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5-1985

Petrology and Mineralogy of Tertiary(?) Volcanic Rocks of Snowville Area, Utah, and Tertiary-Quaternary(?) Volcanic Rocks of Table Mountain and Holbrook Areas, Idaho

Yunshuen Wang Utah State University

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Recommended Citation Wang, Yunshuen, "Petrology and Mineralogy of Tertiary(?) Volcanic Rocks of Snowville Area, Utah, and Tertiary-Quaternary(?) Volcanic Rocks of Table Mountain and Holbrook Areas, Idaho" (1985). All Graduate Theses and Dissertations. 3830. https://digitalcommons.usu.edu/etd/3830

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]. PETROLOGY AND MINERALOGY OF TERTIARY(?) VOLCANIC ROCKS OF SNOWVILLE AREA, UTAH, AND TERTIARY-QUATERNARY(?) VOLCANIC ROCKS OF TABLE MOUNTAIN AND HOLBROOK AREAS, IDAHO

by

Yunshuen Wang

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

ACKNOWLEDGEMENTS

My sincere thanks go to my wonderful teacher, Dr. Donald Fiesinger, for his assistance in the field , and detailed instructions for each step of lab work. His patience, time, and ideas made this thesis possible. Special thanks go to Dr. Clyde Hardy and Dr. Peter Kolesar, for their critical review of the thesis, valuable suggestions for field work and their assistance in preparation of this thesis. My appreciation also goes to my fellow classmates, Amanda Shearer Fullerton, Bruce Scarbrough, and Mark Olesen, in particular, their friendship and encouragement made my study possible. I would also like to thank Dr. Judith Ballantyne, Bob Robison, and Bud Voit for their helpful suggestions and discussions. Special thanks are extended to Or. William Leeman for his generous supply of trace element and strontium isotope data, and valuable suggestions for this thesis. My appreciation goes to Departments of Geology, University of Calgary, Rice University, and the University of Utah, for the use of electron-microprobe equipment. I am grateful to the Department of Geology, Utah State University for the award of the J. S. Williams Graduate Fellowship. Finally, to my parents, for their understanding, sufficient financial support and encouragement, I am deeply indebted. Yunshuen Wang iii

TABLE OF CONTENTS Page

ACKNOWLEGEMENTS •••.•••.••••••••••.••••••••.••••••.•••••••••••••••••• i i

LIST OF TABLES •••••••.•.•••••.••.•••.••••.••••.••••••••••••••••••••. v

LIST OF FIGURES ••••••••••••.•••••••••••••••••••••••••••••••••••••••• vi ABSTRACT •.••••••••••••••.•..•••••••••••.••••••••••••••••••••••.•••• vii

INTROOUCT ION .••••••••••••.••••••••••••••••••••••••• ••••••••••••·•••• General Statement •••••••••••••••••••••••••••••••••••••••••••••••• 1 Location and Accessibility ••••••••••••••••••••••••••••••••••••••• 3 Geologic Setting •••••••••••••.••••••••••••••••••••••••••••••••••• 3 Previous Investigation •••••••••••••••••••••••••••••••••••.••••••• 4 Field Work ••••••••••••••••••••••••••••••••••••••••••••••••••••••• 6 Sampling and Analytical Methods ••••••••••••••••••••••••••.••••••• 6

FIELD RELATIONSHIPS ••••••••••••••••••••••••••••••••••••••••••••••••. 10 PETROGRAPHY ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 16 Snowville Basalt ••••••••••••••••••••••••••••••••••••••••••••••••• 16 Table Mountain Basalt •••••••••••••••••••••••••••••••••••••••••••• 18 Holbrook Basalt •••••.••••.••••••.••••••••••••••••••••••••..•••••• 21 MINERALOGY •••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 24 01 ivine •••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 24 Augite ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 30 Plagioclase •••••••••••••••••••••••••••••••••••••••••••••••••••••• 36 Iron-titanium Oxides ...... 39

CHEMISTRY AND CLASSIFICATION ...... 46

Major Elements ...... 46 Trace Elements ••••••••••••••••••••.•••••••••••••••••••••••••••••• 54

PETROGENESIS ...... 61

Geothermometry ••••••••••••••••••••••••••••••••••••••••••••••••••• 61

Fe-Ti oxide geothermometry •••••••••••••••••••••••••••••••••••• 61 01 ivine-cl inopyroxene geothermometry ...... 63 Spinel-olivine geothermometry ...... 64 Oxygen Fugacity ...... 65 iv

TABLE OF CONTENTS (CONTINUED)

Origin ••••••••••.••••••••••••••••.•.••.•••••••.•••••••••••• . ••••. 67

Crystal fractionation •••••••..•••••••••••.•••••••.••••••••••.. 67 Crustal contamination ••..••••.••.••••••••••••••••••••••••••••• 72 Partial melting ••.•.•.•••••••••••••••.•••.•••••••••••.•••.•••• 73

CONCLUSIONS •••.•••.••.•••••••••••.•••••••.•••••.••••••••.••.•••••••• 76

General Statement •••••••••••••••••••.•••• • • •••.••••••••••• ••••••• 76 Snowville Basalt ••••.••••••••••••••••••••••••••••••••••••.••••••• 76 Table Mountain Basalt ••••••••••••••.•••••••••••••••••••••.••••••• ?? Holbrook Basalt •.••••••••••••••••••.•••••••••••••••••••••.••••••• 77 REFERENCES •••.•••••••••••••••.••••••••••.•••••••••••••••••.••••••••• 78 v

LIST OF TABLES Page 1. Locations of analyzed samples ...... 8 2. Modal analyses of samples (volume percent) ....•.••.•..••••.•••• 23 3. Representative microprobe analyses of olivine ••.•.....•.....••• 25 4. Representative microprobe analyses of augite •.•..•.•..•..••..•. 34 5. Representative microprobe analyses of plagioclase . .•••.••... •.• 40 6. Representative microprobe analyses of magnetite and ilmenite ••.•••.••••••.•••••..••..•.••..••...••••••••••••••• 43 7. Representative microprobe analyses of chromite •••••.•....•••••• 44 8. Whole-rock chemical analyses and CIPW norms ••..••••••••••• ••••• 47 9. Average whole-rock analyses for representative lavas •.•....•.•• 48 10. Representative trace element analyses •••.••.•.••••••••••••••••• 57 11. Rubidium and strontium abundances and strontium isotopic composition .••.••.••••.••••••. •••..••••..••..•.•.•...• 59 12. Temperatures of equilibration and oxygen fugacities •.•.•••••••• 62 13. Results of fractionation models tested ••••.•..••••••••••••••••• 70 14. Percentage of partial melting required to derive Table Mountain and Holbrook basalt from hypothetical mantles of pyrolite, garnet peridotite, and spinel lherzolite composition •••••••••.•••••••• 75 'Ji

LIST OF FIGURES Page 1. Map of northern Utah and southern Idaho ••••••••••.•••••...... ••• 2 2. Index map of study areas and sample locations .•.•.•...•...•••••• 7 3. Snowville lava flow at contact with tuff •••••••••...•..•..•.•••. 11 4. View of Table Mountain ..... ••• ..•••....•...... ••.•...... •..... 13 5. Table Mountain flows at contact with tuff ..•••••.•.• ••...... •..• 14 6. Photomicrographs of representative textures •..••••••..•....•...• 19 7. Electron- microprobe analyses of olivine plotted in terms of molecular percent Fo and Fa ..••••••••.•.•...•.....••••• 28 8. CaO and FeO content of zoned olivine phenocrysts from volcanic rocks ••••••••••••....•..••••••••••••..•..••••..••. 31 9. Electron-microprobe analyses of pyroxene plotted in terms of molecular percent Fs, En, and Wo •.••••••••••...... •.... 33 10. Weight percent of silica versus weight percent of alumina in pyroxene .•••..••...... • ••• ....•.•...••.•.•••.•.•.• 37 11. Alz versus weight percent of Ti02 in pyroxene ••..••.•.••..••..•• 38 12. Electror}-microprobe analyses of feldspar plotted in terms of molecular percent An, Ab, and Or •.••••••••••••• ••• ••••• 41 13. AFM diagram ..•.•..•...•..•.•••.•.•••• ••• ••••••••••••...... •••••• 51 14. Al kal i-sil ica variation diagram ••...••..•• •• •••.••••• •• •• •.• ••• • 52 15. Compositions of basalt plotted on Di-01-Qtz diagram ••••••••••••• 56

16. Oxygen fugacities (-log10 l plotted against temperatures .•••••••• 66 17. Variation diagram (Harker type) of Si02 versus other oxides ••... 69 vii

ABSTRACT

Petrology and Mineralogy of Tertiary(?) Volcanic Rocks of Snowville Area, Utah, and Tertiary-Quaternary(?) Volcanic Rocks of Table Mountain and Holbrook Areas, Idaho

by

Yunshuen Wang, Master of Science Utah State University , 1985

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

Basalt flows occur in the Snowville area of north-central Utah and the Table Mountain and Holbrook areas of south-central Idaho. All basalt flows are aphanitic in groundmass, and contain olivine, plagioclase, augite, and opaque oxides. They can be distinguished by texture. Snowville basalt has predominantly subophitic to intergranular textures. Table Mountain basalt is fine grained, with

stumpy groundmass plagioclase and equant i1 menite crystals. Holbrook basalt has pilotaxitic to intergranular textures, with the presence of plagioclase phenocrysts and characteristic exsolution lamellae in Fe-Ti oxides. The olivine grains in Holbrook area are intensely oxidized to Fe-Ti oxides. viii

Snowville basalt contains olivine phenocrysts (Fo88 -Fo44 ) in a

groundmass of olivine (Fo63 -Fo47 l, augite (Wo 42 -Wo36), and plagioclase (Ann-Ans2l · The lower flow unit of Table Mountain basalt contains

olivine phenocrysts (Fo8a-?l in a groundmass of augite (Wo44 En44 Fs12l, and plagioclase (Ansa-An4al· The upper flow unit of Table

Mountain basalt has olivine phenocrysts (Foa2-Fo6sl, plagioclase

phenocrysts (An73-An 67 ), and plagioclase groundmass (An54-Anssl· The Holbrook basalt is composed of olivine phenocrysts (Fo6rFos7l and

plagioclase phenocrysts (An6a-An 43 ) in a groundmass of olivine (Fosg­ Fos3l, augite (Wo39 En44 Fs17l, and plagioclase (An67-An3sl· The basalts of the Snowville and Holbrook areas, represent petrographic, mineralogical, and chemical characteristics of both olivine-tholeiitic basalt and alkali-olivine basalt, whereas Table Mountain upper and lower flow units show their affinity with alkali­ ol ivine basalt. Chemically, basalts from these three areas are consistently high in silica, magnesium, and alkali content. The

Snowville basalt has a high Ba content and high strontium isotope ratio.

Fractional crystallization models indicate that the basalt flows from the three different areas are genetically unrelated. The testing also suggests that the upper and lower flow units of the Table

Mountain area are not genetically related. The basalts of the three areas also can not be evolved from the basalts found at Kelton, the

Rozel Hills or Black Mountain. Basalts of the Snowville area have consistently higher magnesium and silica contents than Snake River basalt, Kelton area basalt, and Rozel Hills and Black Mountain basalt, i x

indicating that they may represent what was initially a very primitive basaltic lava. High Ba content and strontium isotope ratio indicate that the Snowville basalt was contaminated by crustal material. Table Mountain and Holbrook basalt may have formed as a result of partial melting from a pyrolite or garnet peridotite mantle.

(83 pages) INTRODUCTION

General Statement

The Snowville area lies in north-central Utah, near the Idaho border, east of Snowville, Utah (Figure 1). The volcanic rocks of this area have been mapped by Adams (1962), and cover an area of about 125 square kilometers. Lavas of the Snowville area are basalt of

Tertiary(?) age (Adams, 1962 ; Doell ing, 1980). The study of the

Snowville basalt was concentrated on the roadcuts of the Rattlesnake

Pass area.

Tertiary-Quaternary(?) volcanic rocks occur in the vicinity of

Table Mountain and Holbrook, Idaho (Bond, 1978; Rember and Bennett,

1979) (Figure 1). The amount of exposed basalt in Table Mountain is quite limited, being less than 2.5 square kilometers. Holbrook basalt covers an area of no less than 13 square kilometers, but only the area east of North Canyon was studied.

