The Physical Volcanology And Petrology Of The 3400 Ybp Caldera -Forming Eruption Of Aniakchak Volcano, Alaska

Abstract

Zoned ignimbrites are often interpreted as overturned "snapshots" of a compositionally zoned magma chamber. While ignimbrite zonation requires the spatial and temporal coexistence of diverse magmas, the origin of both the zonation and the diversity are poorly understood. While many studies have developed in situ differentiation models, these models do not explain composition gaps, commonly observed in the ignimbrites. Aniakchak Caldera on the Alaska Peninsula was formed approximately 3,400 years ago. The eruption began with a plinian phase that deposited a thin rhyodacitic pumice fall deposit, and valley-filling rhyodacitic ignimbrites. A second phase is represented by a massive ignimbrite sheet (probably resulting from column collapse) comprising both rhyodacite and andesite. The eruption culminated in the collapse of the volcanic edifice and the deposition of a lithic-rich andesitic ignimbrite. The andesite (~60 wt. % SiO2) emplaced in the caldera-forming eruption, appears to have been produced by magma mixing. Although crystal-poor (~8 wt. % crystals), the andesite contains a bimodal plagioclase population and rare grains of both quartz and olivine. Self-contamination, wherein residual liquids from a crystallizing marginal boundary layer are reincorporated into the hybrid magma during successive basalt or rhyodacite injections, probably led to deviations from a binary mixing trend. The rhyodacite (~70 wt. % SiO2), the most silicic magma erupted at Aniakchak is also crystal-poor (~2 wt. % crystals), but contains a single plagioclase population. The crystal poor nature, simple mineralogy, and silicic composition of the rhyodacite imply an uncomplicated history. Geochemical models indicate partial melting of mafic plutons and fractional crystallization of a basaltic magma contributed to the rhyodacite erupted in caldera-forming eruption. It appears that the andesitic composition was maintained by repeated injections of small volumes of both basaltic and rhyodacitic magmas. Each successive mixing event induced an increment of self-contamination in the andesite chamber. Increased pressure associated with an unusually large influx of rhyodacite, however, immediately induced an eruption, before significant mixing occurred. After the rhyodacitic magma was evacuated, and now destabilized the roof collapsed, leading to the eruption of the residence andesite.