteen
epizonal plutons ranging in composition from syenogabbro to alaskite; it is
exposed over an area of 6,000 to 7,000 km2 (Klepper and others, 1971). The
batholith is an elongate body of approximately 100 km by 50 km; the largest
pluton, the Butte Quartz Monzonite, makes up over 7596 of the exposed batholith
(Klepper and others, 1971). Estimated thicknesses for the batholith range from
less than 5 km (Hamilton and Myers, 1974) to over 15 km (Klepper and others,

1971). Radiometric age dates for the batholith range from approximately 78 to

68 Ma; the Butte Quartz Monzonite was probably emplaced from 76 to 74 Ma ago
(Tilling and others, 1968).

Tilling (1973) presented major element Chemical and isotopic evidence for
recognition of two magma series in the Boulder batholith, the main (or potassic)
series and the sodic series. The main series, represented primarily by the plutons
in the northern and central parts of the batholith, is Characterized by higher K20
and lower Nazo contents at any given SiOz content. In contrast, the plutons in
the southern part of the batholith, most of which comprise the sodic series,
contain less K20 and more Na20. The distribution of Rb, Sr, Th, and U, and of
lead, oxygen, and hydrogen isotopes among the plutons shows the same two groups
(Tilling, 1973, 1977). In a trace element study of the main and sodic series plutons
in the southern part of the batholith, Lambe (1981) presented variation diagrams
for Nb, Zr, Pb, Cu, and Ba against SiO2 which also exhibited different patterns in
groups of samples representing each magma series.

Collectively, the isotopic data and chemical analyses have been interpreted
as indicating a genetic relationship between the Elkhorn Mountains Volcanics and
the Boulder batholith, confirming inferences from earlier geologic observations
(best summarized by Robinson and others, 1968, but also discussed in Klepper and
others, 1957, and Robinson, 1963). Doe and others (1968) concluded that the lead
isotopic data for the batholith and the volcanics (represented by only two samples)
are compatible with the inferred genetic relationship between them. Tilling
(1973, 1974) presented chemical and isotopic evidence comparing the Elkhorn
Mountains Volcanics with the main and sodic series of the batholith and concluded
that the volcanics are more similar to the "more mafic rocks of the plutonic main
series and are distinctly different from any rocks of the sodic series" (Tilling,

1974, p. 1925). The trace element studies of Lambe (1981) are also consistent

with a genetic relationship between the Elkhorn Mountains Volcanics and the

plutonic main series.

GEOLOGY OF THE STUDY AREA

Much of the preserved remnants of the Elkhorn Mountains Volcanics lies in
the region known as the Elkhorn Mountains, located south of Helena, Montana, and
north of the Jefferson River between the Missouri and Boulder Rivers (Figure 2).
The area around Crow Peak, near the former mining town of Elkhorn in the
southern Elkhorn Mountains, was chosen for this project after evaluation of
existing geologic reports (specifically, Klepper and others, 1957; Becraft and
others, 1963; Smedes, 1962, 1966; Klepper, Ruppel and others, 1971) (Figure 3).
W. R. Greenwood (pers. comm., 1981) provided expanded versions of some of the
measured sections in these U.S. Geological Survey Professional Papers as well as
additional measured sections. After preliminary examination of all available
information, the final choice was made by considering a) the relative structural
simplicity of the Crow Peak area, b) the availability of good exposures of the
volcanics, c) the comparatively mild contact-metamorphic effects of the
batholith to the west and the subvolcanic sills and dikes within the area, d) the
limited hydrothermal alteration, and e) the accessibility of the area. The study
area (Figure 2) is located in the Helena and Deerlodge National Forests in
Jefferson County.

Little or no unequivocal physical evidence of the eruptive sources of the
primary volcanic materials is preserved at the present level of exposure of the
Elkhorn Mountains Volcanics-Boulder batholith complex. However, the structural
relationships in the vicinity of the Skyline Mine and the adjacent distribution of a
thick welded tuff of the middle member (see Figure 3) may indicate part of the
structural rim of a cauldron (U.S.G.S—U.S.B.M. Open File Report 78-325, 1978,

p. 67). Specifically, the northwest-trending Skyline fault may be the southern

112°

 

 

 

 

 

 

 

   

 

 

 

 

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Figure 2. Map of remnants of the Elkhorn Mountains Volcanics in the
Elkhorn Mountains; small box indicates study area.

10

 

 

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lllllllVlllEll

 

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Figure 3. Geologic sketch map of the study area. Traverses
labeled A, B, and C indicate the locations of the generalized
stratigraphic columns in Figure 4. Dashed lines indicate
faults. Modified from Klepper and others (1957).

margin of a structural low within which accumulated intracaldera ash flow
accumulated. This interpretation is consistent with the volcanic stratigraphy of
the study area (Figure 4), and with the geology and structure of the Elkhorn
Mountains volcanics at the northern end of the Elkhorn Mountains
(W. R. Greenwood, pers. comm.).

The oldest rocks in the study area are the lower gray to black shales of the
Cretaceous Colorado formation, overlain with slight angular unconformity by the
Elkhorn Mountains Volcanics. These shales represent the lower part of the
Colorado formation; to the east and north on the flanks of the Elkhorn Mountains,
complete sections of the Colorado are overlain conformably by the volcanics
(Klepper and others, 1957; Klepper, Ruppel and others, 1971; Smedes, 1966).
Immediately to the east of the study area, the volcanic sediments of the Slim Sam
formation occur between the Colorado (conformably) and the volcanics (locally
either conformably or unconformably). The Slim Sam formation consists of
erosional debris from the earliest volcanic products; its textural characteristics
suggest relatively short transport distances from the source (Klepper and others,
1957).

A fairly thick (about 2065 In) section of the lower and middle members of
the Elkhorn Mountains Volcanics is preserved in the study area apparently due to
the pre-depositional erosion surface and to post-depositional deformation.
Klepper and others (1957) interpreted the unconformity below the volcanics as
part of a valley or depression in the Colorado and Slim Sam formations, which
allowed a thick accumulation of the volcanics. Subsequent to deposition, folding
produced a major syncline whose axis follows the crest of this trough
northeastward into the central Elkhorn Mountains (Klepper and others, 1957;
Smedes, 1966). Smedes (1966, p. 24) suggested that the pre-volcanic depression

might have been a valley formed by erosion along the crest of an older fold.

12

Figure 4. Volcanic stratigraphy of the study area. Column labels A, B, and C
correspond to traverse labels in Figure 3. Roman numerals identify units
referred to in the text. Letters and Arabic numbers indicate approximate
positions of sample sites and correspond to sample numbers in Tables 2 and 3.

13

 

 

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m OLDER LAVA

 
 
  
 
 
 
 
 
 
 
 
  
   

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x" WELDED TUFF
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The upper member of the volcanics is not present in the Study area.

The salient physical characteristics of several sequences of volcanic
lithologies in the lower and middle members of the Elkhorn Mountains Volcanics
found in the study area are summarized below. The sequence is illustrated in
Figure 4, annotated with Roman numerals to correspond to the following
description of the units within the sampled section.

The ash-flow sheets and lava flows in both the lower and middle members
contain lithic fragments. In the field, two categories of lithic fragments can be
determined, one of xenoliths that are clearly of volcanic origin, generally because
of porphyritic textures, and one of xenoliths of ambiguous origin, because they are
homogeneous and extremely fine-grained. These latter resemble Chips of chert,
like that present in the Colorado formation. However, they could also be lithic

fragments of aphanitic volcanic rock or of bedded or ash-fall tuff.

Field Characteristics

Lower Member. The lower member of the volcanics is dominated by lava
flows (II, ranging in thickness from about 95 m to 252 m; V, about 37 m thick; and
VIII, about 348 m thick). The lower member also contains five ash-flow sheets,
some of which are associated with underlying ash-fall tuffs, ranging in thickness
from about 3 m to 88 m. The remainder of the approximately 1290 m of the lower
member is characterized by epiclastic (volcanoclastic) sediments, comprising
breccias and conglomerates and their finer-grained equivalents (Figure 4). Some
of these layers are clearly waterlain, as evidenced by, for example, graded
bedding and Channel-fill lenses. A pyroclastic origin is suggested for the others,
particularly the finer-grained units immediately underlying ash-flow sheets.

Unit 11 consists of at least three separate flows which are locally
autobrecciated and/or amygdaloidal. The oldest of these is characterized by

abundant oatmeal-flake-sized (.5 cm diameter) plagioclase and occasional

15

hornblende phenocrysts in an aphanitic grey groundmass. This flow is overlain by
one containing less coarse plagioclase and more abundant hornblende phenocrysts
in a similar matrix. A few lithic fragments and broken plagioclase crystals can be
observed in outcrops of both flows. The other f1ow(s) is laterally discontinuous,
whereas the lower two are distinctive, continuous stratigraphic markers.

Unit V is approximately 37.5 m thick (Figure 4). The texture of its
lowermost 4.7 meters is almost aphanitic with very small plagioclase crystals and
contains abundant lithic fragments. This massive basal layer grades upward into
autobrecciated lava for the remaining thickness.

The lava flows in unit VIII are lithologically similar containing, phenocrysts
of pyroxene and plagioclase in a vesicular, nearly aphanitic matrix. Locally
amygdaloidal, these lava flows are interbedded with thin ash-fall tuffs.

Cooling units cannot be recognized in the ash-flow sheets in the lower
member because of post-depositional alteration. However, separate lithologic and
textural units within the preserved sheets can be recognized; most of these can be
traced laterally throughout the study area. The welded tuffs are the most easily
recognized portions of the ash-flow sheets, exhibiting typical textures such as
well-developed eutaxitic structures.

The oldest ash-flow sheet (I) is also the basal unit of the Elkhorn Mountains
Volcanics in the study area (see Figure 4). The exposures of this unit consist of
highly welded tuffs, with fiamme ranging in size from about 1 cm to about 15 cm
in length, surrounded by abundant broken plagioclase crystals and mainly andesitic
lithic fragments. The exposures in the next ash-flow sheet (111), which overlies the
older lava group (see Figure 4), have large fiamme (up to 20 cm in length) and
sparse lithic fragments. The matrix is rich in plagioclase; crystal-fragment
abundance is uniform throughout the unit. Above this unit is a lapilli tuff which is

interpreted as part of the next overlying ash-flow sheet (IV). The fiamme in the

16

welded tuff in this sheet decrease in size from the highest exposures of the unit to
the lowest. The matrix in the upper portion contains up to 2096 plagioclase
crystal fragments, which decrease to less than 596 in the lower portion. Likewise,
angular lithic clasts up to about 8 cm in diameter are more abundant in the upper
half of the unit than in the lower. Within the primarily epiclastic/pyroclastic
sequence in the middle of the lower member (see Figure 4) is another welded tuff
(VI), which is inferred to be an erosional remnant of a thicker ash-flow sheet.
This interpretation is based on the fact that the unit is welded throughout its
4.7 m thickness and is lacking in non- or partly-welded zones. Just below the
lavas of unit VIII the youngest ash-flow sheet (VII) in the lower member. Fiamme
are most abundant in the middle 32 m of exposures of this unit, lithic fragments
are scarce in its upper 7-8 m, and plagioclase is abundant in the matrix
throughout its thickness.

