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Graduate Student Theses, Dissertations, & Professional Papers Graduate School

1977

Geology of a portion of the Eocene Sylvan Pass volcanic center Absaroka Range Wyoming

Daphne D. LaPointe The University of Montana

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Recommended Citation LaPointe, Daphne D., "Geology of a portion of the Eocene Sylvan Pass volcanic center Absaroka Range Wyoming" (1977). Graduate Student Theses, Dissertations, & Professional Papers. 7097. https://scholarworks.umt.edu/etd/7097

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GEOLOGY OF A PORTION OF THE EOCENE

SYLVAN PASS VOLCANIC CENTER,

ABSAROKA RANGE. WYOMING

by

Daphne D. LaPointe

A.B., Smith College, 1973

Presented in partial fulfillm ent of the requirements for the degree of

Master o f Science

UNIVERSITY OF MONTANA

1977

Approved by

:hairman, Board^^f Examiners

D ^n , Graduate School

Date UMI Number: EP37898

All rights reserved

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UMT Oi«seryAicm PuMiahmg

UMI EP37898 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code

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ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 10 6 - 1346 LaPointe, Daphne D ., M.S., May, 1977 Geology

Geology o f a Portion o f the Eocene Sylvan Pass Volcanic Center, Absaroka Range, Wyoming (60 pp.)

D irector: Donald W. Hyndman

Geologic mapping, thin-section petrography, and whole-rock chemistry were used to determine the Eocene volcanic and intrusive history of a portion of the Sylvan Pass volcanic center in the Absaroka Range of northwestern Wyoming. The layered volcanic rocks of the study area consist of over 600 meters of andesitic flows and flow breccias which interfinger to the west with heterolithologic debris-flow breccias and volcanogenic sediments. There is a pro­ gressive upward change in lithology from dominantly pyroxene andésite lower in the pile to dominantly hornblende andésite higher in the pile. The layered sequence is cut by numerous dikes of hornblende andésite and a few of biotite andésite and dacite. A breccia pipe containing fragments o f both Precambrian metamorphic rocks and Eocene volcanic rocks cuts the sequence. A small d io r it ic stock intrudes the layered sequence, hornfelsing and hydrothermally altering the adjacent rocks. Two episodes of faulting have affected the area: one approximately contemporaneous w ith the volcanic activity and one which postdates it. The Sylvan Pass rocks display chemical trends which are typical of the calc-alkaline rock association and similar to trends of other volcanic rocks of the Washburn and Thoroughfare Creek Groups o f the Absaroka Volcanic Supergroup. Overall variations in lithology and thickness of the un its indicate th a t during mid-Eocene time the study area was located on the west flank of an active volcanic center. Chemical and mineralogical variation of the eruptive rocks from more mafic to less mafic with time may support a possible comagmatic origin of the suite from a differentiating magma chamber.

11 ACKNOWLEDGMENTS

I would lik e to extend my warmest thanks to the members o f my committee, Donald Hyndman, Graham Thompson, and Walter H ill fo r th e ir help in e d itin g th is thesis. Research was p a rtia lly funded by a G rant-in-Aid to Research from Sigma X i, the S c ie n tific Research

Society of North America, to whom I am grateful. I would like to especially thank my husband, Tom Irwin, for his constant encourage­ ment and many helpful discussions of the thesis material.

m TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ...... i i i

LIST OF TABLES...... vi

LIST OF FIGURES...... vi

LIST OF PLATES...... v li

CHAPTER

I. INTRODUCTION ...... 1

Previous Work ...... 1

Regional GeologicFramework ...... 4

I I . LOCAL GEOLOGY...... 10

Layered Volcanic Rocks ...... 10

Basal S e q u e n ce ...... 10

Debris-Flow Sequence ...... 15

Upper S e q u e n ce ...... 17

Breccia P ip e ...... 20

Intrusive Rocks ...... 20

D ik e s ...... 20

D i o r i t e ...... 26

A l t e r a t i o n ...... 31

Volcanic Rocks ...... 31

Diorite Intrusive ...... 32

IV CHAPTER Page

S tr u c tu r e ...... 32

Whole-Rock Chemistry ...... 34

I I I . INTERPRETATIONS...... 45

The Study Area as a Part of the Sylvan Pass Volcanic Center ...... 45

Inferred Volcanic History of the Study Area . . . 48

Petrologic E volution ...... 52

Regional Relationships ...... 53

IV. SUMMARY AND CONCLUSIONS ...... 55

REFERENCES CITED ...... 56

APPENDIX...... 60 LIST OF TABLES

Table Page

1. Modal percentages of minerals in diorite intrusive . . 28

2. Whole-rock chemical analyses o f Sylvan Pass area r o c k s ...... 35

LIST OF FIGURES

Figure Page

1. Generalized geologic map of Absaroka volcanic fie ld . 2

2. Location map of study area in Yellowstone National P a rk ...... 3

3. Generalized stra tig ra p h ie sequence o f Absaroka Volcanic Supergroup ...... 6

4a. Generalized geologic map o f study area ...... 11

4b. Generalized geologic cross section of study area . . 12

5. Stratigraphie section of study area units ...... 16

6. Q-A-P diagram showing d io rite in tru s ive samples. . . 29

7. Chemical variation diagrams for Sylvan Pass arearocks 37

8. Chemical va ria tio n diagram showing Peacock Index. . . 39

9. A-F-M plot of Sylvan Pass area rocks ...... 40

10. Plot of K 2O versus SiOg for rocks of Absaroka- Gallatin volcanic province ...... 41

11. A-F-M plot for rocks of Absaroka-Gallatin volcanic province ...... 43

12. Hypothetical cross section o f a typ ica l Absaroka volcanic center ...... 46

v i LIST OF PLATES

Plate Page

1. Zoned plagioclase la th ...... 14

2. Flow breccia t e x t u r e ...... ^4

3. Hypersthene-andesite f lo w ...... 14

4. Roadcut o f well-bedded volcanogenic sediments. . . . 14

5. Graded beds in volcanogenic sediments ...... 19

6. Fragment o f garnet gneiss in breccia p i p e ...... 19

7. Fragment o f amphibolite in breccia p i p e ...... 19

8. Zoned hornblende phenocryst ...... 22

9. Hornblende glomerophenocryst ...... 22

10. Opaque-rimmed hornblende and non-rimmed augite . . . 22

11. Pseudomorph a fte r o liv in e ...... 22

12. Biotite-andesite d ik e ...... 24

13. Diorite intrusive ...... 24

14. Augite partially replaced by alteration products . . 24

15. Brown hornblende rimmed by alteration products . . . 24

VI 1 CHAPTER I

INTRODUCTION

The Absaroka-Gallatin volcanic province is an extensive belt

of Eocene extrusive and intrusive rocks extending from the Gallatin

Range in southwestern Montana southeastward to the Stratified Primi­

tive Area in Wyoming (Figure 1). This report describes the petrology

and field relations of the Eocene intrusive and volcanic rocks of

approximately ten square kilometers in the vicinity of Sylvan Pass

in Yellowstone National Park (Figure 2). The study area is situated

in the vicinity of one of the Eocene volcanic vents alleged to have

been the source of the surrounding volcanic pile (Chadwick, 1970).

The purpose of this study is to determine the Eocene volcanic and

intrusive history of the study area, and to establish its relation­

ship to the Sylvan Pass volcanic center and its sim ilarities to other

centers in the Absaroka-Gallatin volcanic province.

Previous Work

The Sylvan Pass area was f i r s t studied by Hague, Iddings and

Weed o f the U.S. Geological Survey in the la te 1800's (Hague, and others, 1899). Iddings has provided the most detailed existing account of the general distribution of intrusive and volcanic rocks

1 MONTANA

WYOMING

ïv >“?/»//A tr

i 3 YELLOWSTONE

:< NATIONAL PARK STUOY AR£A

m

Thoroughfare Creek Group V 4 & h 49.3 Sunlight Group 4 0 MILES Washburn Group 60 KILOMETERS

Figure 1. Generalized geologic map of the Absaroka-Gallatin volcanic province showing the extent of the three groups described in the text, X marks the locality of radiometrically dated extrusive rocks. I marks the locality of dated intrusive rocks. Ages are in millions of years. Note that the rocks become younger toward the southeast (from Smedes and Prostka, 1972; Love, 1972). y£LLOwsrcfJ£ n a t io n a l

44.

iffK

YELLOWSTONE '

X.

w Figure 2. Location map showing part of Yellowstone National Park, roads, geographic features, and the location of the area studied in this report. on Avalanche Peak and the surrounding area covered in the present study. No more work was published dealing specifically with the

Sylvan Pass area u n til 1967, although several workers in the Absarokas have referred to i t in d ire c tly (Parsons, 1958; Chadwick, 1970, 1972;

Love, 1969). In 1967 Casella reported on some petrography and re­ connaissance mapping which he had done in the Sylvan Pass area. In

1970, Chadwick included the Sylvan Pass area in his paper delineating two chemically distinct subparallel northwest-trending belts of

Eocene eruptive centers. Smedes and Prostka (1972), published a section of U.S. Geological Survey Professional Paper 729 dealing with the Absaroka volcanics o f Yellowstone National Park. The U.S.

