sign in sign up
<<
Home , Iddingsite, Igneous petrology, Olivine, Shoshonite

Utah State University DigitalCommons@USU

All Graduate Theses and Dissertations Graduate Studies

5-1985

The and of Tertiary(?) in the Harrington Peak Quadrangle, Southeastern Idaho

Amanda Shearer-Fullerton Utah State University

Follow this and additional works at: https://digitalcommons.usu.edu/etd

Part of the Commons

Recommended Citation Shearer-Fullerton, Amanda, "The Petrology and Mineralogy of Tertiary(?) Olivine Trachyte in the Harrington Peak Quadrangle, Southeastern Idaho" (1985). All Graduate Theses and Dissertations. 3831. https://digitalcommons.usu.edu/etd/3831

This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. THE PETROLOGY AND MINERALOGY OF TERTIARY(?) OLIVINE TRACHYTE IN THE HARRINGTON PEAK QUADRANGLE, SOUTHEASTERN IDAHO

by

Amanda Shearer-Fullerton

A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in Geology

Approved:

UTAH STATE UNIVERSITY Logan, Utah 1985 ii

DEDICATION

In memory of my father, James Shearer

(1901 - 1979) iii

ACKNOWLEDGMENTS

My sincere thanks go to Dr. Donald W. Fiesinger, under whose direction this work was done, for suggesting the study and providing valuable suggestions and help in the field, the laboratory, and on the

manuscript. Gratitude is also expressed to the members of my committee: Dr. Clyde T. Hardy for his invaluable help in the field and his critical review of the manuscript and to Dr. Peter T. Kolesar for his critical review of the manuscript. I would also like to thank Dr. Judith M. Ballantyne for her companionship in the field and for her helpful suggestions. Much appreciation is extended to the University of Calgary and to Rice University for the use of their electron microprobe equipment . I am indebted to my fell ow classmates, Yunshuen Wang and Steven Kerr, for their friendship and help in the field. Finally, my special thanks go to my husband, Tod, without whose patience, love, and understanding, the completion of this project would not have been possible. Amanda Shearer-Fullerton iv

TABLE OF CONTENTS

Page

DEDICATION •••••••••••.••••••• • ••••••••••••••••••••••••••••••••• ii

ACKNOWLEDGMENTS •••.••••••••••••••••.••••••••••••••••••••••••••• iii

LIST OF TABLES ...... vi

LIST OF FIGURES ...... vii

ABSTRACT •••••••••••••••••••••••••••••••••••••••••••••••••••••• viii

INTRODUCTION •••••••••••••••••••••••••••••••••••.•.•••••••••••••

Purpose of Investigation • .. .. • ...... • ...... 1 Location and Accessibility ...... • 1 Previous Work • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 3 Regional Setting • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 5 Field Relationships ...... 8 Sampling and Analytical Procedures ••••••••••••••••••••••••••• 14

PETROGRAPHY AND MINERALOGY ...... 15

General Statement •••••••••••••••••••••••••••••••••••••••••••• 15 Olivine •••••••••••••••••••••••••••••••••••••••••••••••••••••• 15 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . • • • • • • • • • • • • • • • • • • • • • 20 Sanidine ...... 24 •••••••••••••••••••••••••••••••••••••••••••••••••• 31 Fe-Ti Oxides ••••••••••••••••••••••••••••••••••••••••••••••••• 32 Ph 1 ogopi te • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 34 AND CLASSIFICATION ...... 35

Introduction ••••••••••••••••••••••••••••••••••••••••••••••••• 35 Naming the Harrington Peak Quadrangle ••••••••••••••••••• 35 Comparison with Other Alkaline Igneous Rocks ...... 38

PETROGENESIS ...... 44

Temperatures of Crystallization ...... 44 Hypothesis ...... 47 Regional Tectonics: Petrol ogic Implications ...... 49 CONCLUSIONS ...... 51 v

TABLE OF CONTENTS (CONTINUED)

REFERENCES ...... 53 APPDJDIX • • • • • • • • • • • . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 59 vi

LIST OF TABLES Table Page

1. Modal analyses of samples (volume percent) ...... 16 2. Representative electron microprobe analyses of olivine .•... 18 3. Average electron microprobe analyses of pyroxene ..•...•.... 21 4. Representative electron microprobe analyses of .... 28 5. Comparison of Sa and K •...... •...... •.• . .•• 30 6. Average electron microprobe analyses of -titanium oxides .•...... •...... •...... •...... •. ••• •.. • 33 7. Whole rock wet chemical analyses and CIPW norms ....•...... 36 8. Olivine-clinopyroxene temperatures of crystallization .•.... 48 9. Sample locations ....•.•...•...... •...... ••••.••.•• 60 10. Analyses of Columbia River Reference Sample ....••... 61 11. Analyses of Italian Ciminites from the Viterbo Region, •...... •...... ••...•....•..•...... ••.....•... 62 vii

LIST OF FIGURES Figure Page 1. Index map of the Harrington Peak Quadrangle, southeastern Idaho ...... 2 2. Sample locations and general geology of the study area •..... 6 3. Cross section of the largest outcrop area .•...... 9 4. Outcrop showing steeply inclined stretched vesicles ...... •. 12 5. Outcrop showing vertically stretched vesicles ..•••••...•..•. 13 6. Electron microprobe analyses of olivine plotted in terms of molecular percent Fa and Fa •....•..•••...... ••..•....• . ... 19 7. Electron microprobe analyses of pyroxene plotted in terms of molecular percent Di, En and Wo .•..•...... •...... 22 8. Weight percent silica versus weight percent alumina in pyroxene . . . • ...... • ...... • • • • . • ...... • • • . . • • . . . . . 25 9. Alz versus weight percent Ti02 in pyroxene ...... •• 26 10. Electron microprobe analyses of feldspar plotted in terms of molecular percent An, Ab and Or ...•...•••..•....•...•...... 29

11. Weight percent Fe 2o3 + FeD versus MgO ...... •...... •.....•. 39 12. Weight percent MgO versus weight percent Si02 for olivine trachyte and !tal ian ciminite ..•....•..•....•..•.•••••••..•. 42

13. Weight percent CaD versus weight percent Si02 for olivine trachyte and !tal ian ciminite ••....•••..•.••••...•.•...•...• 43 14. Weight percent FeD versus weight percent MgO for olivine trachyte and !tal ian ciminite .••••.....•.•.•.•.••.•.•.•••..• 43 15. Two-feldspar geothermometer ....•....•..•.•...•...... •.....•. 45 viii

AB STRACT

The Petrology and Mineralogy of Tertiary(?} Olivine Trachyte

in the Harrington Peak Quadrangle, Southeastern Idaho

by

Amanda Shearer-Fullerton, Master of Science

Utah State University, 1985

Major Professor : Dr. Donald W. Fiesinger Department : Geology

The Harrington Peak Quadrangle is located within the Caribou

National Forest of southeast Idaho. Within this quadrangle are outcrops of olivine trachyte of Pl i ocene(?} age overlying sedimentary rocks of Mississippian to Tertiary age. The region contains thrust faults and 1 ater normal faults (generally trending north-south} formed during Basin and Range extension.

The largest outcrop of olivine trachyte (approximately 1 1/2 X 3 km} probably formed as the result of a fissure eruption. Two other outcrop areas show evidence of being sites of local extrusion.

Whole-rock chemical analyses revealed the olivine trachyte to have moderate amounts of Si02 and Al 2o3, high MgO and CaO, and K20 in excess over Na 2o (approximately 2:1}. Mineralogical characteri sties include microphenocrysts of Mg-rich olivine and diopsidic in a groundmass of Ba-rich sanidine, diopsidic augite, Fe-Ti oxides, and less commonly and/or plagioclase. ix

The olivine trachyte closely resembles the ciminites from the

Viterbo region of Italy and has some petrological and mineralogical similarities to many other continental potassic volcanic rocks. The olivine trachyte may have formed by partial melting of a heterogenous source enriched in incompatible elements during a previous tectonic event.

(62 pages) INTRODUCTION

Purpose of Investigation

The purpose of this thesis is to determine the chemistry, mineralogy, and petrogenesis of the volcanic rocks in the Harrington

Peak Quadrangle, Caribou County, Idaho, and their field relationship to adjacent rock units. The chemical and mineralogical characteri sties are to be compared with similar rocks in other areas.

Location and Accessibility

The volcanic rocks studied are located within the Harrington Peak

7 1/2' Quadrangle in the Peale Mountains of southeastern Idaho

(Figure 1). The Harrington Peak Quadrangle is about 17 km east­ southeast of Soda Springs, Idaho, and 13 km west of the Idaho-Wyoming state line. The quadrangle includes parts of Bear Lake and Caribou

Counties (the drainage divide marks the county line), and it is within the Caribou National Forest. Two main creeks run through the study area: the left-hand fork of Twin Creek, which flows south of the drainage divide, and Slug Creek, which flows north of the divide. The gently rolling hills of the Harrington Peak Quadrangle are heavily vegetated and have an abundant mosquito population in the summertime, making fieldwork difficult. Some outcrops are difficult to find because they are of limited extent; however, an unpaved road cuts through the area of most major outcrops.

There are three outcrop areas where samples were collected for 2

111" 22'30" 111" 15'

~42"37'30''-r------~.------r------,

"'USlN" -· -·-· ..-

-~!~ '-·...... _.,_ n ~ '\ )' I DAIO"""'-1 , L ___ _j ( . ,,,_,,~

~42"30'---~~------~------~

0------1::1======>2 rn ll•• o------i::·=====:::.2 kllom•t•rs

Figure l. Index map of the Harrington Peak Quadrangle, southeastern Idaho. Generalized geol og i c maps sho wn in Figure 2. this study. The largest area of is located at the southernmost end of Slug Valley, approximately 1 1/2 km west of Schmid Ridge; a much smaller outcrop area is just north of Summit View Campground; and a third outcrop area is about 5 km southwest of Summit View Campground and 1 km west of the left-hand fork of Twin Creek. The map by Cressman (1964) shows two other areas of volcanic outcrop; one is about 1 1/2 km north of Summit View Campground and the other is 1 1/2 km southeast of Harrington Peak. Neither of these outcrops could be located in the field.