Middle Member. The contact between the middle and lower members in the
study area may be disconformable (Klepper and others, 1957); this interpretation
is supported by the discontinuous presence of a mostly bedded tuff overlying the
younger group of lava flows in the lower member.

The middle member consists of more than 700 meters of ash-flow sheets.
The lowermost continuous lithologic unit in the middle member is about 284 m of
lithic-rich welded tuff (see Figure 4). This distinctive unit is overlain by another
447 m of variably welded tuffs, ranging from 0.6 m to 110 m in thickness and
alternating with ash fall tuffs, some of which show features of reworking. The
remainder of the middle member is covered by glacial sediments and recent soils
north of the study area. It is not possible to subdivide this sequence into cooling
units on the basis of textures and field relationships, although the presence of a

devitrified vitrophyre approximately midway through the exposure demonstrates a

17

cooling break, as do the several well-bedded ash-fall tuffs in the sequence.
Therefore, the divisions illustrated in Figure 4 are purely lithologic.

Unit V111 and the lower part of the middle member are cut by a pyroxene
porphyry dike of probable andesitic composition. It is likely that this intrusion is
similar in age to the volcanics, albeit somewhat younger. Although Klepper and
others (1957) describe other intrusions in the study area, none are of mappable
size. At least one of these was found on re-examination to be a lava flow (this

work; W. R. Greenwood, pers. comm.).

Summary of Thin Section Descriptions

The important microscopic characteristics of the major volcanic units in the
stratigraphic sequence in the study area are summarized in the following section
as a supplement to the field descriptions and as background for the geochemistry
of the volcanics.

Some textural and mineralogical features are common to the ash-flow
sheets and the lava flows. Both rock types have been contact-metamorphosed in
the aureole of the Boulder batholith. In the ash-flow sheets many primary
characteristics have been modified by devitrification and recrystallization, yet
their general features remain recognizable as Characteristic of ash-flow sheets
(summarized by Ross and Smith, 1961). The alteration processes resulted in many
similarities between different rock types, and the cause of a particular
characteristic cannot always be identified.

There is no primary glass in any of the volcanics, but relict devitrified
shards and fiamme remain. The fiamme are now composed mainly of Chlorite and
epidote, and the formerly glassy groundmass of the ash-flow sheets is generally a
very fine-grained mosaic of quartz and feldspar, with minor amounts of epidote,

Chlorite, and calcite.

18

In all of the rocks, the plagioclase is partly altered to epidote and/or
calcite. The biotite is non-uniform in color, faded from brown to very pale
yellow; it contains abundant opaque inclusions. The primary mafic minerals have
been altered; in some samples, calcite and other minerals occupy the relict
mineral shape.

Because most of the ash-flow sheet samples are from the welded portions of
the sheets, they exhibit similar textural characteristics. Ubiquitous eutaxitic
structure is distorted around crystal and lithic fragments to varying degrees in all
but a few samples. Most samples also contain flattened shard shapes.

The most common lithic fragments in the ash-flow sheets and lava flows are
porphyritic. In some samples, more than one porphyritic type can be observed.
Lithic fragments of welded tuff are sometimes present. Other lithic fragments
are apparently either chert or very fine-grained tuff.

In contrast to the similarities in petrographic characteristics discussed
above, the descriptions below are summaries of the distinguishing characteristics
observed in thin sections of the ash-flow sheets and lava flows. The Roman
numerals in the text correspond to those in Figure 4. Qualitative estimates of
mineral percentages are given for the lava flows. Plagioclase compositions have
not been determined because most of the plagioclase crystals have been subjected
to some form of secondary alteration, such as replacement by calcite or fine-
grained material.

I. Ash-flow sheet: The fiamme are a rusty red-brown color and contain
phenocrysts of plagioclase and biotite. The plagioclase contains abundant acicular
inclusions of apatite. The matrix consists of broken and euhedral plagioclase
phenocrysts, as well as biotite, in an intergrowth of quartz, feldspars, epidote, and
chlorote. The groundmass also contains amygdules and patches of calcite. Some

of the amygdules are dominated by quartz intergrowths. There are three main

l9

types of lithic fragments, one probable lava flow, aphanitic with extremely small
plagioclase laths, and a second similar type, coarser-grained with plagioclase and
K-feldspar. The third lithic fragment type is very fine-grained and may be of
sedimentary or volcanic origin.

11. Older lava flow group: The lowermost flow in this group is characterized
petrographically by abundant plagioclase phenocrysts (about 4096), hornblende
(less than 596), and clinopyroxene (5-1096), and opaques (about 1096) in a green-
brown matrix of Chlorite, epidote, calcite, and very small opaques. The overlying
lava flow is dominated by large, sieve-textured plagioclase phenocrysts (about
5096) and lesser amounts of clinopyroxene (about 1096), hornblende, and opaques
(both 5-1096), again in a green-brown matrix of Chlorite, epidote, calcite, and very
small opaques. The pyroxene and hornblende are both partly altered, especially
around their margins, with opaques concentrated along cleavages and fractures.
Many of the plagioclase laths are broken.

Ill. Ash-flow sheet: The fiamme contain plagioclase and opaque
phenocrysts. Some of the largest fiamme are streaky and patchy in grey and
green colors, which may reflect the present state of preservation of mixed
pumice. The gray-green matrix of Chlorite/epidote and quartz and feldspar
intergrowth likewise contains plagioclase and opaque phenocrysts. No lithic
fragments are obvious in outcrop, but in thin section at least three types are ssen,
a lava flow containing fine-grained plagioclase laths, a possibly sedimentary type
dominated by minute quartz grains, and fragments of welded ash-flow tuff.

IV. Ash-flow sheet: The fiamme consist of Chlorite and epidote and are
phenocryst-poor, containing only an occasional plagioclase lath. The gray-brown
matrix of Chlorite/epidote, opaques, quartz, and feldspar also contains plagioclase
phenocrysts, most of which are broken. Several types of lithic fragments are

present, including angular pieces of plagioclase porphyry similar to the lava flows

20

in II. A second probable lava flow type contains abundant hornblende crystals.
Fragments of welded ash-flow tuff are present as well.

V. This plagioclase porphyry also contains phenocrysts of clinopyroxene
(about 1096) and biotite (less than 596) in a very dark matrix of biotite, opaques,
epidote, Chlorite, quartz, and calcite.

VI. Ash-flow sheet: The fiamme contain no phenocrysts. Plagioclase
phenocrysts are present in a greenish matrix dominated by Chlorite and epidote.
Lithic fragments of earlier lava flows and welded ash-flow tuffs dominate the
groundmass.

V11. Ash-flow sheet: The fiamme occasionally contain plagioclase
phenocrysts, but are mainly phenocryst-free. Plagioclase and rare KLfeldspar and
biotite are present in the matrix, which consists of opaques, quartz and feldspars.
Lithic fragments are of two main types, plagioclase porphyries and welded tuffs.

VIII. Younger lava flow group: The lava flows of this group are all
clinopyroxene porphyries, containing 15-2096 pyroxene. Less abundant
phenocrysts of olivine (about 596), plagioclase (about 596), and opaques (5-1096) are
also present. The groundmass is composed of felty plagioclase and pyroxene and
is sprinkled with opaques. The olivine crystals are rimmed with pyroxene.

Between the units numbered VIII and IX are about 284 meters of lithic-rich
welded tuff. Samples from this stratigraphic interval were not submitted for
chemical analysis because of the abundance of lithic fragments. Microscopically,
the unit contains epidote-rich, phenocryst-free fiamme. The devitrified matrix
contains plagioclase and alkali-feldspar crystals and fragments, vugs filled with
calcite, quartz, and feldspar intergrowths, and a variety of rock fragments.

1X. Ash-flow sheet: The lower part of this sheet is distinguished by reddish
color and by dark, wavy laminations. These thinly laminated bands are

concentrations of oxidized opaques and quartz. The degree of distortion and

21

stretching has apparently destroyed all former shard shapes in this portion of the
sheet. The upper part of the unit contains fiamme rich in K-feldspar and quartz.
Phenocrysts in the groundmass include plagioclase, K-feldspar, biotite, opaques,
and extremely murky pyroxene; the groundmass itself is composed of aggregates
of these minerals dominated by quartz and feldspar. Distortions in the
groundmass around phenocrysts and fragments indicate the former presence of
flattened shards in the matrix. This unit is virtually lithic-free, except for an
occasional fragment porphyry or welded tuff.

X. Ash-flow sheet: The fiamme are composed of Chlorite, with crystals of
K-feldspar and opaques. The major phenocryst in the matrix is plagioclase; the
remainder of the matrix consists of intergrown quartz, opaques, biotite, and
K-feldspar. The lithic inclusions in this unit are again plagioclase porphyry and
welded tuff.

Between units X and X1 is another lithic-rich, partly welded tuff unit, which
most likely is the base of another ash-flow sheet, because it overlies a thin ashfall
tuff; the contact between the two is reworked. In thin section, rocks of this unit
show a very fine-grained greyish-green matrix of Chlorite and epidote containing
crystals and fragments of plagioclase, K—feldspar, and opaques. The fiamme are
composed of Chlorite and epidote.

XI. Ash-flow sheet: The base of this sheet was probably originally a
vitrophyre, as suggested by the squashed and deformed shard shapes in the matrix.
Approximately 15% broken and zoned plagioclase crystals are present, as well as
opaques and minor amounts of biotite. Above the base the sheet is highly welded
tuff, with K-feldspar and strongly altered plagioclase (to calcite) crystals and
pieces amongst the deformed shards. Spherulites are also present. The unit

contains lithic inclusions of plagioclase porphyry and welded tuff, whose

22

abundances increase upward in the section; the uppermost portion of the sheet is
too lithic-rich for representative sampling and chemical Characterization.

X11. Ash-flow sheet: The fiamme in this unit are relatively small and
consist of Chlorite and opaques. Distortions in the matrix texture indicate the
strongly welded nature of the rocks, although individual shards are rarely
discernable. Plagioclase and K-feldspar are the dominant phenocrysts in the
matrix of quartz, feldspar, biotite, Chlorite, and epidote. The unit contains very
few lithic inclusions.

XIII. Ash-flow sheet: The base of this sheet is highly welded; the fiamme,
composed of Chlorite and epidote, are few and lack phenocrysts. The matrix
consists of a quartz, feldspar, Chlorite, epidote intergrowth containing plagioclase
and K-feldspar phenocrysts, as well as opaques and occasional hornblende. A few
lithic fragments are present and increase upward through the sheet.

Approximately 116 meters of ash-flow sheet material are exposed above the
base of ash-flow sheet XIII before glacial deposits cover the remainder of the
middle member. This highest exposed portion of the volcanic section is
particularly altered, presumably because of the gold, silver, copper, and lead
mineralization (U.S.G.S.—U.S.B.M. Open-file Report 78-325, 1978) in the vicinity
of the Ballard Mine. For this reason, no samples higher than the base of ash-flow
sheet XIII were chemically analysed. No details of their petrography are
noteworthy, but all samples show the textural characteristics of welded ash-flow

tuffs.