Geological Survey Geologic Map o f Yellowstone National Park (1972) includes the Sylvan Pass area, as w ill the Geologic Map of the Eagle

Peak Quadrangle, 1:62,500 (Smedes and Prostka, in preparation).

Regional Geologic Framework

During the Eocene Epoch two subparallel northwest-trending chains of volcanic vents erupted huge quantities of dominantly andesitic to d a c itic flows and pyroclastics which extended from what is now the Gallatin Range in the northwest to the Stratified Primitive Area in the southeast. These materials engulfed an eroded surface of mainly

Precambrian to Mesozoic metamorphic and sedimentary rocks (Chadwick,

1970; Smedes and Prostka, 1972). The volcanic province extends over

20,000 square kilometers with a volume of over 40,000 cubic kilometers. The area has sustained little deformation since Eocene time, but has

been deeply dissected by fluvial and glacial processes (Smedes and

Prostka, 1972; Brown, 1961).

Smedes and Prostka (1972) named the sequence the Absaroka Vol­

canic Supergroup and subdivided it into three groups: Washburn,

Sunlight, and Thoroughfare Creek, from oldest to youngest (Figure 3)

The volcanic rocks and their associated intrusive rocks become

younger toward the southeast, with the older Washburn Group being

best exposed in the Gallatin Range to the northwest, and the younger

Thoroughfare Creek Group best exposed in the southeastern Absaroka

Range in Wyoming (Smedes and Prostka, 1972). The d is trib u tio n of

these groups is shown in Figure 1. The entire volcanic sequence com­

prises over 3400 m of stratigraphie section, although no more than

1800 m of the section may be seen a t any one lo c a lity (Smedes and

Prostka, 1972).

The Washburn Group is seen only in the G allatin Range and in

northern Yellowstone Park. It has been dated by the potassium/argon

method at about 49 m.y. old (Smedes and Prostka, 1972). It attains

thicknesses of over 900 meters near vent areas and consists mainly

of hornblende- and pyroxene-andesite volcaniclastic rocks with minor

basalt flows and rhyodacite ash-flow tuffs. This group comprises

Hague's Early Acid Breccia and part of his Early Basic Breccia. In

places, the top of the Washburn Group is overlain by the Sunlight

Group with erosional unconformity; elsewhere the two groups inter­

fin g e r. 44.4 -47.1 m.y. old WIGGINS Coarse biotite-hornblende-andesite breccias, thoroughfare f o r m a t io n minor flows and volcanogenic sediments. C R E E K < 46.1 -49.3 m.y. old. Up to 600 m. th ick GROUP Fine-grained, green to brown, ash-rich t e p e e t r a i l volcanogenic sediments. 44 to 49 m.y. old. f o r m a t io n over 1800 m. 47.9 -48.5 m.y. old. Over 600 m. thick thick Massive, resistant, mafic, alluvial TWO OCEAN volcanogenic material. FORMATION

48 m.y. old. Up to 600 m thick. LANGFORD Light-colored volcanogenic sediments FORMATION and dark andesitic volcanic breccia.

Dark pyroxene-trachyandesite flows and flow breccias with some potassic basalts TROUT PEAK

SUNLIGHT ^ TRACHYANDESm GROUP

48 m.y. old up to 2000 m, th ick Dacite to trachy andésite volcanic WAPITI breccias and volcanogenic sediments FORMATION

Dark andesitic flow breccia and volcanogenic LAMAR RIVER sediments with minor mafic flows. WASHBURN \ FO RM A T ION GROUP

49 m.y. old CATHEDRAL Light colored volcaniclastic rocks: up to 1000 m. CLIFFS fine-grained alluvial facies volcanogenic th ick FORMATION sediments.

Figure 3. Generalized stra tig ra p h ie sequence o f the Absaroka Volcanic Supergroup in eastern Yellowstone National Park. The Sunlight Group of Early to Middle Eocene age (about 48 m.y. old) consists of dark pyroxene andésite flows and flow breccias with some potassic basalts in its easternmost extent. The two most prominent units are the Wapiti Formation (part o f Hague's Early Basic

Breccia) and the Trout Peak Trachyandesite (Hague's Early Basalt

Flows). These two formations attain a total thickness of as much as

1800 m just east of Yellowstone National Park along State Highway 16,

(Nelson and Pierce, 1968). The Wapiti Formation consists of dacite

to trachyandesite volcanic breccias, volcanogenic sediments, and lava

flows. The facies vary laterally as a function of topography and distance from the vent. The relative proportion of vent facies breccia

to alluvial facies sediment increases upward in the Wapiti Formation.

The overlying Trout Peak Formation consists primarily of trachy­ andesi te lava flows with minor intercalated volcaniclastic rocks

(Nelson and Pierce, 1968),

The Thoroughfare Creek Group overlies and interfingers with the

Sunlight Group. From bottom to top it consists of the following for­ mations: Langford, Two Ocean, Tepee Trail, and Wiggins, ranging in age from middle to late Eocene (48 to 45 m.y. old). These units are best exposed in the rugged peaks o f southeastern Yellowstone National

Park and the southeastern Absaroka Range (Smedes and Prostka, 1972;

Love, 1939; Wilson, 1963, 1964). 8

The Langford Formation is a sequence of up to 600 m of dominantly light-colored volcanogenic sediments and somewhat darker volcanic breccias with minor flows. These comprise the Late Acid and Late Basic

Breccias of Hague. Near Cathedral Peak, the Langford is dominantly alluvial with the proportion of vent facies material increasing south­ ward toward Sylvan Pass and Eagle Peak, two possible Eocene vent areas.

Lithologies of the Langford Formation are well-represented in the study area of this report.

The Two Ocean Formation unconformably overlies the Langford and is very similar to it in composition, consisting mainly of alluvial volcanogenic material with minor vent facies breccias which increase their proportion southward. The Two Ocean Formation is, however, more mafic, massive, and resistant, forming prominent c liffs .

The Tepee Trail Formation unconformably overlies the Two Ocean and consists of green to brown fine-grained ash-rich volcanogenic sediments. It occurs south and east of the Park in sequences up to

600 m thick. Its age ranges from middle to late Eocene, the younger age based upon paleontological data from its easternmost occurrence.

The Wiggins Formation unconformably overlies the Tepee T rail, forming the prominent vertical c liffs which top many of the ridges in the southernmost Abasaroka Range. It is composed of coarse­ grained biotite-hornblende andésite breccias and minor flows (Hague's

Late Basalt Sheets), and volcanogenic sediments. It is the youngest u n it in the Absaroka Volcanic Supergroup and has been age-dated at

45 to 47 m.y. old, placing it in the middle to late Eocene.

The area studied in this report consists of approximately ten square kilometers near Sylvan Pass along the East Entrance Road of

Yellowstone Park. The area is bounded by Avalanche Peak to the east.

Cub Creek to the north and west, and Clear Creek to the south

(Figures 2, 4). Geologic mapping of the area was done on an enlarged topographic base during the summer o f 1976. The fie ld mapping was done on a scale of 1:6000; the map in Figure 4 has been reduced from the field map to a scale of 1:18,000.

The Eocene volcanic rocks in the study area belong to the Langford

Formation of the Thoroughfare Creek Group o f the Absaroka Volcanic

Supergroup described above. In the study area, the Langford For­ mation consists of dominantly light-colored vent facies and alluvial facies material associated with the Sylvan Pass volcanic center.

