Dacites of Bunsen Peak, the Birch Hills, and the Washakie Needles, Northwestern Wyoming, and Their Relationship to the Absaroka Volcanic Field, Wyoming and Montana

Dacites of Bunsen Peak, the Birch Hills, and the Washakie Needles, northwestern Wyoming, and their relationship to the Absaroka volcanic field, Wyoming and Montana

L. L. LOVE ] A. M. KUDO i Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 D. W. LOVE J

ABSTRACT minor amounts of potassium-rich mafic raphy of the Absaroka Volcanic Super- rocks known as shoshonites, absarokites, group. Chadwick (1970) subdivided the Three porphyritic dacite plugs from the and banakites. volcanic field into two structurally con- western Absaroka volcanic belt in north- Recent work has supplemented the excel- trolled northwest-trending belts of volcanic western Wyoming have been studied, and lent observations of iddings (1899), Larsen centers, which he called the Eastern Ab- their petrography, chemistry, and ages are (1940), and others. Smedes and Prostka saroka and Western Absaroka belts. The treated in terms of the regional igneous (1972) identified numerous volcaniclastic Eastern Absaroka belt has more potassic geology of the Absaroka volcanic field. facies and summarized the regional stratig- igneous rocks than the Western Absaroka The dacite from the northernmost plug studied, Bunsen Peak, is 47.6 ± 1.9 m.y. old, as dated by the fission-track method on apatite; the Birch Hills dacite is 40.5 ± 2.6 m.y. old, as dated by the fission-track method on apatite; and the dacite from the southernmost plug, Washakie Needles, is 38.8 ± 1.6 m.y. old, as dated by the fission-track method on sphene. It appears from the dates available that the oldest igneous activity in the Absaroka volcanic field occurred at the northwestern end about 53.5 m.y. ago. The activity migrated to the southeast, ending about 38.8 m.y. ago at the Washakie Needles. The Absaroka volcanic field has been subdivided into two belts. The western belt is composed of normal calc-alkalic igneous rocks, and the eastern belt is composed of potassium-rich rocks. When the available analyses of the province are treated in terms of the system quartz-plagioclase-orthoclase, it becomes apparent that the rocks of the two belts lie on two distinct differentiation trends. The trend for rocks of the western belt is best explained by fractional crystallization of plagioclase from an intermediate magma. TTie trend for rocks of the eastern belt is best explained by crystallization of both plagioclase and potassium feldspars. The mafic members of the eastern belt rocks are similar to shoshonitic rocks.

INTRODUCTION

The Eocene Absaroka volcanic field straddles northwestern Wyoming and adjacent parts of Montana. The regionally extensive volcaniclastic rocks, flows, and intrusions are predominantly a suite of calc- alkalic andesites, dacites, and rhyolites with Figure 1. Location of study areas and major mountain ranges.

Geological Society of America Bulletin, v. 87, p. 1455-1462, 12 figs., October 1976, Doc. no. 61010.

1455

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LEGEND

Contact Dashed where approximate

Strike and dip of bed

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Qh Hot Spring Deposit Qy Yellowstone Grp. Tuff, Rhyolite, & Basalt

Td Dacite

Is Sepulcher Fm.

1 "» I

Cretaceous Sedimentary Rocks 6000:

Thermopolis Shale & Cloverly Fm.