23

SAMPLING AND ANALYTICAL METHODS

All of the lava flows, the ash-flow sheets, and the possible ash fall units
immediately below welded tuffs were sampled. Thus, sampling included a) lavas
that represent vent and fissure eruptions of small volume that may have
originated from any depth within the magma chamber(s), and b) pyroclastics that
represent much more voluminous, caldera-forming evacuations of the upper levels
of the magma chamber(s). Large-volume pyroclastic eruptions may form calderas
and potentially could provide evidence of any zoning present in the magma
chamber(s) Prior to such an eruption. An ash fall unit below a welded tuff could
preserve the most highly differentiated top of the magma, if both were the
product of the same ash-flow eruption.

Alteration of samples must be considered in any geologic sampling scheme
and was a particularly acute problem in this project. All of the samples Show
some contact metamorphic effects associated with the intrusion of the Boulder
batholith, although the location of the study area was chosen in part to minimize
post-eruption alteration processes. The maximum metamorphic facies seen in this
section of the volcanics is lower greenschist; typical mineral assemblages include
epidote and Chlorite. In the lava flows the olivine and pyroxene crystals are
commonly badly altered and the plagioclase cores may be replaced by calcite.
Amygdaloidal outcrops were avoided. The ash-flow sheet samples present the
greatest difficulties because even under the best of Circumstances secondary
alteration processes such as devitrification, vapor-phase processes, and hydration
may strongly affect the rock chemistry. Samples from the welded parts of the
sheets should theoretically show minimal effects of these processes because of

their low porosity relative to unwelded parts of the ash flow. Exposures

24

exhibiting obvious evidence of secondary alteration (e.g., lithophysae) were
avoided as a matter of course. The age of the Elkhorn Mountains Volcanics
proscribes any preserved glass, although evidence of glassy textures remains.
Despite the care used in sampling, it would be naive to suggest that no effects of
thermal metamorphism or secondary alteration occur in the rocks, and the
chemical analyses must be evaluated with these qualifications in mind.
Furthermore, the sampling approach introduced some biases into the chemical
trends through the stratigraphic section, because no unwelded tuff and very little
partly welded tuff were sampled.

In the laboratory, further winnowing resulted from elimination of all
samples containing abundant lithic fragments. Where necessary, lithic fragments
and amygdules were cut out of selected samples before grinding. It was not
possible to remove fiamme physically from the welded tuffs, and bulk rock
samples were used. Most samples were crushed and ground between porcelain
plates and in an agate mortar; however, some samples were crushed between
stainless steel plates and this is believed to have caused spurious Cr results.
Welded tuffs containing secondary calcite were leached with glacial acetic acid
and sodium acetate.

Major element and selected trace element abundances in the lava flows
were determined by inductively coupled plasma emission methods (ICP) at
Barringer Magenta, Ltd., Toronto. Other trace elements contained in the lavas,
including the rare earth elements, were measured by instrumental neutron
activation analysis (INAA) at Lawrence Livermore National Laboratory. The
ash-flow samples were analyzed for major elements and for selected trace
elements by ICP and for trace elements by INAA at the U.S. Geological Survey,

Reston.

25

Estimates of accuracy were obtained from analyses of standards (Table l).
The fact that Barringer Magenta, Ltd., LLNL, and the U.S.G.S. labs obtain
analytical results that agree well with accepted international standards can be
interpreted as indicating that inter-laboratory differences have not contributed

significantly to compositional differences between the sample groups.

26

 

 

 

 

 

 

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28

CHEMICAL COMPOSITIONS OF THE ELKHORN MOUNTAINS VOLCANICS

Fifteen samples of lava flows and twenty-three samples from ash-flow
sheets were analyzed (results are shown in Tables 2 and 3). The analyses of the
lava flows in Table 2 are arranged in stratigraphic order and are numbered from
1 through 15, youngest to oldest. Table 3 contains analyses of welded tuff
samples from the ash-flow sheets. Like those of the lava flows, these are
arranged according to stratigraphic position. Analyses of welded tuffs are
lettered from A through X (there is no B), with sample A being the youngest at
the tap of the stratigraphic column. The numbers and letters assigned to the lava
flow samples and ash-flow sheet samples (Tables 2 and 3) are used in the following
figures presenting chemical relationships where needed to indicate specific
samples and also in Figure 4, which shows the relative positions of these samples
in the stratigraphic sections previously described. All of the chemical analyses
(Tables 2 and 3) were recalculated anhydrous and were normalized to 10096 in
order to compare the entire suite on the same basis.

The Elkhorn Mountains Volcanics-Boulder batholith association has been
described as calc-alkaline (Tilling, 1973). However, of the samples from the study
area, the older lava group (#10 through 15) and some of the ash-flow samples (A,
C, O, P, Q, R, T) fall into the tholeiitic field of Miyashiro (1974) in Figure 5.

Figure 5 also shows that the new analyses do not represent any of the highly
silicic volcanic rocks reported in previous studies (i.e., Knopf, 1957; Klepper and
others, 1957; Ruppel, 1963; Smedes, 1966; Robinson and Marvin, 1967; Tilling,
unpub. data). These silicic samples occur in welded tuffs of the middle member.
There are at least two explanations for the lack of such samples in the new

analyses. Highly silicic volcanic rocks may have been present in the unstudied,

29

 

 

 

 

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31

 

 

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Table 3. Chemical analyses of ash-flow sheets.

 

 

 

 

9K9: 68.9
A120, 16.8
Pcififi, 1.8
FCC) 1..
Ilgs> .7
Chi) 1..
llazg) ..1
1(2F9 ...
TR): .7.
1553, .09
mu.) .1.
Taco lama

In 1.90
Ch: 2.9
C? 9.0
ca 2.9
(:u 1.
111 7.8
N1 n.d.
nu: 1.0
Sb 1.0
Sc 8..
St .00
T. 1.3
Th 16..
11 3.9
V 28

Y 29
Zn .0

It 310
L: 96.8
(36 10.
5". 7.9
Eu 1.97
Th .83
‘Yb 2.8
Lu ...

 

66.9 63.6 6..8 6..9 69.7 66.9 66.7 66.6 67.. 6..1 68.7
18.. 17.6 17.7 17.9 16.7 17.8 17.3 17.2 16.3 17.2 16.9
1.3 2.. 2.1 2.7 2.3 2.1 1.9 1.9 1.6 2.8 3.0
2.6 1.8 2.0 1.. 1.1 1.. _ 1.2 1.0 1.6 1.7 ..
1.0 1.9 1.6 1.3 2.9 1.2 1.1 1.1 1.3 1.6 .9
2.0 ..6 3.6 ..9 2.6 1.6 1.7 2.7 ..6 ..1 1.2
3.2 3.6 2.9 2.6 2.6 ..1 ..9 ..0 1.7 2.9 3.2
3.. 3.8 ..3 3.9 9.. ..0 ..6 ..9 ..3 ..3 9.6
.78 .79 .80 .83 .8. .80 .66 .62 .79 .77 .79
.22 .10 .29 .23 .20 .16 .19 .20 .21 .31 .11
.09 .13 .09 .13 .07 .07 .07 .08 .10 .11 .06
1370 1780 1970 1600 1370 1.00 1.30 1.00 1760 1270 1.60
9.6 6.0 6.0 9.6 ..9 9.9 9.0 ..6 ..9 7.2 2.8
19.9. 19... 6.3 13.6. 7.6 36.9. 9.9 9.7 9.9 19.19 6.3
2.1 2.9 1.6 2.6 3.. 9.2 2.9 1.2 2.9 3.1 2.9
0.4. 10 13 18 12 10 13 n.d. 10 .6 21
9.3 9.6 9.9 9.2 6.2 6.2 6.3 6.. 9.8 9.6 7.7
8 97 n.d. 6 8 9 n.d. n.d. . 9 6
106 106 110 108 1.9 113 118 123 136 129 199
.. .3 .9 .9 .9 .3 .. .3 .. .9 .6
10.6 11.1 11.2 10.3 10.1 9.0 8.7 8.9 10.. 10.9 8.7
600 1000 77 1200 .90 .30 .60 .70 680 630 360
.9 .9 .9 .8 1.0 1.0 1.0 1.0 1.0 .9 1.3
11.1 11.3 11.3 10.9 12.9 11.. 11.7 12.1 11.8 11.9 16.3
2.6 3.8 2.6 2.8 3.0 2.7 2.8 2.9 2.8 2.8 3.6
66 89 100 79 98 62 70 99 76 7. .1
23 2. 23 23 29 22 21 21 2. 23 23
7. 79 79 70 71 63 97 98 79 77 91
2.0 270 2.0 290 3.0 270 2.0 280 290 2.0 290
.7.2 .9.6 .7.9 .7.2 .7.8 .7.9 .9.0 .7.9 .9.9 .9.3 92.9
8. 88 89 89 89 89 8. 8. 89 87 93
6.9 7.3 6.9 6.8 7.3 6.. 6.7 6.6 7.2 7.1 6.8
1.72 1.82 1.72 1.79 1.6. 1.97 1.99 1.90 1.73 1.69 1.9.
.70 .76 .86 .68 .72 .69 .80 .80 .71 .69 .69
2.3 2.9 2.7 2.3 2.6 2.3 2.7 2.9 2.6 2.9 2.3
.37 .38 .38 .39 ..1 .39 .39 .39 .39 .37 .38

 

32

Table 3 (continued).

