Christiansen, and three authors, reported in 1988 in Recent Advances in the Geology of Granite_Related Mineral Deposits, that Cenozoic magmatism of the eastern Great Basin, western U.S.A., produced a number of aluminous anorogenic (A-type) rhyolites, many of which contain topaz and are rich in U, Be,
Rb and F; such rhyolites could be called rare-metal rhyolites. A small granite pluton that is chemically and temporally equivalent to these rhyolites is preserved in the Sheeprock Mountains of west-central Utah. The pluton ranges from biotite monzogranite to biotite-muscovite syenogranite (IUGS). Ilmenite, magnetite, apatite, zircon, monazite, fluorite, beryl, and thorite are magmatic accessory minerals; in a few specimens topaz and muscovite are post-magmatic accessories. The pluton was emplaced at a shallow level.
Sparse greisen-bordered fractures contain Sn and W minerals. The pluton has a Rb-Sr isochron age of 20.9 ± 1.2 Ma with an initial 87Sr/86Sr ratio of 0.7064. Slight contamination of rocks collected from the pluton's margins is indicated by elevated initial Sr-isotope ratios. Three geochemical groups can be identified within the texturally varied but geochemically zoned stock. Group I consists of highly evolved specimens from the core of the pluton with high Rb/Sr (40-90) and low La/Yb^ (3). Relative to the other two groups, Group 1 specimens are depleted in Th (50 ppm, this and other values below are maximums), Sr (20 ppm), Ba (100 ppm), Zr (125 ppm), Hf(9 ppm), LREE(100xchondritic), U, Ti, Fe, Mg, Ca, K, and P. Group 1 specimens are enriched in Na, F (0.39 wt% maximum), Sc (3 ppm), Be (50 ppm), Li (260 ppm), Ta (33 ppm), Nb (110 ppm), HREE (35 x chondritic) and Y (60 ppm). Specimens from Groups 2 and 3 come from a "collar" that partially surrounds the core but the distribution of these two groups overlaps with one another.
The geochemical effects of weathering, assimilation, and metasomatism are visible to varying degrees in the pluton; nonetheless, most of the chemical variation appears to be the result of fractional crystallization. The compositions of Groups 2 and 3 produce divergent arrays on plots of two elements with
different bulk partition coefficients (e.g. Rb and Sr). This is interpreted as resulting from the preservation of complementary liquid (Group 2) and solid (Group 3) compositions. Trace element models suggest that in situ fractional crystallization produced these trends. The data suggest that evolved liquids
(Group I) were displaced inwards while cumulates (Group 3) and the products of incremental episodes of equilibrium crystallization (Group 2) accumulated on the walls of the magma chamber.
The genesis of the parental magma probably involved partial melting of granulites in the lower crust as the result of the decomposition of F-rich biotite. From trace element models of batch partial melting, small degrees of melting (2.5% to 15%) are inferred to be important in the generation of the parental
magma; subsequent strong fractionation occurred in the upper crust. A geochemically anomalous source is not required for the generation of granites like the Sheeprock granite that are rich in incompatible trace elements.
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