東北日本弧，青麻火山の形成史と主成分化学組成

Abstract

The Aoso volcano is one of the representative stratovolcanoes of the Aoso-Osore volcanic zone in the northeast Japan arc. The volcanic activity of the Aoso volcano can be classified into two stages; the stages are separated by a caldera collapse event. The volumes of the products are 3.6 km3 (earlier stage) and 0.8 km3 (later stage). During the initial part of the earlier stage, thin lava flows (olivine orthopyroxene clinopyroxene basaltic andesite, SiO2=54.7-55.3%), block and ash flows (olivine bearing orthopyroxene clinopyroxene andesite, SiO2=57.9-58.5%), and thick lava (orthopyroxene clinopyroxene andesite, SiO2=59.0-59.9%) were extruded. The products construct either a conical shaped stratovolcano or series of stratovolcano. After that, scoria flows (orthopyroxene clinopyroxene andesite, SiO2=60.2%) and pumice flows (quartz orthopyroxene clinopyroxene hornblende andesite, SiO2=62.1-62.3%) spewed out, which might have caused the caldera at the top of this volcano. All the rocks during this stage can be plotted as linear lines on the co-variant diagrams. During the initial part of the later stage, pyroclastic rocks (olivine bearing hornblende orthopyroxene quartz dacite, SiO2=68.2-69.8%) were erupted, and lava domes grew. The lava domes can be separated into two types. One consists of clinopyroxene bearing orthopyroxene quartz dacite (SiO2=66.6-67.0%), whilst the other is clinopyroxene orthopyroxene dacite (SiO2=64.4-66.8%). All the eruptive rocks of this stage characteristically include mafic inclusions (quartz olivine bearing basaltic andesite and andesite, SiO2=53.8-57.5%). Each type of lava dome displays a distinct chemical trend in the covariant diagrams. All of the later stage products have higher SiO2 contents than the earlier stage products. Some of the later stage products, especially the pyroclastic rocks, were not plotted on the extrapolated lines of the trend drawn by the earlier stage products.