Petrography and whole-rock geochemistry of pre-Aso lavas from the caldera wall of Aso volcano, central Kyushu

Abstract

Pre-caldera volcanism in the Aso area (2.2–0.43 Ma), central Kyushu, is characterized by effusive eruptions of multiple lava flows. To clarify the chemical evolution of the magma chamber beneath the Aso area, we investigated the petrological characteristics of these lavas where exposed in the caldera wall.
The pre-caldera lavas are divided into eight types with distinct petrographic and compositional characteristics: A. cpx-ol basalt, B. ol-2px andesite, C. ol-hb-2px andesite, D. 2px andesite, E. hb-2px andesite, F. hb andesite, G. 2px-hb dacite, and H. bt-hb rhyolite. Incompatible trace element modeling demonstrated that these eight types did not originate via simple fractional crystallization.
The phase assemblages and abundances of phenocrysts of the pre-caldera andesite-rhyolite differ from those of the caldera-forming and post-caldera andesite-rhyolite. In addition, the pre-caldera andesite-rhyolite contain relatively low concentrations of incompatible trace elements compared to the caldera-forming and post-caldera andesite-rhyolite. These observations may indicate that the physical conditions and/or chemical compositions of the source materials that gave rise to the andesite-rhyolite magmas differed between the pre-caldera and caldera-forming stages.