 

 

 

 

 

 

 

 

 

”‘63:“ VI: V! N 111 1
m N o p o a s r u v v x
11.2 Elementsl, weight 96
.9102 6..1 69.7 92.6 61.6 61.9 67.9 60.1 63.8 6... 66.6 6..8
A1203 17.1 17.2 12.1 17.2 17.9 19.9 17.1 16.2 17.2 12.2 17.0
9.20, 1.9 1.9 9.2 9.9 9.7 2.6 2.2 2.1 2.2 1.1 .20
9.0 2.6 1.1 9.1 2.2 2.9 1.6 9.9 1.2 9.1 9.9 9.7
“go 9.1 1.1 2.2 2.1 1.2 1.1 2.1 2.1 1.6 1.9 1.9
c.o 2.6 2.1 1.0 1.2 9.9 9.9 9.6 9.9 9.9 2.0 1.0
«.20 1.9 1.2 2.0. 2.2 1.9 1.1 2.1 1.6 1.1 1.2 1.2
xzo 2.9 7.2 6.9 1.9 7.9 1.2 2.7 9.2 9.9 9.1 1.1
1102 .22 .22 .29 .21 .20 .29 .96 .29 .90 .22 1.1
920, .12 .36 .11 .92 .99 .91 .19 .92 .99 .96 .90
11190 .21 .10 .21 .19 .12 .07 .17 .09 .12 .11 .19
Trace 210411211222, m
2. 912 1290 1190 1290 1710 1710 768 901 1120 996 1960
c. 19.2 11.1 19.1 19.2 11.1 6.9 19.9 9.7 2.9 7.9 9.2
c: 12.9- 17.6- 90.00 96.1- 99.9- 9.9 26.30 12.1 9.9 10.2 12.1-
c. 2.2 2.1 1.9 1.1 1.9 1.2 99.9 11.9 7.6 2.6 6.6
Cu 21 97 99 10 99 92 19 21 12 19 16
111 9.9 1.1 1.9 1.1 9.2 6.1 1.0 9.2 9.2 9.2 2.1
Ni 17 11 99 9o 90 17 17 2 9 6 11
Rb 29 161 191 106 190 111 111 126 109 101 16.
s. .9 .9 1.9 .9 .9 .2 1.2 n... .1 .2 .1
Sc 12.9 11.2 12.6 12.6 11.1 10.6 17.6 19.7 12.9 12.9 12.1
Sr 170 190 700 680 990 720 200 610 790 610 690
1'. .6 .6 .6 .6 .9 1.1 .6 .9 1.0 1.0 2.1
n. 9.9 6.2 6.6 6.0 6.0 12.2 6.6 9.6 9.2 10.1 19.2
0 1.9 1.7 1.7 1.6 1.7 2.7 1.1 1.7 2.1 1.9 1.1
v 110 160 190 160 110 120 220 110 100 110 120
Y 20 20 21 12 16 21 20 29 29 29 21
Zn 22 76 96 27 79 63 99 29 21 22 17
z: 120 190 910 990 120 360 210 260 210 260 110
1.. 29.9 91.0 90.2 29.1 90.9 11.6 92.9 19.7 11.9 16.2 79.1
c. 99 99 97 91 91 22 99 20 20 29 126
Sm 9.0 9.2 9.9 9.1 9.0 6.7 6.1 7.9 7.1 7.1 2.9
Bu 1.92 1.29 1.10 1.99 1.22 1.11 1.90 1.20 1.72 1.67 1.62
n» .97 .91 .62 .60 .61 .29 .68 .22 .21 .22 .20
Yb 1.9 1.9 2.1 2.0 2.0 2.9 2.2 2.6 2.9 2.6 2.2
1.11 .92 .90 .99 .91 .90 .99 .96 .11 .11 .1o .99
Anolyoo. normalbed 20100” anhydrous.

1 a All major elements by 1GP, except FeO by titration, and 1'102 and P20, oolorirnetrically (U.S. Geolog1cal
Survey, J. Gillian, Analyst).

2 . A11 trace elements by INAA (U.S. Geological Survey, G. A. Vandleos, Analyst), except Cu by flame atomic
absorption, N1 and V by graphite furnace atomic absorption, and Sr, Y, and Zr by ICP (U.S. Geological
Survey, 1. D'Angelo, Analyst).

'Cr analyses suspected of contomination.

33

 

 

 

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1\
c3 _ 3“ 91
b A
{A *
L0 _ 91 ,,
(\l to X a! 91
O + +
.. 9+
(n g — +
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In 1—
LO
Q A 1 1 1 1 1 _1
U30 1 2 3 4 S 6 7 8
F E D "’ / M GD

MIDDLE MEMBER RSH—FLDN SHEETS
LDNER MEMBER RSH-FLDN SHEETS
YDUNGER LRVR FLDN GROUP
OLDER LRVR FLDN GRDUP

DTHER RNRLYSES

3K+BXI>

Figure 9. Plot of 510 against FeO*/MgO of all available Elkhorn Mountains
Volcanics analyses. Dzata from Tables 2 and 3, Mutschler and others (1976a
and b), and Tilling (unpub. data). CA = calc-alkaline, TH = tholeiitic.

34

altered upper part of the middle member or in the covered region to the north of
the study area (Figure 3). This possibly implies that the ash flows in the middle
member became more silicic with time. An alternate explanation is that the

highly silicic volcanic rocks are present in the study area, in either the non-or
partly welded portions of the ash-flow sheets or in poorly exposed parts of the
stratigraphic section, that is, in the unsampled portions of the volcanic section.

In this case, the silica variability might be entirely within ash-flow sheets.

Major Elements

Lava Flows. The chemical analyses of the lava flows fall into two groups
(Table 2) and it is convenient to refer to these groups as the younger lava flow
group (VIII and V) and the older lava flow group (II) in the following discussion.

The two groups of lava flows in the lower member differ from one another
in both major and trace element abundances (Table 2). The younger group (#1-9)
is distinctly less evolved in terms of SiO2 content in comparison to the older
group (#10-15).

The major element variation diagrams of the lavas (Figure 6) illustrate the
following characteristics of the two lava flow groups:

1) The SiO2 contents of the younger lava flow group are generally lower
than those of the older group. Silica ranges from 52.3 to 59.6 weight 96 in the
younger group and from 57.7 to 62.5 weight 96 in the older group.

2) The Al203, NaZO, TiOz, and, to some degree, P205 contents of the
younger group are generally lower than in the older group. However, the overall
ranges in T102 and P205 are very small, indicating no statistically significant
difference in the abundances of these oxides.

3) The CaO, FeO, and MgO abundances of the younger lava flow group are
generally higher than those of the older group, consistent with the lower SiO2

contents in the younger lavas and with their less evolved character.

35

.n 0.53.... e. no 338.3
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8
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+ + .m. as 0
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a a z s 8
B
1 S 1 m + + Z
0 .L D

36

‘1) The K20, NaZO, and CaO abundances in the younger lavas are highly
variable. The older lavas are uniformly high in K20 (about l1.0%), with the
exception of sample 14, and in NaZO (about 3.496), and are low in CaO (see
Figure 7).

The variable distribution of K20, NaZO, and CaO within the younger lava
flow group could indicate mobility of these elements. Element mobility in
volcanic rocks has been documented previously (Lipman, 1965; Noble, 1967).
There is no consistent enrichment of K20 with loss of NaZO, as would be expected
from other studies (Lipman, 1965; Noble, 1967), nor is the variability of CaO in
the leached samples any different from that in the non-leached samples. In the
Elkhorn Mountains Volcanics, mobility was probably due to the combined post-
depositional alteration processes of de-vitrification, hydration, contact
metamorphism and others. Systematic linear trends of the other oxides plotted
against SiO2 are interpreted as indicating that they have been relatively
immobile.

Ash-flow Tuffs. Variation diagrams of the major element oxides in the ash-
flow sheets (Figure 7) can be used to illustrate several differences between ash-
flow sheets from the lower member (analyses N through X) and those of the
middle member (analyses A and C through M).

I) In general, the highest $102 and NaZO contents are found in the middle
member. Variability in NaZO may be due to Na2 mobility.

2) A1203, MgO, CaO, K20, and MM) ranges are similar in all the ash—flow
sheet samples, although the distributions of A1203, MgO, and MnO are
considerably restricted in comparison to those of CaO and K20. The possibility of
mobility must be kept in mind when assessing the K20 and C30 variability.

3) Foo and P205 are both generally less abundant in the middle member,

with some exceptions. Although the ranges of FeO and Fe203 concentrations of

37

 

 

 

 

 

 

 

 

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mowbq ooz om. mama.

59 62 as ea 71
SI02

56
Figure 7. Major-element variation diagrams of the ash-flow sheets. Data of

Table 3. Symbols as in Figure 5.

the two members overlap, the variability of FeO and Fe203 in the middle member

is less than in the lower member.

Trace Elements

The samples were analysed for Ba, Co, Cr, Cs, Cu, Hf, Ni, Rb, Sb, Sc, Sr,
Ta, Th, U, V (not measured in the lava flows), Zn, Zr, and the rare earth elements
(Tables 2 and 3). Some important differences in the abundances of some of the
trace elements can be noted, especially when these elements are plotted against
thorium (Figure 8). Thorium was used as reference because I) it does not appear
to be mobile during crystallization and alteration of silicic lavas and ash flows
(Rosholt and Noble, 1969; Rosholt and others, 1971), 2) analytical precision is
good, and 3) it is present over a range of concentrations. Because thorium is an
incompatible element during differentiation processes, it also is an excellent
indicator of the degree of chemical evolution of a suite of cogenetic igneous
rocks.

Except for sample 13 (indicated on Ni plot), the older lava group contains Th
greater than 8 ppm, in comparison to the younger group less than 8 ppm.
Chromium, Co, Ni, and Sc are strongly bimodal in distribution between the two
major lava groups (Figure 8). Furthermore, in the younger lava group these
elements show sharply negative slopes against Th. As will be discussed below,
such a trend in plots of compatible elements against an incompatible element is
consistent with a history dominated by processes of crystal-liquid equilibrium,
either partial melting or fractional crystallization.

Except for samples X and 5 (indicated on Ni and Ta plots), the lower
member ash-flow sheets contain less Th than those in the middle member
(Figure 8). Sample S is similar to the middle member ash-flow sheet samples in

Th content, and sample X is the most Th rich in the entire sequence. All but two

39

180
l

 

 

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+ #44- x
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2 S 8 11 14 17 20 2 S 8 11 14 17 20
TH TH

Figure 8. Plots of Ba, Ce, Co, Cr, Cs, Hf, Ni, Rb, Sc, Sr, Ta, and Zr against Th.
Trace element data of Tables 2 and 3. Roman numerals, Arabic numbers, and
letters correspond to sample groups and analyses of Tables 2 and 3 and Figure ‘1.
In plots of Co and Sc, symbol 12 indicates ash-flow sheets IV and VII. In plots of
Rb, Ba, Sr, and Cs, a solid box indicates lava unit V, solid triangles indicate ash-
flow sheet X11, and dashed outlines indicate ash-flow sheets III and VII.

#0

 

 

 

 

 

 

 

 

 

 

 

 

 

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41

samples from the middle member cluster between 10.5 and 12.5 ppm Th; the
exceptions are samples A and M, containing 16.4 and 16.3 ppm Th, respectively.

Plots of the compatible elements Sc and Co, against Th show that
samples N, O, P, Q, R (unit VII), and possibly T (unit IV) fall on a generally
separate trend from the other samples of ash-flow sheets (Figure 8). Most of the
remaining samples from ash-flow sheets show decreasing abundances of these
elements with increasing Th. Sample X, however, is consistently off of either
possible trend. The plots of Ce, Hf, Ta, and, to a lesser degree, Zr (Figure 8) show
good linear variation with Th in all of the samples. In lower member sample 5,
the concentrations for all four of these imcompatible elements are similar to
those of the ash-flow sheets in the middle member. The concentrations of Ce, Hf,
Ta, and Zr increase with Th and with time, except in sample X, from the base of
the stratigraphic sequence, which consistently shows the most evolved character
of all the ash-flow sheet samples.

The possible mobility of K20, NaZO, and CaO resulting from post-eruption
alteration, seen in the scatter of points in Figures 6 and 7, may also be
accompanied by mobility of related trace elements as well. All three major
elements are plotted against Th in Figure 9. Evidence for mobility is indicated by
wide ranges of these elements in lava group VIII, especially of K20, and in ash-
flow sheet VIII (Figure 9). Rubidium may have been mobile in ash-flow sheets III,
VII, and at least sample C of sheet XII, and in lava group VIII (Figure 8). Barium
has apparently been mobile in ash-flow sheets III, VII, and XII (Figure 8).
Strontium may have been mobile in sheets III, VII, and XII (Figure 8). Cesium
mobility is indicated in lava group VIII and II, ash-flow sheet 111 and IV, and
possibly throughout the entire middle member (Figure 8). It is important to
(‘distinguish the scatter within individual units, best explained by mobility of- the

particular elements during post-eruption alteration, from the overall positive

42

Figure 9. Plots of K O, Na 0, and CaO against Th. Data of Tables 2 and 3.
Solid boxes indicate IaQa grodzp VIII. Ash-flow sheet VII Indicated by It . Other
symbols as in Figure 5.