Figure 2. Geologic map and cross sections of Bunsen Peak. belt. Peterman and others (1970) investi- groups. Exposures of andesitic volcaniclas- and are considered to be of early middle gated the lead and strontium isotopes of the tic rocks near the oldest plug, Bunsen Peak, Eocene age on the basis of pollen analyses. Absaroka volcanic rocks and concluded may belong to the Washburn Group, but The youngest plug, the Washakie Needles, that they had formed from an inhomogene- obscure field relations make it impossible to intrudes the Wiggins Formation in the ous unradiogenic source. Investigations by determine if the Bunsen Peak mass intruded Thorofare Creek Group. Included with the Nicholls and Carmichael (1969) and Prost- these volcaniclastic rocks or was buried by description of the Washakie Needles is that ka (1973) were concerned with the descrip- them. Smedes and Prostka (1972) mapped of Dome Mountain, a mile to the north. tion and origin of the potassium-rich mafic the volcaniclastic rocks near the Birch Hills lavas. and indicated they were undivided Ab- Bunsen Peak Dacite Our study is concerned primarily with saroka Volcanic Supergroup. These same three intrusive porphyritic dacite plugs be- rocks have been given the name Hominy Bunsen Peak is located in Yellowstone longing to the calc-alkalic suite of the West- Peak Formation (J. D. Love, L. B. Leopold, National Park, 2 km south of park head- ern Absaroka belt (Fig. 1). Described here and D. W. Love, 1975, written commun.) quarters at Mammoth (Fig. 2). The intru- are the petrography, chemistry, and age of these plugs and their relation to the regional TABLE 1. AVERAGE MODAL ANALYSES OF DACITES FROM BUNSEN PEAK, petrologic patterns. A model utilizing the THE BIRCH HILLS, WASHAKIE NEEDLES, AND DOME MOUNTAIN phase relationships found in the quartz- orthoclase-albite-anorthite system explains B.P.'* B.H.* W.N.* D.M.'1 the potassium-poor and potassium-rich 72.0 56.3 58.7 trends found by Chadwick (1970). Also in- Groundmass 80.7 cluded are data on the absarokite- Plagioclase phenocrysts 14.8 21.5 28.9 24.7 shoshonite-banakite association. Biotite phenocrysts 3.8 3.9 4.8 4.4 Quartz phenocrysts 0.7 2.6 0.9 0.8 DESCRIPTIONS OF THE THREE DACITE PLUGS Amphibole phenocrysts 6.7 8.8 Inclusions 2.4 2.6 The three plugs are associated with rocks Total 100.0 100.0 100.0 100.0 of the Absaroka Volcanic Supergroup, Number of thin sections which is composed, from oldest to countedf 9 5 7 6 youngest, of the Washburn Group, Sunlight Group, and Thorofare Creek Group Note: Modal analyses in volume percent. (Smedes and Prostka, 1972). There is ex- * B.P. = Bunsen Peak, B.H. = Birch Hills, W.N. = Washakie Needles, and D.M. = Dome Moun- tensive areal overlap and stratigraphic in- tain. terfingering between rocks of adjacent f There were 1,000 points counted on each thin section.

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sive mass forming the peak is a light-gray grained plagioclase (An17 to An:l5), sanidine fine-grained dacite with phenocrysts of (Or(J8 to Or7:l, Fig. 3), and quartz. Opaque plagioclase, quartz, and biotite. Modal and minerals are present in minor amounts, and chemical analyses of the dacite are given in accessory minerals include euhedral apatite Tables 1 and 2. and zircon. The plagioclase phenocrysts range in size Apatite from the dacite was dated by the from 1 to 3 mm and commonly are euhe- fission-track method (Naeser, 1967) as 47.6 dral. Most grains have albite and Carlsbad ± 1.9 m.y. old. An unpublished K-Ar age of twins and oscillatory zoning. Electron mi- 32.0 ± 0.9 m.y. was obtained by Curtis croprobe analyses of these phenocrysts (H. W. Smedes, 1972, personal commun.).

