Magma generation processes for large, zoned ignimbrites of Aso volcano, SW Japan: insights from geochemical variation of melt inclusions and groundmass

Abstract

Aso volcano in southwest Japan has produced four repeated caldera-forming eruptions over the last 270,000 years, each generating compositionally zoned ignimbrites that transition from silicic to more mafic magmas. To understand the magmatic processes behind these chemical and thermal zonations, we analyzed major and trace element compositions of melt inclusions and groundmass glasses. Our results reveal three distinct melt types: high-K silicic (HK-S), high-K mafic (HK-M), and medium-K (MK) melts. The HK-S and HK-M melts dominate the silicic and mafic units, respectively, while the MK melt is a minor component in the mafic units. Combining experimental petrology and mass balance modeling (mostly focusing on rare earth element compositions), we propose the following magmatic evolution: (1) The HK-M magma formed in a mid-lower crustal MASH zone through crystallization of basaltic magma and/or partial melting of basaltic rock; (2) this magma ascended and differentiated in a shallow upper crustal reservoir, generating the HK-S melt; (3) subsequent melt extraction from crystal mush, coupled with HK-M magma recharge, created a compositionally zoned shallow reservoir. The heat from the recharge also triggered partial melting and remobilization of the cumulate mush, producing the MK melt. These processes collectively explain the systematic zonation observed in Aso’s ignimbrites.