The major lithologies occurring in the area are andesitic flows, flow breccias, debris flow material, volcanogenic sediments, related dikes, and small intrusive bodies. CHAPTER II

LOCAL GEOLOGY

Layered Volcanic Rocks

Basal Sequence

The lowermost stratigraphie unit exposed in the area consists

of a sequence of andesitic flows and flow breccias referred to below

as the basal sequence. I t is best exposed on the lower southwestern

slopes of Avalanche Peak and on Elk Ridge (Figures 4a,4b). The

base of the sequence is not exposed, but on Avalanche Peak the ex­ posed thickness is about 270 meters, thinning to less than 10 meters to the west, where it interfingers with debris flow material. The

flows consist of reddish to greenish grey porphyritic augite-

hornblende andésites with brecciated tops and bottoms. Zoned plagio­ clase laths (Plate 1) 1 to 2 mm in length and of composition Ang2 to An^i are abundant and define a moderate preferred orientation.

Small augite phenocrysts and glomerophenocrysts are common and are characteristically rimmed with hornblende. Small euhedral to sub- hedral phenocrysts of brown hornblende are common and rims of fine opaque iron oxides are common. These phenocrysts occur in a very fine-grained to glassy groundmass rich in disseminated opaques and tiny plagioclase microlites.

10 UNITS EXPLANATION

Undifferentiated Quaternary glacial deposits, talus, alluvium

te flows and flow-

H -f5 eccia locally [enic sediments and

! flows and flow

ite flows

[uartzdiorite in- ise jments o f Precam-

iite, bioti te

inferred ; buried

ined stra ta

in Figure 4b. Contour interval = 400 feet Datumis mean sea level

Figure 4a. Generalized geologic map of the study area. P P iC E !Z

IT

00 00 >0 )0

) 0 13

Individual flows within the sequence average 15 to 20 meters in thickness. In some flows, brecciation persists throughout the flow; the fragments are monolithologic in a flow-foliated matrix of the same lithology (Plate 2). These breccias are identical in composition to the unbrecciated flows, and are thought to be the products of auto- brecciation of the material during extrusion and flow.

From 25 to 50 meters below the upper surface o f th is sequence of flows and flow breccias, there are three thin, 1 to 2 meter-thick, discontinuous but very distinctive flows of dense black hypersthene andésite, separated from each other by a few meters of flow breccia.

These dark flows consist of about 65 percent fine groundmass rich in plagioclase microlites, fine magnetite, tiny pyroxene grains and some glass. Set in this groundmass are 20 percent plagioclase phenocrysts, An^y to An^g, and about 1 mm in length. Normal com­ positional zoning is common, and the plagioclase is somewhat altered to fin e s e ric ite and carbonate. About 15 percent small augite and hypersthene phenocrysts occur in subequal amounts, in some cases as glomerophenocrysts (Plate 3). Original phenocrysts of brown horn­ blende have been replaced by pseudomorphs o f fin e opaque iron oxides.

Hypersthene is moderately pleochroic and is partially replaced by chlorite and actinolite. These mafic hypersthene andésite flows are useful in correlating volcanic units across faults and areas of poor exposure. Plate 1. Photomicrograph of oscillatory Plate 2. Photomicrograph of monolitho­ zoned plagioclase lath (X-nicols). logic flow breccia.

- V APPROX. 5 METERS F

Plate 3. Photomicrograph of hypersthene Plate 4. Roadcut of well-bedded volcano­ andésite flow. genic conglomerate and sandstone. 15

Debris-Flow Sequence

A sequence composed mainly of massive debris-flow deposits over­ lies and locally interfingers with the basal sequence; it is referred to below as the debris-flow sequence (Figures 4, 5). This sequence varies in thickness from about 75 meters on Avalanche Peak to over

300 meters on Elk Ridge in the western part of the area studied. In­ dividual layers within the sequence are very discontinuous and were probably controlled by local topography at the time of th e ir depo­ sitio n. The debris flow deposits are typically massive, unsorted, nearly horizontal layers of heterolithologic, angular to subrounded volcanic fragments ranging in size from several meters across to very fine particles. Most commonly the fragments visible in hand specimen are from 1 mm to 5 cm in diameter. The larger breccia frag­ ments normally do not touch, but are supported by the matrix of finer fragmental material. The debris flow deposits vary in handspecimen color from dark reddish brown to lig h t greenish grey. The fragments are composed of diverse lithologies of non-vesicular pyroxene andésite and hornblende andésite, volcanogenic sediments and other breccias.

Individual flows average about 15 meters in thickness.

In several areas the debris flow units grade upward into a conglomeratic breccia containing a substantial percentage of well- rounded boulders and cobbles. In one lo c a lity on the ridge west of

Avalanche Peak, seven consecutive sequences of this breccia grading 16

•-TCT A > UPPER

V 4 k f SEQUENCE y > A > _ Dominantly hornblende-andesite 4 T , j, f. 4 4 T A 4 4 r flows and flow-breccias with minor

V 4 V augite-andesite flows and flow-breccias )■ m

4 4 A 4 & à 4 V A 4 A

4 V "4 t T V r V, 4 4 > 4 A A W 4 ^ ■» > , 4 A» A 4 4 A V 4 < 4 4 4 A 4 d e b r is- f l o w 4 V SEQUENCE

Massive heterolithologic breccia locally grading upward into conglomeratic breccia and volcanogenic sediments.

BASAL SEQUENCE

^n44 4“4-4.^4 4 4 4 4 4 4 -^'ZS'ïâ-A^ Dominantly augite-andesite flows and *44444 4 4 Z ^ 4 4 ^44 44444/ flow-breccias with minor thin mafic j 4 4 4 4-4.^4 4 4 4 4 4ZT4^4.4^4 ^ hypersthene-andesite flows r<: . 4 4 4 44Z T 4'4.4_g^ 4 4 4 4 4 4 4 t>%,4'-4.A.^ 4 4 4^^^4VLA 4 4 4 / .44444444444 4“^A-4 4 4 4 4 4A 4 4 4 4 ^ 4 4 4 4/ /

m . .'3'^ï>a-AA4 4 4 4ÏÎÏ-4 .4 H YPERSTHENE ANDESITE FLOW

4 A ^

Figure 5. Generalized stratin*-anh-i among the units in th Ï section showing interrelationships ' tne study area. 17 upward into conglomerate can be distinguished, each sequence being from 15 to 30 meters thick (Figure 4). Fragments of petrified charred wood occur within the breccias of these layers.

At the western end of Elk Ridge, the heterolithologic debris- flow breccia characteristically grades upward into the interfingers with thin layers of water-deposited volcanogenic sandstone and s ilt - stone. About 4 km farther west, roadcuts expose thicker layers of well-bedded, interlayered volcanogenic sandstone, siltstone and con­ glomerate containing abundant fragments of petrified uncharred wood

(Plate 4).

Near the top of this sequence of debris-flow material and volcano genic sediments, is a thin black hypersthene-andesite flow identical in lithology to those described above, near the top of the basal sequence (Figures 4, 5).

Upper Sequence

On Avalanche Peak and on the higher parts of Elk Ridge, the debris-flow sequence interfingers with a westward-thinning sequence of reddish-colored flows and flow breccias referred to as the upper sequence. This sequence ranges in thickness from 200 meters on

Avalanche Peak to less than 10 meters on Elk Ridge. The top of the sequence is not, however, exposed, and upper portions of the sequence have probably been eroded away. The flow units are reddish-grey and consist of monolithologic subrounded fragments within a flow-foliated 18 matrix of the same lithology. The flow rock contains small plagio- clase phenocrysts 2 to 3 mm long as well as fine opaque iron oxides forming pseudomorphs after hornblende. Some fragments are glassy and contain fewer phenocrysts; these fragments may represent pieces of the chilled margin of the flow which were incorporated into it during flow and autobrecciation.

The debris-flow material described above continues to inter­ finger with this upper sequence at higher levels on Avalanche Peak, where the heterolithologic breccia forms a south-dipping wedge-shaped layer. Within this debris flow layer, is a restricted lens of dark red to grey water-deposited volcanogenic sandstone and siltstone.

Several sequences of graded beds occur within this lens along with other sedimentary structures such as load casts and flame structure

(Plate 5).

Above the wedge of debris-flow material, the flow and flow-breccia sequence changes color to a lig h t greenish-grey porphyritic hornblende andésite. Near its base where it overlies the debris flow, the frag­ ments are more diverse in lithology than the typical monolithologic flow breccia. This is probably attributable to incorporation of some fragments of the underlying rocks by the lava as i t flowed over the surface. Plate 6. Photomicrograph of breccia pipe matrix and fragment of garnet gneiss.

Plate 5. Graded beds of volcano­ genic sandstone and siltstone on Avalanche Peak.

