Mineralogical and isotopic evidence for phenocryst-matrix disequilibrium in the Garner Mountain andesite

Petrographic, trace element and isotopic evidence demonstrates that magma mixing preceded the eruption of the Garner Mountain andesite. The flow contains reversely zoned plagioclase phenocrysts and amphibole pseudomorphs composed of plagioclase, clinopyroxene, orthopyroxene and opaque oxides. Partially resorbed quartz grains are also present. In contrast to the isotopically uniform matrix, plagioclase phenocrysts have ⁸⁷Sr/⁸⁶Sr ratios that correlate negatively with matrix Sr and positively with matrix Rb abundances. These observations demonstrate increasing isotopic disequilibrium between the plagioclase and matrix in the more evolved varieties of the flow.Plagioclase phenocrysts and matrix are assumed to record fractionation-assimilation events in different parts of the magma chamber. Early formed plagioclase phenocrysts crystallized under AFC conditions close to the roof of the chamber and were subsequently entrained in a liquid mixture composed of evolved interstitial liquid held in the partly crystallized roof zone and newly injected parental magma.