Mineralogy and Petrology of Lava Flows (Tertiary-Quaternary) in Southeastern Idaho and at Black Mountain, Rich County, Utah

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All Graduate Theses and Dissertations Graduate Studies

5-1981

Mineralogy and Petrology of Lava Flows (Tertiary-Quaternary) In Southeastern Idaho and at Black Mountain, Rich County, Utah

Barbara J. Puchy Utah State University

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Recommended Citation Puchy, Barbara J., "Mineralogy and Petrology of Lava Flows (Tertiary-Quaternary) In Southeastern Idaho and at Black Mountain, Rich County, Utah" (1981). All Graduate Theses and Dissertations. 3817. https://digitalcommons.usu.edu/etd/3817

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]. MINERALOGY AND PSTROLOGY OF LAVA FLOWS (TERTIARY - QUATERNARY) IN SOUTHEA STER N

IDAHO AND AT BLACK ~fOUNTAIN, RICH COUNTY, UTAH

Barbara J . Puchy

A thesis submitted in partial fulfillment of the requirements for t he degree

of MASTER OF SCIENCE

Geology

Approved:

UTAH STATE UNIVERSITY Logan, Ut ah

1981 ii

ACKNOWLEDGE~ENTS

A sincere appreciation is extended to Dr . Donald W. Fiesinger, whose guidance , assistance, and patience made the completion of this study possible.

I wish to thank Dr . Clyde T. Hardy and Dr. Peter T. Kolesar of my Graduate Committee for their critical review of the thesis and helpful suggestions.

I would also like to express my appreciation to the Department of

Geology and Geophysics, University of Utah and especially Dr. William

Nash and Dr . Stan Evans for the use of the electron- microprobe in the mineral analyses. I am grateful to the Depar tment of Geology, Utah State University for financial assistance in the form of Teaching and Research Assist antships .

Lastly, to my parents , for their continuous encouragement, support and understanding, I am deeply indebted .

Barbara J. Puchy iii

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ii

LIST OF TABLES v

LIST OF FIGURES vi

ABSTRACT vii

INTRODUCTICN

General Statement 1 Purpose of Investigation 1

VALLEY FLOWS OF SOUTHEASTERN IDAHO 3

Location and Accessibility .•... . . •...... 3 Geologic Setting ...... •..••...... , . . . 3 Previous Investigations ...... • ...... 7 Paleomagnetism . . . • ...... • . . . . • . . • ...... • . • . . • . . . . . 7 Petrography . • . . . . • . . . . . • • . . . . . • . . . • . • • ...... • . . • . . . . . • . . . . . 8

Basalts . • • . • . . . • • . • ...... • . . . • . . . . • • ...... • . . . . 8 Alkali trachyte . . . . . • ...... • • ...... • ...... • . . . . . • . . . . ll

BASALT OF BLACK MOUNTAI N, RICH COUNTY , UTAH 14

Location and Acc essibility . . • ...... • . . . . . • ...... 14 Geologic Setting . . • • . . . . • ...... • . . • . . . . • ...... • . . • . . . . • • . . • 14 Previous Investigations . • •.•... .• ...... • . ..•...... •. . .. 17 Field Description . . . . • ...... • ...... • . . • . . • • . . • . . . . . • 18 Petrography ...... • . • • . • . . • • . . . • . . . . . • • . . 21

MINERALOGY 22

Sampling Procedur es . . • . . • • • . . . • . . • • . . . . • . • . . . . . • . . . . . • . • • • . 22 Analytical Techniques .•.•.• • ••..•...... •.•.•• • .• • .• .. • •• .. • 22 Plagioclase • • • • . . . . • • . • . . • . . • . . • • . • . . • • • • . . . . . • ...... • . • • . • 24 Alkali Feldspar . . . • • . • . . . • . . • • • • • . • • . . . . . • . • • . . • . • • . • . . • • . • 28 Olivine . . • . . • ...... • . . • • . • . . • ...... • • . • . . • . . . . • . . • . • • • . 28 Pyroxene . . . . . • • • . • . • . . . . . • . . . . . • • . • . . . . . • . . . • . • . . • • . • • • • . . . 33 Or thopyroxene Xenocrysts . • ...... • . • . . . • . . . . . • . • • . • • . . . . • 45 Fe- Ti Oxides . . . • . • • . • • ...... • • • • • . • ...... 46 iv TAB LE OF CO NTENTS (continued) Page

PETRO LOGY ··· ···· ··· ·· ··· ······ ··········· ·· ···· ···· ·· ·········· 49 Chemi stry and Classificati on ...... •..•.•...... • . • . .• .... . 49 Temperatures of Cr ystallization ...... 57 Petrogenesis ...... • ...... • . • • . . . • • . . • . . • • ...... • • . . 62

l"ractional crystallization ...... • . . • ...... • . . . 62 Par tial melting of the mantle ...... 63 CO NCLUSIONS ·· ··· · ...... ········· ·· ·· ······ ···· ··· ··· ···· ·· ··· ·· 66 REl"ERENCES . ······· ... ······· ·· ····· · ······ ···· ···· ······ ··· ·· ·· 68 APPENDIX ········· ·· ······ ········· ······ · ··· ·· ·· ···· ··········· 72 v

LIST OF TABLES Page Table 1. Modal analyses of samples (volume per cent) 9

2. List of sample codes and locations 23

3 . Ave rage microprobe analyses of plagioclase 25 31 4. Average micropr obe analyses of olivine s. Average microprobe analyses of pyroxene 36 6 . Average microprobe analyses of magnetite and ilmenite 47

7. '"hole- rock chemical analyses and CIPW norms 50 8 . Temperatures of crystallization and oxygen fugaci ties 61

9 . Standards used in microprobe analyses, Universit y of Utah Department of Geology and Geophysics ..• .••• . .•...... 74 vi

LIST OF FIGURES

Figure Page

l. Samp le location map of southeastern Idaho

2 . Sample location map of Black Mo untain 16

3 . Geol ogic map of Black Mount ain 20

4. Electron micropr obe analyses of feldspar plotted in terms of mo lecular pe r cent An , Ab , and Or 27

5. Electron microprobe analyses of olivine plotted in terms of molecul ar percent Fo and Fa . •. •.••...... •...... •. . . . 30

6 . CaO and FeO contents of olivine 3 4

7. Electr on microprobe analyses of augite pl otted in terms of molecular per cent Di, En , and Fs ... • ...• ...... 38

8 . Weight percent silica plotted against we i ght percent alumina for pyroxene . . . . . • • . • . . • ...... • • • • . . . . . • . • • • . . . . 41

9 . Pl ot of Alz versus weight percent Ti0 for pyroxene ..• . ..• •. . 44 2 10. Alkali- silica variation diagram 54 11 . Compositions of lavas plotted on a Ne - Di - 01- Qz diagram 56

12 . AFM diagram 59 vii

ABSTRACT

Mineralogy and Petrology of Lava Flows

(Tertiary- Quaternary) in Southeastern

Idaho and at Black Mountai n ,

Rich County, Utah

Barbara J . Puchy , Master of Science

Utah State University, 1981

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

Lava flows of Tertiary- Quaternary age occur in Enoch Valley, Upper

Vall ey , and Slug Valley in southeastern Idaho . The basalts in Upper

Valley and Enoch Valley contain olivine (Fo to Fo J, plagioclase (An 69 37 62 to An ) , augite and Fe - Ti oxides . The lava in Slug Valley lacks 39 plagioclase, but contains sanidine (Or to Or J with a trace of biotite 70 56 and amphibole, and thus, has been termed alkali t rachyte .

Black Mountain , on the eastern side of Bear Lake, northeastern

Ut ah, is capped by basalt . Mineral s present include oli vine (Fo 83 to Fo J, plagioclase (An to An J, augite, and magnetite . 72 71 53 Chemically , the basalt of Enoch Valley is comparable to olivine tholeiite of the Snake River Plain , as it contains olivine and hypersthene in the norm . The basalt of Upper Valley contains a gr eater viii amount of Si 0 and K o and l ess MgO than thol eiit e of the Snake River 2 2 Plai n . This basalt cont ains nor mative quartz and hypersthene and is cl ass i fied as t holeiit e . The pr esence of nepheline and olivine in the norm of t he basalt from Black Mountain indicat es that i t is an alkali- olivine basalt . The lava from Slug Valley contains high K and

Mg, moderate Si , and low Al and Na. It is similar to lamproites of orenditic affinity,

The t emperatures of crystallization calculated from coexisting olivine and pyr oxene, r ange f r om 1, 01 5 degrees C to 996 degrees C for the val ley basalts, and range f r om 1,021 degrees C t o 1 , 00 2 degrees C for t he alkali t rachyte. The temperature calculated for the basalt sample from

Black Mountain is 1,015 degrees C. The temperatures estimated using co existing magnetite and ilmenite range from 1,021 degrees C to 978 degrees

C for the valley basalts. The proposed origin of the Enoch Valley basalt is that it is a direct product of partial melting of a mantle of pyrolite composition.

Fractionation, during ascent of the magma, could possibly have produced the Up per Valley lava. The basalt on Black Mountain was possibl y derived as the result of partial melting of a pyrolitic man t le as well, but due t o differences in mineralogy and normative constituents, it seems to be unrelated t o the valley basalts. The or igin of the Slug Valley alkali trachyte is uncertain . This lava may have been generated from a mica peridotite mantle and is possibly related to the Leucite Hills lava in

Wyoming. (73 pages) INTRODUCTION

General Statement

Lava flows of Tertiary- Quaternary age occur in several narrow northwest- trending valleys in southeastern Idaho . The origin of these lavas and their relationship to the larger Blackfoot lava field and the Snake

River Plain lavas to the west are uncertain . Previous study of magnetic anomalies in the· area indicates that some valley flows exhibit reversed magnetization (Mabey and Ori el, 1970), wher eas the lava flows to the west exhibit positive magnetization .

Black Mountain, a conspicuous peak of the Bear Lake Plateau on the eastern side of Bear Lake, Utah, is capped by basalt. Previous investigators have considered the or igin and structure of the basalt

(McClurg, 1970; Richardson, 1941), but to the present time no detailed petr ological and mineralogical study of the basalt has been done.

Purpose of the Investigation

The objectives of this study are: ll) to determine the chemical and mineralogical characteristics of the lava which occurs within Upper

Valley, Enoch Valley, and Slug Valley in southeastern Idaho·; (2) to verify the magnetic polarity of the southeaster n Idaho lavas; [3) to compare these valley lavas with the basalt of the Blackfoot lava field and the Snake River Plain; (4) to deter mine the chemical and minera logical characteristics of the basalt capping Black Mountain; and (5) to determine the distribution of the basalt on Black Mountain and its 2 relationship t o t he underlying rock unit s. The valley flows of south eastern Idaho will be described separatelyfromthe basalt of Black

Mountain . The sections on mineralogy and petrology will integrate data from both to facilitate comparison. 3

VALLEY FLOWS OF SOUTHEASTERN IDAHO

Location and Accessibility

The lavas studied are l ocated within t he Lanes Creek Quadrangle and in the nor th- central par t of the Harrington Peak Quadrangle (Figure

1) . Up per Valley is situated approxi matel y 18 miles to the northeast of

Soda Springs, Idaho . Enoch Valley and Wooley Valley lie approximately

10 miles to the north of Georgetown, Idaho. The valleys are within the

Caribou National Forest in Caribou County , Idaho.

The study area is accessible from t he north along Idaho Route 34 and from the south and west along U.S. JON . Road s within the valleys are dirt and gravel and provide access to wi thin 0 .1 mile of t he out crops studied .

Geol ogic Setting

Upper Valley , Enoch Valley, and Slug Valley lie wi thi n the western

Middle Roc ky Mountain physiographic pr ovince . The Basin and Range physio graphic pr ovince lies to the west; the Snake River Plain physiographic province is to the north (Fenneman and Johnson , 1946) . The valleys , drained by the Blackfoot River and its tributaries. which flow into the

Snake River, lie in the Columbia River drainage system (Cressman, 1964).

