Geology of Hachimantai volcanic field and temporal-spatial variation of the magma compositions

Abstract

Northern part of Hachimantai volcanic field, northeastern Japan, comprises four stratovolcanoes, Hachimantai, Chausudake, Nishimoriyama, and Maemoriyama volcanoes, and many flank volcanoes, Appidake, Fukenoyu, Gentamori, Ebisu-Daikoku-mori, and an eroded volcano. K-Ar dating and geological survey revealed that Hachimantai and Chausudake volcanoes erupted about during 0.7-1 Ma, Ebisu-Daikoku-mori volcano erupted about 0.4 Ma, and ages of the products of Maemoriyama volcano are younger than 0.4 Ma. Although compositional range of the rocks from whole of the field is wide, chemical variation of rocks from individual volcano is relatively small. There is little correlation of chemical compositions with age, oppose to distinct spatial variation in the volcanic field, that basaltic lavas distribute in the eastern and northern part, and andesite lavas in the southern and western part. Spatial ⁸⁷Sr/⁸⁶Sr variation, that decreases from east to west, is corresponded with the trend of whole of the arc. The magmas were derived from lithosphere which heterogeneity was resulted in the spatial ⁸⁷Sr/⁸⁶Sr variation.