The purposes of this study are: 1) to determine the field relationships of the volcanic rock units in the study areas and collect representative samples; 2) to determine the petrographic and chemical characteristics of the different volcanic rock units; 3) to determine if the volcanic stratigraphic sequence of the Snowville area was controlled by fractionation; 4) to compare the volcanic rocks of the

Snowville area with the volcanic rocks of Table Mountain and Holbrook,

Idaho; 5) to compare the petrographic and chemical characteri sties of i N

M M M SMILES ~ SKILOMETERS

" v.. .=

Figure 1. Map of northern Utah and southern Idaho. 3

these vol canic rocks with other Tertiary-Quaternary volcanic rocks from the Snake River Plain (Idaho), the Rozel Hills-Black Mountain area (Utah), and the Kelton area (Utah); and 6) to determine petrogenesis and establish models of origin for the volcanic rocks of the Snowville area, Table Mountain area, and Holbrook area.

Location and Accessibility

The study areas (Figure 2) consist of the southern part of the

Rattlesnake Pass quadrangle, northeastern part of the Snowville quadrangle, northwestern part of the Cove quadrangle, and southwestern part of the Holbrook quadrangle. The Snowville area is located five kilometers east of Snowville, Utah (Box Elder County). The Table Mountain area is located eighteen kilometers southwest of Holbrook, and eighteen kilometers northwest of Snowville. The Holbrook area is situated eight kilometers southwest of

Holbrook, Idaho (Oneida County). All outcrops are accessible by ranch roads for a vehicle with sufficient ground clearance.

Geologic Setting

The study areas 1 ie in the eastern part of the Basin and Range

Province. The Basin and Range Province is an area characterized by having unusually high heat flow and a thin lithosphere. It is also the most active tectonically, as evidenced by an abundance of faults and fault scarps with young displacement. Faults of known late Cenozoic age, with movement in Quaternary time, are characterized by steep dips and relatively high structural relief {Eaton, 1982).

Basaltic rock units in the Snowville area overlie Paleozoic

rocks, and in some place they are underlain by thick accumulations of

Tertiary tuffaceous sandstone and conglomerate {Adams, 1962). The area

exhibits typical fault-block structure. The normal faults offset

Quaternary lacustrine or alluvial deposits {McCalpin, 1984).

Cress {1983) described Curlew Valley as a graben that trends

north to northeast and is partially filled with late Miocene Salt Lake Formation. Deposits of 1 ate Quaternary pluvial Lake Bonneville overlie the Salt Lake Formation. Deposits of Lake Bonneville are offset by

faults on the east side of Curlew Valley.

Previous Investigation

Early investigation of the Tertiary stratigraphy of northern Utah and southeastern Idaho was undertaken by Smith {1953). His map showed

the distribution of Tertiary volcanic rocks and orogenic sediments of

northern Utah and southern Idaho, and indicated the limit of the Snake

River basalt flows. He reported that flows within the Snowville area are the southward extension of the Snake River basalt flows of mainly

1 ate Pliocene age.

The Summmer Ranch and North Promontory Mountains were first mapped in detail by Adams {1962). As part of his study,_ he described

basalt flows, about 9 miles east of Snowville, Utah as dark gray to 5 black in color, having phenocrysts of olivine and labradorite, glass, augite, and magnetite in an aphanitic groundmass. The basalt is younger than the Salt Lake Formation and older than Lake Bonneville deposits and was assigned to a late Pliocene age (Adams, 1962). One stronti urn isotope analysis has been reported for the Snowville basalt as 87Sr/86Sr=0.7104 (Leeman, 1970). Three samples from the Snowville area were collected and chemically analyzed by Fiesinger (1976, unpublished data). A comprehensive study of the geology and mineral resources of Box Elder county was provided by Doelling (1980). He described rocks in the Summer Ranch (Hansel) Mountains and North Promontory Mountains as dark gray to black, mostly vesicular, basalt flows . The groundmass is principally labradorite and glass, and a small amount of olivine is usually present. Previous investigations of the Table Mountain and Holbrook areas are restricted to statewide geological maps of Idaho. The basalts of Table Mountain and Holbrook are mapped as lower Pleistocene to Pliocene basalts with associated tuffs and volcanic detritus. They form part of a discontinuous string of volcanic rocks extending southward from the Snake River Plain (Bond, 1978), but are mapped as Cenozoic pediment gravel on the geologic map of the Pocatello quadrangle, Idaho (Rember and Bennett, 1979). There are no age dates from any of these three locations, and there is obviously a question as to the ages of the volcanic rocks, Tertiary or Quaternary. Field Work

Field work was done during the summer of 1983 to determine the extent of basalt outcrops in the area, and to select areas for sampling

for chemical and mineralogical study. The number of flow units, and

the contac ts between flows were examined. The relationships between flow units and Tertiary tuffs, Oquirrh Formation, and major faults were

determined. Samples of representati ve major flow units were collected

for further study. Locations of samples are given in Figure 2 and Table 1.

Sampling and Analytical Methods

Fresh samples were taken from major basal tic flow units

representing the various lava fields. Forty-one thin sections were

made for preliminary petrographic study. Eight samples with

representative textures were selected for further study. Samples

selected are relatively free of oxidation, alteration, and vesiculation. Modal analyses consisted of mineral determination at 1000

points using a spot interval of 0.1 millimeter.

Whole-rock chemical analyses of major elements were obtained by using the gravimetric, volumetric, and colorimetric methods described by Maxwell (1968). Alkalies were determined by using an atomic absorption spectrophotometer. Trace element determinations were made by using ion-coupled plasma-sourced emission spectroscopy at Rice Figure 2. Index map of study areas and sample 1ocati ons. Rattlesnake Pass area (A), Table Mountain area (B), Holbrook area (C). Table 1. Locations of analysed samples. Sa11ple No. latitude longt·tude locatton

SUV83·34 41° 55' 11" N 112° 37' 44" w Snowville area, ridge tope of a sfngle flow, east of Snowv1l le, Utah [5580'). SUV76-5 41° 54' sa• N 112° 37 ' 26" w Snowville area, west of Rattlesnake Pass roadcut.

SUV83-26 41° 55' 19" N 11Z0 34 1 36" w Snowville area, northwest of Rattlesnake Pass north side roadcut, top flow unit of four flow untt sequence. SUV83-36 41° 53' 34" N 112° 34' 5" w Snowville area, west of Rattlesnake Pass, the flow unit in contact with Paleozoic carbonate [5360']. SUV76-4 41° 53' 45" N 112° 32' sr w Snowville area, second lowest flow unit from bottodl of Rattlesnake Pass south side roadcut, identical to second flow unit of north side roadcut. SUV76·1 41° 53' 48" N ll2° 32' 57" Snowv111e area, northside of Rattlesnake Pass roadcut, top flow unit of fhe flow unit sequence. SUV83-18 41o 52' so· N ll2° 32' 40" Snowv111e area, gully south of Rattlesnake Pass south side roadcut, northeast of SUV83-16, tdentfcal to bottom flow unit of south side of Rattlesnake roadcut . SUV83-16 41° 52'50" N ll2° 32'40" W Snowville area, gully south of Rattlesnake Pass south stde roadcut, the flow unit tn contact wtth tuff [5260'] .

SUV83·1 42° 5' 3" N 112° 52' 22• w Table Mountain area, upper flow untt. SUV83-31 42° 5' 3" H 112° 52' 23" w Table Mountain area, bottom flow unit. SUV83·32 42° 7' 38" H 112° 44 1 58" w Holbrook area, bottom of 1 single flow unit [5720'].

()0 9

University. Mineral analyses were obtained using the SEMQ-ARL eight channel electron probe micro-analyzer at the University of Calgary and ETEC microprobe at Rice University. Phenocryst phases and groundmass plagioclase were analyzed systematically from core to rim, with an average of five spot analyses per grain. Groundmass phases were analyzed by taking two to three spot analyses per grain. Correction procedures used on microprobe data follow the method of Bence and Albee ( 1968) and Albee and Ray ( 1970). 10

FIELD RELATIONSHIPS

Basalts in the Snowville area range from dark gray to brownish black in color. The basalt flow units are dense at the bottom, grade upward to vesicular and become reddish, oxidized, and extremely vesicular at the top of flow units. The texture of the groundmass is aphanitic. The vesicles show no preferred orientation. The lava sequence is well exposed in the Rattlesnake Pass roadcuts of Interstate 84 west of exit 17. Five flow units were identified on the north side of the roadcut, with the flows ranging from lm to 10m in thickness. The south side of the Rattlesnake Pass roadcut exhibits three flow units, with thicknesses ranging from 3m to 4m. Outcrops 1/3 mile west of Frankl in Hill were observed to have four flow units. Thickness of those flow units ranges from 2m to 3m. Oquirrh Formation underlies basalt flows in some areas. In a gully south of Rattlesnake Pass, Oquirrh Formation is overlain by pale greenish-yell ow tuff with carbonate pebbles which grades upward into pale reddish-brown tuff. At the basalt contact, moderate-red tuff is overlain by a vesicular, moderate-red, oxidized basalt flow which grades upward to a less-vesicular section (SUV83-16,-18; Figure 3). Oquirrh Formation also underlies basalt on top of the small hill immed i ately southeast of the Hansel Valley exit of Interstate 84 (SUV83-36). The relationship between basalt flows and the Salt Lake Formation has not been clearly demonstrated, although the Salt Lake 11

Figure 3. Snowville lava flow at contact with tuff. Location of sample SUV83 -16. Note extreme vesiculation in lower part of flow .

.J 12

Formation underlies basalt on the ridge crest east of Snowville (SUV83-

34).

On the south side of the Rattlesnake Pass roadcut, well­ developed terraces are cut into the basalt flows, and the Lake Bonneville Group overlies the basalt. The basalt is younger than the

Oquirrh Formation and older than Lake Bonneville deposits. A normal fault, well-exposed in the south face of the Rattlesnake Pass roadcut, shows down-to-the east normal offsets in channel fills and superposed soils which overlie Tertiary basalts (Me Cal pin, 1984). A second fault, 2 kilometers further west, at the Hansel Valley exit of Interstate 84, shows shattered and altered Paleozoic carbonates emplaced against Tertiary basalt, Quaternary alluvium, loessy colluvium, and volcanic ash (McCalpin, 1984). These faulting events in the Snowville area occurred after the eruption of the basalt flows. At Table Mountain, there are two flow units each underlain by pale orange tuff (Figure 4 and 5). The tops of the tuff are slightly welded and oxidized to a moderate- red color. Each tuff unit is approximately three to four meters thick. The lower tuff unit overlies tuffaceous sediment and conglomerate of the Salt Lake Formation. The upper flow is mostly flaggy, with autobreccias occurring locally near the top. Although the upper flow is quite irregular in thickness, the lower flow has a uniform thickness of approximately 3m. The lower flow is oxidized and vesicular at both top and bottom, with the center being flaggy to massive, with subhorizontal flow banding. In a traverse along the ridge east of North Canyon in the 13

Figure 4. View of Table Mountain. Upper flow is represented by cliff face; lower flow is represented by dark band of outcrop immediately below. View looking northeast. Figure 5. Table Mountain flows at contact with tuff. A: lower flow; B: upper flow. 15

Holbrook area, tuffaceous conglomerate and tuffs are found between ridges of 1 ava. The weathered 1 a vas of the Holbrook area are 1 i ght gray. Fresh flow units are dark gray, range from dense to vesicular and are 3m thick. No flow sequence was determined in the Holbrook area, but the upper-most flow was sampled (SUV83-32). PETROGRAPHY

Snowville Basalt

Vesicular basalt from the Snowville area ranges from dark gray to

brownish black. The basalt exhibits a variety of textures, including

ophitic, subophitic, intergranular, intersertal, and diktytaxitic.

Olivine is the predominant phenocryst in all the samples, while

olivine, augite, plagioclase, and opaque minerals are the major

constituents in the groundmass.

The olivine phenocrysts in basalt occur as cumulophyric,

subhedral, equidimensional crystals, commonly containing opaque

inclusions. The phenocrysts are altered partly to reddish-brown iddingsite, especially on the edge or core of crystals and in fractures; narrow, fresh rims on the altered crystals were also

observed. Embayments of the phenocrysts are commonly fi 11 ed with groundmass plagioclase. The size of the olivine phenocrysts ranges

from 0.25 to 2.4mm in diameter. Olivine also is commonly present in the groundmass assemblage as equant, anhedral crystals in an

intergranular arrangement with plagioclase. Sizes range from 0.06 to

0.30mm in diameter. Most of the olivine groundmass grains are slightly altered to iddingsite , but sample SUV83-16 was altered intensely. An opaque mineral coating is present on grains in SUV83-36 and SUV83-16.

Olivine occurs as microphenocrysts in sample SUV83-18. The microphenocrysts are subhedral, equant grains with Fe-Ti oxide rims, and generally present as intergranular filling. Olivine 17

microphenocrysts range i n length from 0.15 to 0.30mm. Olivine grains

with distinct grain size smaller than phenocryst and larger than groundmass are described as mi c rophenocrysts .