Previous Work

The lava in the Harrington Peak Quadrangle was originally described by Mansfield {1927} and Cressman (1964) as olivine basalt based on its appearance in hand specimen. Because the lava lies conformably on the Salt Lake Formation, the lava was considered to be no younger than Pliocene (Cressman, 1964). Puchy (1981) and Fiesinger and others (1982) called this lava alkali trachyte using Streckeisen's (1979) classification, because of the presence of sanidine and lack of plagioclase in the rock samples collected. Puchy petrographically described the lava as resembling a fine-grained shonkinite after Williams and others (1982) and similar to the potassium-rich rocks of orenditic affinity (Sahama, 1974), although less potassic. This lava also contains magnetite, which is uncommon in orenditic rocks (Carmichael, 1967b). Puchy (1981) and Fiesinger and others (1982) concluded that the lava was not related to basaltic 4

in Caribou Co unty, Idaho.

Other volcanic rocks of the region include the Blackfoot, Willow

Creek, and Gem Valley lava fields, and small valley lava flows found

within Enoch, Upper, and Wooley Valleys (Fiesinger and others, 1982).

The volcanic rocks were described as olivine tholeiite with minor

tholeiite and tholeiitic . Previously, Bright (1963, 1967)

described the Gem Valley lavas as porphyritic olivine basalt.

Potassium-argon dates of the Gem Valley basalt (Armstrong and others,

1975) gave an age of 0.1 ± 0.03 million years. Unpublished potassium­ argon (whole rock) age dates for basalt from Wooley Valley and the

Fox Hills (approximately 25 km north of the study area), gave ages of

2.2 :t 1.0 million years and 3.2 ± 1.6 million years, respectively (R. David Hovland, U. S. Management Service, Moscow, ID, personal communication).

Alkaline intrusive rocks crop out at Mount Caribou in Bonneville

County, Ida ho, approximately 60 km north of the study area in southeastern Idaho. These intrusives were described by Anderson and

Kirkham (1931) as monzonite, shonkinite, , aplite, monzonite porphyry, prophyry, porphyry, and 1 atite porphyry. Huntsman (1978) described some igneous rocks in the same

Caribou Mountain area as andesite flows, hornblende-feldspar andesite porphyry , feldspar-hornblende andesite porphyry, and ­ monzodiorite, all of Eocene age. Huntsman (1978) proposed that these igneous rocks were evolved during the Challis-Absaroka magmatic event, whi c h affected much of Idaho during Eocene time. 5

Regional Setting

All of the rocks in the Harrington Peak Quadrangle are sedimentary

except for the volcanic rock discussed in this study. These

sedimentary rocks range in age from Mississippian to Tertiary and most

are gently folded (Cressman, 1964).

The Harrington Peak Quadrangle is in the Middle Rocky Mountain physiographic province, but 1 ies close to the eastern boundary of the Basin and Range Province (Fenneman and Johnson, 1946) and was 1 ikely affected by the tectonics of that region. Recent studies show that the

fundamental geological and geophysical characteristics of the Basin and

Range Province can be found well beyond the boundaries imposed by Fenneman and Johnson (1946), which were based on physiography alone. Eaton (1982) described the Basin and Range domain as extending across eastern California, all of Nevada, western Utah, eastern Oregon, southern Idaho, western Montana, and western Wyoming. Southeast Idaho has been undergoing Basin-Range extension from Eocene to the present along a series of major normal faults (Royse and others, 1975). The study area in the Harrington Peak Quadrangle is characterized by north­ south trending normal faults (Figure 2). Many of the north-south valleys in southeast Idaho are half-grabens bounded on the east by normal faults (Royse and others, 1975). Royse and others (1975) suggested that many of the normal faults in this structural area of

Wyoming, Idaho, and northern Utah, flatten with depth into the older, underlying thrust planes (such as the Meade thrust fault in the study 6 puI\ 1I \. I k:=='===::l2 ...... ,. f N 8

area. Portions of the major thrusts had an earlier compressive motion

of thrust faulting then later extension resulting in norm~ faulting

(Royse and others, 1975).

Field Relationships

The 1 arges t outcrop area of trachyte is 1 oca ted in the northern

part of the Harrington Peak 7 1/ 2" Quadrangle approximately 2 km

northeast of Harrington Peak and 1 1/2 km west of Schmid Ridge. This

occurrence consists of discontinuous outcrops on a hill and covers an

oval-shaped area which is approximately 1 1/2 km east-west X 3 km

north-south. A north-south trending normal fault to the east and one

to the west of the 1 argest outcrop area are both down faulted to the west. The fault, east of the flow, is probably younger than the flow, whereas the fault to the west is pre-1 ava flow in age (Figure 2), but

is concealed by Quaternary sediments (Cressman, 1964). Oriel and Platt

(1983) showed a fault on their map trending north-south through the

eastern half of this outcrop area, down faulted to the east north of

the outcrop area and down faulted to the west south of the outcrop

area. This fault is concealed by volcanic rock and Quaternary

sediments. The age of this fault is unknown, probably pre-dating the

flow and having recurrent movement which post-dates the flow.

The lava of this largest outcrop area probably formed as a result

of a fissure eruption with the fissure trending north-south along the

highest ridge near the western edge of the main flow (Figures 2 and 3).

Although highly eroded, there is some evidence of this being the site of extrusion because of steeply inclined stretched vesicles in the EXPLANATION

Quaternary alluvium

Tertiary {Pliocene?) inferred contact olivine trachyte

~ Tertiary ----- Salt lake Fonnation inferred fault

Permian and Pennsylvanian Wells Formation and

Pennsyl vanf an Wells Formation 1 imestone and sandstone

Figure 3. Cross section of the largest outcrop area (see Figure 2). 10

------~~~------~ ~

. ••

0

0

i ! 11

rocks along this highest area of the hill (Figure 4). There are also

scori aceous red boulders sea ttered along the hi 11 top.

There are no outcrops of trachyte west of the western fault, but

many are found east of the hill top. All of these outcrops dip to the

east. The trachyte extends east of Slug Creek, forming the distal part

of the flow. The trachyte found at this outcrop ranges in texture from

highly vesicular to very fine grained and dense. Here the flow is

thinning, evidenced by Tertiary Salt Lake Formation exposed at the

roadcut.

The outcrop just north of Summit View Campground seems to be a

site of local extrusion. This outcrop is small (approximately 2 m X 2

1/2 m), somewhat circular in plan, and has vertically stretched vesicles (Figure 5). The outcrop is located near the bottom of a

drainage and may be a dike exposed due to erosion. There is a north­

south trending normal fault down to the west 1/2 km north of the outcrop and another normal fault approximately 1 km southwest of the outcrop.

The third outcrop area, approximately 5 km southwest of Summit

View Campground, is near a ridgetop and consists of highly eroded,

vesicular trachyte. This outcrop seems to be on a block between two

northwest-trending normal faults, both down to the west. The trachyte

on the southeast part of the outcrop area show evidence of vertical jointing, and in places shows apparent columnar jointing. Just west of

this outcrop, also on the ridgetop, are scattered boulders of trachyte, one as large as 2 1/2 m X 2m, with most averaging about 3/4 m in diameter. This trachyte is intermixed with boulders of Paleozoic rock, 12

Figure 4. Outcrop showing steeply inclined stretched vesicles . Width of outcrop appro ximately 1 m. 13

Figure 5. Outcrop showing vertically stretched vesicles. Width of total outcrop pictured is approximately 2.5 m. 14 indicating that the trachyte was emplaced, eroded, and mixed with Paleozoic boulders prior to incision of adjacent canyons during the Pleistocene.

Sampling and Analytical Procedures

Samples were taken from all three areas of trachyte outcrop in the Harrington Peak Quadrangle. After preliminary petrographic study, seven samples were selected for further study based on the 1 oca ti on within the outcrop area, lack of intense , and textural variations: SCV83-1, SCV83-3, SCV83-8, SCV83-11, SCV83-16, SCV83-17, SCV83-18. Whole-rock chemical analyses were obtained using gravimetric and colorimetric methods as well as UV-VIS spectrophotometry and atomic absorption spectrophotometry (Maxwell, 1g68). Chemical analyses of pyroxene and olivine were obtained using the ARL-SEMQ 8-channel electron-probe microanalyzer at the University of Calgary, and chemical analyses of and Fe-Ti oxides were obtained using the ETEC autoprobe at Rice University. Correction procedures used on microprobe data follow the methods of Bence and Albee (1968) and Albee and Ray (1970). 15

PETROGRAPHY AND MINERALOGY

General Statement

The trachyte ranges in texture from dense to highly vesicular and is generally aphanitic. Vesicles are commonly stretched and some contain secondary calcite. Fresh surface color ranges from light gray to dark gray, to reddish brown when scoriaceous. Modal analyses of 1000 points on each sample are presented in Table 1. All of the samples are microporphyritic with microphenocrysts of augite and olivine totalling 7% to 15 %. The groundmass is mostly sanidine and pyroxene with some samples containing a few flakes of phlogopite. Plagioclase is present in the groundmass of several samples. All of the samples have abundant Fe-Ti oxides. Undifferentiated groundmass consists of birefringent materials or alteration material too small for identification.

Olivine

01 ivine is present as microphenocrysts in all of the samples, but was not found in the groundmass of any of the samples analyzed. The microphenocrysts are subhedral to anhedral, are pale yellow green in thin section, and range in size from 0.20 to 2.0 mm in diameter. Most of the olivine microphenocrysts are with inclusions of indicating the olivine and chromite formed at the same time.

