K-Ar Age of the Tamagawa Welded Tuffs and Related Rocks from Hachimantai Geothermal Field, Northeast Japan

Abstract

The Tamagawa Welded Tuffs are distributed in and around the Hachimantai geothermal field, northeast Japan (Figs. 1 and 2). The total volume of the tuffs is estimated over 500km³, and the magma reservoir can be expected as a heat source of the geothermal field still now . The Tamagawa Welded Tuffs are devided into Dacite, Upper Rhyolite, Middle Rhyolite and Lower Rhyolite Welded Tuffs in descending order (Table 1). And some part of the underlying Tertiary sediments are composed of dacite and rhyolite tuffs (Fig. 3). Twenty potassium-argon ages (Table 2) established the age range of the Tamagawa Welded Tuffs and the underlying tuffs as from about 2.0 Ma to 1.0 Ma and from about 8.2 Ma to 2.4 Ma respectively. The K-Ar dates and magnetic polarities of the volcanic units agree with previously published geomagnetic-reversal time scale (Fig. 4). The Dacite (0.9-1 .2 Ma), the Upper Rhyolite (1.2-1.7 Ma) and the Lower Rhyolite (1.7-2.0 Ma) Welded Tuffs are correlated to the Matsuyama epoch and the Middle Rhyolite Welded Tuff (1.6-1.7 Ma) to the Olduvai event.