Augite is the only pyroxene in these rocks. It commonly is in an

ophitic to subophitic relationship with plagioclase. Augite commonly

is associated with altered groundmass olivine. Undulose extinction of

augite crystals is present in SUV83-26 and SUV83-16, indicating that

the grains were strained. Groundmass augite grains are as 1 arge as 2.0mm in diameter.

Plagioclase is commonly dominant in the groundmass of the

Snowville basalt. It has a lath-like form and is randomly oriented. Plagioclase crystals are subhedral to euhedral and range in size from

0.10 to 0.80mm in length. These crystals exhibit albite, carlsbad, and carlsbad-albite twinning. Undulose extinction is commonly present in stumpy strained grains. Microprobe data indicate that the lath-like crystals and the stumpy strained grains are identical in composition. Snowville basalt contains opaque minerals in the groundmass. Magnetite grains have a characteristically well-developed cubic form, whereas ilmenites are most commonly elongate or needlelike. The iron­ titanium oxides are disseminated throughout the groundmass. Grain size of magnetite is as small as dust, and is as 1 arge as 0.01mm in diameter. Ilmenites are generally larger , ranging in size from 0.05 to

0.30mm in diameter. A spinel phase is found as euhedral inclusions in olivine phenocrysts, implying that the spinel phase was the first mineral to crystallize. These inclusions in olivine phenocrysts range 18

from 0.08 to 0.24mm in diameter. Reddish-brown glass is found in

Snowville basal~ It is generally interstitial in intersertal texture with plagioclase laths.

The Snowville basalt is characterized by: 1) predominantly sub­ ophitic to intergranular textures; 2) spinel-phase inclusions in olivine phenocrysts; and 3) olivine phenocrysts and groundmass partly

altered to iddingsite (Figure 6 A).

Table Mountain Basalt

The Table Mountain 1 avas are dark gray to dark greenish gray in col or and consist predominantly of olivine, augite, plagioclase and opaque oxides (magnetite and ilmenite) with minor biotite. These rocks show a textural gradation from orthophyric to intergranul ar.

Subhedral, equant olivine is the predominant phenocryst present in Table Mountain basalt. The phenocrysts are partly altered to reddish-brown iddingsite, especially at the edge and in the center. Fe-Ti oxide i ncl us ions are commonly present in olivine phenocrysts.

Embayments of the phenocrysts commonly are filled with groundmass plagioclase. 01 ivine phenocrysts range from 0.60 to 2.0mm in 1 ength.

Olivine microphenocrysts are present in Table Mountain lavas, occurring as subhedral, equant grains with Fe-Ti oxide inclusions. Alteration to iddingsite also is present in olivine microphenocrysts on edges and in fractures; olivine microphenocrysts are as large as 0.40mm in diameter. 01 ivine also occurs as anhedral, equidimensional grains in the 19

·,~ ~ ~ . 1 ,Ji (;,.I '.._.._., J ··~· · . , ...... 'd. • ~ , .. \ . ~" '"" .. ' ~ ... - . .li. • ~ J,, :. • • ., • • '"' :. . .A • • :"'Il. • • # . . ... ~ . . ,. , . . , J. ·.i'-. . ~• .. ~~~ ~ ~-e· ~~ .. ~ ~ '1~~- • , . '• e • I ,.._...... ' .. . ~\ ., ;'!""'. ~,'. .,•._...... '\ ,. ·, ' ' . .·_,:; ··· ;·1/J \ ·:~ ·. ;~ti:,.~J

• • "'• r ·~}1. ~ •f \'.,~ o . " ~t ~. •.. : .I .-r ~..f ' ,ft'. ·' , - ~ . ·J · ~ ( "' ,...... ~ . v,~ t_· • -; ._ ' · · J ,.,- ~v ,. ~ ~ .. r · lW • •• "~'r ,. ! ·~ · • I ~ . ~I . ,,.... t ·, 1·.: . - '\

c D

Figure 6. Photomicrographs of representative textures. A: Snowvfll e area (SUV83-18); 8: Upper flow unit at Table Mt.; C: Lower flow unit at Table Mt.; 0: Holbrook area. (width of photos 8 and C equal to O.S•m; width of photos A and 0 equal to 3.lnm). 20

groundmass. These grains are intensely al terated to iddingsite. The

olivine in the groundmass is very fine grained, ranging from 0.015 to O.OSmm in diameter.

Groundmass augite appears only in SUV83-31, and is present as

anhedral, elongated grains. The grain sizes are from 0.04 to 0.24mm in 1 ength.

Plagioclase is the only feldspar found as a groundmass

constitutent in the Table Mountain basalt. Most plagioclase grains in

SUV83-1 are stumpy and gathered in clusters. The groundmass plagioclase

in sample SUV83-31 occurs as lath-shaped crystals with stumpy strained

grains present occasionally. Undulose extinction is predominant. The 1 ath-shaped crystals range from 0.03 to 0.2mm in 1 ength. The

groundmass plagioclase laths in SUV83-31 generally have an intergranular texture with augite and olivine. Few phenocrysts of plagioclase appear in SUV83-1, they occur as stumpy discrete grains

with undulose extinction. The grain size ranges from 0.7 to l.Omm in diameter.

Opaque oxides are the major constituents of the groundmass in

Table Mountain basalt. t~agnetite and ilmenite both occur as equant, subhedral grains; the abundances of each were not determined. Grain

sizes of opaque minerals in Table Mountain basalt range up to 0.04mm in 1 ength.

Small subhedral flakes of biotite were i dentified in sample

SU¥83-31. It only appears along the vesicles, indicating it was one of the last minerals to crystallize (LeMaitre, 1962) •. Biotite grain sizes range from 0.04 to 0.15mm in diameter. 21

Table Mountain basalts are characterized by : 1) very fine grained

groundmass constituents and lack of conspicuous augite in SUV83-l; 2)

trace amount of biotite in sample SUV83-31; 3) stumpy plagioclase groundmass in sample SUV83-l; 4) abundance of Fe-Ti oxides with ilmenite in equant form (Figure 6 B and C).

Holbrook Basalt

Dark gray Holbrook lavas consist predominantly of olivine, augite, plagioclase and opaque oxides (magnetite and ilmenite) with lesser amounts of glass. Several textures can be observed, including intergranular, intersertal, poikilitic, and pilotaxitic. 01 ivine phenocrysts are subhedral, equidimensional grains with alteration to

iddingsite in fractures. Equant Fe-Ti oxide grains are found as

inclusions in olivine phenocrysts. The size of olivine phenocrysts

ranges from 0.4 to 1.3mm in diameter. 01 ivine also occurs as anhedral grains in the groundmass, and the grains are partially altered to

iddingsite. In some cases olivine phenocrysts and groundmass are partly or almost completely oxided to opaque oxides. The grain size of groundmass olivine ranges from 0.03 to O.OBmm in 1 ength.

Augite is commonly present in the groundmass assemblage of the

Holbrook basalt as small (less than 0.15mm), discrete, anhedral grains in intergranular arrangement with groundmass plagioclase.

Plagioclase occurs as euhedral, skeletal laths in microphenocrysts. Twinning of plagioclase microphen ocrysts includes 22

albite, carlsbad, and albite-carlsbad twins. Undulose extinction is dominant. The grain size ranges from 0.08 to 0.35mm in length.

Microphenocrysts of plagioclase generally contain oval-shaped inclusions (opaques, feldspar, glass, etc.). Groundmass plagioclase in

Holbrook basalt occurs as subhedral laths which are commonly flow­ aligned, and range from 0.10 to 0.35mm in length. Groundmass

plagioclase exhibits albite, carlsbad, and carlsbad-albite twinning.

Undulose extinction is common.

Fe-Ti oxide minerals in Holbrook basalt occur as equidimensional, anhedral, 0.02 to 0.16mm crystals in the groundmass, and as a replacement of olivine or augite. Tiny subhedral inclusions in olivine

phenocrysts are also present. Most of the larger grains exhibit exsolution lamellae in reflected light. Glass in the Holbrook basalt occurs as dark brown patches in the angular space between pl agi ocl ase laths. The Holbrook basalt is characterized by: 1) predominantly pilotaxitic to intergranular textures; 2) presence of plagioclase phenocrysts; 3) characteristic exsolution lamellae in Fe-Ti oxides; 4) olivine oxidized to Fe-Ti oxides (Figure 6 D).

Modal compositions of analyzed basalt lavas from the three areas are given in Table 2. 23

Table 2. Modal analyses of samples (volume percent).

S uowv i 11 e bault Table Moun to~ In llolbrook

bdSollt bHJlt

SUVOJ-34 SUVI\3-26 SUVfl]-36 SUYfl J - 1 6 SUVBJ - lU SUYI\J-3 1 SUYBJ-1 SU 'IBJ-Jl

Ph tno c r y ~ t s Oli v 1 n e 4.7 3 . 8 4 .o 4.5 3.4 '· ' 5.7 6.1 PI,; r; i , . ..; ; • ~ e o.u 0.0 o.o o. 0 O. tl c.u (.l.l) 1.H

Htcrophenocrysts

0 1 i •d;)e o.o o.o o.o o.o 1.6 o. 0 3.7 o.o

,,. 4.7 3.8 4.0 4.5 5.0 9.0 9.4 9.0

Ground~nass

01 tv i ne 1.3 12 .o 11.3 14.2 6.9 14.6 39.2 7.1

Plaglocl.ue SG.8 36 . 1 44.1 4 7 .o 57.1\ 3').6 22-6 55.1 "uglte 29.0 18.6 16.9 16.0 20.2 7.7 o.o 14.) Biotite 0.0 o.o o.o o.o o.o o. 3 o.o o.o Cpa que OJtldes 6.0 7.8 ll.O a. a 3.6 12.0 1Z.4 8.1 Undlfferentloilted 0.0 0.0 o.o o.o 0.0 JG . O 16. 4 o.o Gd111s

Glass 1.1 19.6 10.7 9.5 6.5 0.0 o.o 6.3

,,. 95.3 96.2 96.0 95-5_ 95.0 ~0. 2 90.6 91.0

AI tert•tl parts of ul hl"t" crystals wr.re cuuo tt'rl " fresh ul lv I ne. 24

MINERALOGY

01 ivine

01 ivine is present in a gradation of sizes from phenocryst, to microphenocryst, to groundmass in both Snowville and Table Mountain basalt, indicating that olivine crystallized throughout the cooling history of the basalts. The average compositions are presented in

Table 3.

01 ivine phenocrysts and microphenocrysts show a normal zoning trend of increasing iron from core to rim. The extent of zoning in phenocrysts, microphenocrysts and groundmass is illustrated in Figure 7. In general, phenocryst zoning overlaps that of the groundmass, with groundmass compositions being more iron-rich. Sample SUV83-16 shows more limited compositional variation in groundmass olivine than other samples from the Snowville area, and may reflect a different cooling rate. Olivine phenocrysts and groundmass in SUV83-16 show more intense deuteric alteration than other Snowville samples. Fresh cores and fresh edges in phenocrysts are very difficult to find, and the central parts of groundmass grains areal so intensely altered to iddingsi te.

Haggerty (1976) suggested that in single cooling units maximum high temperature oxidation develops towards the interior of lava flows rather than at the upper and lower cooling flow units. The preferred

1 ocation demostrates that volatiles were trapped or tended to accumulate in zones of prolonged cooling. Intensely altered olivine Table 3. Representative microprobe analyses of olivine.