There need be only a small amount of Cr 2o3 in melts rich in potentia 1 olivine or pyroxene to allow for early precipitation of chromian (Irvine, 1966). Olivine commonly has iron oxide or as rims Table 1. Modal analyses of samples (volume percent).

Sample Microphenocrysts Groundmass number Augite 01 ivine Sanidine Augite Opaques Phl ogopi te Und. Other SCV83-1 5.2 6.1 43.3 22.6 9.1 - 13.6 SCV83-3 5.4 9.6 45.3 17.3 15.1 tr 6.4 gdms pl ag - tr SCV83-8 5.3 5.4 45.5 28.5 11.6 3.6 SCV83-11 3.7 4.0 45.7 10.6 31.3 tr 4.5 SCV83-16 0.8 6.7 46.5 29.6 10.7 tr 5.6 SCV83-17 3.3 10.0 32.5 17.9 11.0 tr 1.3 gdms plag - 24.0 SCV83-18 4.0 7.4 48.8 21.5 6.8 5.0 5.7 unidentified red min. - tr

und. =undifferentiated

..... "' 17

or along fractures. In one sample (SCV83-ll) olivine microphenocrysts

are almost completely replaced by Fe-Ti oxides. 01 i vine

microphenocrysts are occasionally cummulophyric. Average analyses of

olivine microphenocrysts are presented in Table 2. Structural formulas

were calculated using the program of Jackson and others (1967). Figure

6 is a bar graph illustrating zoning from core to rim. Zoning is very

limited (Fogo to Fo75• molecular percent ), indicating that

the olivine equilibrated with the 1 iquid prior to solidification. As

noted in Figure 6, these are very -rich indicative of

a high temperature environment.

Simkin and Smith (1970) suggested that CaO content in olivine is

indicative of the crystallization environment. They found that olivines with greater than 0.10 wt % CaO may have crystallized as shallow intrusives or as extrusives, and those with less than 0.10 wt%

CaO represented a deep-seated origin. Simkin and Smith's (1970) data, however, revealed that there is some overlap between these two groups of rocks and that 0.10 wt% CaO is not an infallible separator of plutonic and near-surface ol ivines. CaO content of olivine from the

Harrington Peak Quadrangle trachyte ranges from 0.08 wt % to 0.21 wt %.

Stormer (1973) stated that ol ivines from relatively siliceous lavas (such as Hawaiian tholeiites) show little or no CaO enrichment from core to rim and that decreasing or constant content may indicate crystallization of olivine by cooling at a constant pressure.

As Table 2 indicates, the olivine generally has little or no enrichment of CaO from core to rim. This indicates that the olivine cooled at depth at a constant pressure. Table 2. Representative electron microprobe analyses of olivine. scV-1 stV-3 scv-3 sCV-8 scV-8 scv -11 scv-11 scv-16 sCV-17 scV-17 scV-18 SCV-18 MPC MPC MPR MPC MPR MPC MPR MPC MPC MPR MPC MPR

Si02 40.20 40.02 39.42 40.43 40.60 41.09 39.70 40.09 40.89 39.78 39.95 39.50 FeO 17.74 15.97 19.82 15.07 15.21 11.37 20.16 18.86 14.63 20.02 17.92 20.66 MnO o. 31 0.27 0.36 0.37 0.37 0.17 0.35 0.34 0.24 0.38 0. 31 0.38 MgO 43.19 44.18 40.98 44.64 44.48 48.15 40.91 42.50 45.82 41.43 43.02 40.72 CaO 0.16 0.14 0.14 0.09 0.08 0.14 0.21 0.16 0.15 0.17 0.15 0.14 Al 2o3 ------0.06 Total TOT:OO" 10().5'"8" Til1f.72" T!l'lf.blY TO"lJ.7il m TIJT:"1"9" TOT.95 TilT:71 T1lT:7S m TITI:4D Number of ions on the basis of 4 Si 1.005 1.003 1.005 1.008 1.011 1.004 1.007 1.004 1.005 1.004 1.002 1.004 ~1g 1.609 1.650 1.557 1.659 1.651 1. 753 1.546 1.586 1.679 1.558 1.609 1.542 Fe2 o. 371 0.335 0.422 0.314 0.317 0. 232 0.428 0.395 o. 301 0.422 0.376 0.439 Mn 0.007 0.006 0.008 0.008 0.008 0.004 0.008 0.007 0.005 0.008 0.007 0.008 Ca 0.004 0.004 0.004 0.002 0.002 0.004 0.006 0.004 0. 004 0.005 0.004 0.004 Sum VI 1.99 1.99 1.99 1.98 1.98 1.99 1.98 1.99 1.99 1.99 1.99 1.99 Molecular percent endmembers Fo 81.08 82.59 78.51 83.97 83.82 88.15 78.11 74.17 79.97 72.30 75.08 71.06 Fa 18.70 17.21 21.30 15.91 16.08 11.68 21.60 26.75 20.75 28.39 25.41 29.30 La 0.23 0.20 0.19 0.12 0.11 0.18 0.29 0.25 0.25 0.26 0.23 0.21 MPC = m1crophenocryst core, MPR = m1crophenocryst r1m.

..... "" 19

SCV83 -1 ~------?

SCV83-3

SCV83-8 u

SCV83 -11

c SCV83-16 L-.__ -?

c SCV83-17

SCV83-18

Fo lr-----~~~------~~.------~.~~------~~±'------~1 Fa Molecular Percent

Figure 6. Electron microprobe analyses of olivine plotted in terms of molecular percent Fo and Fa. Dashed lines where i nferred . c ; core, r = rim of olivine microphenocryst. 20

Pyroxene

Diopsidic augite is present in all samples both as

microphenocrysts and as a groundmass phase. Augite microphenocrysts

are pale yellow green in thin section, are anhedral to euhedral, and

range in size from 0.20 to 2.0 mm in diameter. Cummulophyric

microphenocrysts are prevalent, are usually zoned and/or twinned, and

often have inclusions of magnetite. Groundmass augite is abundant,

occurring in diffuse laths averaging 0.16 mm in length or as pale

green needles randomly distributed with feldspar. Representative

microprobe analyses are given in Table 3. Zoning in the samples is

limited, with the augite plotting close to the Di-Hd join. The

limited zoning indicates that the microphenocrysts had sufficient time

to reequilibrate prior to solidification. There is no apparent

chemical difference between the groundmass augite and the augite

microphenocrysts. Figure 7 illustrates the augite compositions

analyzed.

The CaO content of the averages 21.5 wt %, with 1 imited

variation from 20.7 wt% to 22.0 wt %. The FeO and MgO contents of

the pyroxene also have a narrow composition r ange from 5.3 wt% to 7.0

wt% with an average of 5.9 wt% and from 16.1 wt% to 17.9 wt% with

an average of 16.9 wt %, respectively. Minor elements (Ti, Mn, and Na)

show little variation in the analyzed samples, however the Ti02 content

is slightly higher in groundmass pyroxene than in t h e microphenocrysts. Table 3 . Average electron microprobe anal yses of py roxene .

SCV83- SCV83- SCV83- SCV83- SCV83- SCV83- SCV83- SCV83- SCV83- SCV83- SCV83- lG lMPC 3MP 8MP llMP 16MP 16N 17MP 17G !8MP !8G

SiO 51.79 52.89 5!.23 50.70 5!.80 51.59 49.88 5!. 56 5!.10 50.92 50.44 Algg3 2.67 2.05 2.86 3.43 2. 51 2. 63 3.90 2. 74 2.89 3.11 2. 74 Ti 2 o. 76 0.58 0. 80 0.93 0.84 0. 93 1.41 0.75 0. 92 0.95 0.86 MgO 16.95 17.89 16 . 64 !6. 12 17 . 24 17.30 16.25 17.10 17.01 16.67 16 . 34 FeO* 5. 74 5.65 5.81 6. 15 5.30 5. 37 7.00 5. 74 6.84 5. 95 5. 77 MnO 0.12 0.15 0.13 0.13 0.11 0.13 0.14 0. 13 0. 15 0.13 0.!3 CaO 21.57 20.72 21.73 21.95 21.74 21.70 20.79 2!.80 21.02 21.85 2!.65 Na 2o 0.16 0.22 0.17 0.18 0.18 0.18 0.24 0.18 0 . !8 0 . 19 0.16 Total ~ TI!lJ.T5" "99.17 ~ ~ ~ "W,bf TlllJ.OO TlllJ.IT w.-77 ~

Number of ions on the basis of six oxygens

Si 1.908 1.933 !.898 !.881 !.907 !.899 1.854 1.898 1.886 1.883 !.896 AllY 0.092 0.067 0. 102 0.119 0. 093 0.101 0.147 0.102 0.114 0.118 0.104 AlVI 0.024 0. 021 0. 023 0.028 0.016 0.013 0.024 0.017 0 . 012 0. 018 0.017 Ti 0.021 0.016 0.022 0.026 0.023 0.026 0.039 0.021 0. 026 0.027 0.024 Fe+2 0.177 0 .!73 0.180 0.!92 0. !63 0.!65 0 . 217 0.177 0.211 0. !84 0.181 Mn 0.004 0.005 0.004 0.004 0.004 0.004 0.004 0.004 0. 005 0 . 004 0.004 Mg 0.931 0.975 0.919 0.892 0.946 0 . 949 0.900 0.938 0 . 936 0 . 919 0.915 Ca 0.851 0.811 0.863 0.873 0.857 0.856 0.828 0.860 0.931 0.866 0 . 872 Na 0.012 0.015 0.0!2 0.013 0.013 0.013 0.018 0 . 013 0.013 0.013 0.012

Sum VI 2.02 2.02 2.02 2.03 2.02 2.03 2.03 2.03 2.13 2. 03 2.03 Molecular percent endmembers \lo 43 . 45 41.43 43.97 44 . 64 43 . 60 43. 43 42 . 56 43.54 42.02 43.98 44.29 En 47.52 49.76 46.85 45.61 48.11 48.17 46.28 47.51 47.30 46.67 46.49 Fs 9.03 8.81 9.18 9. 76 8. 29 8.40 11.17 8. 95 10.67 9. 35 9. 22

N G = groundffiass, MPC- r.ncrophenocryst cluster, MP = m1crophenocryst, N- needles (groundffiass) *Total iron reported as FeO

Figure 7. ~lectron microprobe analyses of pyroxene plotted in terms of molecular percent Di, En and Wo. Fields as defined by Deer and others {1966}. mp; microphenocryst, c ; microphenocryst cluster, g ; ground­ mass, gn ; groundmass needles. 23 24

Aluminum content in cl inopyroxenes may be used to determine the host type (Kushiro, 1960; Le Bas, 1962). The three host magma types are designated as: 1) nonalkaline including tholeiitic, high alumina, and calc-alkaline; 2) normal alkaline; and 3) peralkaline and are displayed on a silica versus alumina plot (Le Bas, 1962). Figure 8 shows that most analyzed pyroxenes from this study are clearly in the non-alkaline field. The exception is the groundmass needles in sample

SCV83-16 which plot in the normal alkaline field.