VD

Plate 7. Photomicrograph of breccia pipe matrix and fragment of amphibolite. 20

Breccia Pipe

A pipe-like body of heterolithologic breccia about 15 meters in diameter cuts across the layered volcanic rocks on Elk Ridge, west of Avalanche Peak (Figures 4a, 4b). The breccia contains unsorted subrounded fragments of metamorphic basement rocks sim ilar to Pre- cambrian rocks exposed at the surface in the northern fringe of

Yellowstone National Park and in the Beartooth block to the north.

Up to 30 percent of the fragments consist of amphibolite, quartzite, garnet gneiss, along with mica-quartz schists and gneisses (Plates

6, 7). The matrix consists of comminuted fragmental m aterial, and the selvages of fragments show insignificant reaction with the matrix m aterial.

Intrusive Rocks

Pi kes

Intrusive into the volcanic sequence are a number of dikes and irregularly-shaped porphyritic bodies ranging in width from less than a meter to more than ten meters. Most appear to have been ve rtica lly emplaced and some have been intruded along faults. There is no single dominant dike orientation, although N 70° to 80° E and N 40° to 60° W are prominent directions. The dikes do not exhibit any clearly defined radial or concentric pattern within the study area. All dikes observed cut across the layered volcanic rocks; at no place do the dikes appear to be capped by later flows. 21

Hornblende-andesite dikes. Most of the dikes are porphyritic hornblende andésites containing 20 to 65 percent phenocrysts.

Plagioclase is by far the most abundant phenocryst material; i t comprises 5 to 50 percent of the dike rocks as small phenocrysts, commonly 0.5 to 2.0 mm in length. Normal and oscillatory zoning is common, as is Carlsbad and Albite twinning. Plagioclase also occurs as tiny microlites in the fine-grained to glassy groundmass of most of the dike rocks. The composition of plagioclase as determined by flat-stage optical methods (J_x, YAOlO) ranges from An^^ to An^^, averaging An^g- Plagioclase is generally resorbed and partially altered to a fine-grained mixture of carbonate, sericite and epidote- group minerals.

Hornblende, in the form of small, 0.2 to 5 mm subhedral to euhedral phenocrysts, comprises up to 20 percent of most of the dike rocks. Brown hornblende is more common than is green hornblende, and many grains are zoned from brown cores to green rims (Plate 8).

Fine opaque iron oxides form rims of varying thickness on the horn­ blende grains of a few of the dike rocks. In some cases the original hornblende is completely oxidized leaving only the pseudomorph of fine opaques. Most of the hornblende grains exhibit resorption textures in which parts of both the phenocryst and the oxidized rim are embayed and/or replaced by chlorite, a ctin o lite , calcite, opaques and rarely b io tite . Glomerophenocrysts of hornblende, with or without plagioclase laths, are common in many of the dike rocks (Plate 9). 0.5 MM

r *-r Plate 8. Photomicrograph of zoned horn­ Plate 9. Photomicrograph of hornblende blende phenocryst with rim of opaque oxides glomerophenocryst.

$

/ MM ro ro

Plate 10. Photomicrograph showing horn­ Plate 11. Photomicrograph of pseudomorph blende with thick oxidized rim and augite of serpentine, chlorite, and magnetite without thick oxidized rim. after o livine. Small hypersthene grains rim the pseudomorph. 23

Augite is less common and less abundant in the dikes than is hornblende. It occurs in some of the dike rocks as small subhedral subrounded phenocrysts and glomerophenocrysts. It may comprise as much as 5 percent of a given sample, but more commonly averages about

1 percent. The grains range from 0.2 to 2.0 mm in diameter, and are

in many cases pseudomorphed by fine-grained a ctin o lite , calcite, and epidote. Unlike some of the hornblende phenocrysts, the augite grains do not generally display rims of iron oxide (Plate 10).

Quartz is commonly present in the dike rocks as small (

locally cracked. These grains are generally much larger than the groundmass grains, and may be xenocrysts picked up by the magma from host rocks.

What appear to be pseudomorphs after phenocrysts of olivine are seen in a few of the dike rocks. These consist of serpentine, ch lo rite , and magnetite, forming pseudomorphs which are rimmed with small grains of augite or hypersthene (Plate 11).

Parallelism of plagioclase and hornblende phenocrysts defines a flow texture in some of the dikes. This is most apparent near the margins of some of the larger dikes, where some compositional flow differentiation within the groundmass of the rock is visible as streaks or bands of varying composition and phenocryst content. •• ^ tL ' ''\'"r%Nk.^: Plate 12. Photomicrograph of b io tite - Plate 13. Photomicrograph of d io rite andesite dike. intrusive.

no 0.5 MM -p* Plate 14. Photomicrograph of a ctin o lite Plate 15. Photomicrograph of brown replacing augite. hornblende p a rtia lly rimmed by a ctin o lite and magnetite. 25

Biotite-andesite dikes. A few of the dikes which intrude the volcanic pile in this area are composed of biotite-andesite. The major difference between these dikes and the hornblende-andesite dikes is that the former contain up to 5 percent subhedral rounded phenocrysts of b io tite (Plate 12). The hornblende in these dike rocks is typ ica lly strongly resorbed or completely altered to pseudomorphs of chlorite, carbonate, magnetite, and locally epidote and b io tite . These dike rocks contain no augite. Apart from the presence of biotite, the absence of augite, and the strong alteration of hornblende, these dikes are sim ilar in texture and mineralogy to the hornblende-andesite dikes described above.

Chemically, the only notable distinction between the two types of dike rocks is that the biotite andésites have somewhat greater average s ilic a content (62 wt percent) than do the hornblende andésites

(60 wt percent). The biotite andésite dikes all trend NS^W to N 25°W.

They appear to have been emplaced late in the Eocene volcanic history of the area, as they consistently cut across faults and across horn­ blende-andesi te dikes.

Dacite dike. One dacite dike (70.3 percent SiOg) was found to crosscut the volcanic pile, following a northeast-trending fault. It is discontinuous and its exact time of emplacement relative to other dikes is uncertain, although i t does cut across some of the hornblende- andesi te dikes. The rock is strongly oxidized and the hornblende is nearly gone except for the oxidized rim of fine opaques. Small b io tite 26 phenocrysts are p a rtia lly replaced by chlorite. The rock has abundant

0.5 to 2.0 mm plagioclase phenocrysts which contain magnetite inclusions, in contrast to most of the plagioclases in other dike rocks which are magnetite-free.

Diori te

Also intrusive into the volcanic sequence is a small stock-like body of d io rite to quartz d io rite at the western end of Elk Ridge

(Figures 4a, 4b). Its exposed dimensions are approximately 600 meters

N-S by 1500 meters E-W. I t is cut by a northwest-trending fau lt near its western end, and its easternmost exposure is terminated by another northwest-trending fault. Its northern and western boundaries are intrusive into hornfelsed volcanic breccias and volcanogenic sedi­ ments. To the south i t is overlain by Quaternary Lava Creek Tuff and by the alluvial deposits of Clear Creek.

The intrusive rocks are phaneritic medium- to fine-grained diorites and quartz diorites which to the northwest grade into a more porphy­ ritic andésite. In the field, the diorite is distinguished from the dike rocks by its coarser grain size and by its general lack of prominent phenocrysts. The porphyritic phase, however, is indis­ tinguishable from the dike rocks. Rounded weathered outcrops are common in the coarser-grained diorite, whereas blocky, strongly jointed outcrops are common with the finer-grained d io rite . Fracture sur­ faces weather to a rusty brown color. 27

Modal percentages of minerals in the intrusive body are pre­ sented in Table 1, and are plotted on a quartz - alkali feldspar - plagioclase diagram in Figure 6. Most of the rocks fa ll into the diorite or quartz diorite fields with minor scatter into the mon- zodiorite and quartz monzodiorite fields (as delineated by Streckeisen,

1967).

These intrusive rocks are all characterized by an abundance of subhedral laths of plagioclase (Plate 13) which comprise 46 to 68 percent of the rock, averaging about 62 percent. Many grains exhibit normal or oscillatory zoning as well as twinning according to the

A lbite, Carlsbad, and Pericline laws. The plagioclase composition ranges from andesine (Angg) to labradorite (An^g) with an average composition of An^g. The presence and type of compositional zoning of the plagioclase is variable even among the grains of a single sample. Resorbed margins are common and alteration of calcic zones to zoisite and calcite is common.