SU¥83-34 SUVSl-34 SU¥83-34 SUVBJ-34 SUVAl-26 SUVBl-26 SUVSJ-26 SUVSl-26 SUV83-36 SU¥83-36 PC PR GC GR PC PR GC GR PC PR s1 o 2 40 .12 36.03 37.31 35 . 97 40.21 36.32 36.61 34 .az 40.44 35.59 M90 43.98 22.57 30.87 21.32 H.4J 23.99 27 . 27 18 . 92 44.73 24.39 reo 17.19 41.98 32.60 43.29 1 &.86 39.89 36.77 46.71 16.53 40.08 MnO O. ZJ 0.66 0.47 o. 71 0.27 0.57 0.48 0. 72 0.18 0.59 c.o 0 .19 0. 22 o. 26 o. 22 0.18 o. 23 0. 28 0.40 0.18 0.£0 ------Toul 101.71 101.46 101.57 101.51 ------101.95 101.00 101.41 101.57 102 .06 100.85 Nu•bers of ions on the basts of 4 oxygens

S1 1.000 1.018 1.005 1.022 0.998 1.020 1.008 1.010 1 .000 1.004 Mg 1.633 0.950 1.238 0.903 1. 644 1.004 1.119 0. A18 1. 649 1.026 re•Z 0 . 358 0.99Z o. 7JJ 1.029 0. 350 0. 937 0 . 846 I. 133 0 . 342 0 . 946 Mn 0.005 0.016 0.011 0 . 017 0.006 0.014 0.011 0.018 0 . 004 0.014 c. 0 .005 0.007 0.007 0.007 0. 005 0.007 0.008 0. 012 0 . 005 0.006 yYI 2.00 I. 97 I. 99 1.96 2.01 I. 96 I. 98 I. 98 2. uo 1. 99 Ato111fc r-Utos Mg+2 82.01 48.93 62 . 79 46.75 82.45 51 0 73 56.93 41 . 93 82 .8) re•2 52.03 17.99 51 . 07 37.21 53.25 17.55 48.27 43.07 58.07 17 . 17 4 7. 91

"' 26

:;; 0 :;; 0 :::: 0 ~ ~ 0 w 0 0 ::: ;; ~ ~ - ~ ~ :;; ::; :::: :

~ ;;: ::; ;:; - c 0 00 ~ ~ : ;; N ~ ;; - 0 :;: ::; 0 0 ~ ~ 0"' ~ ~ ~ 0 - :;; :::

N 0 ::; :;; :;: 0 00 ~ 0 :: :: 0 0 00 0 0 ::: :;; :: 0 0 0 M N M 0 - :;, ':;

~ ;:: ~ 0 0 0 ~ ~ :: 0 ~ M ;; - :::: 0 0 :5 ;;: u 0 0 0 0 N ~ :; :: 0 :;: ::

Q :;: ;;; ;;; 0 ~ 00 0 c 0 ~ ~ ~ 0 0 00 0 0 0 ~ Q ~ 0 ~ N ~ M N ~ 0 - - :::: :

00 0 0 ;;: ':; ~ - ~ ~ ::: 0 0 ;; ~ ~ :;; u 0 0 0 0 00 ~ ::; :;; ~ :::: 0 - :;

:;; ;:: :;: ~ 0 00 0 ;:; 0 00 "'0 "'0 0 :; ':; ~ 0 0 0 0 0 ~ ::: - 0 - - - :; ~

0" 0 0 0 ~ 0 0 ~ ~ q ~ ~ :: ~ 0 ~ M 0 0 ~ 0 0 0 0 0 ...: 0 0 0 ..: ~ q - 0 ';; - - :;; -

-;; 00 - ~ 0 "0 ::; ;;; - :;; 0 z ~ ;; 00 0 0 0 ::; ::; OJ ,;. - 0 ::l ;; 0 0 0 0 0 ~ q 0 - :; ~ c: - ~ - +-' c: ;:; 0 g 0 "'~ 0 0 0 ~ 0 0 0 ;c :;; v ::: 0 0 0 -0 0 0 0 ~ :!: N :::: 0 - - M M ';;

OJ

:;:; ON 0 0 0 ": 0 0 f- ~ . u .;; ~ , ~ ~ "' " "' ~ .:: ~ Ta ble 3. (continued)

SUVBJ-18 SUVSJ-18 SUVBJ-31 SUVBJ-1 SUVSJ-1 SUVOJ-1 SUVSJ- 1 SUVBl -32 SUVBJ-32 SUVBJ-32 SUVBJ-32 GC GR PC PC PR HPC HPR PC PR GC GR s1o 2 37.08 34 . 76 41 . 41 39.80 31.73 37.85 36.73 38.20 36 . 73 37.30 36.18 H90 28.52 20.95 48 . 33 43.95 32.59 34.33 28.76 33.19 27.37 28.22 24.82

FeO 35 .60 44 . 51 11.82 17.11 30.97 28.70 35.75 29.37 36.79 35 . 46 39 . 77

HnO o. 54 o. 73 0.1 2 0. 23 0.49 0.49 0. 63 0. 34 0. 58 0. 50 0. 66 CaO 0 .26 o. 29 0 . 15 0 . 20 o. 22 o. 20 0. 24 0. 26 0. 19 0.20 0 .1 9 ------Total 102.00 101.24 101.83 101.27 102 . 00 101.57 102.11 101 . 96 101 .66 101 . 68 101.62

Numbers of tons on the basts of 4 oxygens

Sl 1 . 008 1.001 1.004 0.996 1.002 0. 999 0.999 1. 006 I. 008 I . 015 1.009

M9 1.1 55 0.899 1. 746 1. 639 1.290 1.093 1 . 166 1.082 1 . 120 1.145 1 . 032 re•2 0.809 1.072 o. 240 0 . 358 o. 688 0. 887 0. 814 0. 890 0. 845 0.807 0.928 Mn 0 .012 0 . 018 0.003 0.005 0. 011 o. 011 0.015 0 . 008 0. 014 0 . 012 0.016

Ca 0.008 0.009 0 .004 0 .005 0.006 0.008 0.007 0.007 0.006 0.006 0.006 yYI 1.98 2 . 00 l. 99 2.01 2.00 2. 00 2.00 l. 99 l. 99 1. 97 1.98

Ato~a1c ratios

Mg•2 58.81 45.62 87.93 82.07 65 .22 68.07 58.91 67.22 57.01 58 . 65 52.66

re•.2 41.19 54.38 12.07 17.93 34.78 3 I. 93 4 I. 09 32.78 42.99 4 I. 35 44.80

P•phenocryst HP•11tcrocryst G•ground• ass C•111ost Hg-rtch core co111post t1on R•111ost fe-rich rim compost tton * SUV83 - 31 and SUY83-1 from Table Mountain area. SUV83-32 fro11 Holbrook area.

.....N 28

SNOWVILLE BA SAL T c p SUV33-34 c R G

p SUV83-26 c R c R G c p SU\133-36 R

c G n

c SUVBJ-16 R

CGR

p SUVSJ-18 c CMPR R c G R

TABL E MOUNTAIN BASALT c SUV83-31 L-... ..?

p SUi'B3-1 c R C R MP

HOLBROOK BASALT p R SUV83-J2 c ~ C R G

fo 20 40 60 ao fa Figure 7. Electron-microprobe analyses of olivine plotted in terms of molecular percent Fo and Fa. P=phenocryst ; MP=microphenocryst; G=groundmass; C=most Mg-rich core composition; R=most Fe-rich rim c~mposition. 29

phenocrysts and groundmass in sample SUVS3-16 are consistent with the maximum high temperature alteration of interior lava flows described by Haggerty ( 1976).

Phenocryst olivine in Table Mountain basalt sample SVUS3-31 is more magnesium rich than that in other basaltic lavas, with a core composition as high as Foss· Phenocryst and groundmass compositions in Snowville basalt show more iron-rich rims than Table Mountain basalt and Holbrook basalt, with compositions as low as Fo 42 • Olivine phenocrysts in the Holbrook basalt are the least magnesium rich, having core compositions around Fo67• The compositional variation of olivine is more restricted in the Table Mountain and Holbrook basalts.

Simkin and Smith (1970) reported on microprobe analyses of natural ol ivines from different kinds of igneous rocks. Their work indicated that most plutonic ol ivines have less than 0.10 wU. CaD, whereas those from extrusive and hypabyssal rocks contained more than

0.10 wt't CaO. This suggested that pressure of crystallization plays a major role in determining Ca content. A study by Stormer (1972) suggested that the calcium zoning trends in olivine correlated with the rock type for a volcanic rock suite from northeastern New Mexico.

Using data from a variety of other sources, Stormer (1973) illustrated that microprobe analyses of olivine phenocrysts from nephel ini te and basanite 1 a vas show zoning toward calcium enrichment. Tholeiitic and other more siliceous 1 avas show 1 i ttl e or no such calci urn enrichment, but more extensive magnesium-to-iron zoning. Consideration of reaction between clinopyroxene and olivine indicates that silica activity as 30

well as pressure is an important variable. Dl ivines that show strong calcium enrichment trends could be interpreted as resulting from pressure release during crystallization, whereas a magnesium-to-iron trend with no increase in calcium would reflect crystallization with stable pressure conditions (Stormer, 1973).

CaD and FeD content of zoned olivine phenocrysts from the three areas are shown in Figure 8, with the core and rim compositions joined by straight lines. Figure 8 also shows the calcium and iron content of the cores and corresponding rims of ol ivines from a wide variety of volcanic rock types. Dlivines of this study show little enrichment of calcium but a strong iron enrichment trend, indicating their affinity with more tholeiitic rock types. Constant calcium content may also represent a stable pressure crystallization environment.

Augite

Augite is conspicuously present in the groundmass of all basaltic lavas except SUV83-1. Compositions and zoning trends are shown in

Figure 9, and representative analyses are presented in Table 4. The clinopyroxene trend of most samples reflects stable equilibrium partitioning of Ca, Fe, and Mg between the precipitating material and melt (Smith and Lindsley, 1971). Clinopyroxene in sample SUV83-36 shows a "quench trend" characterized by Fe-Ca substitution, with Mg constant

(Smith and Lindsley, 1971). Smith and Lindsley (1971) stated that the

"quench trend" apparently reflects a metastable crystal-liquid partition caused by rapid crystallization. Augite of Table Mountain 31 J

0.4

0 CJ v ~,_ o.J ::

o.2

o.1

20 30 40 so WT% feO

Figure 8. CaO and FeO content of zoned olivine phenocrysts from volcanic rocks. The core and rim compositions are joined by straight lines. filled circles: Snowville area; filled triangles: Table Mountain area; filled squares: Holbrook area; open circles: nephelinite (Stormer, 1972); open triangles: alkali basalt (Stormer, 1972); crosses: tholeiitic basalt (Moore and Evans, 1967) 32 Figure 9. Electron-microprobe analyses of pyroxene plotted in terms of molecular percent Fs, En, and Wo. Open fields represent compositional ranges. Lines illustrate the Skaergaard trend for calcium-rich pyroxenes (Wager and Brown, 1967). Numbe r s on the diagram are sample numbers. For sample SUV83-36, dots represent divergent analyses. 33

;; :::

ul;:; ¢ c 34

Table 4. Representative microprobe analyses of augite.

SUV8l-J4 SUVSJ-26 SUVSJ -36 SUVA) -16 SUVBJ-18 SUVSJ-31 SUVBJ-32

Si Oz 51.50 Sl.ll 51 .13 49 . 52 50.76 49.80 51. 18

AI 2o3 2.39 2.40 2.31 3.41 2. 20 4. 27 2. JZ

Ti Oz 1.09 1.22' 1.08 I. 65 1.13 1.07 0 . 94

H90 15.34 15.20 16.15 1 ), 68 15. so IS . 18 15 . 45

reo 10 . 48 11.34 11.4 5 II .36 11 . 04 7 . 44 9. 94

HnO 0.25 0 . 26 0.26 0. 25 o. 26 0.13 0 .22 CaO 18.81 18.57 17.55 19.80 18.69 21.51 19. so HdzO o. zo 0.17 o. 16 0. 23 o. 19 0. 34 0. 30

Total 100 . 06 100.27 100.09 99.90 9? . 77 99.74 99 .as

Number of fo ns on the bHi ~ of 6 O'l.y')ens

Si 1. 919 I. 908 1. 908 I. 868 1. 905 1. 855 1. 912 Al IV 0.081 0.092 0 . 092 0. 132 0.095 0.145 0.088 AI y I 0 . 024 0.014 0.010 0.019 0.002 0.042 0. 014 Ti 0.031 O.OH 0.030 0 .047 o. 0)2 0.030 o. 026 Hg 0.852 0 . 846 0. 898 0 . 769 0.867 0.1143 0 . 860 Fe 0.327 0 . 354 0.357 0. 358 0 . 34 7 0 . 232 o. 311 Mn 0 . 008 0 . 008 .o.ooa 0 . 008 0 . 008 0.004 0.007

Ca 0. 751 o. 743 0. 702 0 . 800 0. 752 0.858 0 . 781

Na 0 . 014 0 . 123 0.012 0. 017 o. 014 0.025 0 . 022

SUII VI 2. 007 2. 122 2 . 017 2.018 2.022 2.034 2.021 Hol ecul ar percent end •e•bers

En 44 . 15 43.53 45.89 39.89 44 .1z 43.60 44.08

Fs 16 . 93 18.23 18 . 26 18.59 17.63 11.99 IS . 92 Wu 38.92 311.2-1 35.85 41 . 52 38.25 44.41 40 ~ 00

SUY83-l1 fro• T.:abl~ llovnt.tin •rea. SUY83-32 fro• Holbrook o1ru. 35

basalt (SUV83-31) is of a more Ca-rich composition, and generally trends toward hedenbergite, similar to that observed in SUV83-16.

Groundmass augite in Holbrook basalt shows 1 imited zoning, with an iron-enrichment trend similar to Skaergaard (Wager and Brown, 1967).

