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Hybrid Origin of the Absarokite-Shoshonite- Banakite Series, Absaroka Volcanic Field, Wyoming

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Hybrid Origin of the Absarokite-Shoshonite- Banakite Series, Absaroka Volcanic Field, Wyoming HAROLD J. PROSTRA U.S. Geological Survey, Denver, Colorado 80225 Hybrid Origin of the Absarokite-Shoshonite- Banakite Series, Absaroka Volcanic Field, Wyoming Note: This paper is dedicated to Aaron and Elizabeth origin of potassic rocks, including shoshonites. Waters on the occasion of Dr. Waters' retirement. Recent work on the type shoshonites and absarokites includes chemical analyses and a study of their stratigraphy (Nelson and Pierce, ABSTRACT 1968), a careful study of their mineralogy and Textural and mineralogical features of pot- chemistry (Nicholls and Carmichael, 1969), ash-rich basaltic rocks of the absarokite- and data on the lead and strontium isotopes shoshonite-banakite series strongly suggest that (Peterman and others, 1970). most of the large crystals and aggregates in The present paper, based primarily on a these rocks are xenocrysts and microxenoliths, study of 29 thin sections of shoshonite, four of not true phenocrysts as was previously thought. absarokite, and two of banakite—all from the A hybrid origin, involving assimilation of Absaroka field—focuses attention on critical gabbro by high-temperature syenitic magma, textural and mineralogical features of these is proposed. rocks that indicate a hybrid origin. INTRODUCTION PETROGRAPHY AND MINERALOGY The names absarokite, shoshonite, and banak- A particularly striking feature of the type ite were coined by Iddings (1895) for potas- shoshonites and absarokites is the abundance sium-rich rocks of basaltic aspect that occur of large crystals of plagioclase, pyroxene, and as lava flows, flow breccias, and dikes through- olivine, many of them more than 1 cm long. out the Absaroka volcanic field in northwestern When viewed in thin section, these "pheno- Wyoming (Hague and others, 1899). These crysts" are seen to be aggregates of several rocks appear to be coarsely porphyritic; the grains (Figs. 1 through 8). Most of the aggre- absarokites contain large grains of olivine and gates are monomineralic, but some are made up augite, and the shoshonites contain abundant of two or more mineral phases. The margins of plagioclase as well. Banakites are essentially the aggregates, where they are in contact with feldspar-rich shoshonites that generally contain the groundmass, commonly are ragged, rounded some biotite. All three rock types are part of a and, in some cases, embayed. The aggregates gradational series distinguished by their color are clearly out of equilibrium with the ground- index and chemical composition (Iddings, mass, for they have been caught in the process 1895). Their high potash content (2.5 to 5.7 of being disaggregated and not in the process percent) is due to abundant groundmass sani- of growth. dine. Iddings (1895) reported leucite in some The large plagioclase grains are of labradorite absarokites and banakites, but none was found (An47_68). They typically have oscillatory and during the present study. normal compositional zoning that is very weak These rocks remained obscure petrological in contrast to the pronounced zoning in pheno- curiosities until Joplin (1965, 1968) pointed out crysts of most extrusive porphyritic rocks. the world-wide occurrence of similar potash- Some large grains are virtually unzoned (Fig. rich rocks and proposed that they all be re- 5) but they are commonly rimmed by sanidine. garded as members of a single rock association— The augite grains likewise are nearly unzoned, the shoshonite association. The petrogenesis of and many contain exsolution lamellae of ortho- these rocks has not been adequately explained. pyroxene. Some of the hypersthene grains were Bell and Powell (1969) reviewed and evalu- found to contain exsolved augite (Nicholls and ated the hypotheses attempting to explain the Carmichael, 1969). These exsolution features Geological Society of America Bulletin, v. 84, p. 697-702, 8 figs., February 1973 697 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/697/3429069/i0016-7606-84-2-697.pdf by guest on 26 September 2021 698 H. J. PROSTKA Figure 1. Photomicrograph of a typical shoshonite. Figure 2. Photomicrograph of a typical microxeno- Crystals of olivine (ol), augite (px), and ragged corroded lith of labradorite in shoshonite. Note the subparallel clumps of plagioclase (pi) are enclosed in a trachytic orientation of the grains, the irregular anhedral contacts groundmass of plagioclase microlites, small grains of between them, the corrosion and disaggregation along augite, olivine, Fe-Ti oxides, and accessory biotite. some grain boundaries, and the very limited composi- Plane-polarized light. Sample P-263 F. tional range shown by