Melt inclusions in the quartz phenocrysts of rhyolites from Topaz and Keg Mountains, Thomas Range, Utah

The quartz phenocrysts of rhyolitic rock that have erupted over the last 40 m.y. in the Thomas Range, west-central Utah, preserve glass inclusions that may contain trapped liquidus phases. These glass inclusions, analyzed by electron microprobe, show the melts of the Joy Tuff (crystal tuff member), Dell Tuff, and Topaz Mountain Rhyolite to be peraluminous potassic rhyolites, the latter enriched in fluorine (as much as ~1.9 percent). The older units (Joy and Dell Tuffs), tuffaceous calc-alkaline rhyolites, contain trapped Mn-rich aluminous clinopyroxene, zircon, fluorapatite, feldspar(?), titanite(?), and quartz. The anorogenic Topaz Mountain rhyolite preserves evidence of a ternary feldspar and magnetite (as well as quartz) on the liquidus. The potassic and peraluminous nature of the melts is consistent with anatexis of a crustal sequence. The distribution of fluorine, unlike that of chlorine, is heterogeneous in the Topaz Mountain Rhyolite. In this F-rich unit, primary β-quartz gave way to primary α-quartz. Both Ca and Fe were added to the bulk rocks following their devitrification.