Two sequences of rhyolite lava flows and associated pyroclastic deposits are exposed in the Thomas Range (6 m.y.) and at Spor Mountain (21 m.y.) in west-central Utah. Both contain topaz indicative of their F-enrichment (>0.2%) and aluminous nature. The rhyolites are part of the bimodal sequence of basalt and rhyolite typical of the region. Moderate changes in major elements coupled with large variations in trace elements in vitrophyres from the Thomas Range are generally consistent with fractionation of observed phenocrysts. Especially important roles for the trace minerals are suggested. Nonetheless, disagreement between major and trace element models regarding the degree of crystallization as well as improbably high DTa and DTh suggest that some diffusive differentiation involving the migration of trace elements complexed with volatiles also occurred or that monazite, titanite or samarskite fractionation was important. Higher concentrations of F in the evolving Spor Mountain rhyolite drove residual melts to less silicic compositions with higher Na and A1 and promoted extended differentiation yielding rhyolites extremely enriched in Be, Rb, Cs, U, Th, and other lithophile elements at moderate SiO2 concentrations (74%).

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