Augite comprises up to 18 percent of the diorite as small,1 to

3 mm,subhedral grains and clumps of grains in te rs titia l to the larger plagioclase laths. It is often embayed and partially to wholly re­ placed by fibrous lig h t green amphibole, carbonate, and in some cases, chlorite and epidote (Plate 14). Hypersthene occurs very rarely in the diorite as small grains included with augite in glomeropheno­ crysts. I t is moderately pleochroic pale pink and beige. In many cases i t is rimmed by augite. Table 1. Modal percentages of minerals in 16 samples of a diorite intrusive near Sylvan Pass. Data are based on point counts of approximately 1500 counts per thin section.

• EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR- EPR. EPR- 104 43 44 98 101 102 113 127 150 287 289 295 298 301 304 130

Plagioclase 63.9 70.4 66.1 46.1 66.1 67.7 63.0 61.2 58.7 66.1 65.7 63.4 62.7 50.3 61.7 65.3

Augite 7.5 6.1 5.6 18.1 0.0 9.3 5.6 0.2 18.3 0.0 4.2 3.2 5.8 7.9 7.6 0.0

Hornblende + Actinolite 16.4 15.3 6.2 18.9 13.5 6.2 15.7 16.1 1.1 15.0 13.5 18.0 6.8 7.4 8.3 12.2

Biotite 0.0 1.7 1.0 5.5 1.5 0.5 2.2 0.3 4.5 4.5 0.9 0.5 0.9 1.1 5.4 8.2

Chlorite 2.4 1.4 2.3 4.0 5.2 1.4 0.5 3.4 6.5 0.4 0.4 4.2 5.8 5.1 0.4 0.2

Epidote 0.8 0.0 0.6 1.2 1.9 0.3 0.3 0.6 1.7 0.1 0.0 1.0 0.8 0.1 0.3 0.0

Quartz 3.0 0.9 4.1 0.3 5.9 8.6 2.4 10.0 0.3 5.5 4.5 2.2 7.8 9.2 5.9 12.4

Calcite 0.0 0.0 0.2 0.1 0.9 0.0 0.0 0.8 0.6 0.0 0.0 0.3 0.0 1.2 0.1 0.0

Magnetite 2.7 3.5 4.1 4.9 1.0 2.5 2.9 3.5 3.1 2.6 2.5 4.6 3.7 6.0 4.8 0.2

Apatite 0.5 0.1 0.8 0.9 0.5 0.3 0.3 0.1 0.0 0.4 0.3 0.2 0.9 0.6 0.5 0.5

K-Feldspar 2.8 0.8 6.0 0.2 3.4 5.3 7.0 4.0 5.3 5.5 8.2 2.4 4.8 8.9 4.9 0.9

Unidentified Alteration Products 0.0 0.0 3.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.2 0.0 0.0 Total 100.0 100.2 100.0 100.2 99.9 100.1 99.9 100.2 100.0 100.1 100.2 100.0 100.0 100.0 99.9 99.9 ro 00 29

QUARTZ. 4- AI OA'Z OOlOR/ I'E

MO.yZOOlORiTE.

O/OR/Tà

Q = QUA R TZ A = ALKALI FELDSPAR P = PLAGIOCLASE

Figure 6. Q - A - P diagram (a fte r Streckeisen, 1967) showing modal compositions of 16 samples of the d io ritic intrusive in the western part of the study area. 30

Brown hornblende is common, especially as ragged, subhedral, 1 to 5 mm phenocrysts in porphyritic d io rite . It also occurs as smaller grains in the equigranular diorite where it comprises up to 15 per­ cent of the rock. It is generally strongly resorbed with reaction rims of fine-grained actinolite, chlorite, carbonate, epidote, biotite, opaques, and apatite (Plate 15). Actinolite is its most common alteration product, and in many rocks has entirely replaced the phenocrysts of brown hornblende.

Biotite averages about 2.5 percent of the diorite, where it most commonly occurs in intergrowths with other alteration products a fte r hornblende or augite. Rarely i t occurs as large, ragged, dark red-brown laths p o ik ilitic a lly enclosing grains of plagioclase, augite and magnetite.

Quartz and potassium feldspar are present in small quantities as anhedral intergrowths interstitial to pyroxene and plagioclase.

Quartz and potassium feldspar are subequal in amount, totaling 1 to

18 percent of the d io rite . Most samples contain 3 to 5 percent of each of these minerals. The potassium feldspar in these rocks is sanidine.

Apatite and magnetite are common accessories lib e ra lly dis­ seminated throughout the d io rite . Magnetite also occurs as an a l­ teration product of hornblende and augite. 31

Alteration

Volcanic rocks. The strongest alteration of the volcanic rocks appears to be associated with the diorite intrusive body in the western part of the study area. Intensity of the alteration decreases zonally away from the contacts of the intrusive. In the zone im­ mediately adjacent to the intrusive body, the volcanic breccias and volcanogenic sediments are blackened and indurated; original out­

lines of breccia fragments are obscured and in many cases are visible only on a sawed, polished surface of the rock. The finer sediments are blackened and clinkery, resembling a fine featureless hornfels, although some sedimentary structures such as fine graded beds are s t i l l visible in a few cases. In thin section the original mineralogy of these rocks is indeterminate; however, secondary a lb itic plagio­ clase is present as well as carbonate, opaques, epidote group minerals, and zeolites (?), all extremely fine-grained.

Farther from the contact of the diorite is a zone of less severe alteration. Most of the volcanic breccias and volcanogenic sediments here are greenish-grey in color in contrast to the more common reddish and purplish greys of the unaltered volcaniclastic rocks.

Breccia fragments and sedimentary structures are d istin ct even in handspecimen. In thin section, however, the alteration is quite apparent. Chlorite, epidote, and carbonate are very abundant, com­ pletely replacing the primary mafic phenocrysts. Carbonate, 32 epidote and clay minerals partially replace the plagioclase. Most of the dike rocks cutting the volcanics around the intrusive body are likewise strongly altered, containing abundant chlorite, epidote, and carbonate pseudomorphic after the primary mafic phenocrysts.

D iorite intrusive. Within the d io rite intrusive its e lf, a l­ teration appears to have been dominantly deuteric in nature. Augite and brown hornblende are rimmed by or wholly replaced by fine-grained

intergrowths of actinolite, chlorite with a little biotite, mag­ netite, apatite, and carbonate, Plagioclase laths are in many cases patchily altered to zoisite, carbonate and clay minerals. The al­ teration is somewhat stronger in the marginal porphyritic phase of the intrusive body than it is in the equigranular phaneritic phase.

In the porphyritic phase, the plagioclase is strongly altered and the mafic minerals are commonly completely replaced by the alteration products described above.

Structure

The rocks of the Sylvan Pass area are cut by numerous faults

(Figures 4a, 4b), which appear to be vertical or nearly vertical wherever their attitude is discernible. Three northwest-trending

faults cut the western portion of Elk Ridge; they have orientations of N40°W, N25°W, and N25^W. Four more faults cut the eastern portion of Elk Ridge; these have orientations of N40°E, N5^E, N15^ - 20^W,

and N40^E. In addition there appears to be a fa u lt along the valley 33 which separates Elk Ridge from Avalanche Peak, and which has an orientation of N45°E. Displacements along several of these faults are demonstrably vertical. Information available on the others is compatible with vertical displacement.

The faults fa ll into two general categories: those which were approximately contemporaneous with the Eocene intrusive and volcanic a c tiv ity , and those which occurred somewhat later. The four faults which cut the eastern portion of Elk Ridge fall into the first category. Of these, the oldest is the N40^W fault which offsets the layered volcanic rocks by 50 to 75 meters, and is in turn cut by later faults and dikes. The N5°E fau lt occurred next displacing volcanic layers by only a few meters, east side up. The N40^E fa u lt cuts the previous two and displaces volcanic layers by about 25 meters, east side up. A discontinuous dacite dike has intruded along this fa u lt, and two N80°E dikes of hornblende andésite cut across it .

Finally, the N15° - 20^W fa u lt cuts across both the N40°W and the

N40°E faults. It vertically offsets the volcanic layers by about

50 meters, west side up, and a 15-meter-wide biotite-andesite dike has been emplaced along i t . This area of complexly interrelated faults and dikes is thought to demonstrate roughly contemporaneous faulting and volcanic activity during the building of the volcanic pile.

The three northwest-trending faults which cut the western portion of Elk Ridge are thought to post-date the major period of Eocene volcanic 34 a c tiv ity in the area. They cut across all Eocene volcanic and intrusive rocks and are nowhere observed to be cut by later faults or dikes. Vertical displacement of volcanic layers along the eastern­ most of these three faults is estimated to be about 50 meters, east side up. The middle fau lt truncates the d iorite body and offsets rocks on either side by at least 200 meters, west side up. Displacement along the westernmost of the three faults is not known. It is possible that this set of la te r faults occurred during the waning stages of

Eocene a c tiv ity in the area, or they may be related to some of the easternmost effects of subsidence of the Quaternary Yellowstone caldera.