The Webster Range forms t he eastern boundary of Upper Valley

(Figure 1). The Grays Range forms the western boundar y of Upper Valley and t he eastern boundary of Enoch Valley . The Wooley Range is l ocated to the south of Enoch Valley and to the north of Wooley Valley .

The Blackfoot lava field and the Aspen Range bound the valleys Figure l . Sample location map of southeastern Idaho. Sample location

numbers are shown· in triangles . Hachures indi~ate lavas .

Lower dashed box indicates the Harrington Peak Quadrangle;

upper dashed box indicates the Lanes Creek Quadrangle. 5 6 collectively to the west.

The valleys are a result of tight asymmetrical folding and fault ing in the region (Mansfield , 1927). Normal faults , often inferred under valley fill, separate the valleys and strike northwest-southeast.

Fault relations in Slug Valley suggest several periods of normal faulting

(Cressman, 1964) . The fault on the eastern side of the valley occurred after deposition of the Salt Lake Formation and probably later than the lava. The fault to the west of the mass of lava seems to predate t he lava, however. Faults to the north of this outcrop are thought to be older than the flow, but exposures are so poor that the actual relations cannot be determined (Cressman, 1954).

The valleys are part of a zone of imbricate thrust faults in the southeastern corner of Idaho known as the Bannock Thrust zone . Block faulting in Tertiary and Quaternary time cut the thrust zone and formed the graben valleys (Armstrong and Cressman, 1963). The extensive Black foot lava field lies to the west of the valleys and consists of basaltic lava flows which were erupted late in the Pleistocene epoch (Armstrong ,

1953). According to Mansfield (1927), the Blackfoot lava field is an example of the interfingering of the Snake River Plain with the bordering mountains. Mabey and Oriel (1970) report two cycles of volcanism and deformation in the central part of the Blackfoot lava field as suggested by reversed magnetization of these flows. Near the center of this field three rhyolitic volcanic cones, China Hat, North Cone, and South Cone, are located along a northeast linear trend . These cones are dated by

Armstrong and others (1975) at about 100 ,000 years old and are thought to be older than the basalt Qf the Blackfoot lava field which surrounds them. 7 Previous Investigations

Mansfield (1927 ) , in an investigation of the geol ogy of south eastern Idaho, described the lavas found in the Soda Springs region, and considered the basalt contained i n .the Lanes Creek Quadrangle to have poured out from l ocal sources. Some of these basaltic masses, such as those contained in Enoch Valley and Up per Va lley, may be connected under sur face cover, but this connection cannot safely be assumed (Mansfield,

1927 ) .

Cressman (1964 ) described a patch of lava~ one mile in diameter, exposed in Slug Valley , as a vesicular olivine basalt. The source of t his lava is not known, but since the more recent basalt in the Soda

Springs area issued f rom fissures and cr aters i n Bear River Valley, the sour ce of thi s lava was pr obably wi thin Slug Valley. Because of the confoC'ma bl e relation wi th the Salt Lak e Formation, the lava is believed to be no younger than Pliocene in age (Cressman , 1964 ) .

Mabey and Oriel (1970) made gravity and aeromagnetic surveys of t he

Soda Springs region . They also prepared a generalized geologic map of the Soda Springs region which shows the distributi on of the lava flows sampled in this study.

Paleoma~ etism

Gravity and magnetic surveys by Ma bey and Oriel (1970) were us ed to determine prevolcanic structures· in the Soda Springs region. They found that some of the valley .flows in this ar ea are characterized by reversed magnetization. Negative magnetic anomalies in Lower, Wooley, and Upper

Valleys, and the southern end of Enoch Valley were produced by reversely magnetized basalc . Positive magnetic anomalies were produced by normally magnetized basalt in the northern end of Enoch Valley . 8

Magnetic polarity of the basal t flows of this study was determined using a Model 70 Fluxgate Ma~etometer manufactured by Geo - Cal. Ro ck samples were marked f or orientation in the field to identify north and south poles, and checked to determine normal or reversed polarity. The

basalt in the northern part of Upper Valley and in the northern part of

Enoch Valley were found to have normal magnetization; the basalt in the southern part of Upper Valley was found to have reversed magnetization .

No consistent pattern was obtained for samples from Slug Valley.

Since there have been 20 or mo re reversals in t he Earth's magnetic field since Eocene time (Doell and Cox, 1962), specific ages of these l avas cannot be determined from the magnetic polarity . The variation in magnetic po larity among the different valley flows, however, does in dicate that the flows are not equivalent in age.

Petrogr aphy

Basalts

The basalt samples from Enoch Valley (8PV78- 5 and BPV78 - 10) and

Up per Valley (8PV78 - 7 and 8PV78-8) range from dark ~ray to black . All samples except BPV78- 5 are vesicular. They exhibit intergranular to subophitic textures and are porphyritic or cumulophyric in part . The samples from Upper Valley have the lar gest phenocrysts , up to 2 . 0 mm in length.

Plagioclase is the dominant mineral in the val ley basalt samples, ranging from 37 percent to 51 percent by volume (Table 1 ) . Phenocrysts are common in 8PV78- 10 (10 volume percent) and are present in lesser amounts in all other basalts (1 to 2 volume percent). The phenocrysts Table 1. Modal analyses of samples (volume percent)

Enoch Valley Upper Valley Slug Valley Black Mt . BPV78 - 5 BPV78-10 BPV78- 7 BPV78-8 BPV78- 2D BP V78- Sl BPV78- BM5 Phenocrysts Olivine 2 .9 8 . 8 2.7 2. 2 10 . 5 10 . 9 12.5 Plagioclase 1.4 10.1 2.3 0 . 9 Augite 0 . 2 3. 5 o. 7 0 . 7 2.4 6 . 6 0 . 5 Sum 4 . 5 17 . 4 5.7 3. 8 12.9 17 . 5 13 .0 Groundmass Olivine 4.3 1.7 6 . 6 3. 2 1.2 0 . 4 0.4 Plagioclase 45 . 5 36 . 3 41.5 37 . 8 2 . 2 1.3 36.0 Sanidine ------39 . 7 38.8 Augite 1.4 15 .8 5. 6 5 . 0 4 . 8 4.6 5 . 5 Opaque Oxides 23 .9 15 . 9 23 . 0 23 .1 9.7 16 . 8 20 . 9 Undiff Gdms 20 .4 12.9 17.6 27 .1. 28.2 20.7 24.2 Biotite ------1.3 tr. Sum 95 . 5 82 . 6 94 . 3 96.2 87 .1 82 . 5 87.0 "' 10 range up to 1.5 mm in length and are found in cumulophyric clusters with olivine and pyroxene phenocrysts . Groundmass plagi oclase is in the form of small , subhedral, randomly oriented laths.

Most of the phenocrysts and some of the groundmass plagioclase are twinned according to Carlsbad, albite, or Carlsbad- albite laws. Pheno crysts commonly exhibit no rmal concentric zoning. Zoning is most conspi cuous in the large corroded crystals found in BPV78- 7 and BPV78 - 10 . The plagioclase in the groundmass shows no evidence of zoning in thin section .

Olivine is present as phenocrysts and in the groundmass of all basalt samples . Subhedral to euhedral phenocrysts range up to 2 mm in length . The phenocrysts are colorless in thin section, but are commonly altered to reddish brown along the edges and in fractures. Inclusions of euhedral magnetite and chromite (?) are present in the olivine pheno crysts in most samples . No evidence of zoning in the phenocrysts or in the groundmass is visible in thin section. The phenocrysts commonly exhibit a cumulophyri c texture, interlocking with phenocrysts of plagio clase and pyroxene . Groundmass olivine is present as intergranular fill ings and ranges from 0 .1 mm to 0 . 7 mm in diameter . The olivine in t he groundmass shows less alteration than the phenocrysts.

The clinopyroxene, augite, is present as phenocrysts in BPV78 - 10.

It occurs in the groundmass of all basalt samples. The augite is light br own to pale-green. Subhedral to euhedral augite phenocrysts in BPV78 - 10 range up to 1.8 mm in diameter and are commonly cumulophyric with plagio clase and olivine.

Magnetite and ilmenite are present as microphenocr ysts and as small disseminated crystals in the groundmass of all basalt samples. Magnetite is more abundant than ilmenite in most samples. The total abundance of ll opaques ranges from 16 percent in BPV78- l0 to 24 pe rcent in BPV78 -8 .

The microphenocrysts range up to 0 .4 mm in diameter; the magnetite and ilmenite in the groundmass average 0 .08 mm in diameter . Magnetite crystals generally exhibit an equidimensional cubic form . The ilmenite crystals are generally present as slender needle- like laths. The magnetite and ilme nite are commonly unaltered. Interstitial glass is present in samples

BPV78- 7, BPV78-8 and BPV78- l 0 . The glass is found t hroughout the gr ound mass and occurs as diffuse patches which commonly contain micr olites .

Alkali t rachyte

The lava in Slug Valley has been ter med an olivine basalt by Cressman

(1964) . The essential minerals of a basalt are a calcic or intermediate plagioclase and pyroxene (Carmichael and others, 1974; Spack, 1962).

Because of the presence of sanidine and lack of significant plagioclase in the rock samples collected (BPV78 - Sl and BPV78 - 2D) , this lava is mo re appropriately cal led an alkali trachyte (Streckeisen, 1979) . Petr ogr aphi cally, the lava in Slug Valley is similar to a fine- grained shonkinite

(Williams and others, 1954 ) . It shows similarities to the potassi um - rich rocks of orenditic affinity as described by Sahama (1974). Alka line rocks of the potassic suite show a weight percent K20 in marked excess over that of Na These rocks are subdivided into two groups: 2o. (l) the orenditic rocks, those that are saturated , and (2) the kamafugitic rocks, those that are undersaturated . The Slug Valley samples are saturated

(quartz normative) ·with a K: Na ratio of approximately 2 :1, which suggests these rocks have an orenditic affinity. However, BPV78 -Sl and BPV78 - 2D are less potassic than the "average" orendite of Sahama (1974). They also contain magnetite, which is uncommon in rocks of orenditic affinity 12

(Carmi chael , l967c) . Therefore, the Slug Valley samples can, at best, be classified as alkali trachyte wi th orenditic affinity.

The alkali t rachyte samples are vesicular in part, and ligh t gray weathering to light brown. In the field, they cannot readily be distin guished from basalt. Sanidine is the dominant feldspar and its pr esence was confirmed by staining (Heinrich, 1965) and x- ray diffraction. In

BPV78- 2D , the sanidine is present in t he gr oundmass as randomly oriented mi cr olites (averaging 0 . 1 mm in length) and as subhedral to anhedral pokilit i c plate- like laths up to l mm in length. The sanidine in BPV78 -Sl is also in the for m of randomly oriented microlites and larger laths .

The laths, however, are not as massive as in BPV78- 2D and only range up to 0.5 mm in length.

Olivine is present as phenocrysts and in minor amounts in the ground mass of the t r achyte. Phenocrysts are subhedral to euhedral and range up to 2 mm in diameter. As in basalt, the phenocrysts ar e colorless in thin section, but show alternation to a reddish brown on edges and along fractures . The phenocrysts in BPV78- Sl and BP V78- 2D are commonly cumulophyricwit haugite phenocrys ts. Olivine in the groundmass is pr esent as small granules ranging up to 0 . 4 mm in diameter . No evidence of zoning in olivine is observed in thin section.

Augite:. is present as phenocrysts and in minor amounts in the ground mass. The phenocryst s are colorless in thin section, range up to 2. 2 mm in length and are commonly cumulophyr ic with olivine phenocr ysts and other augit e phenocrysts. Cleavage t races are visible in the phenocrysts .

Augite in the groundmass is present as small inter gr anular fillings.

Augi te shows little alteration . 13

Magnetite and ilmenite are present in the gr oundmass of both t ra chyte samples . The abundance of opaques is greater in BPV78-Sl (17 pe r cent) than in BPV78- 2D (10 percent) (Table 1 ) . Magnetite and ilmenite crystals in t he trachyte are smaller ~~an t hose in basalt , averaging 0 . 02 mm in diameter . Magnetite forms well- shaped cubes , ilmenite is commonly lath- like or tabular and is not _as abundant as magnetite . Exsolution of some of the magnetite and ilmenite in t he t rachyte is observed in polished section .

