抄録
The Asahi Mountains, being located near the southwestern end of Northeast Japan, are important to understand the geological structure of Japan, because it has been commonly said that the Tanakura Tectonic zone is the boundary between the Southwest Japan and the Northeast Japan as regards pre-Neogene system. This paper will show an outline of the geology and petrography in the southwestern part of the Asahi Mountains. Low grade metamorphic rocks and various kinds of plutonic rocks are distributed widely in the southwestern part of the Asahi Mountains. The low grade metamorphites are named the Takenosawa formation which might be derived principally from Paleozoic and Mesozoic strata. They are composed mainly of pelitic, psammitic and siliceous phyllites and their alternating rocks. Judging from their mineral assemblages, this formation has been subjected to greenschist facies metamorphism before the Cretaceous plutonism. The plutonic rocks are divided largely into two major groups, namely the older and the younger stages, based mainly on the relationships with the Suesawagawa rhyolitic welded tuff and the related volcano-hypabyssal rocks. These welded tuff and volcano-hypabyssal rocks and some of the older-stage plutonic rocks are thermally metamorphosed by the younger-stage plutonic rocks. The older-stage plutonic rocks, furthermore, can be divided into five concordant masses: Nishiasahi basic to intermediate complex, Ohasahi granodiorite, Ohtama granodiorite, Nakadake granodiorite and Sagami adamellite. The younger-stage ones, however, can be divided into seven concordant or partly discordant masses (Bakeana granodiorite complex, Heishiro adamellite, Miomote adamellite, Hiraiwa adamellite, Ito adamellite, Fine-grained diorite, and Biotite granitic rocks), and one perfectly discordant mass (Kakunara adamellite). The Takenosawa formation and the plutonic masses except for the Kakunara adamellite mass are, more or less, deformed by the remarkable mylonitization. The Kakunara adamellite mass, however, is chiefly affected by the cataclastic deformation, and the mylonitization occurs in some places.