Whole-Rock Chemistry

The intrusive rocks of the study area range in composition from basaltic andésite to dacite, with the majority of the analyses ex­ hibiting chemical trends typical of calc-alkaline rock associations.

Table 2 presents ten new whole-rock chemical analyses from the Sylvan

Pass area: four of the d io rite intrusive, four of the dikes, one monolithologic flow breccia, and one heterolithologic debris-flow breccia. In addition are partial analyses (SiOg, CaO, Na20, and KgO) for four more dikes. Information on analytical methods and accuracy is included in the Appendix. The composition of average c ircum-Pacific andésite (Taylor, 1969) is included in Table 2 for comparison. Most of the Sylvan Pass intrusive rocks are chemically Table 2. Chemical analyses of volcanic and intrusive rocks from the Sylvan Pass Area. Analyst: W. L. Lehmbeck, Skyline Labs., Inc., Tucson, AZ. Average andésite - average of circum-Pacific calc-alkaline andésites from Taylor, 1969, p. 60.

Hornblende Andésite Debris- Taylor's Dikes Biotite Andésite Flow Flow average Diorite ______(Weight Percent) Dacite Dikes Breccia Breccia andésite EPR EPR EPR EPR EPR EPR EPR EPR EPR EPR EPR EPR EPR EPR 150 104 304 287 334 398 358 244 237 168 234 233 189 328

SiOg 54.3 47.2 57.5 59.8 54.0 61.0 61.2 63.2 70.3 61.8 61.8 63.2 55.2 58.8 59.5

Al^Os 16.1 16.6 17.4 17.0 17.4 17.0 17.0 16.6 14.4 15.7 17.2

4.9 3.4 4.6 3.6 5.0 5.0 3.3 3.4 4.9 5.7

FeO 2.7 3.4 2.8 2.5 3.3 0.5 2.2 1.1 2.8 0.65 6.1

MgO 5.0 3.9 3.7 3.0 4.4 2.6 3.0 2.2 4.9 3.5 3.42

CaO 7.5 6.0 6.6 5.3 5.9 5.2 4.8 4.0 2.0 5.6 4.2 4.0 5.2 4.8 7.03

Na^O 4.0 4.2 4.3 4.2 4.2 4.0 4.4 4.6 4.2 3.8 4.0 3.8 2.4 3.4 3.68

%20 1.9 1.6 1.8 2.2 1.6 2.0 2.2 2.0 2.4 2.0 1.9 2.0 3.5 2.3 1.60

TiOg 0.83 0.80 0.73 0.53 0.87 0.43 0.50 0.40 0.60 0.50 0.70

Total 97.23 97.1 99.43 98.13 96.67 97.73 98.6 97.5 ■ 93.95 95.35 99.23 CO cn 36 similar to Taylor's average andésite, although the average CaO content of the Sylvan Pass rocks (5.1 percent) is somewhat lower than that of Taylor's andésite (7.03 percent). Values for ferric and ferrous iron are somewhat erratic and probably reflect variations

in near-surface alteration.

The chemical compositions of the volcanic breccias fa ll within

the range delineated by the intrusive body and the dikes with respect

to SiOg, MgO, and CaO, but they are anomalously high in KgO and low

in Na20 relative to the intrusive rocks. These variations may be attributable to contamination associated with extrusion or surface processes, although no process of contamination which would increase

^20 at the expense of Na20 is immediately apparent. As the volcanic breccias appear to have been affected by surface alteration and con­ tamination, they have generally been omitted from consideration in the variation diagrams.

Figure 7 shows the variation of MgO, CaO, Na20, TiO^, AI2O3, and

K2O with increasing Si02 for the Sylvan Pass rocks. CaO and MgO

show an essentially linear decrease with increasing SiO^, whereas

Na20 does not change sig n ifica n tly. K2O shows a slight linear increase with increasing Si02 content. Figure 8 is a combined plot of CaO and of Na2Û + K2O versus SiO^. The value of Si02 at which these

two trends intersect, the Peacock Index, has been used in the past

to delineate the calc-alkaline association (Turner and Verhoogen, 37

5 . 0 W T°/a ^ Q MgO

8 0 4- -h 6 0 O o

CaO % 4.0 □ B

5.0 □ — - -fh N o p O 4 .0 0

JO 2.5 _____ □ - KoO - 2.0 4- s 1.5 □ " - V

0.8

T / a 0.6

+ ' □ 0.4 □ "

18 f DIORITE □ 4- □ DIKE I 7 4- QD 4- □ 4- 16 - 1 1 1 1 55 6 0 65 70 WT°/o S i Op

Figure 7 Marker diagrams showing variation of MgO, CaO, Na20, K2O, Ti02, ^^2^3 with Si02 for intrusive rocks' of the study area. 38

1960, p. 78). This value for the Sylvan Pass rocks is approxi­ mately 58 percent SiOg, which fa lls between the lim its of 56 and

61 percent SiO^ which define the calc-alkaline association.

Jakes and White (1972) characterize the typical calc-alkaline andésites as rocks containing 52 to 62 percent SiO^, 15 to 19 per­ cent AlgOg and 1 to 3 percent KgO at 58 percent SiOg. They have

K20/Na20 ratios of 0.35 to 0.75, show l i t t l e or no iron enrichment and slight increase in total alkalies with increasing Si02 content.

The Sylvan Pass andesitic rocks exhibit all these chemical charac­ te ris tic s . They contain 54.0 to 61.2 percent SiOg, 16.1 to 17.0 percent AI2O3, and approximately 1.8 percent K2O at 58 percent SiO^.

They have K2Û/Na20 ratios of 0.38 to 0.57, and they show a slight increase in total alkalies with increasing Si02 (Figure 8).

Figure 9 shows an A-F-M diagram showing the lack of iron enrichment in the Sylvan Pass rocks. This trend is typical of the calc- alkaline association, as can be seen by comparison with the super­ imposed iron-enrichment trends for island arc shoshonitic, tholeiitic, and calc-alkaline associations (Jakes and White, 1972; Kuno, 1969).

The Sylvan Pass rocks plot along chemical trends similar to those of many other rocks of the Absaroka-Galla tin volcanic province.

Figure 10 is a plot of K2O versus SiÛ2 for rocks from the following areas: Sylvan Pass (this report); dacites from Birch H ills , Washakie

Needles and Bunsen Peak, (Love, 1972, 1976); the S tratified Primitive 39

A L K A L I - CALC- [ CALCIC CALCIC O 8.0 ALKALIC

- f □ □ D- 9 \ i 6 . 0 ip o □ \ I ■

§ c ' ■ 4 0 I O CJ O

2.0

□ NcLpO ^ K2 O

55 60 65 70 WT % S/O2

Figure 8. Variation diagram for CaO and (Na20 + KgO) versus SiÛ2 for intrusive rocks of the Sylvan Pass area. The Peacock Index, the value of Si02 at which the two trends intersect, is 58, which fa lls between the boundaries of 56 and 61 percent Si02 which define the calc-alkaline association. / t h o l e iit ic

CALC-ALKALIC

SHOSHONI TIC

F = FeO f 0.9 Fe^O . M □ DIKE

4- d io r it e A — N0 2 0 -h K^O

A breccia M = MqO

Figure 9 A-F-M plot of rocks of the study area with superimposed fields for tholeiitic, calc-alkaline and shoshonitic associations from island arcs (Jakes and White, 1972). The calc-alkalic field overlaps the shoshonitic field. # Sunlight Area (Parsons, 1939). 6 . 0 - a Wapiti Formation and Trout Peak Trachyandesite (Nelson and Pierce, 1968) 5.0 □ À / # e A Absarokite, banakite, and shoshonite / from various locations in the Southern / Absaroka Range (Hague, and others, 1899). 4.0 ▲

% A Electric Peak, Sepulchre Mtn. and the Gallatin Range (Hague, and others, 3.0 - 1899).

y I 0 % □ Northern Gallatin Range and A Emigrant Peak (Chadwick, 1969). ▲ / • O Birch H ills, Bunsen Peak, and n 2.0 ■ Washakie Needles (Love, 1972). /

K. X Stratified Primitive Area (Ketner k • / and others, 1966). 1.0 Sylvan Pass Area (this report).

5 0 55 60 65 70

WT 9 o 5 / O g Figure 10. Plot of K 2O versus SiÜ 2 for volcanic and intrusive rocks of the Absaroka- Gallatin volcanic province from the indicated localities. 42

Area, (Ketner and others, 1966); Emigrant Peak and the northern

Gallatin Range (Chadwick, 1968); Electric Peak and Sepulchre

Mountain (Hague and others, 1899); the Wapiti Formation and Trout

Peak Trachyandesite (Nelson and Pierce, 1968); the Sunlight area

(Parsons, 1939); and several absarokites, shoshonites, and banakites from various lo ca litie s throughout the southern Absaroka Range

(Hague and others, 1899).

