Christtiansen, and three co-authors, reported in 1988 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 (125ppm), Hf(9ppm), 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.
The authors concluded that in spite of recent advances in analytical and theoretical techniques, the construction of unique geochemical models for the
origin and evolution of granitic magmas is extremely difficult, if not impossible. In many cases we can at best demonstrate that it is possible that a specific rock evolved by a certain set of geologic processes. In this paper, we have attempted to erect one such possible model that shows how the internal variation of the Sheeprock granite arose and how the parental magma was generated. To do so we have made explicit assumptions regarding the elemental composition of parental materials and the bulk partition coefficients that govern their distributions between crystals and liquids.
Within the limits imposed by the uncertainties in these assumptions, the authors have shown that relatively simple models of batch partial melting and fractional crystallization can explain the geochemistry of the Sheeprock granite and by analogy other aluminous rare-metal granites. As a source for the Sheeprock granite, we have called upon partial melting of a depleted, "granulitic" protolith in the lower crust. An important requirement of the model is that the amount of partial melting remains small (2.5°7o to 11%). The decomposition of small amounts of refractory F-rich biotite will yield small quantities of F-rich melt.
The extensional tectonic setting of the eastern Great Basin (and in other areas where A-type granites occur) may have been a prerequisite to the separation of small melt fractions from crystalline restite. This sort of tectonic setting may have also set the stage for the intrusion of hot mafic magmas into the crust.
Mafic magmas may have acted as a heat source for partial melting. Subsequent high level fractional crystallization enhanced the depletions and enrichments of some elements. The preservation of cumulates on the walls of the pluton carries important consequences for interpretations of the mechanism of crystallization of granitic magmas.
Magmas rich in fluorine and incompatible trace elements (e.g. Rb, Li, U) can be derived from high-grade metamorphic rocks that have relatively low concentrations of these elements and low Rb/Sr ratios. The ratios of element concentration in the probable source to that in the evolved magma are in the order of 20 to 30 for incompatible elements. If these enrichments also hold for economically important elements (Be, Ta, Sn, W, Mo, U, etc.) then efficient magmatic accumulations of such elements to protore status are within reason. Geochemically "enriched" or anomalous crustal sources are not required for the generation of rare-metal granites such as the Sheeprock granite.
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