Biotite and amphibole are present in minor amounts in both t rachyte samples . The biotite forms small laths and plates which range up to

0 .99 mm in length and commonly are broken . The biotite is pleochroic, light brown to yellow brown . The amphibole , less common t han biotite, forms accicular to prismatic grains up to 0.1 mm in length . The amphibole is deep red to brown and shows weak pleochroism. 14

BASALT OF BLACK MOUNTAI N, RICH COUNTY , UTAH

Location and Accessi bility

Black Mountain, situated between No r th Eden and South Eden Canyons , t wo miles east of Bear Lake, lies in the Mi ddle Rocky Mo un tain physio gr aphic province (Fenneman and Johnson, 1946) . I t is located i n the west central section of the Sheeppen Creek Quadrangle and l ies i n ~ ic h County ,

Utah (F igure 2). Black Mountain lies approximately 12 miles north of

Laketown , Ut ah, arid is located approximately three miles sou t~ of the Utah

Idaho border .

Black Mountain is accessible from dirt and gravel roads which connect with U.S. 89 to the north and Ut ah State Highway 30 to the south . The road leading to Black Mountain is steep and rugged in par t , making access difficult.

Geologic Setting

Black Mountain lies approximately si x miles southeast of t he No r th

Eden delta which extends into Bear Lake . East of Bear Lake, a no rth south trending fault scarp formed of Jurassic sandstone rises steeply f rom lake level (5,923 ft.) to over 7,000 f t . Eastward, the topogr aphy flattens to form the Bear Lake Plateau which extends to the east f or approximately ll miles into the Bear River Valley . Black Mou ntain is t he highest po i nt in the area at 7,696 feet , but has low relief as it rises 696 f eet above the surrounding plateau sur face. Figure 2 . Sample location map of Black Mountain . Dotted block

indicates Sheeppen Creek Quadrangle . Triangle indicates

sample location of BPV78 - BM5 . 16

1 1 1"23' 1 1 1"0 1. 4 ?' 0 7 'r----.---..-.,------...--,4 2" 07.

BEAR LAKE

BEAR PLATEAU LAKE

....,,...'d,...o-ho---< Wyoming

Ulc h

41"•!~------~-----~~ 1 1 l' 23 ' 1 1 1"0 1. 17

The lower member of the Wasatch Formation (Twl) is early Eocer.e in age (Armstrong and Cressman, 1963) , and occurs as a flat- lying cap forming the surface of undissected parts of the Bear Lake Plateau. It covers most of the eastern and southern parts of the drainage area of

No r th Eden Creek and surrounds Black Mo untain . It consists predominately of banded, variegated mudstone interbedded with claystone and marlstone and is often a shade of red (O riel and Tracey , 1970) . An identifiable unit of the lower member of the Hasatch Formation (Tw2) is ~

(Oriel and Tracy, 1970) . Black Mountain is capped by basalt of Tertiary

Quaternary (?) age (McClurg, 1970).

Previous Investigations

Previous investigators have speculated on the origin of Black Moun tain (Richar dson, 1941; McClurg, 1970). Richardson (1941) concluded that

Black Mount ain is an erosional remnant of a lava flow of igneous rock which formerly occupied a more extensive area . He stated that the vent from which the lava was issued is not known . He described the basalt as bluish black , weathering to brown gray and composed of small crystals of lime- soda feldspar and augite with phenocrysts of olivine embedded in a glassy groundmass.

McClurg (1970) did a study of the drainage area of No rth Eden Creek to determine the provenance of minerals in the sediment of Bear Lake.

He disagreed with Richardson and concluded that Black Mountain seems to be the northern rim of a former crater that encircled the volcanic vent from which the basalt was issued. He identified the main constituents 18 of the basalt as olivine, labradorite, and augite . McCl urg dated the

basalt on Black Mo untain as post- Wasatch and t hus Eocene or younger.

Field Description

Black Mountain covers an area of approximately one square mile.

It has gentle slopes, which steepen to a conspicious peak . Black

Mountain consists of .isolated outcrops of basalt with talus covering the souther n face and basalt i c float extendi ng from the base of the outcrop do•m to the 7,100- foot contour (Figure 3 ) .

The basalt i~ massive, black and weathers to light br own and gray .

At the top of Black Mountai n, the basalt outcrops are extensively jointed.

The columns of basalt show no preferred orientation and may have been ro tated, thus a direction of flow canno t be established . An unusual knobby, crumbly texture has been observed in all outcrops of basalt and is associ ated with a denser, fine- grained , shale- like texture . Zeolites f ill vesi cles in the basalt in a limited area on the southeastern face of t he mountain. The zeolite was identified by x- ray diffract ion as natrolite and was found only at this locality. Flow structures ar e not visible in the field on any basalt outcrop . In a saddle at t he top of Black

Mountain , breccias consisting of fragments of sedimentary r ocks set in a basaltic matrix are found. Lithic fragments include carbonate rock, quar tz pebbles, chert and siltstone. This ar ea stands out because of its distinctive white color observable from the south . Other breccias are found extending downslope from the saddle and near t he area of zeolites .

Localized occurrence of the breccia in and adjacent to t he saddle at the top of Black Mo untain suggests that Black Mountain is an eruptive center which incorporated pieces of the olaer rocks of the Wasatch Fo rmation when Figure 3 . Geologic map of Black Mou ntain . TQbc = Tertiary- Quater nary

basalt colluvium; TQb = Tertiary- Quaternar y basalt outcrop .

Tw l, Tw2 = Ter tiar y Wasa tch Fm ., lower members; dotted

line indicates limit of basalt float. 20

0 .5 ' K I L :-

CONrOUR INT E~ VA L 2C FEET 21

the lava e r upted . Ta lus cons isting of chunks of bas alt col umn s covers

the southern face of Black Moun t ain. Float material s urrounds t he mo un t ain and i t s extent is indicated on t he map by a dotted line.

Petrography

The basalt on Black Mountain is dark gray to grayish black and wea t hers brownish gray . The basalt is holocrystalline, containing

phenocrysts of ol ivine set i n a f i ne gr oundmass of plagioclase laths, olivine, augi te and magnetite .

Plagioclase is t he most abundant mineral in BPV78- BM5 . It occurs in the gr oundmass as small, randomly oriented laths, averaging 0 . 08 mm in length. The plagioclase seems to be twinned and zoned, but this is diff i cult to discer n in t hi n secti on due t o the small size of the c rys t als .

Subhedral to euhedral olivine phenocrysts range up to 2 mm in diamet er and occur in cumulophyric clusters . The olivine phenocrysts are colorless in thin section and are extensively fractured. Olivine is pr esent in minor amounts as small granules in the groundmass. Augite is present in the gr oundmass as small disseminat ed cr ys t als , averaging 0 . 03 mm in diamet er.

Optically, the augite is pale brown to pale green. Magnetite occurs as subhedral phenocrysts and is abundant in the groundmass as small cubic shaped crystals. Phenocrysts range up t o 0 . 3 mm in diameter; groundmass magnetite averages 0.02 mm in diameter . No ilmenite is pr esent in

BPV78- BM5. 22

MINERALOGY

Sampling Pr ocedures

Samples were t aken from the flow units within Uppe r Valley , Enoch

Valley , Wooley Valley, and Slug Valley (rigure land Table 2) . Several samples were collected from each locality. Samples were also t aken from basalt outcrops of Black Mo untain (rigure 2 and Table 2 ) . Thin secti ons were prepared and used in the selection of samples for detailed analysis.

Those samples with the least alteration and most representative of the valley lavas and lava of Black Mountain were chosen for further analysis .

Analytical Techniques

Mineral analyses were completed on seven samples: BPV78- 5,

BPV78- 10, BPV78- 7, BPV78-8 , BPV78- Sl , BPV78- 2D , and BPV78- BM5 . The ana lyses were conduc t ed wi t h an ARL EM~ SM electron microprobe at t he

Department of Geology and Geophysics, University of Utah. In each sample , 30 to 50 spots were analyzed per mineral. Bence and Albee (1968) and Albee and Ray (1970) have outlined correction procedures for data obtained from the electron microprobe . These procedures have been incorporated into the computer program, SLAVE (Nicholls and others,

1977), which has been used to reduce the microprobe data in the present study. The standards used for each analysis are presented in Appendix .

Samples BPV78- Sl and BPV78 - 2D were pitted and did not take a good polish. Therefore , the results of their analyses were unsat isfactory in some cases , but have been presented . 23

Table 2 . List of sample codes and locations

Enoch Valley

BPV78 - 5 Basalt , Henry cutoff, 0 . 1 mi . eas t of the Enoch Valley road, (lll027' W. , 42°56 ' N. ) .

BPV78 - 10 Basalt , east of the Enoch Valley road 0 .1 rr.i . south of the Henry cutoff , ( lllo27'W., 42°55'N . ) .

Upper Valley

BPV78 - 7 Basalt , in the central part of Upper Valley, east of Lanes.Creek road, 4 mi . east of the Diamond Creek road. (lll019'W . , 42050 ' N:).

BPV78 -8 Basalt, in the northern part of Uppe r Valley , Eas t of Lanes Creek road . (lll020 ' W. , 42°57 ' N.) .

~lug Valley

BPV78- 2D Alkali trachyte, road cut in Slug Valley western side of Slug Creek road , 2 mi. north of Summit View Camp ground . (lll019'W., 42°36 ' N. ) .

BPV78-Sl Alkali trachyte , top of flow (7624 ' ) in Slug Valley, 0 .8 mi . west of Horseshoe Springs , Slug Valley road. (lll019 ' W. , 42°36 ' N. ) .

Black Mountain

BPV78- BM5 Basalt, from flow, southern side of Black Mou ntain. (lll013'W . , 42°57'N. ). 24

Plagioclase

Plagi oc lase phenocrysts we re analyzed in basalts, BPV78 - 7 and

l BPV78 -10 . They show a range in composi t ion from labradorite (An62 to andesine (An J. Groundmass plagioclase is abundant in all basalts 46 and ranges from An to An , also in the labradorite and andesine fields. 62 39 The average compositions of plagioclase are presented in Table 3.

Zoning patterns of plagioclase are shown graphically in Figure 4.

The ranges in zoning are related to gr ain size and therefore to crystal lization history or rate of cooling. In the fine- grained basalt , BPV78 - 5, plagioclase shows only a limited amount of zoning within the labradorite field, whereas in the coarse-grained basalt s, BPV78-8 and BPV78- 10, the plagioclase is more extensively zoned from labradorite t o andesine.

The plagi oclase in these basalts lies in the range of plagioclase composi tions reported for the Snake Ri'ver Plain basalts (Stout and Nicholls,

1977; Leeman and Vitaliano , 1976).

In the basalt from Black Mountain, BPV78- BM5, plagioclase occurs as small laths in the gr oundmass; no phenocrysts of plagioclase are pr esent .

The plagioclase is zoned from An to An , in the bytownite to labra 71 53 corite fields . The average plagioclase analysis is given in Table 3.

The zoning trend of the plagioclase is illustrated in Figure 4 . The

!.n content of the plagioclase in BPV78 - BM5 is more calcic than that of the basalts in the valley flows and the basalts from the Snake River

Plain (Stout and Nicholls, 1977; Leeman and Vitaliano , 1976). This suggests a higher temperatuPe of crystallization . Table 3. Average mi croprobe analys es of pliJ.gioclase

Enoch Valley Uppe r Valley Black Mt.

BPV78- 5 BP V7 8- 10 BP V78-10 BPV76-7 BPV7 8-7 BPV78-8 BPV78-BM5 p p G p G G G Weight percent oxides

CaO ll.77 12 .67 . 11.86 11 . 71 11 . 65 11.22 13 . 48 Na2o 4.67 4. 22 4. 59 4.82 4. 67 4 . 93 3.65 K 0 2 0.52 0.38 0 .49 0.50 0 . 60 0 . 58 0 . 30 Weight percent end members An 58.37 62. 86 58 . 81 58 . 08 57.76 55 . 65 66 . 89 Ab 39.55 .35 . 7l . 38.90 39.75 38 . 77 41.73 30.09 Or 3 . 00 2.26 2.90 2.89 3 . 53 3.41 1.79

Total 100.92 100 .83 100.61 100.72 100 . 06 100.79 99.58 Molecular percent end members An 56 . 49 61 . 03 57 . 12 56.31 55 . 92 53 . 86 65 . 93 J\b 40. 60 36 . 78 40 . 07 40 . 89 . '• 0 . 66 '•2 . 85 32 . 31 Or 2.91 2.19 2.82 2 . 80 3 .42 3 . 29 l. 76

P ; phenocryst G ; groundmass

f\) \Jl Figure 4. Electron microprobe ana lyses of feldspa r· plot t ed in

t er·ms of mol ecul ar percent An , Ab, and Or . Open c i rcles

indicat e average phenocryst composition ; sol id circ l es

indicate a verage gr oundmass composition. Also shown ar e

the normative f eldspar co111pos ition , in molecular percent. 27

0 N

<: <

<: <( 28

Alkali Feldspar

Sanidine is present only in the Slug Valley lavas, 8PV78 -Sl and

8PV78- 2D, as small microlites and plate- like laths . Because of the pre sence of sanidine, these samples are classified as alkali trachytes . The results of the microprobe analyses for Ca, Na , and K were low suggesting

the presence of Fe o within the feldspar structure. The occurrence of 2 3 Fe o in alkali feldspars from potassium- rich lavas is relatively common 2 3 (Carmichael , 1967c; Sahama, 1974 ) . Sanidine ranges in composition f rom or to Or . Zoning trends of the sanidine are illustrated in Figure 4. 70 56

Olivine

Olivine is present as phenocrysts and in the groundmass of all ba salt and trachyte samples . The olivine in some samples was exteasively altered and tl1erefore not analyzed . Groundrnass olivine •,;as anal yzed in samples BPV78- 5, BPV78 - 7, and BPV78- l0; phenocrysts were analyzed in

BPV78- 7, BPV78-Sl and BPV78- 2D; microphenocrysts we re analyzed in

BPV78- 8 . Figure 5 shows average compositions and ranges in composition for analyzed olivines . Average compositions are presented in Table 4 .

Olivine phenocrysts in the basalt samples are zoned from Fo68 in the cores to Fo at the margins . Compositions for the groundmass olivine 41 range from ro to Fo . The olivine phenocrysts in the trachyte samples 69 37 are more magnesium rich t han the basalt samples, ranging in composition from Fo to Fo . This is a more restricted range of zoning , possibly 87 71 indicative of a more limited interval of crystalliza tion for the tra chytes than that for the basalts. The compositions of the olivines in the basalts of this study -are comparable to the compositions reported for Figure 5. Electron microprobe analyses of olivine plotted in terms

of molecular percent Fo and Fa . · P = phenocryst; G

groundmass; arrows indicate average composition; bars

indicate the composition range for phenocryst and ground

mass of each sample . 30

ENOCH VALLEY

BPV78-5 G

p BPV78-10 G

UPPER VALLEY p 8PV78-7 G

BPV78-8

SLUG VALLEY p BPV78-2D

p BPV78-SI

BLACK MT. p BPV78-BM5 G

Fo I I I I I I I I I I !-Fa 90 80 70 so !50 40 !0 20 10 Table 4 0 Average microprobe analyses of olivine

Enoch Valley Upper Valley Slug Valley Black Mt 0 BPV78- 5 BPV78- l0 BPV78-l0 BPV78-7 BPV78- 7 BPV78-8 BPV78-Sl BPV78-2D BP V 78-B~J5 BPV78- BM5 G p G p G p p p p G Weight percent oxides FeO 37 o43 32o67 4Li ol3 37 o42 44 o73 41 . 35 17.54 22 o56 20030 27ol8 MgO 26 ol4 30o35 20 o6l 26074 20 o85 23058 42o82 38o28 40086 34 04 8 CaO 1.00 Oo76 Oo90 Oo 7l Oo69 Oo66 Oo"i!l Ooli5 Oo75 1.09 Weight percent end members Fa 53 ol0 46 o35 62o59 53o06 63o42 58o64 24o88 3lo98 28o79 38054 Fo 45o62 52 o97 35o97 46 . 64 36 o38 4lo1ll 74 073 66 080 71.32 60o l 6 La 1.53 1.17 1.38 1.09 1.06 l.OL1 Oo76 Oo69 1.16 1.68 Total l00 o25 100o49 99 o94 100 082 100o86 1000 82 100o37 99o47 101.27 100038 Molecular percent end mem bers Fa 43 o97 37 o40 53o90 43o60 54 oll 49018 18 o61 24 o70 21.60 30 021 Fo 54o53 61.49 44o69 55 o35 44.82 li9 o82 80 o72 74o67 77 o37 68 024 La 1.51 l.ll 1.41 1.05 1.07 1.00 0068 Oo63 1.02 1.55

P = phenocryst G = groundmass w..... J2

the Snake River Plain basalts (Stout and Nicholls, 1977 ; Leeman and

Vitaliano, 1976) .

The CaO content of the olivines in the basalts is strikingly high

with respect to the olivine in the Snake River Plain basalts . Average

CaO content of olivine in the valley samples range from D.66 weight per~

cent to l.DD weight percent; averages for the trachyte samples are some what lower, D. 6D weight percent and D. 45 weight percent respectively.

Simkin and Smith (197D) suggested that t here i s a strong dependence of calcium content on pressure. Their work showed that most olivine from

hypabyssal and extrusive rocks contains greater than D. lD weight percent

CaD . Stormer (l97Jl stated that the initial compositions of the olivine depend on the t ype of magma, specifically the level of silica activity.

He concluded that zoning t r ends can be controlled by the pressure and temperature history of t he host magma . At a constant pressure, the calcium content of olivine should decrease with cooling, and at constant temperature , the calcium content of olivine should increase as t he magma rises.

The CaD trends from phenocrysts to gr oundmass are plotted agai nst

FeD contents in Figure 6 . All basalt samples, with the exception of

BPV78 -7, show an increase in FeO with an increase in CaO. These trends parallel the zoning for alkali-olivine basalt and nephelinite . Sample

BPV78- 7 exhibits a sl!ght decrease in CaO with an increase in FeO content.

This trend roughly appr oximates that of tholeiitic basalt .

In the basalt from Black Mountain, BPV78-BM5, olivine is present as phenocrysts and in the groundmass . The average composition is given in Table 4. The olivine phenocrysts in BPV78- BM5 are zoned from Fo 83 to Fo (Figure 5). The groundmass olivine ranges from Fo to Fo . 72 70 65 rigure 6. CaO and reO contents of olivine . rilled circles = pheno~

crysts ; open circles = groundmass . Lines connect coexisting

phenocrysts and groundmass phases. Olivine from tholeiitic

basalt (Moo re and Evans, 1967), alkali basal t (Stormer,

1972) , and nephelini te (Stormer, 1972), shown for compari-

son. Filled squares alkali basalt; open circles tholeii tic

basalt; open squares nephelinite (Stormer, 1972). 34

ID oBPY78-5

.9 .8 ~~-10 8PV78-SM5 .7 ~8PV78-7 •8PV78- 8 .6

0 0 .5 (..) BPV78-ZD ALKALI BASALT gA~ • NEPHELINITE

10 20 30 40 50 Wt.% FeO 35

This olivine is slightly mo re magnesium rich than that of the previously described basalts , but lies in the range reported for the Snake River

Plain basalts (Stout and Nicholls, 1977; Leeman and Vitaliano , 1976).

Olivi ne in BPV78- BM5 is lower in FeO and higher in CaO content than the basalts in the valley flows. Average weight percent CaO for olivine phenocrysts in BPV78- BM5 is 0.71 , for groundmass olivine , 0. 69. The CaO trend from groundmass to phenocrysts is shown in Figure 6 . The trend of

BPV78 - BM5 oli vines most closely resembles that of oli •1 ines in nephelinite, which is highly alkaline , undersaturated and lacks feldspar.

Pyroxene

Augite is present as microphenocrysts and in the groundmass of both basalt and trachyte. Table 5 presents the average compositions of analyzed pyroxene. Phenocrysts in the trachytes are zoned over a rela tively narrow range, as indicated in Figur e 7. In the trachyte samples , tne compositions of phenocryst rims approach and overlap the groundmass pyr oxene compositions . The augites in the trachytes zone towards the Di - Hd join. Bacon and

Carmichael (1973) noted that this trend is typical of silica under saturated lavas due to gr eater substitution of Ti and Al in the Y site causing a corresponding descrease in Mg and Fe and an increase in the

C3./ (Fe + ~!g) raticr, The augi tes -in. tlle other valley flows. show a trend trpical of basaltic lavas.

The augite of the valley flows ranges from 17.3 weight per cent CaO

Ll the groundmass of the basalt , BPV78 -8, to 22 . 4 weight percent in the microphenocrysts of the trachyte, BPV78- 2D. The augite in BPV78- 8 is tne most extensively zoned with respect to CaO content: 13 to 18 weight Table 5 . Average microprobe analyses of pyroxene

Eurn:: h Va llr y IJ pJ)l"l' Va ll ny S lu t~ Vil 1 Icy IJ)nc k 111., BPV7U-5 Jl['V"(0- 10 m·v·m-·r lli'V70-8 ll!YtHI) lii' V7 0-?IJ BI' V'l0-1'1115 c r. G I' r. I'

S iO 1 51.]'/ 19.b',; )0 . 22 50.33 5 1. ')1 50.09 'j? ·'• ?. 50. 18 '•6 ."J lj Til ~ 1.5 ) l. !)'i 1. 110 tl. ')U 1 1.58 1.1 2 0 . 0 2 1. ~ 7 2.1 • n l:i' OJ 2 . 74 2 .96 2. 64 2 .62 ], _) (, 2 ..,,, Fen• ). 7'• '• .27 6. 57 11 . 12 1.1 . 9'• 13.76 l'!,l)l 6. 12 '( .95 ).C)~ 7 .1 6 6.3) NnO 0. 27 O. JY 0.30 0.'•0 0. l (l 0.19 O. H 0. 17 0 . 17 1'1P.0 14.'• 1 1).09 1.1.90 1'•.06 J(, _flfl 15.0'• l(•, 'f(, I I). 13 11. &5 t:a() 19.0'• JO.O~i 17. 59 17.20 22 . 19 21 . 1'• 2"- . J'l 2 1 .6~ 22.75 N;'l./ ) _£.3(, ~ ~ ~ _!!..:..?!~ ~.!. ..!!.Xi. -P~~ ~ Tn l. a l 101 . 50 wn. 'tO 100.J'• 101.52 100.'•2 99.(,1) 101 .'+'/ 100 . 9(1 1Ul .2'• •To t a l i r·on r ep

Number o f i ons ou the bas i .3 o f nix OX)'{ICIIS

S l .1.695 1.002 1.093 . 1.063 I .117 5 1.071 L . 90;;> l .Ev. s I. 720 A.l TV 0.105 0.1111 0.107 O.ll6 0. 125 0.1.29 0.090 O. I Zi 0 .?8(1 Al VI 0.011• O.OlJ 0.011 0.000 0. 019 0.036 tl.O;"O 0.030 ')'j 0.095 2 0.04) O.O~ iS 11. 0'•2 O.O'.S 0.0?') 0. 129 0.0:>:! 0.0'•1 0 .077 Fe " 0.3'•4 0.'t )l3 0.'t3~ 0 .'•66 II. ) {I(, 0. 2'•8 0 . lU I 0. 220 0 . 259 Htl 0.009 0.012 0 .01 2 0.013 o.ous 0 , 006 o. no,, 0.005 0. 005 Hg 0.79'• 0. 7J5 0. 7Hl. 0. '187 o .(JJ7 0.0)') O.C)09 O.B60 0 .6'18 C;1 o. 705 0. '1.10 0, 7 11 0.69 1• o.nr.r) 0.6'•'• O.ll/ 1 0.054 0 .908 ~I n 0,0]0 0 . 0 ~' 0 0. 026 0 . 025 0.020 0,0.10 0 .019 O.U2't 0 .050 Snrn 1V 2.00() 2 . 01)0 ? . 000 1.999 2 , ()00 2.000 :!.tHlO 2 .000 2.000 ~) UUt Vl 2.019 2 .0.11 2 .