The rocks appear to be d ivisib le into two groups on the variation diagram. First, a roughly linear typical calc-alkaline trend is defined by the rocks of Sylvan Pass, Birch H ills , Bunsen Peak,

Washakie Needles, S tratifie d Primitive Area, Electric Peak, Sepulchre

Mountain, and the northern Gallatin Range. Secondly, a more diffuse group of rocks with higher KgO for a given Si 02 value is formed by the rocks of the Sunlight area, the Trout Peak Trachyandesite, Wapiti

Formation, and the absarokites, shoshonites, and banakites.

Rocks along the normal calc-alkaline trend are those which form the main northwest-trending backbone of the Absaroka-Gallatin volcanic province (Figure 1). It is also noteworthy that these rocks are almost exclusively from only two of the groups of the Absaroka Volcanic

Supergroup: the Washburn Group and the Thoroughfare Creek Group.

Most of the high K 2O rocks are restricted in occurrence to the

Sunlight area just northeast of the Sylvan Pass volcanic center, where they form a small chemically d is tin c t subprovince of the 43

THOLE! IT!C

ALKALIC

SHOSHONITIC %

= FeO -f-

— N q J ^ ^

= MgO

Figure 11 A - F - M diagram for volcanic and intrusive rocks of the Absaroka-Gallatin volcanic province with superimposed fields for the tholeiitic, calc-alkalic, and shoshonitic associations from island arcs (Jakes and White, 1972). Symbols and lo ca litie s are the same as fo r Figure 10. 44

Absaroka-Gallatin volcanic province. Most of these rocks are associated with the Sunlight Group of the Absaroka Volcanic

Supergroup.

Figure 11 is an A-F-M plot of the same rocks plotted in Figure 10

Here there appears to be no clear distinction between the two

groups distinguished in the previous figure. All rocks plot along

the normal calc-alkaline trend of no strong iron-enrichment. In

this respect the high-potassium rocks of the Absaroka Supergroup

d iffe r from the shoshonitic association common to active orogenic

belts which show a lesser degree of iron-enrichment (Figure 9). CHAPTER III

INTERPRETATIONS

The Study Area as a Part of the Sylvan Pass Volcanic Center

Figure 12 is a hypothetical cross-section of a reconstructed

"typical" Absaroka volcanic center, as interpreted from descriptions by other workers in the Absaroka-Gallatin volcanic province, (Smedes and Prostka, 1972; Rubel, 1971; Krushensky, 1962,

1964; Parsons, 1939; Rouse, 1937, 1940; Schultz, 1968; Chadwick,

1966). The volcanic center may be divided into three concentric zones extending outward from the volcanic cone: vent-facies zone, interfingering vent- and alluvial-facies zone, and alluvial-facies zone.

The vent-facies zone is characterized by a predominance of andesitic flow and flow-breccia material with subordinate inter­ fingering coarse-grained debris-flow material. These rocks may exhibit quaquaversal dips away from the volcanic vent.

Farther from the cone is a zone of interfingering vent and alluvial material. Here the amount and thickness of the andésite flows and flow breccias decreases, with a corresponding increase in the debris-flow material. The debris-flow layers characteristically contain fragments of petrified charred wood, indicating deposition by hot volcanic mudflows or lahars. Thin layers of coarse

45 INTERFINGERING VENT ALLUVIAL FACIES AND ALLUVIAL FACIES VENT FACIES w

VOLCANIC VENT STUDY

KILOMETERS

Fine alluvial Coarse alluvial Interfingering andésite Andésite flows and flow breccias material. material. flows, flow breccias, Minor debris flows and coarse Standing Fossil wood debris flows and coarse sediments. fossil trees fragments sediments. No wood Fossil charred wood fragments

Figure 12. Hypothetical cross section of a typical Absaroka volcanic center. -p^ Inferred location of the study area is indicated. CT> 47 volcanogenic sediments are characteristically interbedded with the debris flows. Interfingering among the lava flows, debris flows and sediments is common. Primary dips of the layers flatten away from the volcanic cone and are more strongly influenced by variations in local topography.

Farther from the volcanic cone is a broad zone of alluvial facies material. Here well-bedded alluvial deposits of volcanogenic sediments become dominant. These range from coarse conglomerates and sandstones containing abundant uncharred petrified wood to the more distal facies of fine s ilts and muds which may have engulfed standing trees to form "fossil forests".

Comparison of this hypothetical cross-section with the geologic cross-section of the Sylvan Pass area in Figure 4b indicates that the study area was probably located on the west flank of an Eocene volcanic center in the zone of interfingering vent and alluvial facies material, as indicated in Figure 12. The main vent that was the source of this volcanic material probably lay to the east or slig h tly south of east of the study area.

The location of the study area relative to the volcanic vent

is inferred from several lines of evidence. F irst of a ll, the decreasing thickness of the basal sequence toward the west, coupled with the corresponding increase in thickness of the debris-flow

sequence to the west points toward an eastern source for the material. 48

The same conclusion is indicated by the increase in volcanic sediments toward the west, and by the change from petrified charred wood fragments in the debris flows on Elk Ridge to the abundant uncharred wood in the sediments west of the study area. Evidence from the dips of the layers is inconclusive. Most layers are nearly horizontal, with a few dipping to the northwest and to the northeast; no regional pattern that could be construed as quaquaversal dips was observed in the study area, although such a scheme might exist on a larger scale. Some of the dips may be the result of drag adjacent to fa u lts, or of t ilt in g of the blocks by faulting. It is also possible that local topography at the time of deposition controlled the dips of some of the layers.

Overall change in lithology and thickness of the volcanic units is the most compelling line of evidence for the presence of a major volcanic vent situated east of the study area. It was probably less than one kilometer to no more than three kilometers distant, inferred from the areal extent of lithologies surrounding sim ilar complexes elsewhere.

Inferred Volcanic History of the Study Area

During early Middle Eocene time, the basal sequence of horn- blende-augite andésite flows was extruded in the v ic in ity of what is now Avalanche Peak and Elk Ridge from a source vent to the east.

As the flows were extruded, already solidified crust and margins of 49 the flow were often incorporated into the moving lava, forming brecciated tops and bottoms of the flows, or in many cases forming entire flows of monolithologic breccia. At approximately the same time, multi lithologie volcanic debris was shed from the accumulating volcanic pile as lahars and debris avalanches. These formed west­ ward-thickening debris-flow deposits which interfingered with the basal sequence on Elk Ridge. These sequences continued to accumulate to a thickness of over 350 meters in the study area, although the total thickness may be much greater, as the base of the sequence is not exposed. The extrusion of thin mafic hypersthene andésite flows occasionally broke the monotony of the basal sequence accumulation.

Extrusive volcanic a c tiv ity then appears to have waned for a time while debris-flow material encroached upon and eventually buried the basal sequence to a depth of 50 to 100 meters. In the periods between successive volcanic mudflows and debris avalanches, normal sedimentary processes produced volcanogenic conglomerates and sand­ stones which mantled the debris-flow breccias. Meanwhile, a broad sedimentary apron was shed o ff the accumulating volcanic center.

Extrusive a c tiv ity from the vent to the east was subsequently revived, and the study area was engulfed by hornblende-andesite flows and flow breccias. Over 200 meters of this upper sequence accumulated on Avalanche Peak, thinning somewhat to the west where i t interfingered with debris flows and sediments. High-angle faulting 50 occurred frequently, probably in response to the fillin g and emptying of magma reservoirs and to the accumulating weight of the volcanic pile.

Many of the dikes which cut the layered volcanic rocks now ex­ posed in the study area were probably feeders to stratigraphically higher lava flows which have since been removed by erosion. Judging from the composition of the dikes which fed the flows, there was continued accumulation of hornblende-andesite flows followed by a later period of extrusion of biotite-andesite flows for which the feeders were the late biotite-andesite dikes in the area.

