青麻‐恐火山列 東北日本弧火山フロント

Abstract

Nasu volcanic zone, which has been considered as the Quaternary volcanic front of the Northeast Honshu arc, Japan, is divided into two zones, Aoso-Osore and Sekiryo volcanic zones. The Aoso-Osore zone, including Aoso, Nanashigure, Osore and Easan volcanoes, are situated at the closest side to the trench. Total volume of the eruptive materials of this zone is much less than that of the Sekiryo zone. No hydrous phenocrystic minerals occur in the volcanic rocks of the Sekiryo zone, while hornblende phenocryst is recognized in the andesites of the Aoso-Osore zone. Liquidus temperature of hornblende is kept relatively constant as the water content of magma varies. Accordingly, magmatic temperature of the Aoso-Osore zone is lower than that of the Sekiryo zone. The K2O content of the volcanic rocks of the Aoso-Osore zone is lower than that of the Sekiryo zone at the same SiO2 content, indicating that the nature of magmas of these two zones are different to each other. Thus the volcanic front of the Northeast Honshu are is revised to lie at the Aoso-Osore zone. At the front, volume of magma was small and temperature was low compared with those of the Sekiryo zone. These facts support the hypothesis that the volcanic front represents the limit of magma generation controlled by temperature and/or pressure profile of the wedge mantle of the arc.