Kushiro (1960) demonstrated that clinopyroxene crystallizing from tholeiitic magma which is oversaturated with Si02 has a higher proportion of Si and a lower proportion of Al in the tetrahedral sites

(Z). The analyzed pyroxenes from the trachyte range from 3 to 7 percentAl in the Z sites (averaging 5 percent). Figure 9 shows wt 't

Ti02 versus Alz. Again, most of the samples analyzed plot in the nonal kal ine field as designated by Le Bas (1962). Le Bas (1962) stated that clinopyroxenes from nonalkaline rock types commonly have less than

1.0 wt 't Ti02, whereas cl inopyroxenes from alkaline rocks have greater than 1.0 wt 't Ti 02· All of the pyroxenes in this study, both in the groundmass and as microphenocrysts, have Ti02 of 0.95 wt 'tor less,

(except sample SCV83-16 groundmass needles - 1.4 wt 't, just barely in the normal alkaline field).

Sanidine

Sanidine is by far the most abundant feldspar present (43-57%) and is a major groundmass constituent. Sanidine is generally very fine grained, but a few crystals are as large as 0.45 mm in length. 25

53 elmpc

Nonalkaline

52

11"'ftre ••• ...... e17mp

e3rnp .. 51 •17• 0 ellrnp el mp "' / .u, / ~ / ... / / ~ 50 / / e16n / / / / Normal- alkaline 49

48·+------r------~------~----~ 2 4 5

Figure 8. Weight percent silica versus weight percent alumina in pyroxene . mpc = microphenocryst cluster, mp = micro­ , g = groundmass, n = needles (in groundmass) . Dashed line separates fields defined by Le Bas (1962). 26

8 ' Normal- alkaline ' ' ' ...... 7 ' ' ' ' ' ' ' 6 ' ' ' '

17mp• •ll 1 5 3mp• e16mp

4 Nonalkallne

3+------r------,------~------, 0 I 1

WT % Ti02

Figure 9. Alz versus weight percent Ti02 in pyroxene . mpc ; micro­ phenocryst cluster, mp; microphenocryst, g; groundmass, n; needles (in groundmass). Dashed line separates fields defined by Le Bas (1962) . 27

Sanidine is found as subhedral laths in samples SCV83-1, 3, 8, and 11 and/or as anhedral crystals intergrown to form a tight mesh in samples SC V83-3, 16, 17, 18. The largest outcrop area display euhedral sanidine 1 aths in the groundmass, whereas farther from the implied source area the sanidine is more anhedral due to rapid cooling. The sanidine microl ites are often poikil itic with included pyroxene needles and usually display Carlsbad twinning. Average compositions of the analyzed sanidines are presented in Table 4. Zoning trends are plotted in Figure 10. Because of the substantial amounts of barium in

the samples (up to 4.21 wt %; 1.3 wt % average), eel sian is added to on the Ab-An-Or plot. The groundmass sanidine in most of

the samples is very fine grained, making good microprobe analyses difficult. Some of the sanidine plots on possible mixing lines between plagioclase and sanidine, possibly representing fine intergrowths of plagioclase and sanidine (Allan and Carmichael, 1984). These plots are outside the approximate 1 imit of sol uti on for trachytic feldspar as defined by Tuttle and Bowen (1958). Barium-rich sanidine is fairly common in alkaline rocks such as the Leucite Hills, Wyoming (Carmichael, 1967b); Highwood Mountains, Montana (Larsen, 1941); Shonkin Sag Laccolith, Montana (Nash and Wilkinson, 1971); Yellowstone region, Wyoming (Nicholls and Carmichael, 1969); and Roman Volcanic Region, Italy (Cundari, 1979) ; Barium can replace K, Ca, and Na in feldspars, but most commonly K. Feldspars with BaD contents greater than 2 wt % are considered barium varieties (Deer and others, 1966). The element pair K-Ba have similar ionic radii, electronegativities, and ionization potentials (Table 5) and Table 4. Representative electron microprobe anal yses of feldspar san1 1ne ase lor lab 11o r 11ab 16or 16ab 17or 17ab 18or 18ab 17ab Si 0 64.45 66.65 63 .81 58.97 64 . 71 64.81 64.86 64.94 63.29 63 . 26 51.56 52.91 Al 263 19.31 16.42 19.30 23.39 19.14 19.38 19.46 19.13 19 . 40 19.60 31.26 30 . 38 Fe o3 0.20 0.36 0.42 0.52 0.47 0.21 0.23 0. 22 0.21 0.42 ------Ca 6 0.84 3.02 1.07 4.69 0. 97 1.20 0.96 1.25 0.86 2.02 13.4 7 12.63 ~a o 3.12 4. 59 3. 51 3. 72 3.14 3.94 3. 55 . 4.19 3.06 3.62 3.55 4.14 6 11.21 7.38 10.49 6.06 10.62 10.21 10.73 9.85 11.22 9.86 0.70 0.40 a&o 0.62 1. 29 1.59 4. 21 1.60 0.07 0 . 69 1.19 0. 67 1.04 0.28 0.24

Total ~ '9'9:7lY TmJ.T9' 11l'r."'51 TO'O":Oif '99.lltl" Til'0":4l) Til'D.70 mr:7! '99-M TO'lJ."S2' Til'O":IlJ Number of ions on the basis of thirty two oxygens Si 11.813 12.144 11.730 10 .844 11.814 11.800 11.795 11.801 11.746 11.633 9. 331 9. 543 AllV 4.172 3. 526 4.181 5.069 4.119 4.159 4.171 4.097 4.244 4.248 6.668 6.458 Fe+3 0. 028 0 . 049 0.058 0.072 0.065 0.029 0.032 0.030 0.029 0.058 ------Ca 0. 165 0.590 0.211 0.924 0.234 0.234 0.187 0.243 0.171 0.398 2. 612 2.441 · 8a 0.045 0.092 0.115 0.303 0.005 0.005 0.049 0.085 0.049 0.075 0.020 0.017 Na 1.109 1.622 1. 251 1.326 1.391 1.391 1.252 1.476 1.011 1. 291 1.246 1.448 K 2.621 1. 715 2.460 1.422 2.371 2.371 2.489 2.283 2.656 2. 313 0.162 0.092 SumlV 16.01 15.72 15.97 15.98 16.00 15 .99 16.00 15.93 16.02 15.94 16.00 16'.00 SumVI 3.94 4.02 4.04 3.98 4.00 4.00 3.98 4.09 3.89 4.08 3.94 4 .oo Molecular percent endmembers An 4. 19 14.67 5.22 23.24 4. 88 5.85 4.70 5.95 4.30 9.76 64.66 61.02 Ab 28 . 14 40 . 35 30 . 99 33.36 28.58 34.76 31.47 36.11 27.68 31.66 30 . 84 36.21 Or 66 . 54 42.69 60.95 35.76 63.60 59.27 62.59 55.86 66.79 56.74 3.99 2.33 Cs 1.13 2.29 2.84 7.63 2.94 0.12 1.24 2.07 1.22 1.84 0.51 0.43

or= most or-r1ch, ab = most ab-r1ch N co Ao

.· ..... "

•:·.. .,

---"----'-'----'-'--"---'<--...:.<-~C... (I

Figure 10. Electron microprobe analyses of feldspar plotted in terms of molecular percent An, Ab and Or. Solid curve shows 1 imits of ternary for feldspars in trachytic (Tuttle and Bowen, 1958, Fig . 64, p. 132). "' 30

Table 5. Comparison of Ba and K ions.

charge r e

Ba +2 1.34 0.96 5. 21

K +1 1.36 0.80 4.34

r = ionic radius (A) (Shannon and Prewitt, 1970) e = el ectronegati vi ty (Pauling, 1960) I =first ionization potential (kcal/mol)(Moore, 1958) 31

there fo re are incorporated into K-rich- (Imeokparia, 1981).

Since K and Ba have different c harges, a coupled substitution is

required withAl replacing Si in the tetrahedral sites. K-feldspars

may accomodate up to 15% Ba replacing K without appreciable structural

alteration (Heier and Taylor, 1959). There is a preference for barium

to rema i n in the liquid rather than entering the plagioclase feldspa r

structure, but to enter the alkali feldspar struc ture rather than the

melt ( Smith, 1974). Barium is enriched in magmas during

differentiation, rising in concentration as the rocks become more

acidic, but it rapidly depletes very late in the differentiation

sequence toward the extreme acid end when potassium increases (Nockolds

and All en , 1953; Nash and others, 1969; El Bouseily and El Sokkary, 1975) .