F !t 17.59 ?2. 1•7 21 . 99 23 .96 9. 1•5 12 .76 9. 16 J t. 23 14 . 27 En 1t0,46 37 . )9 )9 .'•7 40. 111 '•6.0'' 42.01t ft6,!l't ~t~•. on Jlt.04 Wo ftl .9~ ,,,,_,,., '•0. j t, JO.SJ 35.90 '•'• · 51 '• '•·'•0 '•'t.61t 50.90

I' = phenocrys t (i = {,Wo undm an::~ w "' figure 7 . Electr on mi cr opr obe analyses of augite plotted i n terms of

mol ecular percent Di , En , and rs . Large open f i e l ds s how

ranges f or each sample; small solid circ les show t he a verage

composiLion f or each sampl e . Li ne illus trates cal i cum- r i c h

trend of Skaergaard intrus i on pyroxenes (Brown and Vincent ,

1963 ). ------38

------39 pe r cent. Einar elements (Ti, Mn and Na) show little variation in the samples analyzed. The r e is a s lightly hi gher weight percentage of these elemen ts in the gr oundmass augite of the t rachytes than in the augite phenocrysts .

Kushiro (1960) and LeBas (1962) showed that clinopyroxenes from tholeiitic rocks tend to contain less Al than those from feldspathoid 2o3 free alkali rocks, and that feldspathoid- bearing r ocks contain even gr eater amounts of Al 0 . LeBas (1962) showed that by plotting silica 2 3 ver sus alumina contents of pyr oxenes from many different magma hosts , one can distinguish three well-defined fields: (l) tholeiitic, high

3lumina and. calc- alkaline, (2) normal alkaline, and (3) pe ralkaline .

In Figure 8 , these t rends are shown with the Kap Edvard Holm intrusion, the Skaergaard intrusion , and the Snake River Plain augites shown f or comparison . The augites of this study show less variability than those of the Snake River Plain. With the exception .of BPV78 - 2D phenocrysts, py roxenes from the valley flows l ie in the alkaline zone.

Carmichael and others (1974 ) demonstrated that low silica activity favors the incorporation of Al in the Z site using the following CaA1 Si o reaction: 2 2 8 pyroxene glass anorthite component

The proportion of the Z site occupied by Al also indicates a distinc- tion between magma types . Only two percent Al is present in the Z sites in augite from olivine tholeiites , but up to 20 percent can be present in augite from lavas with lower silica activity. The pyroxenes of the valley flows range from four to seven percent Al in the Z sites (averaging six percent) . Figure 8. Weight percent silica plotted against weight percent alumina

for pyroxene . Open circles = pyroxene phenocr ysts; filled

circles = groundmass pyroxene. Dashed lines separat e fields

defined by LaBas (1962): 1 =tholeiitic, high alumina, calc -

alkaline; 2 = alkaline; 3 peralkaline . Large open box

indicates pyroxenes from the mildly alkaline Kap Edvard Holm

intrusion (Elsdon , 1971); smal l lined square indicate pro

xenes from the Skaergaard intrusion (B r own and Vincent, 1963 ) :

open squares indicate pyroxenes of the Snake Ri ver Plain

(Stout and Nicholls, 1977 ). -

st , , 2 51 i- / 0 / 0 ~ / / / 50 1- Do ,. BPV78-2D-o ,. ,D 49 '- BPV7S-SI~ / , , BPV 78 -5_. ' (\J 48 1- , 0-BPV78-2D / 3 0 / ·- 8PV78 -7 0 , (j') -e _.BPV78-Sb , / 47 1- BPV78- ·8,' , ~0 BPV78-10 , / .....: / 3 46 i-

45i- .SPV78-BM5

441- -

431-

I I I I I I - I I I 8 9 I 2 3 4 5 6 7 Wt. %AI 0 2 3 LeBas (1962) stated that the pyroxene from nonalkaline rocks gen ~

erally contains less than one pe rcent Ti0 , whereas alkaline rocks have 2 more Ti6 . By plotting weight percent Ti0 ve rses Alz in pyroxene, 2 2 LaBas defined three different fields : (1) nonalkaline, (2) alkaline,

and (3) per alkaline rock types . These fields , along with the Skaer gaar d

intrusion , Kap Edvard Holm intrusion , and Snake River Plain trends are

presented in Figure 9 . Most basalt samples of this study lie in the non

alkaline field . Sample BPV78-l0 which lies in the alkaline zone has a

higher percentage of Ti02 (1 . 95 weight pe rcent) . Pyroxene phenocrysts in BPV78 - 2D lie in the tholeiitic zone with the groundmass pyroxenes plot

t ing i nthe alkaline zone . This trend is also pr esent in BPV78 -Sl . The

basalts of the Snake River Plain show a higher per centage of Ti02 than the basalts in this study. The Na content of pyroxenes in nonalkaline rocks averages 0 .35 2o weight percent whi le that of alkaline rocks averages 0 . 70 we i ght percent

(LaBas, 1962) . The aver age Na content of pyroxene in the basalt samples 2o is 0 .38 weight percent. The average for groundma ss pyroxenes in the alkali trachytes is 0 . 37 weight percent. The Na o content of pyroxene pheno 2 crysts is lower , averaging 0.28 weight per cent. Samples BPV78- 2D and

BPV78 - Sl are alkaline and would be expected to have a higher Na value 2o for pyroxene. This low Na content can be explained by the high potas 2o sium to sodium rat io of the lava , as these two alkali trachytes are potassi c .

In the basalt sample from Black Mountain , BPV78 -B M5 , augite is present in the gr oundmass only. The augite ranges from 6.9 to 9.6 weight percent FeO. This value is approximately six percent lower than the FeO figure 9. Plot of Al vet·ses weigl1t per cent Ti0 for pyroxene . Open circles 2 2 = pyroxene phenoc t'ysts ; filled circl es = ground mass py r oxenes .

Dashed l i nes separate fields defined by LeBas (1 962 ): 1

tholeiitic , high alumina , calc- alkaline ; 2 = alkaline ; 3 = per

alkaline . Large open square indicates pyroxenes of the Kap Edvard

Holm intrusion (Elsdon, 1971); small lined r ectangle indi cates

pyroxene of the Skaergaard intrusion (Hagner and Brown, 1967 );

l ar ge irregula r ol val indicates pyt'oxe nes of the Snake River Plain

(Stout and Ni cholls , 1977 ). 44 '

l()

-:t N I(') ::;: I N CD 0 ,._CD' I 1- > I ~ a... r0 0 CD I ...... e I 3 I I I 0 I I CD,._' I I 5:-Q I N I CD I ., 0 N I \1 (I)' (I)

~li ~ 0 CD -9 CD N (J) CD' I ,._ > a... CD

CD <.D ~ -:t N 0 -:t N 2 1v 45 content of t~e augite in the previously described basalts and slightly higher than the value of groundmass augite in the alkali :rachytes (7 . 6 weight percent FeO) . Mi croprobe analysis of pyroxene in BPV78 - BM5 is given in Table 5. The zoning trend of the pyroxene in BPV78- 8M5 is shown in Figure 7 . The augite is zoned toward and extends beyond the

Di - Hd j oi n. .Samples 8Pif78- 2D and 8PV78 - Sl show similar trends , ••hich are typical of alkaline lavas (Ca r michael and others, 1974).

Augite in BPV78-BM5 averages 0 . 69 weightpercent ~a 2o , near the alka line range of LeBas (1962) . This value is considerably higher than the average valuesofaugites in the previously discussed basalts (0 . 38 weight percent) and those of the Snake River Plain (0 . 31 weight percent to

0 . 53 weight percent) (Stout and Ni cholls , 1977 ).

Pyroxene in 8PV78-8M5 is enriched in .a.1 0 , averaging 8 . 6 weight 2 3 pe r cent and ranging from 5.2 to 10.5 weight percent Al o . On a plot 2 3 of weight percent Si0 vs. weight percent Al o , sample 8PV78 - 8M5 lies 2 2 3 in the peralkaline zone , lower in Sio and greater in Al o content than 2 2 3 all other analyzed samples (Figure 8) .

Proxenes from nonalkaline rocks contain less than one weight percent

Ti0 while alkaline- type rocks contain mo re Ti0 (LeBas, 1962 ) . Ti0 2 2 2 con tent of augite in BPV78 - BM5 is higher than the basalts ( 2 . 7 •,;eight percent vs . 1 . 6 weight per cent). When Ti0 content is plotted against 2 the proportion of Al in the Z site (Figure 9), 8PV78 - BM5 again lies in the peralkaline range of LeBas (1962) .

Orthopyroxene Xenoc rysts

Two orthopyroxene xenocrysts (OPX - 1 and OPX - 2) are present in the basalt , BPI/78 - 8 . An augitic reaction rim surrounds OPX - 1 but not OPX - 2 . 46

The presence of the reaction rim indicates an incomplete reaction be t~; e en the or thopyr oxene and the melt in >Thich it ••as present . Ave r age or thopyroxene compo siti on is En for both OPX - 1 and OPX - 2 . The rim 61 on OPX - 1 has an average composition of Wo En Fs . 33 43 23

Fe - Ti Oxides Magnetite and ilmenite are present in t he groundmass of both valley basalt samples and the alkali trachyte samp les. Both magneti t e and il menite in the alkali trachytes, ( 8?1/78-Sl and BPV78 - 2D) , show evidence of exsolut ion and therefore were not analyzed . Averages for microprobe anal yses of Fe- Ti oxides in other samples are pr esented i n Table 6 . Mole cular end membe r s for the magnetite- ul vospinel (mt - usp or oc. ) solid solution series and the ilmeni te- hematite (il- hm or0\ soli d sol ution series >Tere determined using the method given by Carmichae l (l967a) .

