Very silicic, peraluminous rhyolite lavas from Glass Mountain, Mono County, east-central California, contain 2 to 3 ppm Sr—approximately two orders of magnitude less than the average for silicic igneous rocks—and less than 20 ppm Ba. On the basis of available data on the partition of Sr between coexisting feldspar and melt phases, the rhyolite is found to represent about 25 percent of a hypothetical “parent” magma of silicic composition having a “normal” Sr content of 150 ppm. Available strontium and lead isotopic data on similar nearby Pleistocene rhyolites suggest that the initial magmas from which the rhyolite lavas of Glass Mountain and other Quaternary silicic volcanic rocks of the area were differentiated were derived from mafic or ultramafic source material.

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