Plagioclase

O~y one sample contained abundant plagioclase in the groundma ss

(SCV83-17) and microprobe analyses were conducted on this sample. This

sample has a trachytic texture of lath-shaped feldspar, but the groundmass is too fine-grained to clearly distinguish the plagioclase from the alkali feldspar petrographically. The composition of the plagioclase ranges from An 70 to An 59• As Figure 10 demonstrates, the plagioclase has limited zoning, nearly every spot analysis plots within the field. Plagioclase analyses are given in Table 4 for the most An-ric h and the most Ab - rich point analyses.

Plagioclase was not found as a phenocryst phase; however five other samples had a trace amount of groundmass plagioclase with laths 32

up to 0.35 mm in length. These samples are SCV83-3, 4, 5, 6, and 12,

all of whi c h were found in the largest outcrop area at variable

distances from the possible source.

Fe-Ti Oxides

Magnetite is ubiquitous (7-31%), disseminated throughout the

groundmass of each sample. At least 50% of the magnetite grains

present are ex solved. The magnetite grains are either anhedral or

euhedral, averaging 0.02 mm in diameter. Microprobe analyses are

presented in Table 6 for three samples (SCV83-3, 8, and 17) that

contained apparent homogenous magnetite. Lath-shaped ilmenite is

present but rare, and always ex solved and therefore was not

analyzed. A spinel phase occurs as chromite inclusions in olivine

microphenocrysts in every sample. The chromite is usually anhedral,

although a few tiny cubes were observed. The chromite is extremely

fine-grained and generally homogenous. Microprobe analyses for two

samples (SCV83-8 and 17) are presented in Table 6. are

ideally a solid sol uti on between FeCr204 and MgCr 2o4. but most terrestrial chromites also contain appreciable amounts of other

metallic cations (Lindsley, 1976). Haggerty (1976) stated that

chromium-bearing spinel which occurs as inclusions in crystals is

generally homogenous. Cores of decompose only under extreme

oxidizing conditions and decomposition is restricted to the oxidation of Fe2+ to give a "lattice-type" ex sol uti on texture. This texture was not apparent in any of the samples analyzed.

Molecular endmembers for the magnetite-ulvospinel (B) solid 33

Table 6. Average electron microprobe analyses of iron-titanium oxides.

Magnet1 te Chrom1 te SC VB3- SCVB3- SCV83- SCV83- SCVB3- 3 8 17 8 17

Si02 0.45 o. 36 o. 27 0. 33 0.30 TiO 8.66 2.55 16.65 0.40 7.36 A1 203 1.43 1.67 1.89 7.43 4.52 cr o3 0.16 0.68 0.10 24.96 12.05 Fe 0 82.92 85.78 73.38 58.94 69.31 MnO 0.27 0.33 0.43 o. 54 0.46 NiO 0.02 0.04 0.00 0.28 0.05 MgO 1.26 1.44 2.21 6.22 3.11

Sum ""9'57I7 "9"2.!l5' ~ "9"9.1U "9"T.T6"

Fe o3 50.18 60.59 33.97 38.75 38.92 Fe 0 37.77 31.26 42.86 24.07 34.29 Total 100.20 98.92 98.38 102.98 101.06

Molecular fraction

MT • 7395 .9136 • 5211 USP .2605 .0865 .4789 34

soluti on series were calculated according to the method given by

Carmi c hael {1967a). As Table 6 indicates, the analyzed magnetites are

highly variable, with compositions of USP9, USP26 and USP48. All

three of these samples were collected along the outer perimeter of the

largest outcrop unit {Figure 2), but due to the uncertainty of the

homogeneity of the magnetites analyzed, no definitive conclusions can

be made.

Phl ogopi te

Phlogopite is found in the groundmass in most of the samples as

light brown, pleochroic flakes about 0.08 mm in diameter. Phlogopite

is not seen in SCV83-1, is present in trace abounts in SCV83-3, 11, 16, and 17 , and is only slightly more abundant in samples SCV83-8 and 18. The presence of phlogopite in the groundmass shows that the

concentrati on of {H 2o+ F) was sufficient upon eruption to stabilize

phlogopite. The presence of F and H2o increases the thermal stability

of phlogopite {Carmichael and others, 1974). In the system Mg 2sio4- KA1Sio4-sio2-H20, there is an invarient point where leucite, sanidine, phlogopite, and liquid can coexist {Carmichael and others,

1974). With the early crystallization of ferromagnesian minerals as

microphenocrysts, the concentration of {H 2o +F) could increase causing the phlogopite and sanidine fields to expand and the leucite field to

reduce. This would allow sanidine and phlogopite to be in equilibrium

{Carmichael and others, 1974), as is the case with the Harrington Peak olivine trachyte. 35

CHEMISTRY AND CLASSIFICATION

Introduction

The chemical analyses and the CIPW norms are presented in Table

~ Wet chemical analyses were also run on a basalt standard ARHC0-1

as an experimental accuracy check. Results are sho'wn in Appendix B

along with a previously determined chemical analysis.

Cl assi fyi ng the Harrington Peak Quadrangle 1 a vas is difficult due

to their unique chemistry. Chemical analyses show these rocks to have

a moderate amount of Si02 and Al203, high MgO and CaD, and K2cr in

excess over Na2o (approximately 2: 1). Mineralogical characteristics include microphenocrysts of Mg-rich olivine and diopsidic augite in a

groundmass of Ba-rich sanidine, diopsidic augite, Fe-Ti oxides, a few

phlogopite and/or plagioclase grains.

Naming the Harrington Peak Quadrangle Lava

Different classification schemes yield different names for the

Harrington Peak Quadrangle 1 ava. Baker and others (1974) system

classifies the lava as (DI 45-65, normative plagioclase less

than An3 0 ). Their system utilized Thornton and Tuttle's (1960) differentiation index and normative plagioclase content for

rocks. are typically oligoclase (Williams and others, 1982), mineralogically different than the 1 avas in this study.

Streckeisen's (1979) classification places the Harrington Peak

Quadrangle lavas in the trachyte or alkali trachyte field, which is 36

Table 7. Whole rock wet chemical anal yses and CIPW norms*. Sample SCV B3- SCVB3- SCV83- SCV83- SCV83- SCV83- SCV83- 1 3 8 11 16 17 18

Sio2 54.77 53.90 53.73 54.72 55.64 53.86 55.18 TiD 1.42 1. 20 1.24 1.44 1. 37 1.32 1. 27 Al 203 14. 82 14.43 14.25 14.63 14.59 14.39 14.60 Fe o3 2. 82 3.05 3.52 4. 80 3.35 3.12 3.23 Fe 0 3.45 3.64 3.21 1. 93 2.70 3.60 2. 89 MnO 0.10 0.09 0.13 0.09 0.09 0.09 0.08 MgO 6.31 7.07 7.45 6.00 5.33 7.22 6.61 CaD 6.48 7.38 7. 56 7.09 6. 57 7.30 6.75 Na o 2. 30 2. 36 2.34 2.21 2.26 2.32 2.28 K2 0 4. 85 4.01 4.04 3.80 5.26 4.16 4.86 P205 0.68 0.64 0.63 o. 76 o. 72 0.66 0.69 H20+ 1.02 0.90 1.09 1.01 0.86 0.69 1.16 H20- 0.24 0. 66 0.24 0. 28 0.37 0.31 0. 14 BaD** 0.42 1.33 0.39 0.38 0.42

Total ~ w.n "99.U ~ "99.50" ~ TilQ.Tij q 3. 19 2.55 2. 25 7.80 5.30 2.29 3.60 or 28.66 23.70 23.87 22.46 31.08 24.58 28.72 pl 35.25 36.91 36. 25 37.48 33.26 36.20 34.54 ( abl 19.46 19.97 19.80 18.70 19.12 19.63 19.29 (an) 15.79 16.94 16 .45 18.78 19.12 19.63 19 . 29 di 9.38 12.22 13.28 8.90 10.72 12.08 10.73 (wo) 4.97 6.47 7.07 4.77 5.74 6.40 5.73 (en) 3.95 5.08 5. 83 4.12 4. 94 5.10 4.80 ( fs) 0. 46 0.68 0.37 0.04 0.57 0.20 mt 4. 09 4.42 5. 10 2.34 4.86 4.52 4.68 i 1 2.70 2.28 2.36 2.73 2. 60 2. 51 2.41 hm 3.19 ap 1.61 1.52 1.49 1.80 1.71 1.56 1.63 Total 98.03 97.80 98.13 97 . 51 97.91 98.07 98.47 Sal ic 67.10 63.16 62.37 67.73 69.64 63.07 66.86 Fernie 30 . 93 34.64 35.76 29 . 78 28.28 35.00 31.61 Differentiation Index 51. 31 46.22 45.92 48.95 55.50 46.51 51.61

* All values in weight percent. **Values estimated from microprobe data and modal percentages of feldpars. 37

much more appropriate than mugearite. As described by William s and

others (1982), trachytes are vol c ani c rocks close to saturation in

silica (58-66% Si0 2, generally). Trachytes are light colored and usually porphyritic with of feldspar in a groundmass of

feldspar. The rock has typically greater than 80% K-feldspar (sodic

sanidine or anorthoclase) in the mode. Trachytes sometimes contain a

1 ittle plagioclase (oligocl a se or andesine) and may also contain

fayalitic olivine, clinopyroxene, and/or biotite. Magnesian

olivine is very rare or is xenocrystic (Williams and others, 1982).

This describes the Harrington Peak Quadrangle lava fairly well except

the lava studied does not have phenoc r ysts of feldspar; · modal K­

feldspar is under 60 wt %; plagioclase, when present, is labradorite; and the olivine is definitely forsteritic and does not appear to be

xenocrystic. Petrographically, the Harrington Peak Quadrangle lava

could be called ortho-trachyte (Hatch and others, 1973) , having

neither quartz nor feldspathoids. The CIPW norm for these rocks,

however, indicates that the Harrington Peak Quadrangle lava is

oversaturated with respect to Si02. Because the samples analyzed are

slightly oxidized, Fe0/Fe2o3 values were adjusted to a constant ratio, and the norm recalculated. The results did not affect the silica

saturation appreciably; the samples are still oversaturated.