yield an average composition of An29 (wt This young age may be due to argon loss re- percent) with a range from An^, to An«, sulting from high thermal gradient in the (Fig. 3). Quartz phenocrysts (1 to 2 mm in area, hot spring activity, or proximity to diameter) are usually large ovoid grains cooling members of the Pleistocene Yel- with smooth outlines and slight embay- lowstone Group of rhyolitic rocks. These Figure 3. Compositions of feldspars from the ments. Biotite occurs in euhedral grains as factors may also have affected the fission Bunsen Peak dacite (wt percent). Open circles represent compositions of phenocrysts. Each large as 4 mm. The phenocrysts are red- tracks, making 47.6 ± 1.9 m.y. a minimum point is the average of 10 analyses. Area within brown in thin section. A few prismatic age. the dashed line represents the total range of grains of amphibole replaced by biotite, phenocryst composition. Each filled circle is the chlorite, and opaque minerals were ob- Birch Hills Dacite analysis of a single grain in the groundmass. served in most thin sections. Analyses made with ARL-EMX electron micro- The groundmass of the dacite is com- The Birch Hills are located in the south- probe; 15 kiloelectron volts (keV), 0.02 mi- posed of subhedral and anhedral fine- western corner of Yellowstone National croamps. Park, 14 km east of the Bechler Ranger Sta- TABLE 2. CHEMICAL ANALYSES AND MODIFIED CIPW NORMS OF DACITES tion, in an unsurveyed portion of the Grassy Lake Reservoir 15-minute quad- A* B" C* D* E':' F» rangle, Wyoming (Fig. 4). The Birch Hills do not lie within what is generally consid- Si02 71.27 70.52 70.56 70.24 63.57 63.8 ered the structural limits of the Absaroka Ti02 0.38 tr. 0.31 tr. 0.49 0.46 volcanic field, and the associated Paleozoic ai2o3 15.12 15.85 15.44 17.36 15.83 16.8 rocks were thought by Hague and others Fe2Oa 0.68 2.28 0.77 1.38 1.96 2.0 (1899) to represent the northernmost ex- FeO 1.10 0.36 1.03 0.79 1.63 1.5 tension of the Teton Range. They recog- MnO 0.03 0.09 0.04 0.0 0.06 0.06 nized that the volcaniclastic rocks exposed MgO 0.77 0.09 0.90 0.53 2.79 1.4 CaO 1.92 2.59 1.70 2.74 4.11 4.7 in and near the northern Tetons were cor- Na20 4.22 3.93 4.08 3.69 4.58 4.6 relative with similar rocks in the Absaroka k2o 3.71 3.43 3.56 2.65 3.13 2.6 province. Smedes and Prostka (1972) in- h2o+ 0.44 0.92 } 0.73 0.56 cluded the Hominy Peak Formation and the rI \J.OJ0 UO. /7 1 h2o- 0.00 0.48 j 0.36 0.54 Birch Hills dacite in the Absaroka Volcanic PA tr. 0.17 tr. tr. n.d. 0.35 Supergroup, but because of their isolated so3 n.d. 0.29 n.d. tr. n.d. n.d. location, the position of the Hominy Peak co2 n.d. n.d. n.d. n.d. n.d. 0.05 Formation and the Birch Hills dacite within Total 99.64 99.95 99.79 100.09 99.24 99.33 the supergroup has not been determined. We also believe that they belong to the Ab- Q 25.12 27.51 26.26 29.85 12.34 15.54 saroka Volcanic Supergroup. Or 22.13 20.52 21.41 15.85 18.67 15.58 Ab 28.24 35.73 37.28 33.53 41.52 41.87 The more rugged parts of the Birch Hills An 9.62 11.89 8.59 13.76 13.53 17.77 are underlain by an intrusive body of gray Wo 0.0 0.0 0.0 0.0 2.82 1.29 dacite that has phenocrysts of plagioclase, En 2.15 0.25 2.53 1.48 7.78 3.92 biotite, and quartz. Modal and chemical Fs 0.75 0.0 0.70 0.27 0.58 0.39 analyses of the dacite are given in Tables 1 Hm 0.0 1.16 0.0 0.0 0.0 0.0 and 2. Phenocrysts of plagioclase are Mt 0.72 0.68 0.82 1.46 2.07 2.12 euhedral and have some glomeroporphy- 11 0.53 0.0 0.44 0.0 0.69 0.65 ritic aggregates present. The size of the in- Pr 0.0 0.38 0.0 0.0 0.0 0.0 dividual phenocrysts ranges from 2 to 7 C 0.74 1.52 1.98 3.80 0.0 0.0 Ap 0.0 0.36 0.0 0.0 0.0 0.74 mm. In thin section, Carlsbad and albite Cc 0.0 0.0 0.0 0.0 0.0 0.13 twins and normal and oscillatory zoning are observed. The average phenocryst com- Total 100.0 100.0 100.0 100.0 100.0 100.0 position is An25 (Fig. 5), with a range from Differentiation An20 to An.!5. Quartz phenocrysts are large indexf 85.49 83.76 84.95 79.23 72.53 72.99 (up to 10 mm) ovoid masses. Many are Note: n.d. = not determined; tr. = trace amount present. composites of two or three distinct grains * A, analysis of Bunsen Peak dacite by K. Aoki (1971, written commun.); B, analysis of Bunsen with smooth arcuate junctures. Embay- Peak dacite, Hague and others (1899); C, analysis of Birch Hills dacite, K. Aoki (1971, written ments in the quartz are common, and fine- commun.); D, analysis of Birch Hills dacite, Hague and others (1899); E, analysis of Washakie grained reaction rims are present on some Needles dacite, K. Aoki (1971, written commun.); F, analysis of Dome Mountain dacite, W. R. Keefer grains. Biotite phenocrysts are usually (1971, written commun.), euhedral and red-brown in thin section; f Sum of normative quartz + orthoclase + albite. they range in size from 1 to 3 mm.