1+3

 

 

 

 

 

x - x
A A A A A AA
A A X
vmmfid + ..A and: A I #mAMAAA
Axvx + JAX ++x+x x
X I I I I I
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...... m. d .. ...... u...
9m 9m N6 m.m ...N V m N 3: m w v N
ow: owcz ecu

11 14 17 20
TH

8

Figure 9.

44

correlation of Rb and Ba with Th. This positive trend may be taken as a
suggestion of the original variation of Rb and Ba with Th.

Chondrite-normalized rare earth element (REE) profiles (Figure 10) for the
lava flows, lower member ash-flow sheet samples, and middle member ash-flow
sheet samples have common aspects. None shows a distinct europium anomaly.
The older lava group contains slightly greater REE abundances than the younger
group, which is consistent with the more evolved character of these lavas. The

REE profiles in the middle member are extremely uniform (Figure 10).

Ash-fall Analyses

Although there are different kinds of ash falls, some are directly associated
with the eruption and emplacement of ash-flow sheets, and thus may be products
of the same processes which produced the ash-flow sheets. Because compositional
zoning in ash flow sheets is commonly assumed to reflect pre-eruption zonation in
the magma body (Smith and Bailey, 1966; Lipman and others, 1966; Smith, 1979;
Hildreth, 1981), and because ash falls plainly erupt before ash-flow sheets, an ash
fall might sample the upper highest, most-evolved portions of the magmatic
system. Chemical analyses of ash-fall units (Appendix) immediately below ash-
flow sheets (Figure 4) were obtained in order to examine the possibility of one or
more such ash falls showing the same chemical trends as the overlying ash-flow
sheets, and hence might be co-genetic with that ash-flow. Preliminary inspection
of the data from these rocks permitted no generalizations regarding any genetic
relationship between the ash falls and the ash-flow sheets, and the ash fall
analyses were omitted from the discussion of the geochemistry of the Elkhorn
Mountains volcanics.

The absence of any observable relationship between logical ash fall - ash-
flow sheet pairs may have several implications. If ash falls do sometimes sample

the uppermost levels of the magma chamber, then perhaps the top parts of the

45

Figure 10. Chondrite-normalized rare earth element profiles for the Elkhorn
Mountains Volcanics. (Data of Tables 1 and 2 normalized by values in Haskin
and others, 1968). A. Lava flows. B. Lower Member ash-flow sheets.
C. Middle Member ash-flow sheets. Symbols as in Figure 5.

46

 

 

 

 

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Figure 10.

47

chambers are heterogeneous and not uniformly consistent with gradients present
in the rest of the magma body. Alternatively, any evolutionary processes which
might have been identified may not have been preserved in the ash-fall samples,
which would be most susceptible to alteration processes. Conversely, ash falls

may not sample just the uppermost levels of the magma chamber.

48

DISCUSSION

Interpretation of the chemical variation in the Elkhorn Mountains Volcanics
may be made in terms of: l) the chemical trends in the volcanic sequence, 2) the
evidence of processes affecting the distinct groups of lava flows and ash-flow
sheets, 3) the possible relationships between the lava flows and ash-flow sheets,
and lo) the evolution of the entire system within the time frame represented in the
study area. Finally, analysis of the geochemistry of the Elkhorn Mountains
Volcanics and the Boulder batholith may be made in order to compare the

different processes that influenced the volcanic and plutonic rocks.

Chemical Trends within the Volcanic Sequence - A Single Magmatic System

The chemical trends within the entire Elkhorn Mountains Volcanics sequence
are consistent with the assumption that this sequence is related to the evolution
of a single magmatic system, although not necessarily of a single magma
chamber. The enrichment of the incompatible elements Ce, Ta, Hf, Zr, and Th
(Figure 8) and of Ce and Yb (Figure ll) shows the positive, nearly linear trends of
this sequence of volcanic rocks that range in SiO2 from about 5396 to 6996.
However, the possibility of evolution in a single chamber is virtually precluded by
trends in different groups of lava flows and ash-flow sheets (discussed below), that
are interpreted as indicating that certain processes influenced the evolution of
some groups but not of all. Instead, the magmatic system envisioned here would
have consisted of several separately evolving bodies of magma which together
reached batholitic dimensions, different portions and culminations of which were
tapped to produce the lava flows and ash-flow sheets of the Elkhorn Mountains

Volcanics.

49

Figure 11. Plot of Ce against Yb. Units indicated by Roman numerals.
Symbols as in Figure 5.

50

CE
50 50 70 80 90 100 110 120

40

30

 

A XIII

E]
III
Xw
vm
E01] [3
E] 1E! E)

l L l l J

 

1.5 2.0 2.5 3.0 3.5
YB

Figure 11.

51

A plot of Ta against Th for the Elkhorn Mountains Volcanics compared with
some other zoned systems permits further characterization of this proposed
magmatic system (Figure 12). A noteworthy feature of this comparison is that
the ranges of both Ta (from 0.2 to 2.1 ppm) and Th (from 3.5 to 19.8 ppm) in the
entire suite of Elkhorn Mountains Volcanics, which consists of many lava flows
and ash-flow sheets, are comparable to those of individual ash-flow sheets, such
as LCT and HRT ((Lava Creek ash-flow tuff and Huckleberry Ridge ash-flow tuff,
Yellowstone (Hildreth, 1981)). The relative constancy of the Th/Ta ratio, despite
the many eruptive products, separated in time, provides compelling evidence that
the entire Elkhorn Mountains Volcanics pile was derived from the same, single
system.

Within the single dynamic system, however, different mechanisms of
evolution produced chemical trends in the two lava groups and the ash-flow sheets
preserved in the study area. The roles of some specific processes can be
identified and others can be rejected on the basis of chemical evidence in the
samples.
glidence of Processes Affecting the Distinct Groups of Lava Flows and Ash-flow
Sh_ee_tg

The Ash-flow Sheets. The presence of a variety of lithologies in the lower
member permits recognition of parts of five ash-flow sheets, based on significant
changes in the stratigraphic sequence, as reflected in the alternations between
bedded tuffs, lava flows, and volcanoclastic units. Using the same criteria,
cooling breaks in the thick sequence of ash-flow sheets in the middle member can
be placed below the lithologies from which samples K, J, G, and A were taken
(Figure 4). Therefore, at least five ash-flow sheets can be distinguished in the
middle member in the study area. This conclusion is reasonably consistent with

l) Smedes's statement that at least seven ash-flow sheets can be found in

52

71.11.... 11' Tet/nos n. no!

 

Th/ 7.1.10

 

 

 

 

Th "I!

Figure 12. Plot of Ta against Th for the Elkhorn
Mountains Volcanics compared with some other
eruptive units. Data from Tables 1 and 2, Baker
and McBirney (1985), and Hildreth (1981).
Modified from Baker and McBirney (1985). EMV:
Elkhorn Mountains Volcanics, southwestern
Montana (Tables 1 and 2). KAT: Pumiceaflow
and Novarupta dome, Valley of Ten Thousand
Smokes, Alaska. MAZ: Mazama (Pinnacles)
pumice flow, Crater Lake, Oregon. TWP: Twin
Peaks basalt-rhyolite complex, Utah.
DCT: Devine Canyon tuff, Oregon. PRI: Sierra
la Primavera rhyolite complex, Mexico.
LCT: Lava Creek ash-flow tuff, Yellowstone.
HRT: Huckleberry Ridge ash-flow tuff,
Yellowstone (Hildreth, 1981). Modified from
Baker and McBirney, 1985.

53

complete sections of the middle member in the northern Elkhorn Mountains (1962;
1966, p. 33), and 2) the fact that only the lower part of the middle member is
present in the study area. However, a detailed study of field relations over a
greater region of exposures of the Elkhorn Mountains Volcanics is needed to refine
the number of ash-flow sheets present in the entire volcanic sequence.

The ash-flow sheets of the Elkhorn Mountains Volcanics are most similar to
the "monotonous intermediates" of Hildreth (1981) (Figure 13) in terms of SiO2
concentrations. Most major and trace elements show overall uniformity within
the individual ash-flow sheets (Table 3). Likewise, in considering the system as a
whole, the increases in concentration of the incompatible trace elements Cr, Hf,
Ta, Zr, and Th (Figure 8) are consistent with those for other systems of
intermediate composition (Baker and McBirney, 1985).

Smith (1979) and Hildreth (1981) have suggested that "monotonous
intermediate” ash-flow sheets sample the dominant volume in magma chambers.
If so, the eruption of ash-flow sheets with SiO2 concentrations primarily between
58 and 6996 in the Elkhorn Mountains Volcanics may simply reflect the fact that
the system erupted before development of a highly silicic top.

The Lava Flows. The steep negative slopes of Ni, Cr, Sc, and Co against Th

 

in the younger lavas (Figure 8) are consistent with a history dominated by crystal-
liquid fractionation, either crystal fractionation or partial melting. Crystal
fractionation of the sample (#3) of the least-evolved lava flow from the lava flow
group VIII was modeled assuming equilibrium between the total crystallizing solid

and melt. The expression

EL. + I (Arth, 1976)
ci 1“ + DS 1 - F'

was used, where

C

L trace element concentration of the differentiated liquid

Ci - trace element concentration of the original melt

5‘1

Magma volume erupted

 

 

 

 

 

 

 

 

 

 

 

 

lkm’l
I I Y I’ Y I I
BISHOP TUFF soo
MKLEBERRV RIDGE 25m
LAVA CREEK YUFF umo
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FRACTION TUFF 2500
RAINIER MESA loco
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TWAH SPRING zso
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GROUSE CANYON - . 2m
SPEARHEAD ...A..- no
GRAN CANARIA PI -;...;.“.‘.'90..- .25
ASKJA I075 - .-... .- 02
T ' 7‘0 (is 0'0 53
5102 NIH“

Figure 13. $10 ranges of some selected
eruptive units (after Hildreth, 1981,
figure 1). Shaded area shows SiO range
of ash-flow sheets in the Elkhorn
Mountains Volcanics superimposed on
'(‘monotonous intermediates" of Hildreth
1981 .

55

F' = fraction of liquid remaining

DS = bulk distribution coefficient, calculated from the weight fractions
of each fractionating mineral (w) and its solid-liquid distribution
coefficient (KD); DS = {:1 W1 KDi (Cox and others, 1979).