Washington (1896, 1897) described a volcanic rock from the Viterbo

Region in Italy, formerly called trachyte, which he named ciminite

(pronounced "chiminite"). These rocks were described as porphyritic with phenocrysts of augite, oliv i ne, and sometimes feldspar, in a groundmass of alkali feldspar, with subordinate basic plagioclase, 38

aug i te and accessories of magnetite and apatite. Ciminites are

c ha rac ter ized by sili c a betwee n 54 wt 't and 57 wt 't , moderately high

alumina and iron oxides, high MgO and CaO , and K20 in excess over Na20. Washington (1896) thought that c iminite could be compared with the

absarokite-banakite series of Yellowstone, Wyoming. Figure 11, a plot

of Fe2o3 + FeO versus MgO , shows the Harri ngton Peak Quadrangle 1 avas to be c lose to the 1 ine for the !tal ian c iminite. Using Johannsen's

(1937) classification for igneous rocks, the volcanic rock in thi s

study would also be classified as ciminite. Joplin (1968), in her

review of the association, grouped ciminite with sanidine

absarokite and olivine trachyte. Since the name ciminite has become

obsolete, one of the latter two names is preferable. Although the

min eralogy of the Harrington Peak Quadrangle lavas is similar to

absarokite, the ab sarokites are generally quite mafi c rocks (Nicholls

and Carmichael, 1969). Therefore , the name olivine trachyte is

proposed to be the most suitable name for the Harrington Peak

Quadrangle l avas .

· Comparison With Other Alkaline Igneous Rocks

Petrographically, the Harrington Peak Quadrangle lava resembles a

fine grained shonkinite (Puchy, 1981). Shonkinite is an intrusive

found in the Highwood Mountains, Montana (Larsen, 1941),

Shonkin Sag Lacol ith, Montana (Nash and Wilkinson, 1971), and in

southeast Idaho (Anderson and Kirkham, 1931). Shonkinite is described as having phenocrysts of augite and olivine in a groundmass of Ba-rich

sanidine, augite and magnetite, sometimes having biotite, phlogopite, 39

12

"

10

0 Cll' :E

Figure 11. Weight percent Fe o versus MgO. Italy= Italian ciminites; CA A= 2c~lcalkaline association; ABA= association; TA =tholeiite association; Yellowstone= shoshonite association (Joplin, 1968). 40 pseudoleucite, analcite, apatite, aegerine, and . These intrusives characteristically have K2o in abundance over Na 2o, occur as breccia, dikes and stocks, and are in areas dominated by crustal faulting. They are thought to be of mantle-peridotite origin (Hyndman, 1985).

Another type of igneous rock having some similarity to the Harrington Peak Quadrangle lavas is orendite, which is found in the Leucite Hills, Wyoming (Carmichael, 1967b). Carmichael (1967b) considered the orenditic magma to be of mantle origin. Vollmer and others (1984) compared the Leucite Hills lavas (niadupite, wyomingite, and orendite) to other high potasium volcanics. They found the Leucite Hills volcanics to have negative eNd (neodymium) values which indicate that these magmas are derived from, or contain a contribution from an old source enriched in the ( normalized) LREE (light rare elements) relative to the HREE (heavy rare earth elements). Vollmer and others (1984) stated that negative tNd signatures are common with all high potassium volcanic rocks that have been investigated by them so far. Wyomingite and orendite are strongly enriched in LREE. Vollmer and others (1984) determined that wyomingite and orendite can be derived from a common magma under differing cooling conditions, but madupite has a separate heterogenous source. in other areas have been found to have an orenditfc affinity. Chemical characteristics of orenditic rocks include Ba, Ti, and P enrichment, high K and Mg, moderate Si, relatively low Al and Na, and K20 in excess over Na20. Mineralogical compositions may include forsteritic olivine, phlogopite, diopside, leucite (analcime), 41

sanidine, magnophorite, sometimes quartz, and may contain orthopyroxene instead of olivine (Sahama, 1974). Figures 12 and 13 demonstrate that the Harrington Peak Quadrangle olivine trachyte has an orenditic affinity as do the !tal ian ciminites. Figure 14 of total iron plotted against MgO shows that the olivine trachyte and the ciminite plot between kamafugitic and orenditic rocks. Chemical analyses for the ciminites are from Washington (1897) and are tabulated in Appendix C. 42

20

Kamafugitic S. E. Spain ' '< 15 ' .... \ \ 0 \ c:lll ' \ ::E 10 ' \ ' ' \ ~ ' \ ... Leuc i te Hills ' .. \ ~ ' ,. \ ' ... :-· \ 5 . ' ' ' ' • -~~ ...... '.• • Fitzroy Basin I ----- Orenditic

35 40 45 50 55 60

WT % Si02

Figure 12 . Weight percent MgO versus seight percent Si02 for olivine trachyte (solid circles) and Italian ciminite (stars) . (Ciminite data from Washington (1897), see Appendix C. Dashed lines separate fields for other areas for comparison (Sahama, 1974) . 43

20 Kama fug it i c

15

0 c v 10 Orenditic • • • • ') .., 5 •

35 40 45 55 60 WT

Figure 13. Weight percent CaD versus weight percent SiD for olivine trachyte (solid circles) and Italian ciminite2 (stars) . (Cimin ite data from Washington (1897), see Appendi x C, figure from Sahama (1974)).

~ 15 .."'0 Kama fug it i c

~ 10 0 •• •• 0 ...... ~ 5 •

Orenditic

5 10 15 20 WT % MgO Figure 14 . Weight percent FeD versus weight percent MgD for olivine trachyte (solid circles) and Italian ciminite (stars). (Ciminite data from Washington (1897), see Appendix C, figure from Sa ham a ( 1974)) . 44

PETROGENESIS

Temperatures of Crystallization

The presence of coexisting plagioclase and sanidine in sample

SCV83-17 allows the use of a two-feldspar geothermometer. Brown and

Parsons (1981) presented what they called a correct general form of the

two-feldspar geothermometer, but stated that the accuracy may be

questioned. This geothermometer is presented in Figure 15 for a

pressure of 1 kbar (can be used at a higher pressure by adding 180

C/kbar). All of the isotherms must become tangential to the KD=1 line

(where feldspar pairs come to have the same composition) for

compositions poor in Ab, as the tie 1 ines on an An-Ab-Or plot become

parallel to the An-Or join. The compositions of the plagioclase

members of coexisting pairs must first become richer in Ab before

moving toward the An-Or join. Two feldspar pairs from SCV83-17 are

plotted on Figure 15. One point is for the most Ab-rich

plagioclase/Ab-rich sanidine pair and the other for the most An-rich

plagioclase/Or-rich sanidine pair. Both plot close to, but outside,

the KD=1 line. Brown and Parsons (1981) clearly stated that the

feldspar pairs must be in equilibrium, and that this is difficult to

determine. Brown and Parsons (1981) stated that it is not uncommon for

trachytes to have feldspar pairs giving temperatures above 9100 C.

There are three possible causes for this: 1) this represents feldspar

pairs which grew in equilibrium at low pressure, the feldspar solvus must flat ten with respect to temperature above 910° C, otherwise one 45

z

N Ab.AI

Figure 15. Two-felds pa r geothermometer. sol i d circle -most Ab­ rich pair; open circle - most An-rich pair. (from Brown and Parsons, 1981). 46 would reach temperatures above the melting point of the alkali feldspar phase ; 2) this represents feldspar pairs which reached equilibrium at high pressures in an H20-deficient melt; or 3) this represents non­ equilibrium pairs and can not be used to calculalate temperatures

(Brown and Parsons, 1981). Considering the interlocking texture of the alkali feldspar and plagioclase and the limited zoning of the plagioclase, it would appear that the feldspar pairs are in equilibrium. However, Figure 8 shows that some of the sanidines plot on possible mixing lines between plagioclase and sanidine so the sanidine analysis may not be "pure". Therefore, no temperature can be determined using this geothermometer for the Harrington Peak olivine trachyte.

Powell and Powell ( 1974) derived a gee thermometer utilizing the presence of coexisting groundmass clinopyroxene and olivine. The following iron-magnesium exchange reaction between olivine and calcium­ rich clinopyroxene is the basis for this geothermometer:

Di Fa Hd Fo

2CaMgSi 2o6 + Fe2Si04 2CaFeSi 2o6 + Mg2Si04

Unfortunately, the samples in this study had no olivine in the groundmass. Although Powell and Powell (1974) stated that olivine and clinopyroxene phenocrysts are not to be used with the geothermometer because they may have crystallized at several kilobars, they also stated that homogenous olivine and clinopyroxene could reflect equilibration at or just below the solidus. The general lack of zoning 47

shown by mi croprobe analyses of the augi tes and ol ivines indicates that the microphenocrysts had time to equilibrate prior to solidification (negl ec ting the possible effect of pressure); therefore the geothermometer was applied lvith the results presented in Table 8. The calculated temperatures range from 1006° C for sample SCV83-17 to 1046° C for sample SC V83-16 . Comparing only groundmass pyroxene with olivine mi cro phenocryst rims, temperatures of 1007° C and 1012° C were calculated. Temperatures of coexisting olivine-clinopyroxene for the olivine trachyte was calculated by Puchy (1981) on two samples, with values of 1021° C to 1002° C.

Partial Melting Hypothesis

Most of the recent studies on the origin of potassic magmas have concluded either that they are products of very small amounts of partial melting of an upper-mantle peridotite or that they are products of larger amounts of partial melting of an which has been metasomatic~ly enriched in the incompatible elements and volatiles (Cox and others, 1976; Gupta and Yagi, 1980; Allan and Carmichael, 1984). Appleton (1972) and Miyashiro (1978) felt that potassium-rich lavas were produced from partial melting of a phlogopite-rich mantle, with the phlogopite in the upper mantle containing the potassium, rubidium, and barium. To derive the Harrington Peak Quadrangle olivine trachyte, Fiesinger and others (1982) concluded that 30% partial melting of a mica-peridotite source would be required. The mica peridotite used to represent the mantle composition came from the 48

Table 8. Olivine-clinopyroxene temperatures of crystallization.