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Figure 4. Geologic map and cross section of the Birch Hills.

The fine-grained groundmass is com- An24 to An40 (Fig. 8). Phenocrysts of limetres to 30 cm in diameter. These inclu- posed of subhedral and anhedral plagio- hornblende are commonly euhedral and sions are composed of minerals similar to clase (An 18 to An31), sanidine (Or7:! to Or79, less than 1 cm long. Many hornblende the phenocrysts in the dacite. Phenocrysts Fig. 5), and quartz. No amphibole pheno- grains are rimmed with a border of opaque of hornblende located near the inclusions crysts or inclusions were observed in the minerals, and several poikilitically enclose appear to have been separated from the in- Birch Hills dacite. Apatite from the Birch grains of apatite. A few grains have cores of clusions. More hornblende and opaque Hills dacite was dated by the fission-track biotite. Biotite phenocrysts are usually minerals are found in the inclusions than in method as 40.5 ± 2.6 m.y. old. euhedral and generally less than 3 mm in the host rock. The hornblende, usually in diameter; many are rimmed with opaque subhedral or euhedral grains, has the same Washakie Needles and minerals. Some biotite grains also poikiliti- Dome Mountain Dacite cally enclose apatite. Quartz phenocrysts are large (as much as 5 mm across) and The Washakie Needles are about 85 km show extensive embayment. west of Thermopolis, Wyoming, in sec. 27 The groundmass is composed of micro- and 34, T. 103 W., R. 44 N., and unsur- crystalline subhedral and anhedral grains of veyed portions of the Monument Peak and quartz and feldspar. Electron microprobe Twin Peaks IVi-minute quadrangles (Fig. analyses show that the groundmass 6). The Needles are the topographic expres- feldspars range in composition from an- sion of a dacite plug (Fig. 7). The dacite is desine through anorthoclase (Fig. 8). The composed of phenocrysts of plagioclase, groundmass in the Washakie Needles dacite hornblende, biotite, and quartz in a has not crystallized a potassium-rich groundmass of microcrystalline feldspar feldspar, as have the Bunsen Peak and Birch and quartz. Modal and chemical analyses Hills dacites. Magnetite, titanomagnetite, of the dacite are given in Tables 1 and 2. and ilmenite are minor constituents of the Dome Mountain, adjacent to the Washakie groundmass. Needles, is composed of an almost identical An overlap in composition between Figure 5. Compositions of feldspars from the body of dacite. groundmass feldspars and phenocrysts Birch Hills dacite (wt percent). Open circles represent compositions of phenocrysts. Each point is The phenocrysts of plagioclase are exists in the dacite. Some grains in the the average of 10 analyses. Area within the groundmass are as calcic as — and some are euhedral, and most are less than 1 cm long. dashed line represents the total range of pheno- Oscillatory zoning and Carlsbad and albite more calcic than — the most calcic zones in cryst composition. Each filled circle is the twins are present. Electron microprobe the phenocrysts. analysis of a single grain in the groundmass. analyses of these phenocrysts yield an aver- Rounded mafic inclusions with jagged Analyses made with ARL-EMX electron microp- age composition of An2- with a range from boundaries range in size from a few mil- robe; 15 keV, 0.02 microamps.

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LEGEND © Horizontal bed HD Colluvium •EI Dacite

LULI Wiggins Fm on Tepee Trail Fra

Figure 6. Geologic map and cross section of the Washakie Needles and Dome Mountain.

pleochroic formula as the dacite. Some hornblende crystals poikilitically enclose quartz, plagioclase, or apatite. Biotite, plagioclase, and quartz are also common minerals in the inclusions. Minor amounts of chlorite, sericite, and diopside are present in some inclusions, usually as cores in hornblende. The inclusions contain as much as 50 percent plagioclase, which, when present, is usually a mosaic of equant grains. Some of the plagioclase grains have mottled zones between their cores and rims. Most grains are zoned normally; a few grains are twinned. Opaque minerals, espe- cially magnetite, are present as a fine dust scattered throughout the inclusions and also concentrated near grains of hornblende and biotite. One unusual inclusion with a schistose structure composed of plagioclase, quartz, biotite, and epidote was observed. Sphene from the Washakie Needles dacite was dated by the fission-track method as 38.8 ± 1.6 m.y. old.

