K-Ar ages of some pliocene to pleistocene felsic volcanic rocks around the Japanese Northern Alps, central Honshu

Felsic volcanic rocks of probable Pliocene to Pleistocene age which preceded the formation of young volcanoes of the Norikura Volcanic Chain as Tateyama, Yakedake, Norikura and Ontake Volcanoes, are developed around the Japanese Northern Alps. Five specimens of them were dated by K-Ar method by Teledyne Isotopes, Inc. (Table 3) in order to clarify the late Neogene history of volcanic activity and tectonic development of this area. 1. Omine Group is distributed to the east of the Itoigawa-Shizuoka Tectonic Line at the eastern foot of the Japanese Northern Alps. Two whole-rock specimens from the lower part of the group gave ages of 2.4 +- 0.2 and 2.0 +- 0.2 Ma (Nos. 1 and 2 or Table 3) to be latest Pliocene. 2. Two whole-rock specimens of Nyukawa Pyroclastic Flow Deposit distributed to the west of Norikura Volcano gave 2.5 +- 0.2 and 2.3 +- 0.1 Ma (Table 3, Nos. 3 and 4) to be latest Pliocene. This age is conformable to the stratigraphic relation and its reversed magnetic polarity. 3. A whole rock age of Yugamine Dacite to the west of Ontake Volcano was 0.5 +- 0.1 Ma (Table 3, No. 5). It revealed an occurrence of dacite monogenetic volcano in the middle Pleistocene in the area and that fault movement of the Atera Fault commenced before about 0.5 Ma. (author)