(Powell and Powell, 1974) (T° C at 1 bar)

MPC MPR GR

SCV83-1 1008 SCV83-3 1016 1008 SCV83-8 1020 1019 SCV83-11 1026 SCV83-16 1046 SCV83-17 1019 1006 1012 SCV83-18 1013 1008 1007 MP = m1crophenocryst (CPX), G = groundmass (CPX), C = core (all­ vine microphenocryst), R = rim (olivine microphenocryst) 49

western e nd of the Uinta Arch east of Salt Lake City, Utah (Best and

others, 1968).

Regional Tectonics: Petrologic Implications

Rowell and Edgar (1983) demonstrated a spatial and temporal

relationship for Cenozoic potassi urn-rich . They attributed

this relationship to -related magmatism, utilizing the

relationship observed between increasing depth to the seismic zone and

increased K20 content of 1 avas (c.f. Dickinson and Hatherton (1967) and

Snyder and others (1976)). Interpreting the Harrington Peak Quadrangle olivine trachyte in the time- space relationship observed by Rowell and

Edgar (1983) suggests that the trachyte should be Eocene in age. This is in a contradiction with its apparent stratigraphic age of Pliocene

(see Previous Work and Field Relationships).

As an alternative to paleosubduction, partial melting may be related to Basin and Range tectonics, as the olivine trachyte 1 ies in an area affected by high-angle normal faults, a characteristic of Basin and Range extension (see Field Relationships; Eaton, 1982).

Partial melting is the likely origin for the occurrence of a low velocity zone (LVZ) in the mantle (Anderson and Sammis, 1970; Lambert and Wyllie, 1970). For the central part of the Basin and Range

Province, the LVZ lies just below the , at a depth of 30 km, and is approximately 100 km thick (Archambeau and others, 1969, as cited in

Thompson and Burke, 1974). Smith (1978) showed a similar crustal thickness and similar upper-mantle seismic characteri sties for the 50 eastern Great Basin. Therefore, if the Harrington Peak Quadrangle olivine trachyte is a product of partial melting of mantle material, its origin is probably within the LVZ at a depth in excess of 30 km. 51

CONCLUSIONS

Th e Harrington Peak Quadrangle olivine trachyte is assumed to have

been emplaced during the Pliocene because it rests conformably on Salt

Lake Formation and Paleozoic boulders are intermixed with the olivine

trachyte in an area of high relief. It appears likely, based on the

physiographic location, and the structural relationships, that the

Harrington Peak Quadrangle olivine trachyte was associated with normal

faulting at the easternmost edge of Basin and Range extension .

The crystallization sequence of the olivine trachyte is as

follows : chromian spinel and mg-rich olivine were the first phases to

appear, crystallizing at depth with microphenocrysts of diopsidic

augite following; upon eruption, diopsidic augite, Ba-rich san i dine and

minor plagioclase, phlogopite, and magnetite formed. The very fine

grained texture of the groundmass indicates that the lava cooled

rapidly. The presence of stable phlogopite in the groundmass, shows

that the concentration of (H 20 +F) near surface was relatively high. The unusual chemistry of the olivine trachyte, being rich in

potassium (K 20:Na 2o approximately 2:1) and with appreciable barium in the sanidine, indicates that partial melting of a heterogenous mica peridotite mantle source enriched in K and Ba, and possi b 1 y Sr and Rb, is plausible (Gupta and Yagi, 1980; Cox and others, 1976; Allan and

Carmichael, 1984; Frey and others, 1978; Onuma and others, 1981; and

Hughes, 1982). In addition, Vollmer and others (1984) findings on high potassium volcanic rocks having negative fNd signatures, indicates the magma may hav e been derived from, or contained a contribution from, an old source enriched in the LREE relative to the HREE. 53

REFERENCE S

Additon, M. K., and Seil, R. 0., 1979, Columbia River basalt reference sample: Rockwell International, RHO-SA-93, 25 p.

Albee, A. L., and Ray, L., 1970 , Correction factors for electron probe mi croanalysis of silicates, carbonates, phosphates, and sulphates: Analytical Chemistry, v. 42, p. 1408-1414.

Allan, J. F., and Carmichael, I . S. E., 1984, Lamprophyric lavas in the Colima graben, S W Mexico : Contributions to Mineralogy and Petrology, v. 88, p. 203-216.

Anderson, A. L., and Kirkham, V. R. D., 1"931, Alkaline rocks of the Highwood type in southeastern Idaho : American Journal of Science, 5th series, v. 22, p. 51-68.

Anderson, D. L., and Sammis, C., 1970, Partial melting in the upper mantle : Physics of the Earth and Planetary Interiors, v.3, p. 41- 50.

Appleton, J. D., 1972, Petrogenesis of potassi urn-rich 1 a vas from the Roccamonfina Volcano, Roman Region, Italy: Journal of Petrology, v.

13' p 0 425-456 0

Archambeau, C. B., Flinn, E. A., and Lambert, D. G., 1969, Fine structure of the upper mantle: Journal of Geophysical Research, v. 74, p. 5825-5865.

Armstrong, R. L., Leeman, W. P., and Malde, H. E., 1975, K-Ar dating, Quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho : American Journal of Science, v. 275, p. 225-251.

Baker, P. E., Buckley, F. , and Holland, J. G., 1974, Petrology and of Easter Island: Contributions to Mineralogy and Petrology, v. 44, p. 85-100.

Bence, A. E., and Albee, A. L., 1968, Empirical correction factors for the electron microanalysis of silicates and oxides: Journal of Geology, v. 76, p. 382-403.

Best, M. G., Henage, L. F., and Adams, J. A. S., 1968, Mica peridotite, Wyomingite, and associated potassic igneous rocks in northeastern Utah : American Mineralogist, v. 53, p. 1041-1048.

Bright, R. C., 1963, Pleistocene lakes Thatcher and Bonneville, southeastern Idaho: University of Minnesota, Minneapolis, unpublished Ph.D. dissertation, 292 p. 54

, 1967, Late Pleistocene in Thatcher basin, southeastern Idaho: Tebiwa, v. 10, no. 1, p. 1-7.

Brown, W. L., and Parsons, !., 1981, Towards a more practical two­ feldspar geothermometer : Contributions to Mineralogy and Petrology, v. 76, p. 369-377.

Carmichael, I. S. E., 1967a, The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates: Contributions to Mineralogy and Petrology, v. 14, p. 36-64.

-----, 1967b, The mineralogy and petrology of the volcanic rocks from the Leucite Hills, Wyoming: Contributions to Mineralogy and Petrology, v. 15, p. 24-66.

Carmichael, I. S. E., Turner, F. J., and Verhoogan, John, 1974, Igneous petrology: McGraw-Hill, 739 p.

Cox, K. G., Hawkesworth, C. J., O'Nions, R. K., and Appleton, J. D., 1976, Isotopic evidence for the derivation of some Roman Region volcanics from anomalously enriched mantle: Contributions to Mineralogy and Petrology, v. 56, p. 173-180.

Cressman, E. R., 1964, Geology of the Georgetown Canyon-Snowdrift Mountain area, Southeastern Idaho: U. S. Geological Survey Bulletin 1153, 58 p.

Cundari, A., 1979, Petrogenesis of leucite-bearing lavas in the Roman Volcanic Region, Italy- the Sabatini lavas: Contributions to Mineralogy and Petrology, v. 70, p. 9-21.

Deer, W. A., Howie, R. A., and Zussman, J., 1966, An introduction to the rock forming minerals: Longman Group Ltd., 528 p.

Dickinson, W. R., and Hatherton, T., 1967, Andesitic volcanism and seismicity around the Pacific: Science, v. 157, p. 801-803.

Eaton, G. P., 1982, The Basin and Range Province: origin and tectonic significance: Annual Review Earth Planetary Science, v. 10, p. 409- 440.

El Bouseily, A. M., and El Sokkary, A. A., 1975, The relation between Rb, Ba, and Sr in granitic rocks: Chemical Geology, v. 16, p. 207- 219.

Fenneman, N. M. and Johnson, D. W., 1946, Physical divisions of the United States (map): U. S. Geological Survey, Scale 1:7,000,000.

Fiesinger, D. W., Perkins, W.D., and Puchy, B. J., 1982, Mineralogy and petrology of Tertiary-Quaternary volcanic rocks in Caribou County, Idaho: in Bonnichsen, B., and Breckenridge, R. M., eds., Cenozoic 55

Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 465-488.

Frey, F. A., Green, D. H., and Roy, S. D., 1978, Integrated models of basalt petrogenesis: a study of quartz tholeiites to olivine mel il ites from southeastern Australia utilizing geochemical and experimental petrological data: Journal of Petrology, v. 19, p. 463-513 .

Gupta, A. K., and Yagi, K., 1980, Petrology and genesis of leucite­ beari ng rocks : Springer-Verlag, 252 p.

Haggerty, S. E., 1976, Oxidation of opaque mineral oxides in basalts: in Rumble, D., ed., Oxide minerals: Mineralogical Society of America Short Course Notes, v. 3, p. Hg74-75.

Hatch, F. H., Wells, A. K., and Wells, M. K., 1973, Petrology of the Igneous Rocks: Macmillan Publishing Company, p. 290-393.

Heier, K. S., and Taylor, S. R., 1959, Distribution of Ca, Sr and Ba in southern Norwegian pre-Cambrian alkali feldspars: Geochimica et Cosmochimica Acta, v. 17, p. 286-304.

Hughes, C. J., 1982, Igneous petrology: Elsevier Scientific Publishing Company, 551 p.