Summary

All the dacites have phenocrysts of Figure 7. The Washakie Needles (view northeast, August 1971). plagioclase, biotite, and resorbed quartz. In

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addition, the dacites from Bunsen Peak and the Washakie Needles have phenocrysts of amphibole. Plagioclase is the dominant phenocryst. All specimens have a fine- grained groundmass of feldspar, quartz, and opaque minerals. The groundmass of the Washakie Needles dacite has only one type of feldspar, which belongs to the plagioclase-anorthoclase series, but the dacites from the other two plugs have both plagioclase and sanidine in the groundmass. Amphibole and biotite are present only as phenocrysts. The Washakie Needles dacite has con- spicuous mafic inclusions that could represent either residual unmelted portions of a source rock, contaminating xenoliths, or cognate inclusions. This cannot be resolved satisfactorily at the present time, but the schistose inclusion suggests that the Washakie Needles dacite magma could have been contaminated to some extent. We believe that most of the euhedral plagioclase phenocrysts with oscillatory zoning have crystallized directly from the magma. It is possible that minor amounts of quartz are xenocrysts, but because the groundmass has crystallized quartz, we believe that nearly all the quartz precipitated from the magma at depth prior to the de- velopment of resorption textures. Chemically, the Washakie Needles dacite is the most primitive of the three, having the most calcic plagioclase, the highest percent- age of mafic minerals, the lowest differentiation index (Thornton and Tuttle, 1960), and the greatest amount of normative albite and anorthite. The groundmass has some plagioclase that is as calcic as, or more calcic than, the most calcic zones of the plagioclase phenocrysts. This phenom- enon has been observed in other rocks by Smith and Carmichael (1968) and ex- Figure 9. Generalized map of the Absaroka Volcanic Supergroup with radiometric ages. Solid line plained by Nicholls and Carmichael (1969) is present approximate boundary of Eocene flows and volcaniclastic rocks. Intrusive rocks and erup- as resulting "either from metastable crystal- tive centers shown in black. X marks locality of dated extrusive rocks. I marks locality of dated intrusive rocks. Dashed line is boundary of Yellowstone National Park (from Chadwick, 1970; Smedes and Prostka, 1972; Schassberger, 1972).

lization of the groundmass plagioclase, the dacites described above) compared with and/or a response to a pressure drop from experimental petrologic systems. intratelluric conditions to those of a sur- There is an apparent shift in age of the ficial lava" (p. 55). volcanic activity from the northwest, in the The dacites from Bunsen Peak and the Gallatin Range, to the southeast. Smedes Birch Hills have almost identical mineral and Prostka (1972) inferred this shift in and chemical compositions, which indicate volcanic activity on the basis of strati- that conditions of magma evolution were graphic relationships in the Absaroka Vol- similar over a time period of 7 m.y. and a canic Supergroup. Radiometric ages sub- distance of 105 km. stantiate this interpretation (Fig. 9). The oldest dated extrusion in the northwest is PETROGENESIS OF THE 53.5 m.y., whereas the youngest volcani- DACITES AND ASSOCIATED clastic rock in the southeast is 43.1 m.y. ROCKS IN THE (Chadwick, 1970; H. W. Smedes, 1972, Figure 8. Compositions of feldspars from the ABSAROKA PROVINCE personal commun.). The intrusive rocks as- Washakie Needles dacite (wt percent). Open sociated with the Absaroka Volcanic circles represent compositions of phenocrysts. Before any interpretation of petrogenesis Supergroup (some intrusive rocks in the Each point is the average of 10 analyses. Area within the dashed line represents the total range can be made, the following data should be Gallatin Range apparently predate this vol- of phenocryst composition. Each filled circle is considered: (1) the space-time relationships canogenic sequence; McMannis and the analysis of a single grain in the groundmass. within the province, (2) overall chemical Chadwick, 1964; Ruppel, 1972) follow the Analyses made with ARL-EMX electron micro- trends within the province, and (3) the same trend, the oldest in the northwest, probe; 15 keV, 0.02 microamps. composition of specific rock-types (besides 49.5 m.y. (Chadwick, 1970), and the