uniform extinction along the entire length of the grains. Crossed nicols. Sample P-263 F. Figure 3. Photomicrograph of a plagioclase micro- Figure 4. Photomicrograph of a microxenolith of xenolith containing entrapped grain of augite (px). plagioclase rounded by resorption. The interlocking Corrosion, along the grain boundaries, has begun to grain contacts suggest a plutonic or metamorphic disaggregate this clump. Xenocrysts, freed by this origin, and the wavy ex tinction indicates that the grains process, are scattered throughout the groundmass. have been bent and strained. Crossed nicols. Sample This feature was found in most of the Absaroka P-263 F. shoshonites examined. Crossed nicols. Sample P-263 F. and the compositional homogeneity of the have deformation features: (1) grains that are pyroxenes and plagioclase are characteristic strained and have wavy extinction (Figs. 4, 7), not of volcanic phenocrysts, but rather of (2) pyroxene xenoliths that have a weakly de- slowly crystallized cumulate minerals in mafic veloped mortar texture (Fig. 7) indicating intrusive complexes (Wager and Brown, 1968). mild cataclasis, and (3) strained plagioclase Grain boundaries, especially between plagio- xenoliths that have interlocking grain bounda- clase grains, are mostly anhedral (Figs. 2, 4, 5) ries like those in a metamorphic rock (Fig. 4). as found in extensive adcumulate growth Some of these features are apparent in early (Wager and Brown, 1968, p. 65). In contrast, photomicrographs (Hague and others, 1899) intratelluric plagioclase phenocrysts that have but were not interpreted at that time. clumped together are generally in contact These mineralogical and textural features along euhedral crystal faces. A small proportion strongly suggest that most, if not all, of the of the aggregates or xenoliths in these rocks coarsely crystalline material in these rocks is Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/697/3429069/i0016-7606-84-2-697.pdf by guest on 26 September 2021 ABSAROKITE-SHOSHONITE-BANAKITE SERIES, WYOMING 699 Figure 5. Photomicrograph of a microxenolith of Figure 6. Photomicrograph of an aggregate (px) plagioclase Even extinction of the grains indicates consisting of about equal amounts of hypersthene and virtually no compositional zoning. Mild cataclasis of augite. Most of the grain contacts within the aggregate the plagioclase prior to incorporation in the melt is are euhedral suggesting a cumulate origin, but some of indicated by fractures cutting across several grains but the grains in contact with groundmass show the effects not extending out into the groundmass. Crossed nicols. of incipient disaggregation. There is no olivine in the Sample P-217 A. xenolith even though single olivine crystals (ol) as well as pyroxene grains are abundant in the ground- mass of this absarokite. Crossed nicols. Sample P-70. Figure 7. Photomicrograph of a shoshonite. The Figure 8. Photomicrograph of a shoshonite. The two largest microxenoliths each consist of several inter- plagioclase xenoliths all have about the same composi- locking grains of strained augite with a few small grains tion but their degree of corrosion and disaggregation of olivine (ol). Mild cataclasis is indicated by the wavy varies greatly from clump to clump. This photograph extinction of the larger grains and by the mosaic of shows particularly well how small plagioclase fragments small augite fragments along some of the grain bounda- form by disaggregation of larger grains, aided by ries. The xenolith on the left has a euhedral over- differential flowage of the groundmass while it was growth around the strained composite core, but the still molten. Plane-polarized light. Sample 767-145. one on the right has a much thinner and incomplete overgrowth suggesting either mingling of different magmas or recurrent assimilation during ascent of the advanced stages in which individual plagioclase magma. Crossed nicols. Sample P-60. xenocrysts are being reduced to fragments (Fig. 8). A very few of the shoshonite samples xenolithic in origin. Furthermore, these xeno- (not illustrated) that represent more advanced liths were not in equilibrium with the enclosing stages of dissolution consist almost entirely of melt but were being corroded, dissolved, and groundmass with only scattered remnants of disaggregated. All stages of disaggregation may highly resorbed xenocrysts. Evidently, the be seen in thin section, from the earliest stages groundmass liquid of these rocks was highly where xenoliths are only partly disaggregated corrosive to the xenocrysts. by corrosion along grain boundaries (Fig. 1), The mineralogy of the xenocrysts suggests freeing xenocrysts with ragged edges, to more that they are foreign, for they show no indica- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/697/3429069/i0016-7606-84-2-697.pdf by guest on
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