Partition coefficients for orogenic andesites were used in the modeling of
sample #3 (see Table I). As defined by Taylor (1969), orogenic andesites are
typically hypersthene normative, containing 53-6396 SiOz, less than 1.7596 TiOZ,
and K20 less than (0.1” x SiOz). Table 2 shows that the SiO2 and TiO2 contents
of the Elkhorn Mountains Volcanics lava samples meet these restrictions, except
for sample 3, which contains just under 5396 SiOz. The potash abundances of
samples 1, 5, 7, and 9 are less than the appropriate calculated value; the other
measured K20 contents are too high. Nonetheless, the choice of the distribution
coefficients for orogenic andesites for modeling is reasonable, because of the
probable mobility of K20 discussed previously and because the samples otherwise
meet the criteria. Modeling of separation of 3396 olivine, l1096 orthopyroxene,
2096 magnetite, 596 plagioclase, and 296 clinopyroxene from lava #3 over 2096
crystallization yields predicted trends of Ni, Cr, Sc, and Co that are in agreement
with those of the actual concentrations in the rocks (Figure 14). The particularly
good match of the predicted and actual Ni abundances and the partition
coefficient of Ni in olivine (Table II) suggest that olivine dominated the
fractionation process.

Crystal fractionation in the younger lavas was further modeled using the
method of Cox (1980). This quantitative procedure was designed for modeling
major-element fractionation trends in basaltic rocks. The calculations yield
values for the fraction of liquid remaining and the composition of the residual
liquid, as well as the composition of olivine, plagioclase, and clinopyroxene in

equilibrium with the liquid after specified intervals of fractionation.

56

Table 4.

Partition coefficients used in modelling
(from Gill, 1978).

 

Ni Cr Sc Co
Olivine 12 2 . 27 l . 35
Orthopyroxene 4 2 ' 3 6
Magnetite 8 2 2 8
Plagioclase . 04 13 . 065 . 009
6 30 3 2

Clinopyroxene

57

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58

Fractionation of lava sample 3 was modeled over 2096 crystallization of the
starting composition. Because this method allows modeling of only olivine,
plagioclase, and clinOpyroxene, the same relative proportions of those minerals
were used as those that produced the best agreement of predicted and actual
trace element trends (i.e., 3396 olivine, 596 plagioclase, and 296 clinopyroxene
were normalized to 10096 and became 82.596 olivine, 12.596 plagioclase, and 596
clinopyroxene). This approach is plainly idealized, and absolute agreement
between the predicted and actual major-element concentrations would not be
expected. However, reasonably similar trends in predicted and actual Mg"
(Mg/ Mg + Fe) were found (Table 5); this is good support for the possibility that
fractionation in the younger lavas is dominated by olivine.

Processes Common to Both the Ash-flow Sheets and the Lava Flows. The
EulEu* values provide further evidence for fractionation of the Elkhorn
Mountains Volcanics (Figure 15). As discussed earlier, the Elkhorn Mountains
Volcanics have only slight or no Eu anomalies. For most of the lava flow samples,
the values of Eu/Eu* lie between 0.9 and 1.0, indicating at best only minor
plagioclase fractionation. Two lava flow samples with Eu/Eu* greater than 1
apparently evolved by addition of plagioclase-composition crystals or melt. In the
ash-flow sheet samples, however, there is significant plagioclase fractionation
which increases with increasing differentiation of the liquids. This interpretation
is also supported by the trend seen in Eu/Eu* against Sr (Figure 15).

In contrast, there is no evidence of alkali feldspar fractionation in either the
lava flows or the ash-flow sheets. Increasing Ba abundances in a suite of
differentiated, cogenetic rocks indicate no fractionation of alkali feldspar (Baker
and McBirney, 1985) (Figure 8). Similarly, decreasing Sr can be explained by
fractionation of plagioclase (Figure 8), already indicated by the Eu/Eu* values

(Figure 15).

59

Table 5. Trends in actual and modeled Mg*.

 

 

 

 

Starting compositions: LAVA #3
Mg* (liquid) = Mg/(Mg + Fe) = 78.62
Calculated equilibrium mineral compositions: Fo Di An
92.4 95.3 72.8

Step 1: 9596 liquid remaining
Mg* (liquid) = 75.79 9l .2 90.5 73.2

Step 2: 90.2596 liquid remainin
Mg* (liquid) = 72.08 89.2 93.5 73.5

Step 3: 85.7496 liquid remainin

Step ll: 81.4596 liquid remainin
Mg" (liquid) = 59.38 83.0 89.0 70.2

Final composition: LAVA #6
Mg* = 67.01

 

PF

60

 

 

 

 

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+
C3 01 +
~" m
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LLJO)
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DO A xxAA Ax
“Joe A A x
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‘7 _ A Xx
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C300 450 600 750 900 10501200
SR
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DO)
LUC.)_ m V +Ag
XX A
00 >><< A A
c3 ” x AA
b. I 1 1X 1 A1 %
CD

2 s 8 11 14 17 20
TH

Figure 15. Plots of Eu/Eu* against Th and Sr for The
Elkhorn Mountain Volcanics. Symbols as in Figure 5.

61

Possible Relationships Among the Lava Flows and the Ash-flow Sheets

Two separate trends in plots of $102, MgO, and Fe as Fe203 against Th
(Figure 16) suggest two lines of evolution among the lava flows and ash-flow
sheets in the study area. The fields outlined in Figure 16 show lava flow groups V
and VIII leading to ash-flow sheet IV and VII and lava flow group 11 leading to the
other ash-flow sheets, except 1. Both lines follow increasing differentiation as
defined by Th. Similar pairing is observed in the variation of Ce, Hf, Ta, and Zr
with Th (Figure 8).

Neither possible evolutionary path can be explained by crystal-liquid
fractionation. Attempts to model fractionation from each lava group to the
corresponding ash-flow sheets were not successful.

The possibility of magma mixing being the dominant mechanism has been
evaluated using ratio-ratio plots (Figure 17). Ideally, in Figure 17a, if either lava
flow - ash-flow sheet(s) pair is genetically related by magma mixing, the samples
should plot on hyperbolic curves. The permissibility of mixing as a viable process
in the evolution from the lava flows to the ash-flow sheets can be supported
further if the samples lie on straight lines in Figure 17b. Two possible positions of
hyperbolae and straight lines are shown between the suggested lava flow - ash-
flow sheets pairs in Figure 17. From examination of Figure 17, it is concluded
that simple magma mixing was not a dominant process, although some mixing may
have taken place.

The failure of both fractionation and magma mixing to dominate the
processes producing the chemical changes from the lavas to the ash-flow sheets
implies that the apparent relationships are the results of a combination of
evolutionary mechanisms, not of any single process. The entire system was
almost certainly a dynamic one, evolving by a combination of mechanisms while

receiving injections of new magma at intervals and, just as periodically, losing

62

Figure 16. Plots of SiO , MgO, and Fe as Fe 0 against Th. Dashes outline
field of lava groups V andZVIII and ash-flow shee s and VII. Solid line outlines
field of lava group II and all other ash-flow sheets except 1. Symbols as in
Figure 5.

63

 

 

 

 

 

 

S m A
mom: ma

 

 

Figure 16.

64

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am“ X vi X a VI. 6
3:... B H ...—Ufa H
a 1m a 1m
a a
1m 18
+ 1.6 + 13
0 O

 

 

various volumes in lava and ash flow eruptions. Possibly both crystal
fractionation and magma mixing had minor roles in producing the chemical trends.
In a magmatic system of this size, convection and diffusion would be expected and
could play a major role (Smith, 1979; Huppert and Sparks, 1984). Influx of new
material and eruption of evolved ash flows were most likely periodic through the
early stages of the system. During this early period of evolution, thermal flux
was high and crystallization was limited. The only demonstrable crystal
fractionation was of olivine. This high thermal flux was due to and maintained by
influxes of relatively primitive magma. Later in the volcanic phase, evolution of
the magmatic system by fractionation of plagioclase feldspar occurred.

The Ev_olution of the Entire Magmatic System within the Time Frame Represented
in the Study Area

The evolution of the Elkhorn Mountains Volcanics over the period of time
represented by these lava flow samples and ash-flow sheet samples may be
evaluated in terms of both caldera cycles (Christiansen, 1979) and models for
evolution of high-level magmatic systems (Smith, 1979; Hildreth, 1981) and by
comparison with other ash-flow fields of similar size.

Interpretation of Caldera Cycle Remnants. It has long been established that
eruptions of ash-flow tuffs are commonly associated with caldera collapse
(Williams, 1941; Smith, 1960) and that many calderas have histories of repeated
eruption and collapse. Christiansen (1979) summarized the basically similar
sequence of events in several examples. Is it possible to identify aspects of such
cycles in this sequence of the Elkhorn Mountains Volcanics and to relate them
even in a general way to their possible caldera-source areas?

A summary of the changes in chemistry in the Elkhorn Mountains Volcanics
in terms of the variation of Th abundances with time (Figure 18) provides a

reasonable tool with which to begin answering this question. All of the samples

66

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67

H.

are arbitrarily spaced at regular intervals along the time axis because their exact
ages are not known; however, crosscutting field relations and K-Ar age
determinations of the volcanic and plutonic rocks indicate that the volcanic
activity which produced the lower and middle members took place over
2-3 million years, from about 81 to 76 Ma ago (Tilling, 1973, 19711). The following
discussion is based on Figure 18.

The variation of Th with time for units of the lower member, composed of
intercalated lava flows and ash-flow sheets (Figure ll), may provide a fragmental
record of several caldera cycles. Ash-flow sheetl is likely the product of a
caldera-forming event early in the history of the Elkhorn Mountains Volcanics; it
is interpreted to be the base of an ash flow sheet whose upper thickness was
eroded prior to eruption of the older group of lava flows (II). Lava flow group II
may be pre-caldera lavas to some later ash flow eruption, perhaps represented by
ash—flow sheet III, VI, or the sequence in the middle member (ash-flow sheets IX
through XIII); all of these possibilities could be argued on chemical grounds in
view of the trends in Figure 16. Many chemical aspects of ash-flow sheet IV are
similar to those of sheet VII (see Figure 16, for example); this similarity could
indicate that sheet IV is a product of an eruption from the same source/chamber
that later produced sheet VII. The best preserved cycle in the lower member is
probably the caldera-forming eruption of ash-flow sheet VII; the possible
relationship between this sheet and lava flow group VIII suggests that they are
post-caldera lavas associated with this event. Lava flow V precedes ash-flow
sheet VII in time and could be a remnant of the pre-caldera lavas of this cycle.

The ash-flow sheets in the middle member (IX through XIII) are each
separated from the next-younger sheet by some significant physical break (see
Figure 4). However, many aspects of their chemistry are quite uniform and

similar (note the clustering of analyses L through C in Figure 16, for example).

68

The most highly differentiated samples, M and A, come from the bases of ash-
flow sheets IX and XIII, respectively. Thus, ash-flow sheets IX through XII may be
considered products of a single catastrophic eruptive episode, a complex one that
might have tapped several separate culminations above the larger magma
chamber. According to this interpretation, ash-flow sheet XIII would then
represent the base of another, perhaps also complex, ash- flow sheet.