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o Eastern Belt volcanic activity and has the most potassic along a line trending away from the ' Western Belt rocks. Affinities between some intrusive plagioclase corner toward the Q-Or side rocks in the eastern group and the shosho- (Fig. 11). This trend can be attributed to nites are apparent from Iddings' work plagioclase fractionation of magma of in- (Hague and others, 1899). termediate composition (Presnall and The differences between the belts are Bateman, 1973). The scatter of points in the % <9 clearly illustrated by Iddings' classic work trend reflects differences in the original on Electric Peak, in the western belt, and on compositions of the rocks from different 50 60 70 the dissected Crandall volcano, in the east- emplacement centers of the western belt. Si02 wt% ern belt. In the Crandall area, orthoclase As representatives of the western belt, the gabbros and diorites with high potassium three dacites described in this paper proba- Figure 10. Variation diagram of KzO for intrusive rocks of the Absaroka Volcanic Super- content and chemical similarities to bly were tapped after some fractional crys- group. Sources: Ketner and others (1966), Hague shoshonites are common. The diorites have tallization had occurred. The minor and others (1899), Wilson (1936), Chadwick abundant pyroxenes but little, if any, amounts of phenocrystic quartz, all of (1970), W. R. Keefer (1972, written commun.), hornblende, indicating a relatively dry sys- which show resorption textures, could indi- W. H. Wilson (1972, written commun.), Rubel tem. In contrast, the potassium-poor gab- cate that the magma had reached the (1971), Clarke (1900), McMannis and Chad- bros and diorites of Electric Peak have quartz-saturation surface prior to the tap- wick (1964). abundant hornblende. These rocks have ping process. Subsequent rise of the magma youngest in the southeast, 38.8 m.y. less modal orthoclase than equivalent rocks to near-surface regions, which causes the (Washakie Needles, our data). These dates in the eastern belt. In general, the diorites contraction of the primary phase volume of allow the interpretation of a migration rate from both groups are texturally similar, quartz in response to the pressure drop, to the southeast of about 2 cm/yr. with idiomorphic labradorite. However, may have resulted in the resorption of the Porphyritic dacite intrusive rocks, as il- some porphyritic rocks from Crandall vol- quartz. lustrated by our three plugs, are the most cano, which are transitional between During quenching of the intrusions, the common intrusive rocks in the province. shoshonite and normal basalt, display or- Bunsen Peak and Birch Hills dacites may Sequences of intrusions in some of the erup- thoclase rims around labradorite crystals. have reached the line of intersection be- tive centers of the province range from This feature is not observed in the Electric tween the quartz field and the two-feldspar mafic to felsic (for example, at Independ- Peak rocks. surface (quaternary univariant line in the ence volcano, Rubel, 1971; at Electric Peak, Most of the intrusive rocks of the West- Q-Or-Ab-An tetrahedron), resulting in the Hague and others, 1899), although other ern Absaroka belt (predominantly the rapid crystallization of quartz, plagioclase, areas are dominantly andesitic and show no Washburn and Thorofare Creek Groups) and sanidine. The Washakie Needles dacite, evidence for differentiation. contain more than 75 percent normative however, has crystallized only enough to Chadwick (1970) showed that the cen- plagioclase, potassium feldspar, and quartz have its liquid on the quartz-plagioclase ters of igneous activity define two subparal- and can be analyzed with reference to the surface. Consequently, only plagioclase, lel northwest-trending belts. The rocks of quartz-plagioclase-orthoclase system anorthoclase, and quartz have crystallized the eastern belt are generally more potassic (Carmichael, 1963; Presnall and Bateman, from the groundmass liquid. than those of the western belt (Fig. 10). 1973). The quartz-normative analyses from The difference between the potassium- There is no simple relationship between the the western belt plot within the primary rich rocks of the eastern belt and the "nor- relative amount of potassium and strati- plagioclase field and in this projection lie mal" rocks of the western belt is illustrated graphic position within the Absaroka Vol- canic Supergroup. The middle group (Sun- light Group) has the easternmost centers of

Ab+ An Figure 12. Normative quartz, albite + anorthite, and orthoclase in intrusive rocks of the eastern belt (dot) and quartz normative shoshonites, ab- Ab • An sarokites, and banakites (bull's eye). Samples 118Ar and 118Ag are, re- Figure 11. Normative quartz, albite + anorthite, and orthoclase in in- spectively, the whole-rock shoshonite and its groundmass, from which trusive rocks of the western belt. Bunsen Peak (bull's eye), Birch Hills (solid plagioclase and other phenocrysts have been separated (from Nicholls and diamond), Washakie Needles (open star), Dome Mountain (solid cross). Carmichael, 1969).

