Petrography, K-Ar age, and chemistry of Yoshino-dai lavas in the Aira caldera

Abstract

The Aira caldera was formed about 22, 000 years ago in association with large-scale pyroclastic eruptions of more than 400km^3 felsic ejecta. Petrography was described and major/trace element compositions and K-Ar ages were determined for Aira precaldera lavas in the Yoshino-dai area, in order to examine mechanisms by which a series of the Aira precaldera and climactic magmas were produced. Precaldera lavas in the Yoshino-dai area mainly consist of augite-olivine basalt and two-pyroxene andesite, as previous workers indicated. K-Ar ages of precaldera basalts and andesites are >0.5 Ma and, in contrast, K-Ar age of precaldera rhyolite is <0.5 Ma and eruptions of precaldera dacites and minor pyroclastic flows may have occurred after ～ 0.5 Ma. The precaldera and climactic ejecta show a single chemical trend with the cusp at SiO^2～65 wt. % for some elements. It follows that a series of Aira precaldera magmas was not produced by a single process such as fractional crystallization from basaltic magmas or two-end-member mixing between basaltic magmas and rhyolitic magmas. The generation of a series of Aira precaldera magmas requires at least two processes; production of basaltic to andesitic magmas before ～ 0.5 Ma and of dacitic to rhyolitic magmas after ～ 0.5 Ma. The possibility that the state of the magmatic system beneath the Aira caldera may have changed at ～ 0.5 Ma remains.