LeBas (1962) stated that the sodium contents of clinopyroxene from tholeiitic, high alumina, and calc-alkaline series rocks averaged

0.35 weight percent, which is 1 ower than the clinopyroxene of normal­ alkaline series basalts (average 0.55 weight percent) and the clinopyroxene of per-alkaline series rocks (average 0.70 weight percent). The average Na 2o content in clinopyroxene of Snowville basalt is 0.19 weight percent, which is much lower than the average

Na 2o content of clinopyroxene from non-alkaline basalts. In Table

Mountain and Holbrook basalts, the Na 2o content of clinopyroxene is 0.34 weight percent and 0.30 weight percent respectively , which is also 1 ower than the average Na 2o content of clinopyroxene from non­ alkaline basalt series.

Kushiro (1960) presented a statistical study of chemical compositions of cl inopyroxenes from igneous rocks. He found clinopyroxenes from oversaturated tholeiitic magma tend to have a higher proportion of Si, a 1 ower proportion of Al and a 1 ower proportion of Ti than the clinopyroxene in undersaturated magma.

Cl inopyroxenes which come from alkaline rocks or feldspathoid-bearing rocks should have lower Si, lower Al, and higher Ti proportions.

Verhoogen (1962) proposed that entry of titanium into pyroxene is dependent on the free energy of mixing and is therefore possible only at high temperature. Aluminum in Z-si tes in pyroxenes favors introduction of Ti; Al in the Y-si te does not. Low silica activity favors the solution of Ti in silicates; more titanium is expected in pyroxenes in feldspathoidal rocks.

LeBas (1962) divided host magma into three types on the basis of the silica to alumina ratio and the relationships between Alz and titanium in clinopyroxenes. The three fields are: 1) non-alkaline, which includes the tholeiitic, high-alumina, and calc-alkaline series;

2) normal-alkaline, which includes the alkaline basalt-hawaiite­ mugearite series; and 3) per-alkaline, which includes nepheline-basalt­ tephrite-phonol ite series. Figure 10 illustrates the weight percent silica versus the weight percent alumina in the analyzed clinopyroxenes. Samples SUV83-16 and SUV83-31 plot in the normal ­ alkaline field, while samples SUV83-18, SUV83-26, SUV83-32, and SUV83- 36 lie in the non-alkaline field. LeBas (1962) also stated that in pyroxenes of the non-alkaline types, Ti does not enter the Z group; in the normal-alkaline series, Ti occasionally enters, and in the per­ alkaline series, it frequently enters the Z group. Figure 11 shows values of percent Alz versus weight percent Ti02 for clinopyroxenes of this study plotted in the fields of LeBas (1962). SUV83-16 and SUV83-

31 plot in the normal-alkaline field, with the other six samples plotting in the non-alkaline field.

Plagioclase

Average plagioclase compositions of basalts studied are given in 37

Sl N o n-al k aline

Sl

.~ sn

49 Normol-c l k.::lln ~

4e

47 ../'

46.L------~------~2~------~J~------~4~------~------~6~------~---­ WT% A1 0 2 3

Figure 1~ Weight percen t of si lled ver~u5 weight percent of alumina 1n pyroxene. Dasn ed 1 ines define fie1cs of LeBas (1962). All symbol es used as in Figure 8. 313

14 .

12

10

6

8 J6~ 26 32 • • • .J4

2

0 3 WT ~o Ti02

Figure 11. Al z versus 'lleight percent of TiOz in pyroxene. Dashed 1 ines define fields by LeBas (1952). All symbols used as in Figure 8. The a rrows indicate the general trends of differentiation. 39

Table 5. The compositions were determined by recalculating partial analyses of CaO, Na 2o, and K20 into feldspar endmembers. The plagioclase groundmass compositions of the two samples from

Table Mountain are distinct. SUV83-31, with compositions ranging from labrodorite (An 59) to andesine (An4 7), is more sodic than SUV83-l, which has compositions ranging from An 64 to Anss· Plagioclase phenocrysts in sample SUV83-1 are generally more calcic than groundmass plagioclase. Few plagioclase phenocrysts were found in thin section, indicating that some mechanical process of fractional crystallization, such as filter pressing may have occurred. Groundmass plagioclase in

Snowville basalts is zoned normally in composition, with cores of An 78

(bytownite) to rim as sodic as An 53 (labrodorite). This plagioclase is more calcic and shows a greater range of composition or zoning than

Table Mountain basalt.

The groundmass plagioclase composition in Holb rook basalt is zoned with between An67 (labradorite) and An35 (andesine). It is generally more sodic than the coexisting plagioclase phenocrysts, which are zoned from An7o to An45· Phenocrysts and groundmass pl agi ocl ase laths in Holbrook basalt are more sodic than plagioclase in Snowville and Table Mountain basalts. Representative samples are shown in Figure

12 to illustrate the zoning patterns.

Iron-titanium Oxides

Coexisting magnetite and ilmenite occur as groundmass phases in 40

Table 5. Representative analyses of plagioclase.

SUUl·H SUYIIl·l~ ~11111}-16 SUYHl-l6 SUYBl•H SI/Yfll.}G Hll'll-16 ~HV111·U SUUl-U SUYIIl ·U GC Ga « Gl « CN CC G~ K CR

Wtltht perce11t o•ldtl

1).11 \1.06 11.12 )4.76 \0 . 80

"'20 ),2J J.\8 4 , U

,,, 0. ·~ \It .,~t ptrct•H '"' ..... bll'l ~· .zs 72.4\ 18.61 ,, ll.l1 U.tO 4] . 1S

91-li 101.70 100-ll \00.46 IO I.l7 100 .46 100 .34 10\. , 100.51

llo\t(lll .r perc tilt tnd •t•b,.\ 68.41 sz .oo 70.11 SJ . l& 1t.SQ 21.71 Z7.1\ H .9 l U.40 0.\S

,, Z-57 J,)2 1.9l J,S)

S\IYIIJ-ll 'iU'Il.ll·JI ';IJ't~J-l S U 'f~J-1 SUY~J-1 ~U'Ifol- L SIH9 l-H SUY8l -Jl SUV9l-l2 SIJYAJ.)l Gt G~ ~ C ? ~ 1;~ CR ~C 9M CC G~

11.11 ILJJ

).52 6.2J ).6\

O.l\ O.l9 0.}7 o.st o.zs

48.0 7 7].2 7 65.31 31 . 110 H . Zt 58.11

l- 11 1. 49 5.56

\OO.Z) I CO. ll n . 60 I OO.U ! OO.l9

S8.41 7J.OI U.\S 4).45 67.11 H.U

lO.I"Z Sl.)6 \.40 I.U Z. LZ (1 .16 1.44 "·"s.n

PC•phenocrt1t JR•oloenocryH rl• r.C•9r~"'"'l•us cort CiR•'Iro..,nd••n .-1• SU¥111-l\ 4ftd SUY'Il-1 fro• T~b\1 MunUIII trU. SUUl-Jl rro• Kolbroot uu. Ab L---- Or

Figure 12. Electron-microprobe analyses of feldspar plotted in terms of molecular percent An, Ab, and Or. Filled circles indicate maximum and minimum 1 imi ts of zoning. P=phenocrysts, G=groundmass. Also shown are molecular percent normative feldspar compositions. The curve in the normative feldspar field represents the solidus (Carmichael and others, 1974). Numbers on the diagram are sample numbers. ,. 42

both Snowville and Table Mountain basalt. Representative analyses are

presented in Table 6, with total iron recalculated to weight percent

Fe 2o3 and FeO, based on the method of Carmichael (1967a). Analyses were not made whenever these oxide phases were exsolved, oxidized or

extremely small. Groundmass magnetite is noticeably enriched in Cr2o3. and Al203, whereas the coexisting groundmass ilmenite shows higher MgO. Opaque oxides within olivine phenocrysts are characteristically

high in Cr2o3 and are considered as chromite. Representative analyses of chromite are presented in Table 7. Groundmass magnetite in Table

Mountain basalt ranges from Usp43 to Usp61 , while that of Snowville basalt ranges from Usp44 to USP68• The composition of magnetite from Table Mountain basalt is

similar to that of Snowville basalt. Compositions of the coexisting

hematite-ilmenite solid solution in Snowville basalt range from 92 to

96 molecular percent ilmen i te whereas the hematite-ilmenite

compositions in Tab 1 e Mountain basalt are 91 and 97 molecular percent

ilmenite. Fe-Ti oxide phases of Holbrook basalt were found to be

exsolved and microprobe analyses were not made.

Carmichael and others (1970) proposed that silica activity of

magma and ferromagnesian minerals are related by the reaction:

MgAl204 + Si02 = MgAl2Si06• so that low silica activity in the liquid favors the left-hand side of

the reaction. Carmichael and others (1974) noted that titanomagnetite

in typical basic lavas of low silica activity contains 3 to 5 percent

Al203 and 1 to 3 percent MgO, versus 1 to 2 percent Al203 and 0.5 to 1.5 percent MgO for tholeiitic basalts. The MgO content of magnetites 43

Table 6. Representative microprobe analyses of magnetite and ilmenite. SUUJ.JI SUYIJ-JI SI.IUJ·I "·•~•llh ~UUl-H UI1U-U 'UUl·U SVUJ · II

O.ll O.)l SIOz o.u 0.10 IL\4 TIOz lt.ZI ..... 14.91 Z.lO z.u z.u I. IS .~o1 1 a 1 o.n 0.06 0.06 o.H cr 2o1 "·U 16.SI II. II 11-0l 0 .41 0 . 4) 0 . 48 O.SI O.H ,,, o.tl o.oo o.oo o.oo o.oo ... ,, _,, 9S.I1 91.00 96.01 "•z01 l6 . U \9 . 19 lS . iS )6,JZ ZS.04 17.\0 ,,, 41.61 SO.Sl 4Lll 4).15 4} ,64 ,_,} Tot• l U.ll 91.H 91.29 ,_z, IOO.JZ

"oleculir fri(tiOII1 O.llS6 Q.SSBS O.l9H Q.ilU '" 0.49S6 o.sna o. uts Q.4Jl0 Q,6Q6S

ll•enlte SIIYU·H SU1ill·U SUUJ-l6 S ~ vqJ.\S ~UHJ·I~ :uv~ ~ -n SUY8l-l

:l.ll Slllz o.?u 1).4l o.z; TIOz H.U 41 . 88 S0.\1 49.')6 i 8. U Al zO J o.ao o.oo o.ol a.oo cr2o1 ,,, i J.SJ 45.79 U.10 Q,S'f o.n O.TZ o.u z.ao Z. ll ,,, z.u \.41 l.9Z l.U z.ll ,,, o.oo o.oo o.oo o. oo o.oo o.oo '" o.oo o.oo o.oo ,_,, , .n n.u ,.. U.90 U.H t7.J6 n.ao

l-1' ''l0J )I.Zl 40 . 4) r~o li.U 40.U 42.)0 40 .61 ... u.n u . oo J1,J1 ,.u 100.01 100.1! n.u

Jtoltclllll' fr•cthou 0.9JU 0 .9084 0 . 97U 11• o.nH o.nn o.tH' o.nn O.Ol5f .- 0.07U o.out 0.0654 0 .0401 0.0604 o.otL6

Ft•l.Ft•l rtCficlllHt• ~Ht4 tR tl

Tab 1 e 7. Representative microprobe analyses of chromite.

Chromite SUV83-34 SUV83-26 SUV83-36 SUV83-16 SUV83-16 A

0.25 0.86 0. 4 5 Si0 2 0. 33 0.40 1. 70 1. 86 2. 0 5 27.45 Ti0 2 2.08 21.38 21. OS 2.64 A1 20 3 19. 56 20.50 32.36 33.50 32.56 7. 24 Cr 2o3 31.04 FeO 37.05 29.73 3 2. 16 33.98 57.88 MnO 0. 51 0.37 0.49 0.3 7 0.61 MgO 7. 89 10.44 9. 16 8.86 1. 82 CaO 0.00 o.oo 0.03 0.00 o.oo Ni 0 0. 07 0. 10 0.04 C.03 0.00

Sum 98. 53 95.60 99.93 99.76 98.09

10.54 10.69 4.46 Fe 2o3 14 .52 11. 15 FeO 23.73 19.69 22.68 24.36 53.87 -- Tot a 1 99.91 96.71 99.93 100.83 98.54

Fe+3_Fe+2 recalculated based on the method of Carmi c hae 1 ( 196 7 a). A=Altered chromite. 45

from Snowville basalt ranges from 0.13 to 1.53 and weight percent Al203 from 0.85 to 2.82 weight percent. Magnetite in Table Mountain basalt has 1.14 and 1.53 weight percent MgO and 1.75 and 2.18 weight percent

Al203. The chemical composition of magnetite in Snowville basalt and Table Mountain basalt indicates that these basalts are intermediate between the ranges for low silica activity lavas and tholeiitic lavas.