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in Figures 11 and 12. Some overlap occurs Volcanic Supergroup and in radiometric central Montana: Geol. Soc. America Bull., at the quartz-rich end of the two trends, but ages of extrusive and intrusive rocks. The v. 51, p. 887-948. divergence of the trends toward the composition of the rocks does not bear a Love, L. L., 1972, The dacites of the Washakie feldspar-rich side is significant. The trend simple relationship to this shift in activity. Needles, Bunsen Peak, and the Birch Hills, Wyoming, and their relationship to the for the intrusive rocks of the eastern belt Similarities and differences among the Absaroka-Gallatin volcanic province [M.S. cannot result from the same process as for three dacites and other quartz-normative thesis]: Albuquerque, Univ. New Mexico, the intrusive rocks of the western belt be- intrusions of the Absaroka volcanic field 86 p. cause removal of plagioclase would give a can be explained by using the Q-Or- McMannis, W. J., and Chadwick, R. A., 1964, liquid line of descent and a cumulate trend Ab+An diagram. The rocks of the western Geology of the Garnet Mountain quad- diagonal to the observed trend. However, belt apparently underwent plagioclase frac- rangle, Gallatin County, Montana: should the plagioclase be joined early in the tionation. The rocks of the eastern belt frac- Montana Bur. Mines and Geology Bull. 43, crystallization sequence by a potassium tionated both plagioclase and orthoclase. 47 p. feldspar (that is, the liquid has intersected Although the shoshonitic rocks of the re- Naeser, C. W., 1967, The use of apatite and sphene for fission track age determination: the two-feldspar surface) and simultaneous gion plot similarly to rocks of the eastern Geol. Soc. America Bull., v. 78, p. 1523- fractionation of both plagioclase and potas- belt on this diagram, their geographic and 1526. sium feldspar occurs, the liquid line of de- petrologic relationships are not clear, and Nicholls, J., and Carmichael, I.S.E., 1969, A scent would move away from a point in- their scatter probably indicates contamina- commentary on the absarokite-shosonite- termediate between the Ab and the Or cor- tion. banakite series of Wyoming, U.S.A.: ners and not from the Ab corner. In order Schweizer. Mineralog. u. Petrog. Mitt., v. for this to take place, the parent magma ACKNOWLEDGMENTS 49, p. 47-64. must be enriched in potassium relative to Peterman, Z. E., Doe, B. R., and Prostka, H. F., the parent magma of the western belt. This work was supported in part by the 1970, Lead and strontium isotopes in rocks of the Absaroka volcanic field, Wyoming: Further investigations of the potassium-rich University of New Mexico Research Allo- rocks of the eastern belt may shed light on Contr. Mineralogy and Petrology, v. 27, p. cations Committee. We are indebted to K. 121-130. the origin of the shoshonitic magmas. Aoki, D. Armstrong, G. Conrad, K. Keil, Presnall, D. C., and Bateman, P. C., 1973, Fusion