Comparison of the Elkhorn Mountains Volcanics to the Similar-Sized Timber
Mountain and Associated Calderas Complex. The interpretation of the caldera
cycles in the Elkhorn Mountains Volcanics (Figure 18) can be more complete by
comparing these cycles to those of a system of comparable size whose volcanic
history is better preserved. The Timber Mountain and associated calderas
complex in southern Nevada is suitable for such an analogy. This caldera complex
occupies an area of roughly the same dimensions as those of the Boulder batholith,
which presumably reflect the pre-erosional extent of the nested suite of calderas
related to the eruption of the ash-flow sheets of the Elkhorn Mountains Volcanics.
The study area may thus be compared to the region outlined in the map of the
Timber Mountain and associated calderas complex in Figure 19.

Over a time span of about 5 Ma, between 16 and 6 Ma ago, the Timber
Mountain and associated calderas complex developed 4 to 6 calderas. Each
caldera formed concurrently with the eruption of 1 to 3 predominantly rhyolitic
ash-flow sheets, resulting in at least its ash-flow sheets plus other rhyolitic tuffs
and lava flows; estimated erupted volumes of the ash-flow sheets range from 20
to over 1200 km3 (Christiansen and others, 1977; Scott and others, 1984). The
Elkhorn Mountains Volcanics and the Timber Mountain and associated calderas
complex have approximately the same areal extent; with respect to time span of
volcanic activity and number of calderas and ash-flow sheets, they may also be

similar.

69

 

   
  
     
   
    
  

 

 

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30 MILES

Map of the Timber Mountain and related calderas complex
(after Carr and others, 1985). Stippled area shows region analogous to
Elkhorn Mountain Volcanics study area.

70

Based on the caldera cycles in the Timber Mountain and associated calderas
complex (Byers and others, 1976), a possible pre-erosion scenario for the Elkhorn
Mountains Volcanics preserved in the study area can be constructed (Figure 20).
The sequence of events represented in these highly schematic cross sections
begins after the eruption of ash-flow sheets I, III, and IV and lava flows II and V on
eroded Cretaceous sedimentary rocks. Caldera formation related to eruption of
these ash-flows probably took place to the west (Figure 20-1). Subsequence to
these events, eruption of ash-flow sheet VII and caldera collapse took place,
followed by eruption of lava flow group VIII (Figure 20-2). Figure 20-3 illustrates
the possible configuration of the units at the start of eruption of the ash-flow
sheets in the middle member and associated caldera formation. The final
schematic cross-section in this series shows possible resurgence of an early middle

member caldera (Figure 204).

Comparison of the Elkhorn Mountains Volcanics to the Boulder Batholith

The Elkhorn Mountains Volcanics are coeval with the associated plutons of
the Boulder batholith (Robinson and others, 1968; Klepper and others, 1957;
Smedes, 1966; Doe and others, 1968; and Tilling, 1973, 19714). The chemical
analyses from this study may be compared to analyses of the associated plutonic
rocks in an attempt to improve the present understanding of the relationship
between the batholith and the volcanics. A further goal of this comparison is to
contrast the fractionation processes which influenced the evolution of the
volcanics with those which influenced evolution of the plutonics. It is important
to realize that the plutonic rocks represent crystallized products and not magma,
whereas the volcanic rocks are direct samples of the magma. In volcanic rocks,
the processes that control the evolution of the magma can be directly inferred
rather than having to interpret the magmatic processes from the crystallized

product.

71

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72

Tilling (1973) recognized and defined two magma series in the batholith, the
main series and the sodic series. Rocks from the main series contain more K20
and less NaZO at a given SiO2 content than do rocks from the sodic series.
Because of mobility of K20, Nazo, CaO and related trace elements recognized in
the Elkhorn Mountains Volcanics, comparison of the volcanic and the plutonic
rocks is necessarily limited, and such limitation should be considered in the
following discussion.

The Elkhorn Mountains Volcanics analyzed in this study show great
similarity to the main series plutons when plotted in a KZO-NaZO-Cao diagram
(Figure 21). This affinity of the Elkhorn Mountains Volcanics for the main series
agrees with the conclusion of Tilling (1973, 1974) that the volcanics are
compositionally more like the main series than the sodic series.

Tilling (1973) found that the variation of Rb and Sr with respect to SiO2 also
could be used to illustrate the two magma series. The distribution of Rb in the
Elkhorn Mountains Volcanics is similar to that in the main series; the scatter in
the distribution of Sr, which obscures any affinity of the volcanics for either
series, is probably due to mobility (Figure 22).

The abundances of Nb, Y, Zr, Pb, Cu, and Ba in the plutons of the southern
portion of the Boulder batholith were determined by Lambe (1981) in order to
further confirm and characterize the two magma series. The new analyses of Y,
Cu, Zr, and Ba are plotted with Lambe's (1981) data in Figure 23 against $102.
Yttrium distribution is relatively constant in both the volcanics and the batholith
and does not distinguish either magma series in any respect. Copper in the lava
flows falls rapidly as SiO2 increases; the same pattern is seen in the plutonic main
series, in contrast to the sodic series, where Cu is fairly constant. Zirconium in
the volcanics shows neither the rapid depletion of the main series nor the

uniformity of the sodic series observed by Lambe (1981); it exhibits a much wider

73

K20

  
  

IAIN SERIES

  

some 51111152

 

CaO

Nazo

Figure 21. K 2O--Na O—CaO diagram of the two plutonic main and sodic
series fields2 and The Elkhorn Mountains Volcanics (solid circles)
(modified from Tilling, 1973; volcanic data from Tables 2 and 3).

74

Figure 22. Plots of Rb and Sr against 510 in the Elkhorn Mountains Volcanics
and the plutonic main and sodic series fields (modified from Tilling, 1973).
Symbols as in Figure 5.

75

RB
55 80 105 130 155 180

30

SR
200 400 500 800 10001200

0

 

 

 

 

 

MAIN SERIES
" ." SODIC SERIES
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8102

Figure 22.

76

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Figure 23.

Abundances of Y, Cu, Zr, and Ba in the Elkhorn Mountain

Volcanics and the plutons of the southern part of the Boulder batholith
(plutonic data of Lambe, 1981) in plots against SiOz.

77

range of values in the volcanics and correlates positively with increasing 5102.
The positive trend of Ba abundances in the volcanic rocks parallels a similar trend
in the sodic series. This trend in the volcanic rocks has already been explained as
indicating no fractionation of alkali feldspar. Lambe (1981) attributed Ba
enrichment in the sodic series to late crystallization of alkali feldspar and Ba
depletion in the main series to possibly early crystallization of biotite. The Ba
abundances are thus indicative of processes which influenced the evolution of the
volcanic and plutonic rocks, not of any trace element signature of either plutonic
series or of the volcanic rocks.

Lambe (1981) also presented chondrite-normalized REE profiles for the main
and sodic series. In comparison with the REE profiles of the volcanics (Figure 10),
the plutonic rocks Show pronounced negative Eu anomalies (Figure 211). Eu/Eu*
can be calculated for only two of Lambe's samples, one from the Butte Quartz
Monzonite and the other from the Burton Park pluton; his other samples contain
no detectable Tb. Eu/Eu* for the Boulder batholith are presented in Table 6 with
Eu/Eu* calculated for the volcanics; all Eu/Eu* are plotted against Th in
Figure 25. The Eu anomalies in the plutonic rocks are clearly more negative than
in the volcanic rocks, two of which exhibit slightly positive anomalies. The large
negative Eu anomalies in the plutonic rocks suggest that plagioclase fractionation
played a significant role in the evolution of the batholith. In contrast, the
evolution of the volcanics seems to have been only slightly, if at all, controlled by
plagioclase fractionation.

The strongly incompatible behavior of Zr in the volcanics (Figures 8 and 23)
implies that no fractionation of zircon occurred. The likewise incompatible
behavior of Hf (Figure 8) supports this conclusion. In contrast, the decrease in Zr
with increasing SiO2 in the plutonic main series (Figure 23) is best explained by

crystallization of zircon (Lambe, 1981). An important influence on Zr abundance

78

Table 6. Eu/Eu* in the Boulder batholith and
the Elkhorn Mountains Volcanics.

 

Sample Number EulEu*

 

Plutonlcsl
701-8 .607

#83 . 503

Ash-flow Sheetsz
J“

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U

:0

O

G

l"
u

 

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MOUNi-O‘DOOVQUOUNU-
o
\D
‘0
.—

 

l = Data of Lambe, 1981.
2 a Data of Table 3.
3 = Data of Table 2.

\

79

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81

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« .1 EH? ++21+
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L1_Joo L. X A A
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A MIDDLE MEMBER RSH-FLDM SHEETS
>< LONER MEMBER RSM-FLDM SHEETS
1:1 YOUNG-ER LRVR FLDM GROUP
+ DLDER LRVR FLDM GROUP
0 BOULDER BRTHOLITH

Figure 25. Plot of Eu/Eu* against Th for the Elkhorn Mountains Volcanics
and the Boulder batholith (batholith data from Lambe, 1981).

82

is seen in the experimental data of Watson (1979), that demonstrate a direct
relationship between Zr solubility and rock alkalinity. Watson (1979, p. l115)
states that "only two variables are of major importance in determining whether a
magma is saturated in zircon: 1) the amount of Zr present in the system, and
2) the (Nazo + K20)/."11203 ratio of the liquid." For compositions in which this
ratio is less than or equal to 1.0, zircon becomes saturated in the melt at less than
100 ppm; for values greater than 1.0, Zr solubility increases linearly from 100 ppm
at (Nazo + K20)/A1203 = 1.0 to almost 4 wt 96 at (NaZO + K20)/A1203 = 2.0.
From the Zr abundances and (Nazo + l<ZO)/A1203 ratios given in Table 7 for the
Elkhorn Mountains Volcanics, it is clear that the "peraluminous" ash-flow sheet
samples contain more Zr than should be possible based on experimental data. This
inconsistency is best explained by mobility loss of K20, which has been previously
discussed. Because the present (N320 + K20)/A1203 in the volcanics almost
certainly do not reflect the original ratios and because most of the samples
contain over 100 ppm Zr, it is likely that the parent magma for the Elkhorn

Mountains Volcanics was peralkaline.

Comparison of Volcanic and Plutonic Processes

The conclusion that the volcanic and plutonic rocks evolved along different
paths is supported by the evidence for crystal fractionation. The Eu anomalies in
the plutonic and the volcanic rocks indicate that crystal fractionation of feldspar
was important during the development of the plutonic rocks, but was minor in the
evolution of the volcanic rocks (Figure 25). Although chemical modeling has
permitted the argument for crystal fractionation of olivine in the volcanic rocks
(Figure 13), the role played by this process was not a prominent one and
apparently influenced only the early stages of evolution of part of the magmatic

system.

83

Table 7. Zr and (NaZO + K20)/A1203.

 

 

Lavas (Na20+K 2O)/A1203 Zr ppm

 

 

 

l 1.20 89
2 1.22 153
3 1.22 89
4 1.23 152
5 1.18 86
6 1.22 140
7 1.21 92
8 1.22 97
9 1.18 85
10 1.28 207
11 1.09 185
12 1.12 193
13 1.24 185
14 1.22 149
15 1.18 163 Range: 85-207
Ash-flow sheets
A .55 310
C .23 240
D .31 270
E .44 240
F .33 250
G .38 340
H .38 270
I .28 240
J .39 . 280
K .50 250
L .46 240
M .29 290
N .45 180
O .49 190
P .57 310
Q .44 350
R .44 180
S .32 360
T .43 210
U .53 260
V .39 250
W .39 260
X .23 410 Range: 180-410

 

Analytical data from Tables 2 and 3.