Isolated chromite inclusions in olivine phenocrysts contain Cr203 ranging from 31 to 34 percent, which is lower than chromite of the tholeiitic basalts of the 1959 Kilauea Iki (40 to 46 percent cr2o3l and

1965 Makaopuhi eruptions (35 to 43 percent cr2o3) (Evans and Wright, 1972). Alteration of chromite appears in sample SUV83-16 where the grains are found in olivine phenocryst rims. Decreased Cr2o3 content in altered chromite is correlated with decrease in MgO and Al203 and increase in FeO and Ti o2. 46

CHEMI STRY AND CLASSIFICATION

Major El ements

Wet chemical analyses and CIPW norms of eleven whole rock samples

are presented in Table 8. Chemically, all of the 1 avas studied are

basalt, with Si02 between 49 and 52 weight percent (Streckeisen, 1979). The basaltic lavas of the Snowville area have Si02 contents between 50

and 52 weight percent, which are consistently higher than the basalts

from Table Mountain and the Holbrook area. The lower flow at Table

Mountain has higher magnesium, calcium and phosphorus contents than all

other lavas; SUV83-31 has the highest total water content, in part due

to the presence of biotite. The intensely oxidized Holbrook basalt has

7.73 weight percent of Fe 2o3, which is significantly higher than other samples, and the potassium content is lower than the average Snowville

basalt and Table Mountain basalt.

Average analyses of the basalts from the Snowville and Holbrook

areas, and the analyses of Table Mountain basalt, were compared with

the average analyses of Craters of the Moon lavas ( Stout and Nicholls,

1977), Snake River Plain lavas (Stout and Nicholls, 1977), Glenns Ferry

Formation, Snake River Plain basalt (Pliocene to Middle Pleistocene;

Tilley and Thompson, 1970), Bruneau Formation, Snake River Plain basalt

(Pliocene to Middle Pleistocene; Tilley and Thompson, 1970), and

'average' basalt (Nockolds, 1954) (Table 9).

