30- to 32-m.y.-old rhyolite flows and domes (host rhyolite) capped by rhyolitic ignimbrites (cap ignimbrites). Relatively low crystallization temperatures (700 degrees + or - 50 degrees C) in the host rhyolites, but higher (740 degrees + or - 40 degrees C) in the cap ignimbrites. The host rhyolites are high silica rhyolites (>74% SiO 2 ) that are metaluminous or slightly peraluminous; they have high K 2 O/Na 2 O ratios; are enriched in Sn, U, Th, and Rb; and depleted in Ca, Mg, Ti, P, Co, Ni, Ba, Sr, and Eu relative to high K calc-alkalic rocks. Rare earth element patterns exhibit marked negative Eu anomalies and a slight enrichment of light rare earth elements relative to the heavy elements. These characteristics suggest that the "tin rhyolites" formed as extreme differentiates in high-level magma chambers closely related to caldera development. Initial Sr isotope ratios of the tin rhyolites range from 0.7053 to 0.7078. Differentiation was clearly the major factor in concentrating tin in the Mexican tin rhyolites.--Modified journal abstract.

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