Petrogenesis of rare-metal granites from depleted crustal sources: an example from the Cenozoic of western Utah, U.S.A. ERIC H. CHRISTIANSEN* � Department of Geology, University of Iowa JOHN S. STUCK LESS U.S. Geological Survey . Denver, Colorado MYRA J. FUNKHOUSER-MAROLF and KRISTY H. HOWELL Department of Geology, University of Iowa . Iowa City, Iowa ABSTRACT and the products of incremental episodes of equilibrium crys­ Cenozoic magmatism of the eastern Great Basin, western. tallization (Group 2) accumulated on the walls of the magma U.S.A., produced a number of aluminous anorogenic (A-type) chamber. rhyolites, many of which contain topaz and are rich in U, Be, The genesis of the parental magma probably involved par­ . Rb and F; such rhyolites could be called rare-metal rhyolites. tial melting of granulites in the lower crust as the result of the A small granite pluton tlu!t is chemically and temporally equi�a­ decomposition ofF-rich biotite. From trace element models of lent to these rhyolites is preserved in the Sheeprock Mountall1s batch partial melting, small degrees of melting (2.5 % to 15 %) of west-central Utah. The pluton ranges from biotite mon­ are inferred to be important in the generation of the parental zogranite to biotite-muscovite syenogranite (lUGS). Ilmenite, magma; subsequent strong fractionation occurred in the upper magnetite, apatite, zircon, monazite, fluorite, beryl, and thorite crust. A geochemically anomalous source is not required for are magmatic accessory minerals; in a few specimens topaz and the generation of granites like the Sheeprock granite that are muscovite are post-magmatic accessories. The pluton was rich in incompatible trace elements. emplaced at a shallow level. Sparse greisen-bordered fractures contain Sn and W minerals. The pluton has a Rb-Sr isochron Introduction age of 20.9 ± 1.2 Ma with an initial 87 Srl86 Sr ratio of 0.7064. Rare metal granites have been identified as those with high con­ Slight contamination of rocks collected from the pluton's mar­ centrations of normally dispersed elements such as F, Li, Rb, gins is indicated by elevated initial Sr-isotope ratios. Cs, Sn, Ta, Nb, Zr, and REE (Tauson, 1974; Kovalenko, 1978; Three geochemical groups can be identified within the tex­ the metallogenically specialized granites of Tischendorf, 1977). turally varied but geochemically zoned stock. Group 1 consists Such granites occupy a special place in petrologic thinking of highly evolved specimens from the core of the pluton with because of their unique and often extreme chemical features. high Rb/Sr (40-90) and low La/YbN (3). Relative to the other Two fundamentally different types of rare-metal granites exist two groups, Group I specimens are depleted in Th (50 ppm, - one peralkaline (agpaitic) and the other metalum nous to p r­ this and other values below are maximums), Sr (20 ppm), Ba : aluminous (plumasitic or Li-F). Each type of gramte! has a dIS­ (100 ppm), Zr (125 ppm), Hf (9 ppm), LREE (100 x chondritic), tinctive set of trace element characteristics and associated U, Ti, Fe, Mg, Ca, K, and P. Group 1 specimens are enriched mineral deposits . The peralkaline granites are extremely enriched in Na, F (0.39 wt% maximum), Sc (3 ppm), Be (50 ppm), Li in some trace elements, e.g. Zr , Nb, LREE; whereas the (260 ppm), Ta (33 ppm), Nb (110 ppm), HREE (35 x chon­ aluminous granites are enriched in Rb, Ta, U, and Sn. In dritic) and Y (60 ppm). Specimens from Groups 2 and 3 come . general, these granites appear to be part of the anorogemc group from a "collar" that partially surrounds the core but the dis­ of granites (A-type) identified by Loiselle and Wones (�979) nd tribution of these two groups overlaps with one another. � Collins et al. (1982). In addition to sharing the tectome settl g The geochemical effects of weathering, assimilation, and � in which they are produced, both types of rare-m tal gramte metasomatism are visible to varying degrees in the pluton; : are enriched in fluorine and chlorine. From a revIew of vol­ nonetheless, most of the chemical variation appears to be the canic glass analyses, Christiansen et al. (1983) hav pointed out result of fractional crystallization. The compositions of Groups � that silicic peralkaline magmas are generally CI-nch (FICI < 2 and 3 produce divergent arrays on plots of two elements with 3), whereas aluminous rare-metal granites or rhyolites are different bulk partition coefficients (e.g. Rb and Sr). This is . enriched in fluorine (F/CI > 3). The latter are the SovIet eqUIva­ interpreted as resulting from the preservation of complemen­ lent of the "Li-F geochemical type" and contrast with the tary liquid (Group 2) and solid (Group 3) compositions. Trace agpaitic geochemical type (e.g. Kovalenko, 1978). element models suggest that in situ fractional crystallization In this study, we have examined the rare-metal Sheep rock produced these trends. The data suggest that evolved liquids granite from western Utah, U.S.A., in an effort to formulate (Group 1) were displaced inwards while cumulates (Group 3) a model for the origin and evolution of fluorine-rich aluminous magmas. We conclude that the parental magma of the Sheeprock granite appears to have been derived by small degrees *Now at Department of Geology, Brigham Young University, Provo, Utah, U.S.A. of partial melting of Rb-poor granulites of the lower continen- Granite·Related Mineral Deposits 307 ° f 112 30' LEGEND Qat Alluvium N Tgs p-C Sheeprock Granite Tv Volcanic Rocks Ps Sedimentary Rocks Qat Metasedimentary Rocks p€ • o GrOUP 1 rI' �8 ". • Group 2 o Group 3 �101 •••111 �\ 1,--_...., SlC p-C Sheeprook � Mountains _ J ' GB ; j CP o 1 2 // Utah Km FIGURE 1. Schematic geologic map (after Cohenour 1959, and Christie-Blick 1983) of the Sheeprock granite showing the distribu­ tion of the specimens collected for this study. Symbols are used to distinguish the three geochemical groups discussed in the text. Sample localities without numbers represent unpublished analyses. Locations mentioned in text include CJ - Copper Jack mine; U - Utah mine; ED - Flying Dutchman mine; JC - Joes Canyon (samples from these localities have these labels); SLC - Salt Lake City: GB - Great Basin: CP - Colorado Plateau. tal crust. Decomposition of small amounts of F-rich biotite in middle and late Tertiary), most are Cenozoic. Mid-Tertiary high-grade metamorphic rocks controlled the degree of partial intermediate to silicic pyroclastic rocks, lavas, and plutons were melting. Most of the geochemical variation within the Sheeprock emplaced in a southward migrating volcanic belt that may have granite is the result of in situ fr actional crystallization with mar­ been part of a broad volcanic arc related to lithospheric sub­ ginal accretion of cumulate phases. These processes produced duction at the continental margin (Stewart et al., 1977; Lindsey a marked geochemical zonation in the pluton defined by three et al., 1975; Lipman, 1980). A fundamental change in the nature distinctive groups of specimens. The fractionation of accessory of Cenozoic magmatism occurred approximately 23 Ma (Best minerals played an important part in the trace-element evolu­ et al., 1980) and was marked by the onset of bimodal mafic tion of the magma. and rhyolitic volcanism in western Utah. These A-type granites and rhyolites (many of which contain topaz) are geochemically Geologic Sett,ing distinct from older I(Caledonian)-type (Pitcher, 1982) and S-type Late Cenozoic magmatism of the eastern Great Basin, western granitoids of western Utah and eastern Nevada. Even before U.S.A., produced a number of aluminous A-type rhyolites. this change in magma chemistry, tectonic extension of the Great These rhyolites are part of a bimodal (basalt-rhyolite) suite and Basin in a west-southwest direction Was underway behind a calc­ are contemporaneous with lithospheric exte'nsion in the Basin alkaline volcanic are, which by thi's time was restricted to areas and Range province. Many contain topaz and are rich in Be, near the coast (Zoback et al., 1981; Eaton, 1984). As a result U, Rb, Ta, and F and appear to be the volcanic equivalen,ts of the development and growth of a transforn boundary at the western margin of the continent, the extension direction changed of rare-metal granites (Christiansen et al., 1983, 1986). The erup" , ' tions of these rhyolites have resulted in several small uranium to approximately east-west sometime between 10 and 6 Ma ago and fluorite deposits and an important beryllium deposit within and fo rmed the present fault-controlled topography of the Great the 21 Ma Spor Mountain Formation, Utah (Lindsey, 1982; Burt Basin (Zoback et al., 1981). et al., 1982). A small pluton of chemically anq temporally The Sheeprock granite is a small pluton (ca 25 km2) exposed equivalent granite is preserved in the Sheeprock Mountains, along the southwest flank of the ,Sheeprock Mountailis horst about 60 km to the northeast (Fig. 1). in the eastern Great Basin of west-central Utah (Fig. 1). The The Sheeprock granite is only one of a number of igneous pluton intrudes Late Proterozoic quartzites, slates, phyllites and complexes exposed along the length of the east-trending Deep diamictites as well as Paleozoic sedimentary rocks . Portions of Creek-Tintic mineral belt (Shawe and Stewart, 1976; Stewart its margins are covered by Cenozoic alluvium (Christie-Blick, et al., 1977). This trend is marked by basement highs, plutonic 1982; Cohenour, 1959). Several types of geologic information and volcanic rocks. aeromagnetic highs, and by mineral deposits suggest that the pluton crystallized at a relatively shallow level; generally associated with the igneous rocks. Although the plu­ the lack of a regional metamorphic aureole, the presence of tonic rocks are of a variety of ages (Precambrian, Jurassic, miarolitic cavities, the extreme variations in texture, and the 308 Recent Advances in the Geology of presence of older (Oligocene?) volcanic rocks very near the con­ and Utah Cu-F deposits).