The Snowville area basalt has a higher silica content than

'average' basalt, but both Table Mountain basalts and Holbrook basalt 47

Table 8. Whole-rock chemical analyses and CIPW norms.

~~~~8l-l4 SUfl6-~ SUYU -26 SUU)·li SUHIJ-16 SUUJ·II, SUfl6-4 SUU6 - l SUUl · ll SUUJ-1 SUYil·H

S!.U SI.U 5\.57 5\.U 51.20 SO.tt 49 . ll 49-U

\.37 \.08 !.SO \ , Jl l.ll 1.09 l.SJ 1.40

)4.16 ILSS 14.75 \5.00 S.\7 4,)1 1.7) 2.01 2.0\ \.61'1 l.l9 5-09 5.45 6 . 56 4.34 l - 21 8.62 1.60 0 .16 0.15 0.15 Q.l4 0.14 o.u o.u 6.59 1.27 7.60 6.86 7.27 7.26 8.28 8.74 a.ss 1 - 65 .8.89 8 - ZT 9.58 9 . 09 9-il Z.H 2.91 2.48 2.39 2-ll 2.H 2.30 z.s6 I.U 0.96 l.tZ 1.89 t.n Z.Ol 1.46 \.98 O.J9 0.31 0-l• 0.41 0.41 0.41 0.5\ 0.84 1.06 O.Sl 1.04 0.90 t.lZ \.ll 1.39 o.u O.ll 0.08 o.u 0 . 97 Q, JI

99.4) Tot.al 99.1) 99.30 u.sz 99 . 12 99.92 100.01 100.It 99.96 99.l9

0.4] Z.\2 1.06 \.73 \.97 0 .17 5.61 II.H 9.S7 \2.00 11. 70

4l.59 u.n u.zs u.96 45.15 u.79 U.)l 46.79 24 .79 20.91 \9.S5 \9 .18 19.10 19.46 ZJ,U 2\.U H.6Z n . u 11.1Z lS.It [ ... ~ l u.6L 21.as ZLZJ l4 .l1 Zt . 16 u.u 21.0' zs.n n .u o.oo o.oo 0.00 0.00 IJ.SI 1!.40 \O.U 1).51 [ Wo) '· " 5-H 1.U 1.26 [En) 14.95 ].16 ).64 S.ll 6-21

P'sl 2.93 l.74 2.01 1.41 I .U o.oo ,, ZZ.89 ZO.Zl 24.62 22.62 \9.01 ZJ.30 tz.ZO Z0.09 16.65 I0.\4

lEn\ l\.95 14.95 tz.ll 14.)4 '·" 14.29 0.00 tl.JO 10-14 !Fsl 1.96 z.u 5.11 o.oo o.oo o.oo o.oo o.oo 0 . 00 10.60 o.oo ( fo) 0 .00 o.oo o.oo o.oo o.oo .... IF .a) o.o o o.oo o.oo o.oo 2.11 o.oo o.oo J.n z.•1 ).6\ s.oo s.zo Lll \0.45 2. 96 2.60 2.05 2.85 z.u Z.SI z.u 2.07 Z.91 z.u o.oo o.oo o.oo o.oo o. oo o.oo o.oo o.oo o.sz 0.99 0 .11 0 .97 2.04 1.11 0-"

u.n 9a.zo u .u u.z• u.11 n .u u.11 u.2o u.•t

SL70 S1.l6 ss .n s1.u H.U u.11 sr.n U.H SS.71 , 60.ll F••tc 42 .1 ] 40.62 )9.14 40 . 72 41.71 42 , ,. li.SI ll.H . zt.n 37.61 JZ.U ]2.11 Table 9. Average whole-r~~~n~~ses_!9r representative lavas . 2 3 ' SiOz 51.36 49.41 1\9.]) 49.65 !iU. 03 4(i .89 4 s. 72 45.15 49 . 0 fi 02 1. 30 I. 09 1.5] I. 40 2 . OJ 2.55 3. 06 ) . 47 3 . 32

AI zOJ 14./2 13 .95 14.76 15 .09 14.07 14.74 12.77 12 .55 1 3 . 71 re,o 3 2. 94 ). 59 s .01) 7. 73 2 . fl8 2.14 ) . 06 4.53 3. 07 F;,:O 7.56 5. 45 6. 56 4 .34 9. 05 11. 14 l s. 05 l l. 91 11.28

HnO 0. 16 0. II 0. I Ci 0.16 u. 18 0. 18 0. 26 0. 22 0. 22

HgO 7.06 6. 91 7. 26 6 . 59 6. 34 a. 23 4.15 7.02 3.68

CoO 8. 66 9 . 58 9.09 9.12 10 .1\2 10.06 7 .53 9. 17 6 . 79

Na zO 2.45 ) ,lJ 2. 74 2.93 2 .23 2. 68 3.57 2.40 J. •n

K2o 1.71 l. 98 l. 43 0. 96 0 . 82 0. 65 1. 87 o. 61 2 . 43 PzOs o. 39 0. 86 o. 47 0.41 o. 23 o. 54 2. 20 o. 76 1. 40 Hzo• 1.09 1. 39 0. 5 1 0 .R4 0. 91 0. 33 0. 42 0.69 o. 54 H20· o. 32 0. II 0. 31 0.21 1.22 o. 26

Total 99.84 99 .96 99 . 29 99.43

I. Ave r age Snowville basalt. 2. SUVBJ-31, lower f l ow unit of Table Mount.1in bas.1lt. 3 . SUVBJ-1, upper flow unit of Table Mountain basalt. 4. llolb r ook b.lSalt , s. 'Average' basalts (Nocko1ds, 1954). 6. Snake Rive r Phf n (Stout and tl1chulls , 1977). 7. Crate r s of the Moon {S tout and Nicholls , 1971) . 8. Glenns Fer ry For111ation, Sn olke River Plain basalt (Ttlley and Tompson, 1970). 9. Bruneau Formation, Snake River Plain basalt (Tilley and Tompson, 1970}.

"'00 49

are lower. Basalts from the three different areas show lower titanium and calcium contents than 'average' basalt but higher magnesium and alkali contents. The Snowville and Table Mountain basalts have lower total iron contents than 'average' basalt. Snowville, Table Mountain and Holbrook basalt all show consistently higher silica and lower titanium and total iron contents than Snake River Plain and Craters of the Moon lavas, Snake River Plain basalts from the Glenns Ferry

Formation and the Bruneau Formation. Snowville and Table Mountain basalt demostrate higher magnesi urn contents than basalts of the Glenns

Ferry Formation and the Bruneau Formation. The magnesium content of

Holbrook basalt is also higher than the magnesium content of the

Bruneau Formation basalt. The chemical compositions of lavas from these three different areas are significantly different from Craters of the Moon lavas, having higher aluminum, magnesium, and calcium contents, and lower alkali, total iron, and phosphorous contents.

The differentiation index (DI) of Thornton and Tuttle (1960), defined as the sum of normative quartz, orthoclase, albite, nepheline, leucite, and kaliophilite, is a measure of the "basicity" of a rock, or an indicator of the degree of differentiation from a primitive liquid.

The differentiation index for basalts of this study are presented in

Table 7. The DI values of Snowville basalt vary from 30 to 38, whereas Table Mountain samples have Dis of 32 and 39, and Holbrook basalt has a

DI of about 35. All can be defined as basic (a differentiation index below 50). The DI values of Snake River Plain lavas reported by Stout and Nicholls (1977) range from 24 to 29, but the rocks of Craters of 50

the Moon have Dis greate r than 40, indicating that Snowville basalt,

Table Mountain basalt , and Holbrook basalt are probably more differentiated than Snake River Plain basalts , but less than Craters of the Moon basalts.

An AFM diagram was plotted ( Figure 13) with A (weight percent

Na20 + K20 ), F (weight pe rcent total iron as FeO), and M (weight percent MgO) , recalculated to 100 percent and with the trends for the

Skaergaard Complex, Thingmuli, and Cascade suites (Carmichael, 1964) shown for comparison. The fields of the Snake River Plain and Craters of the Moon lavas are also indicated on this diagram (Leeman and

Vitaliano, 1976). In Figure 13 the Table Mountain basalt shows a strong iron-enrichment trend, while Snowville basalt shows a cluster, near the mafic end of the Cascade trend; both are different from the strong iron-enrichment trend of Snake River Plain basalts.

The CIPW norms of the rocks show the presence of quartz and hypersthene except sample SUV83-31. SUV83-31 sampled from the 1 ower unit of Table Mountain, shows normative olivine and nepheline.

Following the nomenclature of Yoder and Tilly (1962), most of the basaltic lavas are saturated to oversaturated with silica; they contain normative hypersthene with normative quartz and are classified as tholeiite. SUV83-31 is an alkali olivine basalt.

Using the criterion devised by Macdonald and Katsura (1964) for

Hawaiian lavas, weight percent Na 2o + K20 is plotted against weight percent Si02 (Fig. 14). The dashed 1 ine divides tholeiitic from alkali basalt. Figure 14 shows that most Snowville and Holbrook basalts lie in the field of tholeiitic basalt (except SUV76-4), whereas Table 51

Figure 13. AFM diagram. Skaergaard (SK), Thingmuli (TH), and Cascade (CS) trends from Carmichael (1974); Snake River Plain (SRP) (horizontally lined field), and Craters of the Moon (COM) (vertically lined field) from Leeman and Vitaliano (1976). Symbols used as in Figure 8. 52

6 Alkaline/ / / / / / - 5 et•~• / 0 / ~ / ... / •'• High-alumino ~ / ,.... _, 0 ,, 0.. / . :.: 4 + A o 0 0 oA z o A 3 A t.~ / / / / / / '[ / ._--~~4~s__ .__. __ ~~~~s~o~L-~--~~~si---

Sio, !wr%J

Figure 14. Alkali silica variation diagram. Series of Kuno (1g66) separated by solid lines; upper field alkalic, lower field tholeiitic of Macdonald and Katsura (1964) separated by dashed line. Open circles=Kel ton basalt (Voit, 1985), open triangles=Rozel Hills and Black Mountain basalt (Greenman, 1982). Symbols of study areas used as in Figure 8. 53

Mountain basalt 1 ies in the alkali basalt field. The petrographic

study of the rocks from the Snowville and Holbrook areas does not confirm this classification due to the absence of modal Ca-poor pyroxene, the presence of modal augite, and the presence of more than 5 percent modal olivine, which are the characteristics of alkali-olivine basalt {Kuno and others, 1957). High-alumina basalt was stated by Kuno

{1960) to contain higher Al 2o3 {generally higher than 17 percent and rarely as low as 16 percent) and intermediate alkali contents. The high-alumina basalt also may yield a little normative quartz or normative olivine. The two solid lines in Figure 14 determined by Kuno

{1960), denote the general boundaries between the tholeiitic series, the high-alumina series, and the alkali basalt series. Figure 14 shows that samples SUV76-5, SUV83-18, SUV83-26, and SUV83-36 lie in the field of high-alumina basalt, while the other seven samples all lie in the field of alkali basalt. The Al 2o3 contents of samples SUV76-5, SUV83- 18, and SUV83-26, and SUV83-36 are consistently lower than typical high-alumina basalt, but alkali contents are higher. The porphyritic texture of Snowville basalt is different from high-alumina basalt which typically has a nonporphyritic texture.

Coombs {1963) found that an indicator ratio based on normative components of whole-rock analyses can be defined by Hy+2Qz/Hy+2{Qz+Di) in molecular proportions. This ratio indicates the projected course of any liquid from which olivine or olivine and plagioclase are the only phases crystallizing. When compositions are so deficient in silica as to contain normative nepheline, the indicator ratio may be formulated 54

as Ne/Ne+Di in molecular proportions and is considered to be negative.

He also found that an indicator ratio of 0.38 to 0.00 is characteristic of mildly alkaline basalt, an indicator ratio 0.65 to 0.50 is characteristic of tholeiitic basalt, and an indicator ratio 0.50 to

0.39 is transitional from tholeiitic basalt to mildly alkali basalt.

The samples of this study were plotted on a Ne-Di-Ql-Qz diagram (Figure

15). The samples exhibit significant variations, with SUV83-31 plotting in the basanitic alkaline field, SUV76-4 and SUV83-1 plotting in the mildly alkaline field, SUV83-32, SUV76-5, SUV83-16, and SUV83-34 plotting in the transitional field, and SUV76-1, SUV83-18, SUV83-26, and SUV83-36 plotting in the tholeiitic field. Coombs (1963) also stated that any individual analyses showing more than 5 weight percent

Fe2o3 or 5 weight percent H20 have the probability of secondary alteration, and the number should be rejected. The norm for the highly oxidized Holbrook basalt was recalculated with Fe0/(FeO+Fe 2o3) equal to 69.01 the average FeD/( FeO+Fe2D3l ratio of Snowv i 11 e and Table Mountain basalt. The corresponding change in normative mineralogy is shown in

Figure 15, with the two values connected by a dashed line.

Trace Elements

Four trace elements (V,Sr,Zr,Ba) determined by ion-coupled plasma-sourced emission spectroscopy analyses for Snowville basalt are listed in Table 10. Data for different kinds of igneous rocks, analyzed by different methods also are shown for comparison. All samples from Snowville basalt contain relatively high amounts of the 55 Figure 15. Compositions of basalt plotted on Di-01-Qtz diagram (Coombs, 1963). Samples plotted in terms of normative Di, 01, and Qtz, recalculated to 100 percent. Symbols used as in Figure 8. The Fe0/(FeO+Fe203) ratio of Holbrook basalt was recalculated to 69.01 and is represented by the open square. 56

N 0 "" 57

Table 10. Representative trace elements analyses.

SUV83-34 SUV76-5 SUV83-26 SUV83-36 SUVB3-16 SUV83-18 SUV76- 4 SUV76-1

214 225 220 218 222 21R 212 220 Z r 270 219 210 225 244 222 243 226

Sr 555 455 443 4 79 5 71 543 502 431

Ba 1317 937 905 1032 12 82 1158 1267 1022

WPL-104

219

Zr 2 17 385 278 92 106 215 215

Sr 447 70 328 90 321 131 375 43 8 460

Ba 95 1 550 115 1000 380

Values in ppm IC? analys e s by Mr . N. Honjo , Dept. of Geology, Rice University. 1. No rmal granite (Turekian and Wedepchl, 1961). 2. High-Al basalt, K-Tr'ig basalt, Taupo, New Zealand {Taylor, 1969) . 3. Rhyolltes, Thingmuli lavas, Iceland (Carmichael, 1964). 4 . Olivine tholei i tes, McKinney basalt, Idaho (Leeman and Vitalfano, 1976) . 5. Ocean floor basalt-mean (Pearce and Cann, 1973). 6 . Calc-alka li basalt-mean (Pearce and Cann, 1973}. 7. Ocean Island basalt-mean (Pearce and Cann, 1973). 8 . Continental basalt-mean (.Pearce and Cann,l973). 58

large-ion lithophile elements Sr, Zr, and Ba, which are thought to be incompatible trace elements {Carmichael and others, 1974). Salic magmas are enriched in Ba. Nockolds and Allen {1953) proposed that Ba is not depleted in the magma until very late stages of the differentiation sequence. Concentration of Ba in Snowville basalt is as high as 1282 ppm, which is higher than the Ba content of normal granite {Turekian and Wedepohl, 1961) and rhyolite {Carmichael, 1964).

Sr can be admitted easily into the structure of feldspar of appropriate composition {Carmicheal and others, 1974). Abundant strontium in

Snowville basalt is consistent with the high modal amounts of plagioclase. Average Sr content {492 ppm) of Snowville basalt is close to the Sr content of mean continental basalt {460 ppm, Pearce and Cann,

1973). Zr is a 1 arge, highly charged cation which is partitioned more strongly into acidic melt than basic melt {Watson, 1976). Average Zr abundance in Snowville basalt is 231 ppm, which is higher than the mean values for ocean island basalt and continental basalt {Pearce and Cann,

1973) and lower than McKinney olivine tholeiitic basalt {Leeman and

Vitaliano, 1976). The isotopic composition of strontium in Snowville basalt analyzed by Leeman {1970) is presented in Table 11, with the late­

Cenozoic lavas from the Basin-Range Province and adjacent areas in the western United States shown for com pari son. 87SrfB6sr ratio in

Snowville basalt is higher than the ratio of all basaltic lavas listed in Table 10, and identical to the 87sr/86sr ratios of Tertiary adamellite and granodiorite in Vipont Mountain {Compton and others, Table 11. Rubidium and strontium abundances (ppm) and strontium isotopic composition.

WPL-104 9 70-15 SOR COP SRP

Rb 23 4 5 . 2 ----- 12-34 9-44 Sr 365 1182 ----- 3 30-978 1225-270 Rb/Sr 0. 06 0.111 0. 049 0.03 -0. 09 0. 01-0.07 87 s r tB6sr o. 7104 0. 71 04 o. 7066 o. 7032-0.7045 o. 7034-0.7062 o. 7054-0.7076 0.7027-0. 705

~~Pl -1 0 4 Snowville area. Northe<"lstern Basin-Range province (L ~cm,,n, 1970). 9. Tertiary adamellite and grJnodiorite, Vipont t'lountoi n, Ut .-l h (Compton ilnd others, 1977). 70-15. McKinney basalt, Snake River Plain, Jd,lh o {Leeman .1nd Vitaliano, 1976). SUR. Southern Basin-R.Jnge Province (leeman,1970). COP. Colorado Plateau (Leeman, 1970) . SRP. Snake River Plain-Yellowstone National Park. {leeman, 1902). C. Cascade Arc {Leeman, 1982).

"'·o 60

1977). Relatively high 87sr;86sr ratios in basal tic 1 avas commonly result from crustal contamination or arise from isotopic differences in the source regions (Leeman, 1970). 61

PETROGENESIS

Geo thermometry

Table 12 lists pre-eruptive temperature estimates derived from a variety of geothermometers for basalts of this study. A brief discussion of each geothermometer follows.

Fe-Ti oxide geothermometry

Coexisting magnetite-ulvospinel and ilmenite-hematite solid solutions in eight samples were used to determine the temperature and oxygen fugacity of equilibration using the methods of Budding ton and Lindsley (1964), Powell and Powell (1977), and Ghiorso and Carmichael ( 1981).

Budding ton and Lindsley (1964) presented a graphical geothermometer and oxygen barometer for coexisting ilmenite-hematite solid solution and magnetite-ulvospinel solid solution in the system

FeO-Fe203-Ti02. Temperatures obtained are accurate to ± 50°C and the f02 to within one order of magnitude. Powell and Powell (1977) developed an independent iron-titanium oxide geothermometer and oxygen barometer on a thermodynamic basis, using the experimental data of

Buddington and Lindsley (1964). Using the calculation the error in the determined temperature is -t3ooc and the error in oxygen fugacity(­ lnfo2J is 2.0. Ghiorso and Carmichael (1981) wrote a computer program based on graphical interpolation of temperature and oxygen fugacity Table 12. Temperatures of equilibration and oxygen fugacities.

Mt-11 Mt-11 Mt-11 01-Cpx SpInel -01

(Buddington & Lin ds l ey) (Powell & Powell ) (Ghiorso & Carmich;u~l) (Powell.\ Powell) 1 F ab r 1 e s)

0 roc -ro 2 r c - ro2 roc -ro 1 1 °C T°C SUV83-l4 900 12. 5 87 8 11.1 913 11. 0 901 917

SUV83-26 935 12 0 3 918 11 .1 967 11.4 970 !lOU

SUV83-36 1025 10.8 977 i I. 2 1058 I 0. 2 978 917

SUV03-18 suo 14 0 9 799 14 . 9 U52 13 0 5 900

SUV83-16 750 16 .7 749 16.4 835 14 0 3 I 010 876

SUV83-1* 760 17 0 4 689 I 0 . 6 749 16 0 8

SUV83-31 1010 10 0 8 991 10.6 IU 17 10.6 SUV83-32 ------996

Tem perilture in column 4 are at 1.0 Kb water pressure. • SUV83-31 and SUV83-l from Table Mountain area . SUV83-32 from Holbrook. area.

"'N 63