Huntsman, J. R., 1978, The geology and mineral resources of the Caribou Mountain area, southeastern Idaho: Bryn Mawr College, Bryn Mawr, Pennsylvania, unpublished Ph.D. dissertation, 146 p.

Hyndman, D. W., 1985, Petrology of igneous and metamorphic rocks: McGraw-Hill , 786 p.

Imeokparia, E. G., 1981, Ba/Rb and Rb/Sr ratios as indicators of magmatic fractionation, postmagmatic alteration and mineralization­ Afu Younger Granite Complex, Northern Nigeria: Geochemical Journal, v. 15, p. 209-219.

Irvine, T. N., 1966, Chromian spinel as a petrogenetic indicator: Canadian Journal of Earth Sciences, v. 4, p. 71-103.

Jackson, E. D., Stevens, R. E., and Bowen, R. W., 1967, A computer­ based procedure for deriving mineral formulas from mineral analyses: U. S. Geological Survey Professional Paper 575-C, p. C23- C31.

Johannsen, A., 1937, A descriptive petrography of the igneous rocks: University of Chicago Press, v. 3, p. 90-92.

Joplin, G. A., 1968, The shoshonite association: a review: Journal of the Geological Society of Australia, v. 15, p. 275-294. 56

Kushiro, I., 1960, Si-Al relation in cl inopyroxenes from igneous rocks: American Journal of Science, v. 258, p. 548-554.

Lambert, I. B., and Wyllie, P. J., 1970, Melting in the deep crust and upper mantle and the nature of the LVL: Physics of the Earth and Planetary Interiors, v. 3, p. 316-322.

Larsen, E. S., 1941, Igneous rocks of the Highwood Mountains, Montana, Part II. the extrusive rocks: Bulletin of the Geological Society of America, v. 52, p. 1733-1752.

LeBas, M. J., 1962, The role of aluminum in igneous clinopyroxenes with relation to their parentage: American Journal of Science, v. 260, p. 267-288.

Lindsley, D. H., 1976, Experimental studies of oxide minerals: in Rumble, D., ed., Oxide minerals, Mineralogical Society of AmeriCa Short Course Notes, v. 3, p. L-82.

Mansfield, G. R., 1927, Geography, geology, and mineral resources of part of southeastern Idaho: U. S. Geological Survey Professional Paper 152, 453 p.

Maxwell, J. A., 1968, Rock and mineral analyses: John Wiley and Sons, 548 p.

Miyashiro, A., 1978, Nature of alkalic volcanic rock series: Contributions to Mineralogy and Petrology, v. 66, p. 91-104.

Moore, C. E., 1958, Atomic energy 1 evel s: U. S. National Bureau of Standards Circular 457, v. 3.

Nash, W. P., Carmichael, I. S. E., and Johnson, R. W., 1969, The mineralogy and petrology of Mount Suswa, Kenya: Journal of Petrology, v. 10, p. 409-439.

Nash, W. P., and Wilkinson, J. F. G., 1971, Shonkin Sag laccolith, Montana I I. Bulk rock geochemistry: Contributions to Mineralogy and Petrology, v. 33, p. 162-170.

Nicholls, J., and Carmichael, I. s. E., 1969, A commentary on the absarokite-shoshonite-banakite series of Wyoming, U. S. A.: Schweizerische Mineralogische und Petrographische Mitteilungen, v. 49, p. 47-64.

Nockolds, S. R., and Allen, R., 1953, The geochemistry of some igneous rock series: Geochimica et Cosmochimica Acta, v. 4, p. 105-142.

Onuma, N., Hirano, M., and Isshiki, N., 1981, Sr/Ca-Ba/Ca systematics in four volcanoes of Oshima, Izu Islands, Japan: Geochemical Journal, v. 15, p. 315-324. 57

Oriel, S. S., and Platt, L. B., 19B3, Geologic map of the Preston 1° X 2° quadrangl e , southeastern Idaho and western Wyoming : U. S. Geological Survey, scale 1:250,000.

Pauling, L., 1960, The nature of the c hemical bond: Cornell University Press, 3 rd. ed., 644 p.

Powell, M. and Powell, R., 1974, An olivine-clinopyroxene geothermometer: Contributions to Mineralogy and Petrology, v. 48, p. 249-263.

Puchy , B. J., 1981, Mineralogy and petrology of lava flows (Tertiary­ Quaternary) in southeastern Idaho a nd at Black Mountain, Rich County, Utah: Utah State Un iversity, unpublished M. S. thesis, Logan, Utah , 73 p.

Rowell, W. F., and Edgar, A. D., 1983, Cenozoic potassium-rich mafic volcanism in the western U.S.A.: its relationship to deep subduction : Journal of Geology, v. 91, p. 338-341.

Royse, F., Warner, M. A., and Reese, D. L., 1975, Thrust belt structural geometry and related stratigraphic problems Wyoming­ Idaho-Northern Utah: in Bolyard, D. W., ed., Deep drilling frontiers in the central Rocky Mountains: Rocky Mountain Association of Geologists Symposium, p. 41-48.

Sahama , T. G., 1974, Potassium-rich alkaline rocks, in Sorensen, H., ed., The Alkaline rocks, J. Wiley and Sons, p. 96-1~

Shannon, R. D., and Prewitt, C. T., 1970, Revised values of effective ionic radii: Acta Crystallographica, v. B25, p. 925-946.

Simkin, Tom, and Smith, J. V., 1970, Minor-element distribution in olivine: Journal of Geology , v. 78 , p. 304-325.

Smith, J. V., 1974, Feldspar minerals, v. 2: Springer-Verlag, 690 p.

Smith, R. B., 1978, Seismicity, c rustal structure, and intraplate tectonics of the interior of the western cordillera: in Smith, R. B., and Eaton, G. P., eds., Cenozoic tectonics anaregional geophysics of the the Western Cordillera, Geological Society of America Memoir 152, p. 111-144.

Snyder, W. F., Dickenson, W. R., and Silverman, M. L., 1976, Tectonic implications of space-time patterns of Cenozoic magmatism in the western United States: Earth and Planetary Science Letters, v. 32, p. 91-106.

Stormer, J. ~. 1973, Calcium zoning in olivine and its relationship to silica activity and pressure: Geoch i mica et Cosmochimica Acta, v. 37, p. 1815-1821. 58

Streckeisen, A., 1979, Classification and nomenclature of volcanic rocks, , , and melilitic rocks: recommendations and suggestions of the I. U. G. S. subcommission on the systematics of igneous rocks: Geology, v. 7, p. 331-335.

Thompson, G. A., and Burke, D. B., 1974, Regional geophysics of the Basin and Range Province: Earth and Planetary Science Annual Review, v. 2, p. 213-238.

Thornton, C. P., and Tuttle, D. F., 1960, Chemistry of igneous rocks; I. Differentiation Index : American Jounal of Science, v. 258, p. 664-684.

Tuttle, 0. F., and Bowen, N. L., 1958, Origin of granite in the 1 ight of experimental studies in the system NaAl Si 3o8-KA1 Si 3o8-SiOrH20: Geological Society of America Memoir 74, 153 p.

Vollmer, R., Ogden, P., Schilling, J. G., Kingsley, R. H., and Waggoner, D. G., 1984, Nd and Sr isotopes in ultrapotassic volcanic rocks from the Leucite Hills, Wyoming: Contributions to Mineralogy and Petrology, v. 87, p. 359-368.

Washington, H. S., 1896, !tal ian petrological sketches II: The Vi terbo Region: Journal of Geology, v. 4, p. 541-66, 826-849.

-----, 1897, Italian petrological sketches V: summary and conclusions: Journal of Geology, v. 5, p. 349-377.

Williams, H., Turner, F. J., and Gilbert, C. M., 1982, Petrography, an introduction to the study of rocks in thin section: W. H. Freeman and Co., 626 p. 59

APPENDIX 60

Table 9. Sample 1 oca ti ons

Sample Sample Location

SCV83-1 42° 33' 30" N.' 111° 17' 15" w. SCV83-3 42° 36' 25" N.' 111° 18' 30" w. SCV83-8 42° 35' 54" N., 111° 17' 45" w. SCV83-11 42° 36' 20" N., 111° 19' 05" w. SCV83-16 42° 30' 54" N.' 111° 20' 00" w. SCV83-17 42° 35' 30" N.' 111° 17' 54" w. SCV83-18 42° 35' 54" N.' 111° 18' 00" w. 61

Table 10. Analyses of Columbia River Basalt Reference Sample

ARCH0-1 Standard Analysis* This Study**

Si02 52.54 53.19 TiO 2.76 2.55 Al2C3 13.79 13.21 Fet3 13.73 13.94 Mn 0.19 0.23 MgO 2.95 2.82 CaO 6.40 6.25 Na o 3.20 3.14 0 2.52 2.65 ~20 0.90 0.89 H~OfT} 1.24 1.23 BaO 0.35 *** C02 0.03 ***

* analyzed by Additon and Seil (1979); mean weight percent ** weight percent *** not analyzed for 62

Table 11. Analyses of Italian Cimini tes from the Viterbo Region, Italy

SiO 55.44 58.67 57.95 55.00 53.63 55.23 Al 203 18.60 15.07 12.52 14.38 14.17 14.06 Fe o3 2.09 1.46 5.06 Fe 0 4.48 8.35 5;44 9.29 8.07 4.12 MnO 1. 70 Trace o. 57 MgO 4.75 2.97 5.27 7. 72 7.05 4.00 CaO 6.76 8.07 3.80 8.51 8.52 9.34 ~at 1. 79 3.36 3.27 2.25 1.80 2.07 2 6.63 3.50 4.78 2.52 2.03 2.43 H20 0.25 0.82 5.49 0.48 2.01 1.07 T102 0.16 so3 o. 62 so3 o.84 P205 Trace 0.93 1. 33

Total ~ Tlm:1!T TOlJ.72" "ITill":TI Trlll":N rmr:T2" Source Washwgton, 1897