 

 

84

The fact that at best minor amounts of feldspar fractionation occurred in
the development of the Elkhorn Mountains Volcanics suggests that feldspar was
unstable in the source region. Such a conclusion would be consistent with the
lower crustal or upper mantle source model proposed by Doe and others (1968) to
explain the isotope variations in the Boulder batholith, and with the genetic
relationship between the batholith and the Elkhorn Mountains Volcanics.

The lower crust might thus have been the site of partial melting to produce
the volcanic magmas, which were then emplaced and stored at shallow crystal
levels. Minor fractionation of olivine and plagioclase, and perhaps pyroxene, and
magnetite occurred. If magma mixing took place, perhaps in response ‘
replenishment of the magma chamber(s) by partial melts, its e
The plutonic phase of the magmatic system was initiated as cry.
upper portions of the magma system continued and the liquids ev

too viscous to erupt (Smith, 1979; Marsh, 1981)

85

CONCLUSIONS

The goals of this project were:

1) To identify the major processes that led to the chemical diversity of
the Elkhorn Mountains Volcanics, based on interpretation of the major
and trace element variations within the volcanics, and

2) To compare the evolutionary processes inferred from the volcanic rocks
with those recorded in the plutonic rocks.

These goals have been accomplished in the conclusions summarized below.

Interpretation of the chemical trends within the Elkhorn Mountains
Volcanics has shown that 1) some fractionation, primarily of olivine, influenced
the evolution of lava flow groups VIII and V, 2) a small amount of fractionation of
plagioclase occurred in the evolution of the ash-flow sheets - more occurred in
the lava flows, and 3) no fractionation of alkali feldspar occurred.

The relationships among the lava flows and ash—flow sheets were not
dominated by either crystal-liquid fractionation or magma mixing; a combination
of these mechanisms, aided by convection and diffusion, may have occurred.

The chemical trends of the incompatible trace elements within the complete
volcanic section are consistent with the conclusion that all of the Elkhorn
Mountains Volcanics were related by evolution of a single magmatic system. The
magmatic system envisioned consisted of several separately evolving magma
bodies of combined batholithic dimensions, tapped at different depths and
locations to produce the lava flows and ash-flow sheets of the Elkhorn Mountains
Volcanics. The absence of zoning in the ash-flow sheets is interpreted as
indicating that eruption occurred before development of a highly silicic upper

zone in the magma chamber.

86

The Elkhorn Mountains Volcanics are similar to the main series of the
Boulder batholith in K20, CaO, NaZO, and Rb contents. This affinity is consistent
with Tilling's (1973, 1974) conclusions that the volcanic rocks are more closely
related to the main plutonic series than to the sodic series and that the plutons of
the main series evolved from the magma left in the system after each volcanic
withdrawal.

That the volcanic and plutonic rocks nonetheless evolved along different
paths is plain from the available evidence. The plutonic phase, as represented by
the Boulder batholith, appears to have been affected more by feldspar
fractionation. In contrast, very little feldspar fractionation occurred in the
Elkhorn Mountains Volcanics, as only the most primitive lavas can be shown to
have fractionated a small amount of ferromagnesian minerals and the ash-flow
sheets only negligible amounts of plagioclase. Considering that the Elkhorn
Mountains Volcanics and the Boulder batholith preserve different kinds of records
(that is, the Elkhorn Mountains Volcanics preserve a record of the magma,
whereas the Boulder batholith preserves crystallized products), such contrasts are
predictable.

The Elkhorn Mountains Volcanics bear chemical characteristics similar to
those of volcanics produced in modern subduction zone environments. The
relatively constant REE abundances, with variable LREE enrichment, in the
Elkhorn Mountains Volcanics (Figure 10) is typical of calc-alkaline volcanics in
modern subduction zone environments (Condie, 1982), for example. The ranges of
Rb (approximately 35-165 ppm) and Sr (approximately 300-1200 ppm) in the
Elkhorn Mountains Volcanics (Tables 2 and 3; Figure 8) can be interpreted as
indicating a source region in crust of 20-30 (or more) km thickness (Condie, 1973);
calc-alkaline rocks generally are found on crust of comparable thickness over

' i'subduction zones of intermediate depth (100-200 km) (Condie, 1982).

87

The Zr abundances in the volcanic rocks and the experimental data of
Watson (1979) regarding Zr and the (NaZO + K20)/A1203 ratio of the liquid which
can be interpreted as indicating that the parent magma of the Elkhorn Mountains
Volcanics was peralkaline, may also reflect the depth to the source area.
Increasing K20 abundances in subduction-zone related volcanic rocks appear to
correlate positively with the depth to the subduction zone (Hatherton and
Dickinson, 1969; Dickinson, 1970).

Based on the peralkalinity of the magma and the intermediate crustal
thickness and depth to the subduction zone inferred from the Rb and Sr
abundances, it is likely that the source region of the Elkhorn Mountains Volcanics
was more toward the continental interior than are the sources of many other
subduction-related magmas (Hatherton and Dickinson, 1969; Dickinson, 1970).

From the overlapping igneous and tectonic activity (Robinson and others,
1968) and from the compressional nature of the deformation in the Elkhorn
Mountains Volcanics -Boulder batholith region (Schmidt and Hendrix, 1981), it is
clear that the volcanic and plutonic activity took place within a compressional
framework, which, in terms of plate tectonics, is consistent with a subduction
zone environment for the Late Cretaceous igneous activity in southwestern
Montana. Although studies to date of the tectonic setting do not provide
unequivocal evidence of a Late Cretaceous subduction zone to the west, the fact
that the chemical characteristics of the Elkhorn Mountains Volcanics are similar
to those of modern subduction zone calc-alkaline volcanics is consistent with such
a tectonic environment.

The source region of the parent magma of the Elkhorn Mountains Volcanics
and the Boulder batholith was probably the lower crust or upper mantle, based on
the isotope variations in the plutonic rocks (Doe and others, 1968). Such a source

region is consistent with the lack of a significant Eu anomaly in the volcanic

88

rocks, which implies that the source region of the Elkhorn Mountains Volcanics
was not in the stability range of feldspar. If feldspar had been stable in the

source, large Eu anomalies would be found, unless large amounts of melting

occurred.
Because shallow level processes, such as crystal fractionation and magma

mixing, have been rejected as dominant processes in the development of the
Elkhorn Mountains Volcanics, an alternative origin of the chemical trends in the
magmas must be considered. The major chemical characteristics of the magma
could reflect the processes that occurred during the generation of the magma,
probably by partial melting of a lower crustal or upper mantle source, instead of
evolution in shallow-level magma chambers. This possibility is suggested as the
explanation for both the "monotony" of the geochemistry of the Elkhorn
Mountains Volcanics and similar eruptive units (see Figure 15) and their

intermediate $102 ranges.

89

APPENDIX

APPENDIX: CHEMICAL ANALYSES OF ASH-FALL SAMPLES

 

Sample
Number 7 12 6 4 36 33 9 14 3 2

 

m elementsl, 21M
mo2 60.3 64.0 64.6 63.3 63.8 36.1 62.1 63.6 66.3 64.6
111203 17.0 17.4 17.2 16.7 17.7 16.1 18.0 19.9 13.4 20.0

F0203 3.2 2.7 3.3 4.3 3.2 3.7 4.4 1.6 3.3 1.3
PeO 2.7 2.6 3.0 2.9 3.1 4.8 3.3 2.2 1.8 4.0
Mgo 2.7 1.4 1.4 2.1 3.l 4.8 2.1 .9 1.8 1.2
CaO 6.1 3.7 3.0 4.3 4.1 7.9 3.7 2.6 3.1 3.3
Nazo 3.3 3.2 2.1 3.7 1.6 1.0 2.6 3.9 1.9 2.1
K20 1.1 3.6 4.2 1.2 2.1 1.9 2.4 4.2 4.8 2.0
T102 .81 .87 .99 .89 .86 1.2 .90 .73 .82 .80
P20, .48 .33 .14 .13 .28 .30 .16 .23 .16 .21
MnO .17 .10 .13 .09 .13 .27 .08 .12 .23 .04
Trace elementsz, m

Ba 412 1200 1690 743 1090 761 1190 1130 1280 770 .
Co 16.1 10.6 10.6 14.7 14.0 23.7 19.3 7.1 l0.2 8.1
Cr 11.3 9.9 22.1 24.0 23.4 39.0 42.7 3.0 37.1 26.6
Cs 1.1 2.3 4.0 .3 1.0 1.2 9.3 1.3 1.2 4.8
Cu 97 26 29 l8 13 23 33 16 40 21
H! 3.3 2.6 4.9 3.7 4.8 2.3 4.7 7.3 4.1 4.8
Ni 9 8 20 10 6 13 37 n.d. 41 9
Rb 31 104 113 128 49 42 101 93 103 64
Sb .9 .2 .4 .2 .3 .3 .7 .3 .4 .3
Se 10.7 11.8 14.0 13.6 13.8 28.0 17.3 “.3 10.6 18.7
Sr 730 630 640 300 610 390 660 330 380 780
Ta .6 .9 .8 .6 .7 .4 .8 1.2 .6 8
Th 4.4 11.3 9.9 3.7 8.8 4.i 9.6 13.0 6.7 7.8
U 1.0 2.4 1.7 1.4 2.2 1.2 1.4 3.3 1.7 .7
V 140 l40 130 170 200 330 200 80 100 230

Y 18 24 20 20 20 l8 18 28 i7 17
Zn 122 86 79 76 81 132 82 83 69 77
Zr 190 410 210 170 210 240 260 410 140 360

La 34.1 49.i 42.0 29.7 38.7 23.3 38.2 63.7 29.7 30.8
Ge 60 86 78 33 70 47 68 11.3 33 33
Sm 3.3 7.3 6.3 3.1 6.3 3.1 6.0 9.3 4.9 3.1
Eu 1.43 1.83 1.34 1.31 1.36 1.29 1.33 2.39 1.27 1.44
Tb .61 .90 .68 .36 .76 .39 .66 1.19 .33 .62
Yb 1.9 2.7 2.0 2.1 2.3 1.8 1.9 3.3 1.9 1.9
Lu .31 .39 .32 .31 .33 .29 .30 .30 .30 .31

 

Analyses normalized to 100% anhydrous.

1 a All major elements by ICP except Feo by titration and T102 and P20, coiorimetrically (U.S.
Geological Survey, 3. Glillson, Analyst).

2 a All trace elements by INAA (U.S. Geological Survey, G. A. Wandless, Analyst), except Cu by flame
atomic absorption, Ni and V by graphite funace atomic absorption, and Sr, Y, and Zr by ICP (U.S.
Geological Survey, W. D'Angelo, Analyst).

 

9O

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96

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