Origin of felsic magmas of some large-caldera related stratovolcanoes in central part of NE Japan

Abstract

We examined the genesis of felsic magmas of three post large-caldera volcanoes; Aoso, Takamatsu, and Gassan from front to rear arc side, NE Japan. These volcanoes are mainly composed of andesites which were formed by mixing between mafic and felsic end-members. The following data suggest the two end-member magmas are cogenetic. (1) Increasing potassium contents in felsic end-members from front to back arc side can be observed, as well as in mafic ones. (2) Sr isotopic data of each volcano are rather constant in all range of silica contents. Fractional crystallization process of phenocrystic minerals from the mafic end-member can not explain the whole rock compositions of the felsic one. An alternative process to produce the felsic end-members is partial remelting of solidified mafic ones. The trace element composition shows that the felsic end-member can be formed by remelting, leaving hornblende gabbroic residues.