from published, smoothed calibration curves of Buddington and Lindsley (1964). The results of these three methods are shown in Table 11. For most samples, the values determined by the graphical method (Buddington and Lindsley, 1964) are bracketed by the values obtained from the other two methods. In addition, the values obtained by the methods of Powell and Powell (1977) and Ghiorso and Carmichael (1981) are within the limits of accuracy stated for the method of Buddington and Lindsley (1974). Discrepancy is observed for samples SUV83-16 and SUV83-1. Temperatures of SUV83-16 are consistently 1ower than the temperatures of the other samples from Snowville area, and the temperatures of SUV83-1 (upper flow unit of Table Mountain basalt) are also much lower than temperatures of SUV83-31 (lower flow unit of Table Mountain basalt). The texture of iron-titanium oxides in sample SUV83-16 is distinct. Very fine iron-titanium oxide grains are associated with glass in sample SUV83-16, in intersertal texture with plagioclase laths. The low temperature of SUV83-16 may represent lower temperatures of the last stage of cooling. The ilmenite analysis of SUV83-1 (Table 6) has a low total and this may contribute to an unrealistic equilibration temperature.

Olivine-clinopyroxene geothermometry

A geothermometer proposed by Powell and Powell (1974) is based on the iron-magnesium exchange reaction between Ca-rich clinopyroxene and olivine. The geothermometer was calibrated by Powell and Powell (1974) 64

using groundmass olivine and clinopyroxene in lavas for which there are groundmass iron-titanium oxide temperatures. Powell and Powell {1974) reported the uncertainties of this calculation are within ±3ooc at one bar pressure, with a pressure dependence of soc per kilobar. For the present study, temperatures were calculated using the rim compositions of groundmass olivine and average groundmass augite composition. Temperatures range from 9780C to 1010oc for Snowville basalt, a value of 9960C was obtained for Holbrook basalt.

The temperatures using the olivine-clinopyroxene geothermometer are generally higher than the temperatures derived from iron-titanium oxides solid solutions. The much higher temperature determined by olivine-clinopyroxene geothermometry in sample SUV83-16, may be due to the more Mg-rich rim composition of groundmass olivine {see Figure 7).

Spinel-olivine geothermometry

A spinel-olivine geothermometer derived for the parageneses of ultramafic rocks {Fabries, 1979) was also applied to Snowville basalt.

The magnitude of the uncertainty of this geothermometer is about :sooc.

Fabries {1979) stated that differential rates of diffusion and recrystallization may account for the large range of temperatures between 7oooc and 12oooc as determined by various mineral geothermometers in peridotite. The partitioning of Ca and Al in coexisting pyroxenes may yield higher temperature estimates, while the exchange of Mg and Fe between spinel and olivine always becomes 65

effective during the cooling down to relatively low temperatures.

Temp eratures of Snowville basalt determined by the spinel - olivine geothermometer range from 110ooc to 876°C, and are generally lower than

the t emperatures derived by the coexisting olivine-clinopyroxene

geothermometer. The recorded temperatures of Snowville basalt based up

on the spinel-olivine geothermometer may represent temperatures of a later stage of cooling.

Oxygen Fugacity

Estimates of oxygen fugacity obtained from the coexisting Fe-Ti oxide phases of Snowville basalt and Table Mountain basalt are presented in Table 12.

Fudali (1965) experimentally determined oxygen fugacities in equilibrium with the original ferrous to ferri c iron ratios of nine basalts and andesites. He found the range of value at 12oooc to be 10- 8.5 to 1Q-6.5 atm. Oxygen fugacity of Snowville basalt and Table

Mountain basalt, representing consistently lower pre-eruptive T-fo2 points from the method of Buddington and Lindsley (1964), are plotted on Figure 16. Temperatures and oxygen fugacities for Snowville and

Table Mountain basalt are in the expected range for basic 1 ava

(Haggerty, 1976). The temperatures and corresponding oxygen fugacities plot close to the fayalite-magnetite-quartz buffer curve, indicating internal buffering of oxygen by the crystal-liquid assemblage

(Carmichael and Nicholls, 1967). The smooth and progressive decrease 66

/ EXTRUSIVE 8 - / sums ~~ /

10 / /

12 - I I I I e I .2"" 16 I I I

18 -

20 - I I ~ 22 ... I I 1200 500 600 700 100 0 110 0

Figure 16. Oxygen fugacities (-logtol plotted against temperatures. The dashed curves label ed HM and FMQ are the hematite­ magnetite and fayal ite-magnetite-quartz buffer curves, respectively (Eugster and Wones, 1962}. 'Basic' envelope is taken from Haggerty (1976}. Symbols of study area samples used as in Figure 8. 67

in oxygen fugacities with decreasing temperature throughout the

Snowville basalt is similar to the pattern of the Thingmuli series

(Carmichael, 1967b), and to that reported by Stout and Nicholls (1977) for the Snake River Plain 1 avas.

Origin

Crystal fractionation

Green and Ringwood (1967) reported the results of a detailed experimental study on fractionation of natural basal tic compositions under high pressure and high temperature conditions. They stated that the observed sequence of appearance of phases at atmospheric pressure was olivine, plagioclase, and clinopyroxene. The appearance of plagioclase before clinopyroxene is a characteristic feature of the crystallization of basaltic magmas at 1 ow pressures.

Experimental crystallization studies of basal tic magmas of the present study are not available. Where plagioclase occurs both as phenocrysts and groundmass, as in the upper flow unit of Table Mountain and the Holbrook basalt, crystallization under low pressure and shallow depth (less than 15 km) conditions is suggested. Green and Ringwood (1967) also proposed that there are important differences between the enrichment of incompatible elements in a process of wall - rock reaction at low pressure versus wall-rock reaction at high pressure. In a low pressure environment, where plagioclase is 68

the stable phase in the wall-rock mineralogy, strontium will behave as a compatible element and substitute for calcium during plagioclase crystallization or reaction processes. High strontium enrichment of Snowville basalt confirms that Snowville basalt crystallized under low pressure conditions. A variation diagram (Figure 17) has been used to illustrate observed changes in chemical composition possibly due to fractionation processes. A computer program written by Stormer and Nicholls (1978) was used to quantitatively evaluate processes of assimilation and fractionation to account for the Snowville and Table Mountain sequences and the genetic relationships between the three different areas. Whole-rock chemical analyses and microprobe mineral analyses are required (Table 3,4,5,6 and 8) Parent-daughter pairs, added or subtracted phases, and sum of squares are presented in Table 13. Using SU¥83-31 as the Table Mountain parent, it was attempted to derive SUV83-1, the upper flow at Table Mountain; SU¥83-32, the Holbrook basalt; and SU¥83-36, the parental lava for the Snowville sequence; by fractionating olivine and plagioclase. The sums of squares for these relationships are too high, indicating that the differentation hypothesis should be rejected (Stout and Nicholls, 1983). Additional evidence for rejection of the hypothesis that SUV83-1 (upper flow unit of Table Mountain) was derived from SU¥83-31 (lower flow unit of Table Mountain) comes from the mineralogy. Sample SU¥83-1, with lower MgD content than SU¥83-31, Ti ol Mo O ··'L·& . . . • , • . • ~----~----~----~----_j----~ ·•l· - • -L-L---'---'----'---- p.Jo, Ho,O 0 ~ • • .. • • ' DL_ . L _ _ t ___~_j_----.1._ __ l ___ L e ~~· oL.J!_O~ k,O L leO l 4 • • • • • ...... ' . l-- _ , ______, ____ j __ _j_ _ ___, __ .____ "l .. .· Mg O . ·: __. _.______, _.__ . - ...... ' t_ _ , _ .____ _j _ __ , ___j __.____ . j _ _ __t:,_. __ A.I~O l < --·------! CoO ..l " H 4 e • • 16 li ""' "t _· .. ~ . L 'l ~ ~ 1od J\".'!•• ~'u ~' J; ~ : ~' .~ : . • " I ___ __J ______l_ _ _f ~ ~ --- ' --- - - l. :! _ __ _j _____ ' -----'---' --' -----J.----~------U .

Figure 17. Varation diagram (Harker type) of Si02 versus other oxides. The arrow indicate the stratigraphic sequence from Rattlesnake Pass, proceeding from 16 to 76-1.

"'"" Tuble 13. Results of fractionation models tested.

Parent Daughter Added or Subtracted Phases Sum of Squares

SUVS3-31 SUVS3-1 -6.52 Foss Fa12 -10.71 An 59 Ab 40 or1 5.5S23 SUVS3-31 SUV83-32 -S.15 Foss Fa 12 -11.05 An 59 Ab 40 or1 6. 77 S6 SUVS3-3 1 SUV83 -36 -5.73 Foss Fa12 -7.68 An 59 Ab 40 or 1 5.269 9

0" 71

contd ins a higher-temperature pl~gioclase phase. Plagioclase

phenocrysts only appear in SUV83-l, instead of appearing in the more

parental-like flow unit SUV83-31 . Discrepancies in compositional

trends, great variation of eruption temperatures and oxygen fugacity,

and different mineralogies indicate that at Table Mountain, the two

flow units separated by tuff may not be genetically related by a

process of fractionation. For the Rattlesnake Pass sequence within the

Snowville lavas (samples SUV83-16, SUV83-18, SUV76-4, SUV76-1),

inspection of Figure 17 indicates that although Si02 decreases systematically from the base (SUV83-16) to the upper part (SUV76-1), variation of the major oxides is irregular. In addition, the base of the Rattlesnake Pass sequence (SUV83-16) is unlike SUV83-36 or SUV83-

34, samples collected west of Rattlesnake Pass at contacts with Oquirrh

Formation and the Tertiary Salt Lake Formation, respectively. SUV83-

26, collected from a ridgetop west of the Rattlesnake Pass sequence, shows an increase in silica, rather than a decrease over SUV76-1.

Fractionation modeling requires removal of phases forming early in the crystallization sequence, such as olivine phenocrysts. Removal of olivine, with Si02 less than 50 weight percent from SUV83-16, will bring about a decrease in Sio2, but will not satisfactorily explain the irregular change in MgO and FeO. Including plagioclase with olivine in the modeling to account for the transition from SUV83-16 to lavas higher in the sequence, will bring about the observed changes in chemistry, but this requires alternating fractionation and accumulation processes, which are considered to be unrealistic. 72

Crustal contamination

The presence of high Si02, MgO, and alkalies in the Snowville lavas relative to other basalts in the region, such as the Kelton area

(Voit, 1985) and the Rozel Hills-Black Mountain area (Greenman, 1982),

suggests the possibility of crustal contamination of a basalt with low initial Sio2. Basalts from Black Mountain (RPV80-17) and Kelton (BV81- 11) were selected as parent compositions, and tuff from Kelton (BV81-

17) and rhyolite from Grouse Creek, Utah (LV82-4; Scarbrough, 1984) were selected to represent partial melts of crustal rocks. Using the program of Stormer and Nicholls (1978) it was attempted to derive basalt from the Snowville area by removing phenocryst phases from these parental magmas and adding the representative crustal melts. Results of this testing were also unsatisfactory.

Crustal contamination of the basalts in the Snowville area is still suggested. Evidence includes: 1) relatively high Sio2, MgO, Na 2o, and K20 contents; 2) strong enrichment in Ba; and 3) a consistently higher strontium ratio (0.7104) than other nearby basalts of the region. The 87sr;86sr ratio of Snowville basalt (0.7104) is higher than 87sr;86sr ratios in more typical basaltic magma (0.702-0.707;

Carmichael and others, 1974). Herier and others (1965) reported the mean 87sr;86sr value for samples from Tasmanian dolerite as 0.7115.

They stated that the homogeneity and uniform chemical composition of the large masses of dolerite are not favoured by crustal contamination.

The constancy of the Th/K, U/K, and 87sr;86sr ratios throughout the 73

dolerite confirms that the magma was completely homogeneous before crystallization started. Herier and others (1965) suggested that Tasmanian dolerite magmas may have begun to crystallize at crustal levels, with high temperature required. It would be expected that generation of more felsic magma would occur prior to the formation of a doleritic magma if the doleritic magma was formed in the crust. The characteristic high strontium istope ratio of Snowville basalt together with its relatively uniform chemical composition, the

lack of xenoliths, relatively small volume eruption, high Tio 2 content, and the association with underlying felsic tuffs, indicate that the magma was possibly generated from within the crust. Snowville basalt also shows a relatively higher Sr content and K/Rb ratio than Tasmanian dolerite. Other mechanisms, such as the basaltic magma passing through granite or contaminated by underlying felsic tuff should be considered. Additional trace element and isotopic analyses are suggested for further study.

Partial melting

As the Table Mountain and Holbrook lavas have less than 50 weight percent Si02 and do not appear to have an origin as complex as Snowville lavas, the generation of these lavas by partial melting of hypothetical mantle compositions was evaluated. Three hypothetical mantle materials were used to test for sources of magmas SUV83-1, SUV83-31 and SUV83-32: 1) pyrol ite (Green and Ringwood, 1967); 2) garnet peridotite (Yoder, 1976); and 3) spinel 74

lhe rzolite (Bacon and Carmichael, 1973}. The percentages of partial melting required to derive Table Mountain and Holbrook lavas from the hypothetical mantle compositions are listed in Table 14. Using spinel lherzolite as a possible source material, partial melts of only 0.56,

0.40, and 0.82 percent are required to produce SU¥83-1, SU¥83-31, and SU¥83-32 respectively.

Anderson and Sammis (1970} proposed that approximately five percent melt is required for a melt to move upward. With about one percent melting, the heating, further melting and heat transfer by liquid motions will stabilize the region of partial melt. Thus spinel lherzolite is improbable as a source material for Table Mountain and

Holbrook basalt because of the low amounts. For a pyrolite mantle, a range of 6 to 13 percent partial melting is required to produce SUV83- 1, SU¥83-31, SU¥83-32, whereas for a garnet peridotite mantle, a range from 7 to 11 percent partial melting is required. As the larger amounts are more reasonable for migration of liquid, the theoretical mantle material could be either pyrolite or garnet peridotite. Higher potassium contents of SUV83-1, SU¥83-31, and SU¥83-32 suggest that the source material may contain minor phases such as phlogopite. Table 14. Percentage of partial melting required to derive Table Mountain and Holbrook basalt from hypothetical mantles of pyrol ite, garnet peridotite, and spinel lherzolite composition.

Pyrolite Garnet Peridotite Spinel Lherzolite

SUV83-1 8.63 9.74 0.56

SUV83-31 6.46 6.92 0.40

SUV83-32 13.25 11.33 0.82

'-"" 76

CONCLUSIONS

General Statement

Chemical analyses of basalts from the Snowville, Table Mountain,

and Holbrook areas show that they are high in silica, magnesium, and

alkalies. Chemical and mineralogical characteristics indicate that the

Snowville, Table Mountain, and Holbrook basalts are more primitive than

Snake River basalt, or the basalts of Kelton, Rozel Hills and Black

Mounta i n areas. Eval uti on of fractionation models indicates there are no genetic relationships between the basalts from the three areas. Petrographic and chemical characteristics suggest that Snowville, Table

mountain upper flow unit and Holbrook basalts crystallized under low

pressure and at shallow depths.

Snowville Basalt

The volcanic rocks of the Snowville area show characteristics of both olivine-tholeiitic and alkali-olivine basalt. Fractionation processes can not satisfactorily explain the irregular change in the c hemistry of the Snowville volcanic sequence. Trace element and strontium isotope analyses favor a crustal contamination model. 77

Table Mountain Basalt

The two eruptions at the Table Mountain area show no genetic relations between them. Field relationships show that the upper flow unit erupted after the lower flow unit had cooled and was overlaid by tuff. Chemical and mineralogical characteri sties of both the upper and lower flow units suggest their affinity with alkali-olivine basalt. The parental basalt of both Table Mountain basalt flow units probably formed by partial melting of source rocks similar to garnet peridotite or pyrol ite. The high K2o content indicate that the source material may have contained minor phases such as phlogopite.

Holbrook Basalt

Highly oxidized Holbrook basalt represents characteri sties of both alkali-olivine basalt and olivine-tholeiite. Limited variation of mineral compositions and the exsolved iron-titanium oxides suggest a slow cooling rate. A possible fractionation trend is not available with only one detailed analysis from this area. A pyrolite or garnet peridotite mantle containing minor phlogopite is the possible source of material. 78

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