In Figure 12 we have also plotted the C. W. Naeser, and E. I. Smith for their as- relations in the system NaAlSi3Os- compositions of the quartz-normative ab- sistance in the analytical part of the study. CaAl2Si20s-KAlSi308-Si02-H20 and gen- sarokites, shoshonites, and banakites We are grateful to H. W. Smedes for critical eration of granitic magmas in the Sierra (Nicholls and Carmichael, 1969; Iddings, review of the manuscript. William Dun- Nevada batholith: Geol. Soc. America Bull., 1899). There is a nearly complete overlap mire, Chief Park Naturalist, and John R. v. 84, p. 3181-3202. of the eastern intrusive rocks with the Douglass, West District Naturalist, both of Prostka, H. F., 1973, Hybrid origin of the absarokite-shoshonite-banakite series, Ab- shoshonitic rocks. There are apparent the National Park Service, helped secure saroka volcanic field, Wyoming: Geol. Soc. plutonic equivalents of shoshonites at permission to collect rocks in Yellowstone America Bull., v. 84, p. 679-702. Crandall volcano, but there are not clear National Park. Unpublished data supplied Rubel, D. N., 1971, Independence volcano: A geographic or petrologic relationships be- by W. R. Keefer, J. D. Love, H. W. Smedes, major Eocene eruptive center, northern Ab- tween the eastern belt and the shoshonite and W. H. Wilson are greatly appreciated. saroka volcanic province: Geol. Soc. group. Although the bulk of the shosho- This article is based on a master's thesis by America Bull., v. 82, p. 2473-2494. nites, absarokites, and banakites were L. Love (1972). Ruppel, E. T., 1972, Geology of pre-Tertiary erupted from vents in the eastern belt, there rocks in the northern part of Yellowstone are scattered occurrences in the western belt National Park, Wyoming: U.S. Geol. Sur- and in the Hominy Peak Formation. The REFERENCES CITED vey Prof. Paper 729-A, 66 p. Schassberger, H. T., 1972, A K-Ar age of a composition of the bulk rock and quartz monzonite dike in the Kerwin min- groundmass glass within a shoshonite Carmichael, I.S.E., 1963, The crystallization of feldspar in volcanic acid liquids: Geol. Soc. ing district, Park County, Wyoming: (Nicholls and Carmichael, 1969, sample Isochron/West, no. 4, p. 31. 118A) may indicate that the rock under- London Quart. Jour., v. 119, p. 95-131. Chadwick, R. A., 1970, Belts of eruptive centers Smedes, H. W., and Prostka, H. F., 1972, Strati- went early plagioclase fractionation, but in the Absaroka-Gallatin volcanic province, graphic framework of the Absaroka Vol- more data are needed on other absarokites, Wyoming-Montana: Geol. Soc. America canic Supergroup in Yellowstone National shoshonites, and banakites. Petrographic Bull., v. 81, p. 267-274. Park region: U.S. Geol. Survey Prof. Paper analyses of absarokite-shoshonite-banakite Clarke, F. W., 1900, Analyses of rocks from the 729-C, 33 p. rocks (Prostka, 1973) suggest contamina- laboratory of the U.S. Geological Survey: Smith, A. L., and Carmichael, I.S.E., 1968, tion of trachytic liquid with plagioclase and U.S. Geol. Survey Bull. 168, 308 p. Quaternary trachybasalts from southeast- ern California: Am. Mineralogist, v. 54, p. pyroxene. Lead and strontium isotopes in- Hague, Arnold, Iddings, J. P., Weed, W. H., and others, 1899, Geology of the Yellowstone 909-923. dicate contamination or derivation from an Thornton, C. P., and Turtle, O. F., 1960, unradiogenic inhomogeneous source National Park: Pt. II, Descriptive geology, petrography, and paleontology: U.S. Geol. Chemistry of igneous rocks. I. Differentia- (Peterman and others, 1970). The scatter of Survey Mon. 32, p. 1-439. tion index: Am. Jour. Sci., v. 258, p. 664- points plotted for the absarokite- Iddings, J. P., 1899, Absarokite-shoshonite- 684. shoshonite-banakite rocks on the Q-Or- banakite series, in Hague, A., and others, Wilson, J. T., 1936, The geology of the Mill Ab+An diagram may support a contamina- Geology of the Yellowstone National Park: Creek Stillwater area, Montana [Ph.D. the- tion mechanism for producing these rocks. Pt. II, Descriptive geology, petrography, sis]: Princeton, N.J., Princeton Univ., 123 and paleontology: U.S. Geol. Survey Mon. p. + bib. CONCLUSIONS 32, p. 326-355. Ketner, K. B., Keefer, W. R., Fisher, F. S., Smith, D. L., and Raabe, R. G., 1966, Mineral re- MANUSCRIPT RECEIVED BY THE SOCIETY FEB- There is an apparent shift in igneous ac- sources of the Stratified Primitive Area, RUARY 24, 1975 tivity through time from the northwest to Wyoming: U.S. Geol. Survey Bull. 1230-E, REVISED MANUSCRIPT RECEIVED JANUARY 26, the southeast, which is reflected in both the 56 p. 1976 stratigraphic relationships of the Absaroka Larsen, E. S., 1940, The petrographic province of MANUSCRIPT ACCEPTED MARCH 22, 1976

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