CONSTRAINING THE ORIGIN AND INFLUENCE OF ANDESITE IN THE ZONED PURICO IGNIMBRITE, N. CHILE

A feature of large continental magmatic systems is voluminous dacite ignimbrites erupted from upper crustal magma reservoirs. In the Altiplano Puna Volcanic Complex (APVC) of the Central Andes, a major ignimbrite and caldera plateau, magma systems are found to be long-lived and remarkably homogeneous requiring that they be maintained by recharge at depth. The characteristics of the recharge magma are poorly constrained because it is rarely erupted due to density and viscosity barriers in the magmatic system. An exception is the 1 Ma Purico ignimbrite (~100 km³), where andesite appears late in the dominantly crystal-rich dacite climactic eruption and becomes dominant in post-climactic effusive eruptions. We have investigated the andesitic pumices in the Purico ignimbrite to constrain its origin and significance.

The andesitic pumice (60 to 62% SiO₂) and bands in mixed pumice are microvesicular and contain a phenocryst assemblage of plagioclase, orthopyroxene, clinopyroxene, magnetite and ilmenite ± amphibole, biotite and quartz. Glomerocrysts of opx, cpx, and plagioclase are common. Two texturally, compositionally, and isotopically distinct types of plagioclase, small (<500 μm) subhedral to euhedral crystals with high MgO (130-490 ppm) and low ⁸⁷Sr/⁸⁶Sr crystals (0.7076-0.7084) record a hot (>900 °C) andesite magma derived from an ~20 km deep magma reservoir. In contrast, the second type of plagioclase in the andesite appear as broken fragments of larger crystals and have significantly lower MgO (90-240 ppm), higher ⁸⁷Sr/⁸⁶Sr (0.7096-0.7114), and record cooler ~800-900 °C, upper crustal <10 km conditions.

The compositional variations observed in plagioclase crystals from the Purico andesitic pumices record recharge of a previously emplaced upper crustal (4-8 km depth) dacite magma reservoir by andesite coming from deep (~20 km). During ascent, the andesite incorporated crystals from the surrounding upper crustal plutonic bodies. Thus, the andesite itself is significantly modified and is a hybrid between a more mafic parent and upper crustal silicic melts.