The Geology and Geochemistry of Cenozoic Topaz Rhyolites from the Western United States.

High-silica, topaz-bearing rhyolites of Cainozoic age are widely distributed across the W USA and Mexico; they are characteristically rich in F (>0.2 wt.%) and contain topaz crystallized during post-magmatic vapour-phase alteration. Their emplacement followed or was contemporaneous with calc-alkaline and basaltic magmatism in the Basin and Range province, along the Rio Grande rift, and in Montana, and coincided with episodes of extensional tectonism in these regions. Analyses for major and trace elements and REE are tabulated and the petrography and mineralogy of these topaz rhyolites are described. Magmatic phenocrysts include sanidine (Or₅₀), quartz, sodic plagioclase (usually oligoclase) and biotite rich in F and Fe; Fe-rich hornblende or clinopyroxene occur in a few lavas. Common accessory minerals include magnetite, ilmenite, zircon, apatite, allanite and fluorite; garnet, bixbyite, pseudobrookite and hematite occur with topaz in cavities or in devitrified groundmass. The REE patterns are almost flat but have strong negative Eu anomalies. The topaz rhyolites are intimately related to economic deposits of lithophile elements (Be, U, F, Li, Sn). In their age, tectonic setting, mineralogy, chemistry and style of replacement, the topaz-rhyolites resemble the rhyolitic stocks associated with Climax-type Mo deposits, and some may be surface manifestations of such deposits. The topaz rhyolites are distinct from both peralkaline rhyolites and calc-alkaline rhyolites, but are similar to other aluminous rhyolites erupted in bimodal associations with basalts in W USA; they may be the equivalent of the topaz-bearing ongonites of the USSR. They appear to have evolved from partial melts of a residual felsic granulite source in the lower or middle crust of the Precambrian continent.-R.A.H.