The petrogenesis of topaz rhyolites from the western United States

Abstract

High-silica topaz-bearing rhyolites of Cenozoic age are widely distributed across the western USA and Mexico. They are characteristically enriched in fluorine (>0.2 wt.%) and incompatible lithophile elements (e.g. Li, Rb, Cs, U, Th, Be). In addition to topaz, the rhyolites contain garnet, bixbyite, pseudobrookite, hematite and fluorite in cavities or in their devitrified groundmasses. Magmatic phases include sanidine, quartz, oligoclase and Fe-rich biotite. Allanite, fluorite, zircon, apatite and magnetite occur in most; pyroxene, hornblende, ilmenite and titanite occur in some. The rhyolites crystallized over a wide temperature interval (850° to 600° C) at \(f_{0_2 } \) that ranges from QFM to NNO. The REE patterns of most topaz rhyolites are almost flat (La/YbN=1 to 3) and have deep Eu anomalies (Eu/Eu*=0.01 to 0.02). Both parameters decrease with differentiation. Titanite-bearing rhyolites have prominent middle REE depletions. Topaz rhyolites appear to have evolved from partial melts of a residual granulitic source in the Precambrian lower crust. According to the proposed model, the passage of hot mafic magmas through the crust produced partial melts as a result of the decomposition of F-rich biotite or amphibole. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas.