High-potassium intrusive rocks of the Crandall ring-dike complex, Absaroka Mountains, Wyoming A. M. KUDO Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 DAVID E. BROXTON ESS-1, Mail Stop D-462, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ABSTRACT The Crandall ring-dike complex has a gabbro-diorite core in- truded by a shoshonitic ring dike. Both have been cut by granular and porphyritic quartz monzonite dikes. The most abundant rock is medium-grained diorite. In contrast, the ring-dike shoshonite has large phenocrysts of plagioclase and augite in a microcrystalline ground- mass of biotite, Siinidine, plagioclase, amphibole, magnetite-ilmenite, and variable amounts of glass. Texturally, the plagioclase and pyrox- ene appear out of equilibrium with the surrounding matrix in both the ring dike and the intrusive core rocks. The gabbro, diorite, and por- phyritic quartz monzonite are characterized by plagioclases with bi- modal core and rim compositions, indicating that one group is possibly xenocrystic. Trends on variation diagrams for both minor and trace elements cannot fa« traced from the diorite to the quartz monzonite. On variation diagrams, the shoshonite rocks have the widest scatter of data points, which can be explained by the addition of plagioclase and augite phenocrysts. Geochemical modeling is successful in demonstrat- ing that the gabbro can be formed as a cumulate from the diorite by fractional crystallisation but is unsuccessful in relating the diorite and the quartz monzonite by this process. The strongly positive Eu anom- aly in the gabbro supports the cumulate origin; the diorite and sho- shonite have chemical and textural signatures which strongly indicate contamination by plagioclase and pyroxene accumulation. Even the porphyritic quartz monzonite has experienced plagioclase accumula- tion. The nature of the uncontaminated magma is uncertain. Chemi- cally, the diorite is a plutonic equivalent of shoshonite, and support is given to the hypothesis that shoshonites are formed by contamination with plagioclase and pyroxene (Prostka, 1973). INTRODUCTION The Absaroka volcanic province of northwestern Wyoming crops out over 23,000 krn^ and consists mainly of calc-alkalic Eocene andesite flows and breccias (Smedes and Prostka, 1972). The rocks range in com- l> j I Map Area position from basalts to rhyolites, and it was here that Iddings (1899a) first " IDAHO .< • Cody described and namijd the potassium-rich mafic lavas as absarokite, sho- shonite, and banakite from exposures in the northern Absaroka Mountains. Chadwick (1970) postulated two subparallel belts of eruptive WYOMING centers spanning the length of the province. The belts trend northwest, and the rocks of the eastern belt are generally more potassic than those of the Figure 1. Location and geologic map of the Crandall ring-dike western belt. complex, Absaroka Mountains, Wyoming. Symbols are as follows: Our paper is a petrogenetic study of intrusive rocks from the eastern dikes and sills are represented by dark, heavy lines; g = gabbro, d = belt (Fig. 1). The majority of the rocks analyzed come from the dissected diorite, qm = quartz; monzonite, s = shoshonite, Tv = volcanic and Crandall "volcano" (on Hurricane Mesa) of Iddings (1899b). The Cran- volcaniclastic rocks of the Absaroka Supergroup. Additional material for this article, Tables A and B, may be secured free of charge by requesting Supplementary Data 85-19 from the GSA Documents Secret!iry. Geological Society of America Bulletin, v. 96, p. 522-528, 11 figs., 1 table, April 1985. 522 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/4/522/3445046/i0016-7606-96-4-522.pdf by guest on 02 October 2021 CRANDALL RING-DIKE COMPLEX, WYOMING 523 dall volcano has a core of medium-grained diorite with minor gabbro and quartz monzonite surrounded by a ring dike composed of porphyritic aphanite, the composition of which is similar to shoshonites described by Iddings (1899a) or to high-K andesite. Krushensky (1960) performed a detailed pétrographie study of the intrusive rocks, and Pierce and others (1973) published a geologic map of the quadrangle which contains the Crandall pluton. We will discuss the genesis of these rocks using mineral chemistry and major- and trace-element geochemistry of the whole rocks. We hope to show similarities between the shoshonites and the dominant rock type in the intrusion, the diorites. Evidence for some contamination of the Crandall magmas by pyroxene and plagioclase will be presented. FIELD RELATIONSHIPS Two plutons, a ring dike, and swarms of late-stage dikes have intru- sive contacts with the Eocene Wapiti Formation and the Trout Peak trachyandesite (Nelson and Pierce, 1968). The largest intrusion is the Crandall pluton and ring dike on the south-central part of the mesa Figure 2. Photomicrograph of diorite with glomerocrystic aggre- (Fig. 1). The Crandall ring-dike complex is primarily diorite but also gate of plagioclase arranged subparalleUy with jagged contacts and contains rocks ranging from gabbro to quartz