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 crystalli- zation 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 adja- cent parts of Montana. The regionally ex- tensive volcaniclastic rocks, flows, and in- trusions 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/10/1455/3433626/i0016-7606-87-10-1455.pdf by guest on 26 September 2021 1456 LOVE AND OTHERS LEGEND Contact Dashed where approximate Strike and dip of bed Road Co 11 uv ium 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/10/1455/3433626/i0016-7606-87-10-1455.pdf by guest on 26 September 2021 DACITES OF NORTHWESTERN WYOMING 1457 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.