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

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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

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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-

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tion of having crystallized from an alkali.c melt. and (b) a syenitic liquid that invaded the Microprobe studies (Nicholls and Carmichael, cumulates, and disaggregated and partially 1969) indicate that the olivine, pyroxene, and assimilated them. The proportions of syenite plagioclase grains are typical of those found in to gabbro could have been highly variable in ordinary calc-alkaline gabbros. The augite the resulting mix. Furthermore, if the gabbroic grains are not rimmed with aegirine-augite, material was not homogeneous, but came from there are no large crystals of biotite, there are a layered complex, then the relative propor- no biotite rims around the olivine or Fe-Ti tions of mafic minerals to each other and to oxides, and there are no phenocrysts of sanidine. plagioclase could also have varied greatly. In The groundmass of the type shoshonites and this manner, syenitic liquid plus cumulates absarokites consists of variable proportions of from an ultramafic zone would yield absarokite, sanidine and andesine, with lesser amounts of and syenite liquid plus mafic minerals and augite, Fe-Ti oxides, biotite and (or) olivine, plagioclase would yield shoshonite. According to this model, the: proportions of olivine, and accessory apatite and sphene. Residual pyroxene, and plagioclase could all vary inde- glass, present in some rocks, is silica rich pendently of one another as well as indepen- (Nicholls and Carmichael, 1969). The ground- dently of the proportion of syenitic liquid; the mass, exclusive of the xenocrysts, has a compo- resulting highly variable suite of rocks would sition ranging from quartz latite to rhyodacite. be difficult, if not impossible, to explain by Because the groundmass must have been en- simple crystal-liquid differentiation from a riched in the components of calcic plagioclase single parental magma. and pyroxene by resorption, the original liquid must have been more felsic and potassic with a Xenocryst compositions are quite uniform, composition approximating that of trachyte but the degree of xenocryst resorption in rocks or syenite. from different flows and dikes varies; this It is not clear just how a syenitic liquid, difference suggests that successive pulses of which presumably crystallizes at a much lower invading syenitic liquid were at different tem- temperature than does gabbro, coulc effec- peratures, or that the time between invasion of tively assimilate mafic rocks. Initial disaggrega- the gabbro and extrusion of the mix varied for tion of the gabbro might have occurred by different batches. Plagioclases of the same melting an interstitial mesostasis of granitic composition that are: disaggregated to varying composition, but further melting involving degrees in the same rock (Fig. 8) suggest that the labradorite and pyroxene would not be assimilation occurred recurrently or at different expected unless the syenitic melt were espe- levels during ascent of the magma, or that cially hot. Nevertheless, the textural evidence different batches of hybrid magmas mingled. for corrosion and disaggregation is very com- Whatever the situation, the magma must pelling. Perhaps the gabbroic material was not have remained very hot until the time of completely solidified, but was rather a crystal extrusion, for the only reaction relation seen mush that became mechanically disaggregated between xenocrysts and liquid is that of re- by invasion of the syenitic melt. The assimila- sorption. The widespread distribution of indi- tion of gabbroic rocks by syenitic magma is not vidual lava flows indicates low viscosity, which a new idea. Syenite plutons in the Adirondacks may have been related to a high temperature have mafic phases, which, according to Bud- of eruption. dington (1939), were formed by extensive The preponderantly fine-grained trachytic assimilation of gabbro and amphibolite. The and hyalopilitic grcundmass textures suggest resulting hybrid rocks, called pyroxene sye- very rapid cooling after extrusion. Low PH,O nites, have chemical compositions almost iden- during final consolidation is indicated by the tical with that of shoshonite. scarcity of groundmass biotite or biotite rims around the mafic xenocrysts. The magma prob- ORIGIN ably was originally more hydrous, for associ- From the foregoing discussion, it seems evi- ated plugs of syenogabbro, the plutonic equiva- dent that the type shoshonites and relatid lavas lent of shoshonite, contain much more biotite are hybrid rocks composed of (a) large grains of than do the lavas (Hague and others, 1899). labradorite, augite, olivine, hypersthe ne, and Vesicles are common in the lavas, an indication Fe-Ti oxides that crystallized plutonically as that water and other volatiles were lost during cumulates from a calc-alkaline mafic magma; extrusion.

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The ultimate origin of the syenitic melt and Mem. 7, 354 p. the age and depth of origin of the assimilated Hague, A., Iddings, J. P., and Weed, W. H„ 1899, gabbroic rocks are necessarily beyond the scope Geology of the Yellowstone National Park: of this paper. Isotopic studies and analyses of U.S. Geol. Survey Mon. 32, pt. 2, p. 215-355. minor-element abundances now in progress Iddings, J. P., 1895, Absarokite-shoshonite-banak- may help in answering these fundamental ite series: Jour. Geology, v. 3, p. 935-959. questions. Joplin, G. A., 1965, The problem of the potash-rich basaltic rocks: Mineralog. Mag., v. 34, no. 268, If the shoshonite association (Joplin, 1965) is p. 266-275. accepted by petrologists as a valid rock associa- 1968, The shoshonite association-—A review: tion, we must remember that the rocks for Geol. Soc. Australia Jour., v. 15, p. 275-294. which it is named are of hybrid origin, and are Nelson, W. H„ and Pierce, W. G., 1968, The not representatives of a primary potash-rich Wapiti Formation and Trout Peak trachyan- mafic magma. desite, northwestern Wyoming: U.S. Geol. Survey Bull. 1254-H, p. Hl-Hll. ACKNOWLEDGMENTS Nicholls, J., and Carmichael, I.S.E., 1969, A com- mentary on the absarokite-shoshonite-banak- This work was done as part of a comprehen- ite series of Wyoming, U.S.A.: Schweizer. sive study of Yellowstone National Park by Mineralog. u. Petrog. Mitt., v. 49, no. 1, the U.S. Geological Survey in co-operation p. 47-64. with the National Aeronautics and Space Peterman, Z. E., Doe, B. R., and Prostka, H. J., Administration. Reviews of this manuscript by 1970, Lead and strontium isotopes in rocks of D. L. Blackstone, R. S. Houston, B. F. the Absaroka volcanic field, Wyoming: Contr. Leonard, III, and R. E. Wilcox are acknowl- Mineralogy and Petrology, v. 27, p. 121-130. edged and appreciated. Wager, L. R., and Brown, G. M., 1968, Layered igneous rocks: Edinburgh and London, Oliver REFERENCES CITED and Boyd, 588 p. Bell, Keith, and Powell, J. L., 1969, Strontium isotopic studies of alkalic rocks—The potas- sium-rich lavas of the Birunga and Toro- MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 7, Ankole regions, east and central equatorial 1972 Africa: Jour. Petrology, v. 10, p. 536-572. REVISED MANUSCRIPT RECEIVED JUNE 12, 1972 Buddington, A. F., 1939, Adirondack igneous rocks PUBLICATION AUTHORIZED BY THE DIRECTOR, U.S. and their metamorphism: Geol. Soc. America GEOLOGICAL SURVEY

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