Intermediate composition

Abstract: Andesites represent a large proportion of the magmas erupted at continental arc volcanoes and are regarded as a major component in the formation of continental crust. Andesite petrogenesis is therefore fundamental in terms of both volcanic hazard and differentiation of the Earth. Andesites typically contain a significant proportion of crystals showing disequilibrium petrographic characteristics indicative of mixing or mingling between silicic and mafic magmas, which fuels a long-standing debate regarding the significance of these processes in andesite petrogenesis and ultimately questions the abundance of true liquids with andesitic composition. Central to this debate is the distinction between liquids (or melts) and magmas, mixtures of liquids with crystals, which may or may not be co-genetic. With this distinction comes the realization that bulk-rock chemical analyses of petrologically complex andesites can lead to a blurred picture of the fundamental processes behind arc magmatism. Here we present an alternative view of andesite petrogenesis, based on a review of quenched glassy melt inclusions trapped in phenocrysts, whole-rock chemistry, and high-pressure and high-temperature experiments. We argue that true liquids of intermediate composition (59 to 66 wt% SiO(2)) are far less common in the sub-volcanic reservoirs of arc volcanoes than is suggested by the abundance of erupted magma within this compositional range. Effective mingling within upper crustal magmatic reservoirs obscures a compositional bimodality of melts ascending from the lower crust, and masks the fundamental role of silicic melts (>/=66 wt% SiO(2)) beneath intermediate arc volcanoes. This alternative view resolves several puzzling aspects of arc volcanism and provides important clues to the integration of plutonic and volcanic records.

Abstract: The intimate association of basalt, andesite, dacite, and rhyolite within a volcanic center suggests that these rocks are genetically related Individual lava flows that show a gradation in composition may preserve maximum evidence of the magmatic processes producing this association One such flow of rhyolite to dacite composition, Glass Mountain in northern California, was formed by contamination of rhyolite magma as it intruded the basaltic flows of the Medicine Lake Highland shield volcano Although dacite flows and domes commonly show less variation in composition than the Glass Mountain flow, many show similar evidence of contamination by basalt by the presence of abundant basaltic inclusions and phenocrysts and phenocryst clots from those inclusions Similarly, many andesite flows contain rhyolitic inclusions, rhyolitic bands, and phenocrysts appropriate to rhyolite These observations indicate that andesite and dacite are hybrid rocks that are formed when rising primary basalt and rhyolite magmas either become contaminated with the glassy debris of the volcanic pile or mix with each other directly Linear variation in bulk composition, phenocryst assemblages of intermediate rock, and frequency distribution of lava compositions in the southern Cascade Range, Chilean Andes, Taupo volcanic zone, and Tongan Islands support this hypothesis It appears that partial melting usually produces magma of rhyolitic and basaltic compositions and that any subsequent fractional crystallization is of limited importance