At some time following the deposition of the latest layered volcanic and volcanidastic rocks now seen in the study area, a breccia pipe was emplaced on Elk Ridge. It probably originated deep within the volcanic system, below the contact of the volcanic rocks with the underlying Precambrian basement rocks. The breccia pipe may have originated when a deep fracture intercepted a pocket of highly pressurized vapor. The vapor streamed upward through the fracture with a velocity sufficient not only to suspend the rock fragments picked up from the fracture walls, but also to transport them v e rtica lly for a distance of at least several hundred meters to their present stratigraphie level (cf: Reynolds, 1954). The fluidized stream of gas, comminuted particles, and larger fragments plucked additional fragments of the country rock from the walls of 51 the fracture a ll the way to the surface. As the streaming subsided, the pipe fille d with the fallback material: a m ultilithologic matrix- supported breccia containing fragments of various Precambrian meta- morphic rocks as well as Eocene volcanics in a comminuted matrix of fin e r fragmental material. The pipe is evidence that at least part of the country rock underlying the Eocene volcanic pile in this area consists of Precambrian metamorphic terrain. Similar breccia pipes have been described in the Eocene volcanics of the New World d is tr ic t near Cooke City, Montana (Eyrich, 1971).

Also late in the Eocene volcanic history of the study area, a body of d io rite magma intruded the debris flows and volcanogenic sediments of the western part of Elk Ridge. It hornfelsed the rocks immediately adjacent to its contacts, probably driving o ff the water therein, which in turn hydrothermal 1y altered the rocks in a broad zone surrounding the hornfelsed zone. Later northwest-trending faults offset all the Eocene volcanic and intrusive rocks in the area by as much as 150 meters.

Subsequent erosion during the late Cenozoic Era greatly lowered the Eocene volcanic pile and shaped the major aspects of the present topography before the eruption of the Quarternary Yellowstone volcanic rocks. At that time, the Lava Creek Tuff (rhyolitic welded ash flows) was erupted, fillin g in topographically low areas and lapping onto the Eocene volcanic rocks in the study area. Subsequent glacial 52 a c tiv ity further eroded the area and le ft morainal deposits along valley floors and margins. Recent allu vial processes have covered much of the area with a veneer of unconsolidated alluvium and talus.

Petrologic Evolution

Chemical and mineralogical variation in the suite of volcanic intrusive rocks of the Sylvan Pass area favors a possible origin of the rocks by the progressive differentiation of an andesitic magma chamber. Field relations and petrography indicate that the overall mineralogical composition of the erupted rocks changed with time.

This change is documented by the occurrence of the following sequence:

1. Early flows of dominantly augite-andesite with

several more mafic flows of hypersthene andésite

(basal sequence).

2. Later flows of dominantly hornblende-andesite with

minor augite-andesite (upper sequence).

3. Abundant crosscutting dikes (feeders to stratigraph­

ically higher flows) of hornblende andésite.

4. Later dikes of biotite-hornblende andésite.

5. A single occurrence of a la te r dacite dike containing

both b io tite and hornblende.

Chemically, the intrusive rocks of the area exhibit relatively smooth linear trends on chemical variation diagrams as described above 53

(Figure 7). This also supports a possible comagmatic origin for the

suite. The d io rite intrusive, although more mafic than most of the

dike rocks, lies along the same linear trends, and is probably

comagmatic with the rest of the suite. I t appears to have been em­

placed later than some of the more fe ls ic rocks of the sequence, and may represent a later influx of the andesitic "parent" magma. These

various chemical and mineralogical trends imply that the volcanic

and intrusive rocks of the Sylvan Pass area are probably cogenetic,

resulting from successive tapping of a single parent magma at various

stages in its magmatic diffe ren tiation .

Regional Relationships

The volcanic rocks of the Sylvan Pass area are chemically sim ilar to those rocks along the main trend of the Absaroka Range associated with the Washburn and Thoroughfare Creek Groups. These rocks exhibit chemical variation typical of the calc-alkaline association which is characteristically associated with island arcs and active continental margins. The occurrence of this association so far from contem­ poraneous plate margin activity is puzzling at best. The possibility exists that the Absaroka volcanic field did form over an "imbricate subduction zone" as postulated by Lipman and others (1972), although l i t t l e evidence has been brought forward in support of this hypothesis.

Elsewhere in the western states. Eocene andesitic volcanism has heralded the onset of Basin and Range tectonic a c tiv ity (Armstrong,

1970; McKee and Silberman, 1970). It is possible that the 54

Absaroka volcanic field represents a volumetrically larger version of this early Basin and Range volcanism. Perhaps comparison of the

Absaroka volcanic rocks with rocks of sim ilar age and composition of the Clarno Formation in Oregon and the Challis volcanic fie ld of

Idaho will shed some light on their origins and relationships to regional tectonics of the northwestern United States (cf: Chadwick,

1977; Jones and Buffa, 1977; Prostka, and others, 1977; Siems and

Jones, 1977; Taylor, 1977). CHAPTER IV

SUMMARY AND CONCLUSIONS

The rocks in the study area west of Sylvan Pass in Yellowstone

National Park consist of a 550-meter thick sequence of hornblende- and pyroxene-andesite flows and flow breccias and derivative volcani- clastic rocks intruded by hornblende-andesite dikes and by a small dioritic stock. Field relations and overall lateral lithologie variations indicate that the source vent for these materials lay to the east, perhaps 0.5 to 2 kilometers.

Mineralogical and chemical variations upward through the se­ quence suggest that the volcanic and intrusive rocks of the study area are a ll comagmatic. They were probably derived from successive tapping of a differentiating andesitic parent magma.

The rocks of the Sylvan Pass area are chemically and mineralog- ically similar to other intrusive and volcanic rocks associated with the Washburn Group and the Thoroughfare Creek Group of the Absaroka

Volcanic Supergroup, all of which exhibit normal calc-alkaline a ffin itie s . These rocks d iffe r, however, from those associated with the Sunlight Group, which generally have higher KgO content. Rocks of all three groups follow nearly identical iron-enrichment trends.

55 REFERENCES CITED

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, 1977, Overview of early Tertiary volcanism in the northern Rockies region (Abst.): Geol. Soc. Am. Prog. 30th Ann. Mtg., Rocky Mtn. Section, p. 715-716.

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Hague, A., Iddings, J. P., Weed, W. H., 1899, Geology of the Yellowstone National Park, Part II: Descriptive Geology, Petrography and Paleontology: U.S. Geological Survey Monograph 32, pt. 2, p. 269-325: by J. P. Iddings, Ch. VIII - The igneous rocks of the Absaroka Range and the Two Ocean Plateau and of outlying portions of Yellowstone National Park.

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56 57

Jones, R. W. and Buffa, J. W., 1977, Chemistry of early Tertiary age Challis volcanics in south half of the Twin Peaks Quadrangle, east-central Idaho (abst.): Geol. Soc. Am. Prog, 30th Ann. Mtg., Rocky Mtn. Section, p. 737.

Ketner, K. B. , Keefer, W. R., Fisher, F. S., Smith, D. L., and Raabe, R. G., 1966, Mineral resources of the S tra tifie ld Primitive Area, Wyoming: U. S. Geological Survey Bull. 1230-E, 56 p.

Kurshensky, 1962, Volcanic features of the Hurricane Mesa, Park County, Wyoming (abst.): Geol. Soc. Am. Special Paper 68, p. 214.

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Kuno, H., 1967, Origin of Andésite and its bearing on the island arc structure: Bull. Vocanol., vol. 32, p. 141-176.

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Nelson, W. H,, and Pierce, W. G., 1968, Wapiti Formation and Trout Peak Trachyandesite, northwestern Wyoming: U.S. Geological Survey Bull. 1254-H, 11 p.

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, 1964, Geologic reconnaissance of the southern Absaroka Mountains, northwest Wyoming: Part I: the Wood River-Greybul1 River area: Wyo. Univ. Contr. Geology, vol. 3, p. 60-77. APPENDIX

Whole-rock chemical analyses were performed by Skyline Labs, Inc.,

Tucson, Arizona, using the following analytical methods:

(1) SiOg was determined gravimetrically within a reproduci­ b ility of plus or minus 0.5 percent of the amount present.

(2) AlgO^) MgO, CaO, Na 2Û and K 2O were determined by atomic absorption spectrophotometry with a reproducibility of plus or minus less than 5 percent of the amount present.

(3) FepOg and FeO were determined volumetrically with a reproducibility of less than plus or minus 5 percent of the amount present.

(4) TiOg was determined colo rim e trica lly, again with a reproducibility of plus or minus 5 percent.

60