An association between the composition of the oxide phase and the coexisting ferromagnesian silicate minerals have been observed by

Car michael (1967a ) . When present with olivine and augite, the composi tions of the coexisting oxi de phases vary ••ithin r estricted r anges ; the

~ phase is approximately 65 to 74 molecular percent usp (Smith and

Carmichael , 1968) and the ~ phase is approximately four to s ix molecular percent hm (Carmichael, l976b ) . Values for magnetit e analyzed in this study range from usp to usp . Ilmenites range in composi tion from hm 53 69 6 to hm . These ranges in composition for magnetite and ilmenite ar e 9 comparable to values reported by Stout and Nicholls (1977 ) for Sn ake

Ri ver Plain basalts (5 to: ll molecular. per cent hm; 58 to 74 molecular percent usp) . 47

Table 6 . Av erage microprob e analyses of magne t i te and ilmenite

:n;.~ k ~ t. ::: noc:;, "la..i2.e y :: ;:lpe!" 'iall.:y 2? '1 73 - 21·~5· 3P'na~; 3f1i72- l O 3?'i73- 7 3?'!73-d

•• 11 : .JS 1. !.2 2:..13 23 .'?9 22 .: .3 :7. : J J :::(. 2. ~ ? 2 . 20 L. 73 .. 2: .:. . ::6 "· J . : .: : . .30 ') . J7 ).2} j .J.! 57 .... 2 .') ~. ~5 0 . J . :9 j . 5- ~ .55 0 . 52 !2 2 • ..38 :.;; !... 30 l. 70 2. i'9 0. 56 O. J4 J . JO o .~ t. J. s·; ~ . !.2 1.10 l.l: l. ~8 t .a:: ') ,_2 1 . 08 0 . lJ 0 . 12 IJ .U 97.01.'. '1 1 .?2 l'JO . JS 97.53 ~7 .12

22. ;o 17 ::: 1 12."' 27. ';.6 . JI. 51 . 63 "9. ' ' 1..2 . ; : ?9. 7: :.. : 2. .:.::. 19 . 22 ; ; . . -

!!a ~ec:..:e . .37 ~!.. f.Jl·;osp:::el .::: 2':•: .:.;:s

C:r.oc~ tfalle•r Upper '.Jalle:r 1 9?'173- 5 3P'J7 -3 ~ l0 3P'f73 - 7 3? 178--3

0 . ?9 LJ2 1. .. <; l.5~ 47 ' J2 !..2 .57 : . 3C 0 . !... ~ c .-; .J;s J.=c •:. .-.:. ·J . __ ~ . :::. ) .oc o.: .!...... 26 -6. :. 2 45.:2 :. j .2 · ~ J . Sl J . sa 0 . 56 0 . :: 1 } . ·)3 i. £ 4 c. 7) 0 . 56 ':l . :.o 0 .30 O.)J 0 . ?2 0 .36 0 . 90 0 .')5 C.O': o.ou. :) . :J ~ 39. 7J 10!..': 5 100 .29 .: . :a 5.66 5 . 31 ~~3J J J6 • .:. s WQ . !.2 ~C • .!.l 40 . ·~ 6 'i'otal 100 . 58 : 02 . 51 100 .aJ l OO . !.C \·!olec'.llar fr-ac:i:ms Ilr.:.enBe . 9059 . 92~0 :-!~:nat:!. t e . "l 9'-l . 'J 74 0 48

Ca r michael and others (1974) noted that magnetites in basic lava of l ow silica activity f~equently contain mo re A1 0 and ~~ than those 2 3 found in tholeiit ic lava . Typical values given f or magnetite in alkali olivine basalt average three to five weight percent Al and one to three 2o3 weight percent MgO as compared with one to two weight percent Al and 2o3 0 . 5 to 1 . 5 weight percent MgO for magretites in tholeiitic lava. Magnetite composi tions in this study show a range overlapping both lava types . The

Al 0 contents l i e mo st closel y to the range for t holeiitic basalt s and 2 3 the MgO contents lie in the alkali- olivine basalt range.

In the basalt sample from Black Mountain, BP V78- BM5 , magnetite is present as phenocrysts and in thegroundmass ; no ilmenite was detected ,

Average micropr obe analysis of magnetite in BPV78 - BM5 is presented in

Table 6 .

The average end member composition of magnetite phenocrysts and groundmass in BPV78 - BM5 is usp . This is lower than the values reported 47 for the magnetites in Snake River Plain Basalts (Stout and Nicholls,

1977) and the previously discussed basalt samples. Magneti:es i n

BPV78 - BM5 average 3 . 54 weight percent Al and 2 . 7 weight percent MgO 2o3 which lie within the range for alkali- olivine basalt as reported by

Carmichael and others (1974) . 49

PETROLOGY

Chemistry and Classification

Chemical analyses were conducted using the methods outlined by

Carmichael and others (1968) . Results of the chemical analyses and the

CIPH no rms are given in Table 7 .

Based on che chemical analyses , the lavas in this study can be divided into three distinct groups: (11 the basalts from Enoch and

Opper Valley; (2) the basalt on Black Mountain; and (3) the alkali tra chyte from Sl ug Valley . The presence of nepheline and olivine in the norm of 8P1f78 - 8N5 indicates that it is an alkli olivine basalt (Yoder and

Tilley, 1962) . The low Si0 content (45 . 3 weight percent) and high A1 0 2 2 3 content (15 . 7 weight percent ) of the Black Mountain sample also supports this classification. The basalts from Enoch Valley , BPV78 - 5 and BPV78 - 10, are olivine and hypersthene normative and thus are classified as olivine tholeiites (Yoder and Tilley, 1962) . The basalts from Opper Valley, BPV78- 7 and BPV78 - 8, contain normative quartz and hypersthene and thus are classi fied as tholeiites . This is substantiated by the analysis of glass in

BPV78 - 8 whi ch is also quartz normative. The two Slug Valley samples,

BPV78 - Sl and BPV78 - 2D, show only slight chemi cal differences . 8PV78 - 2D contains slightly less FeO , MgO , CaO, and a higher amount of Si02 . Chemically, BPV78 -Sl and BP1f78- 2D are comparable to lamproites of orenditic affinity (Sahama , 1974) . Chemical characteristics of such rocks are high amounts of K and Mg t<~ith moderate amounts of Si, and relatively low Al and low Na contents . Howeve r , the K20 contents of Table 7 . Whol e- rock chemi cal analyses and CI PW norms

Lipper Va 11ey S ln1 ~ Val l •'Y 11\ ;lc k r·rt. En ~x. l! Va ll ey n-1; 1a :;!": BI' V"IO -S t lti'V"I II-:'o IIIY/I\-fll1') IWV'/1\ -5 lli' V'l B- 10 111 ' '1'10- 7 BI 'V'/ll-ll '•5 . 10 '•6 . JS '•'i .(,') •.o.96 )).0'1 (.fJ .'l'• ')3.99 ' •'>. 5(• ~i02 J.l ~O~ ). 1o0 :• . '1'") 6. 1t l l O. qJ 12 . I '• l O.l't 0,11) l.H!I '"6 0.0'1 0. I o.v. O.'• fJ -- ilt\. 11) '1'1, ;.-'I J C)H , 59 T0 La l 9~-'· 5 C)!}.H() 9'J . 'J'• 99. ) 2 99-'"' 2 .JS 6.20 111.',0 \.)6 '•.II') . _,,(! ,,_qo 6 .1 0 10.1 6 12 . 29 \li, 'i7 25. '(I ) l.JU J6 . fl 11 v •. .- n J .l . n pi ,,O.S l II'), ~~~ '•6. I l '•'•. '() t'IJ. l.1 20.90 (-' 0. '/U l l.U9 ( n h) 22 . 17 2'• .05 "'• .?fl 2J.fl(, ';"•· 'l'l 15.BO ) '). (,1 20. Jli ( a n) ?.ll. )1, 2~ . 'i' • 21.02 ~o.ns :l.'•O 10 . 10 I Ii! ------;•_v, -- wo ------ll . l'• ll .99 l ll.'i'/ 19 . t ) di 17. '( 2 l ~.'JJ ll. A7 0.60 'J,:, r) 6. )6 5.?1) 10.00 (uh ) 9.00 '1. 9A 5.')6 '•.l'f 5.01 ':i . ?R '1.0') ( eu) t, , 77 '·. 15 2. 7"1 2 .on I . ? 5 ,,_!)() 0.0 1 2 .01+ ) . fib 3.6) I . 11+ 2.22 0 .61 If" I 1 1 ] (,,9(> 13.9 • 10. '•? ..,, )0 10. •"• I R.h'• -- lo y 111,1,) ( I""!! I ) 2.90 •). '(0 0.'1'• H. 20 -- 12 · '·] I) ,CJO 0. 76 1. 5 1 0.(11 I r ~ J 2.'• 1 '•. '('j -- '1. '19 ------l'• .06 o l 12 . l'• -- -- 10. 6£\ (,,)'• ),1,1, ------t f o l -- -- J.Jf"\ (I' a 5.80 t,_j •, ------I -- ') .06 4 . lll• mL ) . 60 ,,_{ff ).19 ).')2 -- '• ·'·7 ? . ~ 6 ,,_,., (J • ~,{I ,, _06 ,,_ ;..o t, ? .•v, 2. )9 1 1 5 .09 l .hfl 2 . 2'( I .)1 I .fJJ 1.68 I . ~l9 I . 26 -- (, ')_,.,'j ')U.02 6). 2fl H? . 20 6).9 \ ~'· · '/(J '.Sn'" lit: SJ. 'II 5:-! .lU t,;, . 9f> t,() , \A J 'j ,OJ j (,_(.j )1, ·'·7 ('!1 , (> 1 F• ~ mi c '• '>. I I '•0 . 00 1 IIH.II) f)tl . J<) ClH. lf.J Q'/. 75 '/'Q t r+l 91J. ~~ I 9H. '1 • \Jfl.69 90.).\

V1 0 51

3PV78- Sl acd 3PV78- 2D are considerably lower than the averages for orendites, 4 . 6 and 5 . 5 weight percent i n BP V73 - Sl and BPV78 - 2D as compa red wi th ll to 12 cercent in average orendites (Sahama, !9741 .

Because of t he proximity of the Slug Va lley lava to the Leucite

Hills orenditic lavas , a relationship is suggested . However, the rocks of

the Leucite nills have K in excess over Al , which is not the case f or

the Slug Valley lava. Further, the Leucite Hills rocks lack magnetite, which is present in both BPV78 - Sl and BPV78 - 2D . Therefore, although they

share some mineralogic and chemical characteristics, a direct relationship

between the lavas of the Leucite Hills and those in Slug Valley cannot

be assumed .

The basalt samples in this investigation have silica contents

ranging f rom 46 to 51 weight percent , whi ch is comparable to the ran~e

in silica contents for the basalts of the Snake Ri ver Plain (Stout and

Nicholls, 1977; Leeman and Vitaliano , 1976) . The silica content in 8PV78 - 7

and BPV78 - 8 is higher than that of BPV78- 5 and BPV78 - l0 , which is also higher

than that of BPV78 - BM5 . This is reflected in the normative consti tuents ,

from quartz and hyoersthene in BP V78 - 8 to olivine and hypersthene in

BP V78 - 5 and BPV78 - l 0 to olivine and nepheline in BPV78 - BM5 .

The basalt samples from Enoch Valley differ from the basalt

samples from Upp er Valley by having higher amounts of MgO and CaD , and

less K 0 . The remaining amounts of oxides are similar. Chemically , t he 2 basalt of Enoch Valley is comparable to olivine tholeiite of the Snake

Ri ver Plain . The basalt of Upper Valley contains higher amounts of Si02 and K 0, and less MgO than the tholeiitic basalt of the Snake River 2 Plain . 52

Mac~ onal d and ~atsura 11966) distinguished alkali- olivine basal~s

and tholeiitic basalts of Hawaii based on an alkali- silica va riation

diagram. figure 10 shows that the basalts , BPV78 - 5 and 8PV78 - l 0 , lie

in che alkali- oli vine basalt field. The basalt samples BPV72 - 7 and 8PV78- 8

lie in the tholeiitic basalt region, close to the dividing line,

s uggesting these may be t ransitional between the two basalt types . The

Black Moun tain sample , 8PV78 - BM5 plots well into the alkali basalt zone

having a low Si0 content (45 . 3 we i ght percent) and hi gh alkali content . 2 Fo r comparison , the alkali trachytes have been plotted . As expected ,

these lie in the alkaline zone.