monzonite. In the core of the corroded rims. These are surrounded by potash feldspar (black) con- complex, quartz monzonite is found as dikes cutting the diorite, but the taining poikilitic inclusions of plagioclase and pyroxene. Crossed ni- contact between the gabbro and the diorite appears gradational. In some chols. Long dimension = 3.5 mm. areas of the core, the quartz monzonite occurs in large outcrops continuous over tens of metres, suggesting stock-like bodies rather than dikes. The central diorite is almost completely enclosed by a younger, circular ring dike composed of a porphyritic, K-rich andesite or shoshonite. The ring dike, in turn, has been intruded by dikes and dikelets of quartz monzonite, especially in the southwestern exposures of the complex. Some of these dikes project into the stock-like bodies of the quartz monzonite in the core. No quartz monzonite dike was observed outside of the ring dike. These quartz monzonite dikes appear lithologically similar to the quartz monzo- nite of the core and appear to emanate from there, therefore, the sequence of the intrusion is most likely gabbro-diorite, ring dike, and lastly, quartz monzonite. The Crandall ring-dike complex has a width of 3.2 km, and the interior stock measures ~2.5 km in diameter. The thickness of the ring dike varies from <0.5 km to >1.0 km. It is interesting that, although the volumes of the ring dike and the core are approximately equal, the ring- dike rocks are porphyritic with an aphanitic matrix, and the core rocks are medium grained. A zircon fission-track age of 40.2 ±1.7 m.y. has been obtained for a quartz monzonite sample from the core of the intrusion (dated by C. Naeser, 1980, written commun.). Figure 3. Photomicrograph of augite aggregate in ring-dike rock. PETROGRAPHY Long dimension = 3.5 mm. Crossed nichols. Medium-grained granular gabbros, diorites, and quartz monzonites subparallel orientation (Fig. 2). In contact with the interstitial potash contain plagioclase, two pyroxenes, minor biotite, and magnetite-ilmenite feldspar, the rims are corroded (Fig. 2). Subophitic intergrowth of plagio- (Table A).1 There are minor variations in texture, and minor olivine clase and augite is not uncommon. Plagioclase, being the largest, appears (mostly altered) occurs in the gabbro. Some of the quartz monzonites are to be the earliest crystallized. This is unusual for the gabbros if they are porphyritic with a phaneritic groundmass, but others are fine grained and equivalent to absarokites because plagioclase phenocrysts are absent in the aphyric. Some of the diorites have an intergranular texture. The most latter. The aphanitic rocks from the ring dike have phenocrysts of plagio- striking texture, however, in almost all rock types, but especially in the clase and augite set in a hypocrystalline to holocrystalline groundmass. diorite, is represented by early formed, large subhedral to euhedral plagio- The augites occur in aggregates with interlocking grain contacts (Fig. 3) clase laths with jagged rims and stubby pyroxene prisms. These are sur- and in single grains with rounded resorbed outlines. These features have rounded by later interstitial potash feldspar (sanidine?) and/or quartz been observed in shoshonite flows by Prostka (1973) who inferred a poikilitically enclosing the smaller plagioclase and pyroxene grains. Many hybrid origin for these rocks, the plagioclase and pyroxenes being xeno- of the plagioclases occur in monomineralic aggregates in which the indi- crystic or xenolithic. vidual anhedral grains have irregular wavy contacts between them and a MINERAL CHEMISTRY 'Tables A and B are in the GSA Data Repository. Request Supplementary All mineral compositions have been determined on two electron Data 85-19 for free copies. microprobes (ARL-EMX and the JEOL Superprobe 733) which are fully Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/4/522/3445046/i0016-7606-96-4-522.pdf by guest on 02 October 2021 524 KUDO AND BROXTON automated. At least 15 individual grains in each microprobe section have realize that sectioning of rocks will not always expose actual cores for been analyzed for both core and rim compositions. Several step scans microprobing, but at least 25 individual cores were measured for sach across large, zoned plagioclases also were done. Six element analyses for section. The gap occurs for rim compositions as well, so that the bimodal plagioclases, nine for pyroxenes and olivines, and eight for the Fe-Ti distribution appears real. In a sample of a sparsely porphyritic quartz oxides have been corrected by Bence-Albee. monzonite with <1% plagioclase phenocrysts, a bimodal distribution is lacking, however. No calcic plagioclase composition was detected. In this Feldspar sample also, large interstitial sanidine (Or6i to Orgl) has a more Na-rich composition than the smaller interstitial grains which have Or82 to C)r91. The compositions of plagioclase in these medium-grained rocks are Sanidine compositions are Or80_74 in diorite and Org^ in quartz mon- plotted in Figure 4.