Origin of pyroxene-plagioclase aggregates in a rhyodacite

Bodies of pyroxene-bearing rhyodacite in New England, New South Wales, previously considered to be intrusions, are reinterpreted as ignimbritic flows. Small aggregates of pyroxene and plagioclase are inferred to be either compositionally modified crystalline residuals from partial melting or crystal cumulates. An earlier hypothesis involving mixing of solid biotite-diorite with a rhyolitic liquid is questionable, and the origin of the rhyodacite can be explained in terms of recent experimental work on crystal-liquid equilibria under inferred crustal conditions.