Coombs (1963) devised an indicator ratio to show the degree of alka-

linity and differentiation potential based on normative hypersthene,

diopside , and qu artz using the following :

Hy + 2Qz I.R. Hy + 2(Qz+Di)

He found that an indicator ratio of less than zero is typical of alkali

basalts which have nepheline in the norm. A ratio of less than 0 . 39

is typical of mildly alkaline basalts . Tholeiitic basalts characteris -

tically have ratios of 0 . 39 to 0 . 65 . Figu re 11 shows a plot of the

samples under investigation on the Di - 01-Qz - Ne t e t rahedron of Coombs

(1963) . A separation of the basalts into three groups is shown : 11)

the basalt samp le of Black Mountain has a negative ratio and plots in the alkaline zone; 12) the basalt sample from Enoch Valley lies in the mildly alkaline to t ransitional basalt zone; and (3) the basalt samples

from Upper Valley lies in the tholeiitic region with indicator ratios greater than 0 . 39 . Figure 10 . Alkali- silica variation diagram . Fields of tholeiitic and

alkali- olivine basalts defined by Macdonald and Katsura (1964 )

for Hawa i ian l avas . 8 BPV-20 •

7 e8PV78 - SI

6 8PV78 - 8M5 0 • N Alkali - Oliv ine Ba salt ~.. 5 • BPV78- 8 0 8PV78- 7 N 0 4 z BPV 78- 5 BPV78-IO• • 0~ .....: 3 3: Thol eiitic Basalt 2

I I I I _ L ____ _l___.J 45 46 47 48 49 50 51 52 53 54 55 56

U1 -"" wt.% Si0 2 Figur·e 11. Compositions of lavas plotted on a Ne - Di- 01- 0z diagram. All

samples plot t ed j_n terms of nor111aU ve Ne , Di , Hy , and Qz

reca l c ulated t o 100 per·cent (Coombs , 1963) , Alka line

I .;; . flPV70 - BM5 %/. ~0 <7/.foO'/;- 0_.,. 0~ J: ~'l rol) i? <$ .-}) sll/o ·o___. . . BPV7B~5 .5 /0 1)0/ . BPV78 - IO bPV78 - S\ • • BPV7B -2 0 6 . OPV78-7

7 OPV78 -8 _...------~ 0/ , 8 \ 0 ~/ · // · % /(' "'0 \II~ 9 S? ~ I ======-- "------" L " (Mg,Fe l sio (Mg,Fel S10 Si0 Vl 2 4 3 2 "' 57

~gain , a separation of the three lava types is sho•m by a trian gular plot of A (K 0 + Na 0l, f (feO + Fe 0 l, and~ (~gO) (Figure 12) . 2 2 2 3

7he Skaergaard trend of iron enric~~ent and range for the Snake ?iver

Plain lavas are shovm for comparison. The basalt samples, BPV78 - 5 and

BPV78 - 10 lie within the range of the Snake River Plain basalts . Samples

BPV78 - 7 and BPV78 - 8 are more alkaline and lie outside of this region .

The alkali trachyte samples and BPV78 - BM5 are less iron rich and dis- tir.ctly alkaline .

Temperatures of Crystallization

The presence of magnetite and ilmenite in the basalts BPV78- 5 ,

BPV78- 7, BPV78 - 8 and BPV78- 10 permi ts the calculation of temperatures and oxygen fugacities using the geothermometers of Budding and Lindsley

(1964) and Powell and Powell (1977) . Both methods are based on the coexisting magnetite- ulvospinel and ilmenite- hematite solid solutions.

Suddington and Lindsley (1964) used a graphical method to determine the temperature and oxygen fugacity . If the composition of a magnetite solid solution is known in terms of the mole fraction of Fe Ti0 , and the 2 4 coexisting ilmenite solid solution in terms of the mole fraction of

Fe , then the temperature and logarithm of Fo of equilibrium can be 2o3 2 determined . Buddington and Lindsley (1964) assumed that the coexisting

Fe - Ti 6xi des do not depart from the Fe0- Fe - Ti0 system, and give an 2o3 2 error of ~ 30 degrees C for temperature and + 1 for the log of oxygen fugacity . Powell and Powell (1977) used a thermodynamic appr oach based on the geothermometer of Buddington and Li ndsley (1964) . Their me thod allows for a departure from the Fe0- Fe -Ti0 system in calculating the 2o3 2 Figure 12 . AFM diagr· am. Skaergaard (SK) trend fPom Car·mi cllael (1964) ;

Snake RiveP Plain trend (dashed line) and basalt field (SRP)

from Leeman and others , 1976) . 59

2 "'6,.._ .~ ? ~ -,. . ' '-0..0:: Vl 60 mole fraction of end members . Results using both methods ar e gi ven in

Table 8 .

The coexistence of olivine and pyroxene in the groundmass of all samples studied permi ts the calculat ion of temper atures using the geother mome t er of Po we l l and Powel l (1974) . Results ar e shown in Table 8.

This geother momete r is based on the fol lowi ng r eact i on :

The calculated temperature for the basalts range from 1,015 degrees

C to 996 degrees C. Temperatures were also calculated for the alkali trachyte samples . Temperatur es obtai ned for the t r achytes range from 1 , 021 degr ees C to 1, 002 degr ees C; this is within the range for the tempe r atures cal culated for t he basal ts . All olivine and pyroxene temper atures are similar and do not show the variation of Fe - Ti oxide temperatures .

Kudo and Weill (1970) formulated a geothermometer based on the distributi on of al uminum , silica , and alkali s be t ween coe xisting plagioclase and glass usi ng the fol lowing exchange reacti on :

(Na Si0 _ )AlSi o _ + (C aAl0 _ lA1Si o _ 1 2 5 2 5 5 2 5 2 5 5 plagioclase glass

(Na Si 0 _ lA1Si o _ + (C aA l0 _ lA1Si o _ 1 2 5 2 5 5 2 5 2 5 5 l i quid plagioclase

Mathez (1973) was a ble to refine the geothermo me ter of Kudo and

We i l l (1970) and improve the res ults for l avas with calci c plagi oc l ase .

The presence of coexisting plagi oclase and glass i n BPV78 - 8 al lows the calculat ion of temperatur es using both Kudo and Weill (1970) and the revised geot hermome t er of Mathez (1973) . Temperatur es calculated using the two m e t~ods ar e 1 , 032 degr ees C and 1, 088 degrees C respective l y

(Table 8). These values are comparable with the temperatures obtained Table S; Temperatures of crystallization and oxygen fugaciti es .

Mt-Il Mt-Il 01-epx Plag- Glass Fl ag- Glas s (Buddington (Pm•ell & (Powell & (Kudo & (Matl1ez) & Lind1sey ) Powe ll ) Powell) Weill) 0 0 0 Lnro T0 e Lnf0 T0 e T e T e T e 2 2 BI'V78- 5 1008 - 24 ll55 - 20 1001

BP V78-10 1021 - 24 ll45 - 21 1001

BPV78-7 978 - 26 1058 - 211 996

BPV78-8 994 - 25 1068 - 211 1003 1032 1088

BPV78- S1

ol. P. , pyrox. P 1014

ol. P. , P)l r·ox . G. 1021

BPV78- 2D

ol.P. , pyrox.P 100 2

ol. P. , pyrox. G 10J.2

BPV78 - BM 5 1015

,_.0' P = phenocryst G = groundmass 62 from he Fe- Ti oxide and oli·Jine- clinopyroxene geothermometer calcula tions .

The temperatures of crystallization for the lavas in thi s study all lie •,;i thin the ranges of temperatures reported for the basalts of the

Snake River Plain (Stout and Nicholls , 1977 ; Leeman and Vitaliano , 1976) .

Petrogensis

(2) partial melting of a theoretical mantle composition .

Fractional :rystallization

The first hypothesis implies that the lavas in this study may be genetically related thr ough fractional crystallization. The Skaergaard trend from ~ MgO - rich magma towards increasing iron content and finally towards an increasing alkali content is shown on the AFM diagram (Figure

12). This trend suggests that the lavas represent points on a possible liquid line of descent of a parental magma. The basalts in Enoch Valley

(BPV78- 5 and BPV78 - l 0) are compositionally similar to and lie within the range of the parental olivine tholeiites of the Snake Ri ver Plain (Stout and Nicholls , 1977; Leeman and Vitaliano , 1976). This analogy to the Snake

River Plain lava suggests the Enoch Va lley lava may be a parental magma which was fractionated to produce the lava in Upper Valley (BPV78 - 7 and

BPV78- 8) .

To show a liquid line of descent it is necessary to subtract the phenocryst compos itions from possible parental magmas to generate possible daugh ter magma . To test possible pa rent - daughter combinations , a 63 computer program which gives a best fit by least squares mixing was used (Stormer and Ni cholls , 1973) . The feasibility of differentiating the lava in Enoch Valley to produce t hat in Opper Valley and of differen tiating Black Mountain basalt to produce the Slug Valley lava was tested .

Acceptability of results ~as arbi trarily based on a limit of 1. 0 for the sum of the squares of residuals . Fi ve phases (olivine , pyroxene, plagio clase , magnetite and ilmenite ) ••ere subtracted from the par ent of each pair. The following par ent- daughter comb i nations had a sum of squares of

1 . 0 or less: BPV78- 5 , BPV~8 ~ 7 ; BP V78 - 5 , 3PV78 - 8 ; BPV78 - l 0 , BPI/78- 7 ;

BPI/78- 10, BPV78 - 8 . Although the generation of BPV78- 7 and BPV78- 8 from

BPV78 - 5 is mathema tically consistent , there is a discr epancy be tween cal cualted·values and modal amounts of phenocryst phases . In order to derive the basalts , BPV78 - 7 and BPV78 - 8 , from BPI/78 - 5 , significant amounts of py roxene must be added . The deri vation of the Oppe r Valley lava from BPV78 - l0 however, does seem plausible . Results are consistent with the mineralogy and modal amo un ts of ~henocryst phases for this parent-daughter pair.

Differences in composit ion and high values obtained for the sum of squares ter m for the Black Mo untain- Slug Va lley pair reject the possi bility of derivation by fractional crys tallization. This indicates that the alkali trachyte of Slug Valley has a separate origin from, and is un rel ated to, the alkali- olivine basal t of Black Mountain. The high MgO content of :Joth the alkali trachyte and the alkali- olivine basalt pre cludes any relationship wi th olivine tholeiite or tholeiite.

Partial melt ing of the mantle

In order to test the hypothesis that BPV78- 5, BPV78 - l 0, and BPV78 -

BMS were derived as a result of partial melting of the mantle , it is 64

~ecessary to calcula te the percentages of partial melting :rom a theoret

: cal ~antle material. Thi s was determined using the standard addition-

3Ubtractiorc techni que of Bowen (1928) . Two model mantle compos itions were used as approximations for the unknown mantle material: (1) pyrolite

(G reen and Ringwood , 1967, p. 160 , Table 20); and (2) spinel lherzolite

(Bacon and Carmichael, 1973, pg . 2 , Table 1, 1135) . The percentages of partial melting using a spinel- lherzolite mantle composition are 0 . 50 pe r cent (BPV78 - BM5 ) , 0 . 99 percent (BPV78 - l 0) , and 1 .18 pe r cent (VPB78 - 5) .

~·! hen using a pyrolite mantle composition, the percentage s of par tial melt - ing are 6 . 53 percent (BPV78 - 8M5 ) , 6 .15 pe rcent (BP V78 - l 0) , and 11.12 pe r cent (BPV78- 5) . To evaluate these results , partial melting in the upper mantle must be examined. Anderson and Sammis (1970) determined that if only trace amounts of water exist in the upper man t le, the melting temperature will be lowered so that up to one percent partial melting may occur without magma being produced. They also suggest that approximately five percent partial melting must occur before the migration of the mol t en phase takes place.

Therefore, the small per centages of par tial melting of spinel lherzo lite would not be sufficient to produce the lavas in this study . The larger per centages of partial melting of pyrolite suggest that i t is more likely the mantle composition . However ,. before accepting a pyroli te mantle model, factors such as magma viscosity, conduit geometry and possible concentration of fluid phases in the upper mantle in the area of study should be examined .

The origin of the Slug Valley lava is uncertain. Potassium - rich rocks of orenditic affinity are indeed rare and occur i n a few restr icted 65 areas only , including the Leucite Hills, Wyoming , the Fitzroy Basin in

the \~est Kimber ley area of \·/estern Australia , and in southeastern Spain .

These rocks are c0nfined to volcanic or subvolcanic environments and lack

plutonic counterparts (Sahama , 1974). Those who have studied orenditic

rocks do not consider crustal contamination to be s ignificant in modifying

the chemi stry of a lamproite magma; however , a large part of the lamproite

masses have ascended through various sedimentary beds including l i mestones

(Sahama, 1974) . Carmichael (1967c) considered the orenditic lamproiti c

magmas to be of mantle origin . Pride r (1960) proposed derivation of the

Fitzroy Basin lamproites from a mica peridotite magma generated in small

cupolas in a roof of an extended pe ridotite magma mass. He implied that

the chemi cal characteristics of the lamproite material were developed

prior to the ascent of the magma. The best explanation for the origin of

the Slug Valley lava may be tha t it was derived f rom a mica peridotite

which could pr oduce a high amount of potassium . The generation of this

lava is most likely complex and may involve more than one single process . 66

CON CLUSIONS

The lava in this study can be separated into three gruops:

Enoch Valley and Up per Valley basalt

Chemical analyses of the basalt samples in Enoch Valley show that the y have less MgO than, but are otherwise compar able to , basalt of

: he Snake ~i

BPV78 - 7, are quartz normative, and thus are classified as tholeii te.

The mineralogy and alkali- silica variation diagram indicate the Enoch Valley and Upper Valley basalt samples to be transitional between alkali olivine basalt and tholeiitic basalt, having characteristics common to both types of lava .

Proposed for the origin of the Enoch Valley basalt is that i t is most probably a direct product of partial melting of a pyrolitic mantle. Near surface fractionation of the magma could possibly have pro duced the Upper Valley lava.

Black Mountain basalt

The sample of lava from Black Mou ntain is classified as an alkali olivine basalt. Chemically , it has a lower Si0 content and higher 2 amount of Al 0 than the other basalt samples in this study as well as 2 3 67 the choleiitic basalt of the Snake River Plain . It is the only sample in this investigation to contain nepheline in the no rm, whi ch further supports the alkali-olivine basalt classification.

The Elack Mountai n lava was possibly derived as the result of a partial melting of a pyrolitic mantle as well , but because of the differences in mineralogy and normative constituents, it appears to be un related to the valley lava .

Slug Valley alkali trachyte

The Slug Valley samp l es (BPV78 -Sl , BPV78 - 2D) are markedly unique com pared to the other samples in this study in that they are not basaltic .

Based on mineralogy and chemical analyses , they are classified as alkali trachyte , wi th orenditic affinity . The origin is uncer tain . A good fit cannot be obtained for the derivation of this lava by differentiation from any other lava in the region. It may have been generated from a mica

~e rid o ti te mantle , and may be related :o the Leucite Hills in Wyoming . 68

REFERENCES

Albee , A. L., a.nd Ray, L., 1970 , Co rrection factors for electron- probe microanalysis of silicates, car bonates, phosohates, and sulphates : Anal. Che rn ., v . 42 , p. 1408- 1414 .

And erson, D. L., and Sammis , C., 1970 , Partial melting in the upper mantle : Physics Earth and Planetary Interiors, v . 17 , p . 16 - 20 .

Armstrong , F. C., 1953, Generalized composite stratigr?Qhic section for the Soda Springs Quad r angle and adjacent areas in :;)outO'leastern Idaho: in Intermountain Assoc . Petroleum Geo logists Guidebook, 4ch Ann. r:eld Conf .. chart .

Ar mstrong , F. C., and Cressman, E. n., 1963, The Bannock Tj rust Zone . Southeastern Idaho : Am . Jour . Sci ., V. 275, p . 225- 251 .

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: Am. Jour. Sci., v . 275, p . 225- 251.

Bacon, C. R., and Carmichael , I.S . E., 1973 , Stages ir: the P-T path of as cending basalt magma : an example from San Qu entin , Baja Ca lifornia : Contr. Mineral. and Petrol . , v. 41 , p , 1- 22 .

Bence, A. E., and Albee, A. L., 1968 , Empir·ical correction factors for t he electron mi croanalysis of silicates and oxides: Jour. Geol . , v . 76 , p . 382- 403.

Bowen , N. L. , 1928 , The Evolution of the Igneous Rocks : Princeton Un i ve rsity Press , 332 p .

Brown , G. M. , and Vincent, E. A., 1963 , Pyroxenes from l ate stages of fractionation of the Skaergaard Intrusion, East Greenland : Jour . Petrol . , v . 4, p. 175- 197.

Buddington , A. F. , and Lindsley, D. H. 1964, Iron- titanium oxide minerals and synthetic equivalents : Jou r~ Petrol ., v . 5, p . 310- 357.

Carmichael, I.S . E., 1964, The Petrology of Thingmuli, a Tertiary volcano in eastern Iceland, Jour . Petrol., vol . 5, p . 435 - 460 .

1967a, The iron ~ titanium oxides of salic volcanic rocks -----and their associated ferromagnesium silicates: Contr . Mineral. and Petrol, , v . 14, p . 36- 64 .

1967b, The mineralogy of Thi~gmuli, a Tertiary volcano in eastern -----Iceland : Am . Min., v . 52 , p. iB15 - 1841.

1967c, The mineralogy and petrology of the volcanic rocks from -----the Leucite Hills, Hyoming : Contr. Mineral. and Petrol., v. 15 , p, 24 - 66 . 69

Ca r~ic~ael, I .S.S., nampel, J ., and Jack , A. L., 1968, Analytical data o~ the U. S. G. S. standard rocks : Chern . Geol., v . 55, p, 246- 263 .

Carmichael, I . S. E. , Turner , F . .J . , and 1/erhoogen , J . , 1974, I gneous Pe trology . ~-!cGraw - Hill, N.Y . , 739 op .

Coombs, D. S. , 1963, Trends and affinities of basaltic magmas and pyrox enes as illustrated on the diopside- olivine- silica diagram: Min . Soc . Ame r ica Sp . Paper No . 1, p . 227 - 250.

Cressman, E. R. , 1964, Geology of the Georgetown Canyon-Snowdrift Mountain Area , Southeastern Idaho : U.S . Geol . Survey Bull. 1153, 105 pp.

Doell, R. R., and Cox , A. , 1962 , Determination of the magnetic polarity of rock samples in the field : U.S. Geo1. Survey Prof. Paper 450- D, p . 105- 108 .

Elsdon, R., 1971, Cli nopyroxenes from the Oppe r Layered Series, Kap Edvard Holm, East Greenland: t1ineralog. Mag . , v . 38, p . 49 - 57.

Fenneman, N. M., and Johnson, D. W., 1946, Physical Divisi ons of the United States (map) : 0. S. Geol. Survey, Scale l: 7 , 000 , 000 .

Gt'een , D. H. , and Ring.-10od, A. E. , 1967 , The genesis of basal tic magmas : Contr. Mineral . and Petrol., v . 15, p. 103- 190.

Heinrich , E. H. , 1965 , ~Iicroscopic Identification of Minerals: McG raw Hill, N. Y., 396 pp .

Kudo , A. M. , and Vleill, D. F., 1970, An igneous plagioclase geothermo meter: Contr . Mineral. and Petrol ., v . 25, p. 52 - 65.

Kushiro , I ., 1960, Si- Al relations in clinopyroxenes from :gneous rocks: Am . Jour . Sci. , v. 258, p. 548- 554 .

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

Leeman , H. P. , and 1/italiano , C. J . , 1976, Petrology of the McK i nney basalt., Snake River Plain, Idaho: Geol. Soc . Amer. Bull. , v. 87 , p. 1777 - 1792 . Leeman , W. P. 1/italiano , C. J., and Prinz, M. , 1976 , Evolved lavas from the Snake Ri ver Plain : Crater s of the Moon National Monument , Idaho: Contr . Mineral . and Petrol . , v . 56, p. 35 - 60 .

Nabey, D. R. and Oriel, S. S., 1970 , Gravity and magnetic anomalies i n the Soda Springs region , southeastern Idaho: U.S . Gaol . Survey Prof . Paper 646- E, p . 1- 15 .

Macdonald, R. , and Katsura, T., 1964, Chemical composition of Hawaiian lavas : Jour . Petrol ., v. 5, p. 82- 133. 70

Mansfield , G. R. , 1927, Geogr aphy , geology , and miner al resources of part of so~th eastern Idaho : U.S. Geol . Su rvey Prof . Paoer 152 , 453 pp .

~'!a thez, E. A. , 1973 , Refinement of the Kudo - Weill plagioclase thermo meter in the system ~1g0 - Fe0 - Fe o - Si:J~ : Jour . Am . Ceramics Soc . , v . 39 , p . 121- 140 . 2 3 - t'icClur g , L. VI . , 1970 , Sour ce rocks and sediments in Drainage Area of No r t h Eden Cr eek , Bear Lake Plateau, Ut ah- Idaho : Utah State Un i v . , M. S. thesis . t1o ore, J . G. , and Evans, 8. W. , 1967 , The role of oli vine in the crystal lization of the prehistoric Makaopuhi tholei itic lava lake , HaHaii: Contr . Mineral . and Pe t rol . , v. 15, p . 202- 223 .

Nicholls , J ., Fiesinger, D. W. , and !thier, V. G. , 1977 , Fortran IV programs for processing routine electron mic roprobe data : Comput ers and Geosci. , v. 3 , p. 49 -83 .

Or·iel, S. S. , and Tracey , J . I. , 1970 , Uppermost Cret aceous and Tertiary stratigraphy of Fossil Basin , southeastern Wyoming : U.S. Geol . Sur . Prof . Paper 635 , 53 pp .

Po we ll, M. , and Powell, R. , 1974, An olivine- clinopyroxene geot he rmo meter: Contr . Mi neral . and Petrol . , v . 48, p. 249- 263 .

Powell , R., and Po well, M., 1977 , Geother momet r y and oxygen barometry using coexisting iron- titanium oxides : a reappraisal : Mine ralog . Mag ., v. 41 , p. 257- 263 .

Prider, R. T., 1960 , The leucite lamproites of the Fitzroy Basin , Wester n Australia: Jour . Geol. Soc . Australia, V . 6 , p. 71 - 118 .

Richardson, G. B., 1941 , Geology and mineral resources of the Randolph Quad rangele, Ut ah- Hyomi ng : U.S. Geo l. Survey Bull. 923, 54 pp .

Sahama, T. G. , 1974 , Potassium- rich alkaline r ocks : in H. Sorensen (editor ) , The Al kaline Rocks, John Wiley and Sons , p. 96 - 109 .

Simkin, T. , and Smi th , J . V., 1970 , Minor- element distribution in olivine, J . Geo l., v. 78, p . 304 - 325.

Smith, A. L. and Carmichael, I . S. E., 1968, Qu a ternary lavas from the southern Cascades , western U. S. A.: Contrib. Mine r al . and Petrol ., v. 19, p. 212- 238.

Spock , L. E., 1962 , Guide to the Study of Rocks: Harper and Row , N·. Y., 298 p.

Stormer, J . C., 1972 , Mineralogy and petr ol ogy of the Raton- Clayton vol cani c field , northeastern New Mexico : Geol . Soc . Amer . Bu ll., v . 83 , p. 3299- 3322 . 7l

1973, Calcium zoning in olivine and its relationshio to silica -----activity and pressure : geochim et Cosmoch i~. Acta , · v . 37 , p. 1815- 1821.

Storr.1er , J . C: ., and Nicholls , J . , 1978 , Xl.FRAC: A program for the i nter active testing of magmatic differentiation models : Com puters and Geosciences, vol . 4 , p . 143- 159 . Stout , M. Z. , and Nicholls , J ., 1977, Mineralogy and petrology of Quarter nary lavas from the Snake Rive r Plain: Can . Jour . of Earth Sci., v. 14, p, 2140 - 2156 . Streckeisn, A. , 1979, Classification and nonmenclature of volcanic rocks, l&~p r ophyr e s , car bonati tes, and melil itic rocks : recommendations and sugges tions of the IUGS subcow~is si on on the systematics of igneous rocks : Geology , v . 7 , p. 331 - 335 .

Wager , l. . R., and Br own , G. M., 1967, Layered Igneous Rock s: \.J . H. Freeman and Co., San Francisco , 588 pp .

Hilliams, H., Turner, F. J ., and Gilbert, C, M., 1954, Petrography: 'd . H. Freeman and Co ., San Francisco , 406 pp .

Yoder , ~ . S. , and Tilley , C. ~ ., 1962 . Origin of basaltic magmas : an exper imental study of natural and synthetic rock systems : Jour . Petrol . , v . 3, p. 34 2 ~ 3 52 . APPENDIX Tabl e 9 . S tandards used i n micro probe a nalyses , Uni vr·r·s i Ly of Utah , Departmet

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