Pleistocene volcanic rocks from the eastern Itoigawa Region.

Abstract

Volcanic stratigraphy performed at the eastern Itoigawa region revealed 3 formations of volcanic rocks of Sarukura, Eboshiyama and Takanomine in ascending order, with subordinating coarse grained epiclastic formation of Kajiyashiki which is partly interfingered with the Takanomine Formation. K–Ar datings revealed ca. 1.3–1.2 Ma from juvenile dacite volcanic blocks of the Eboshiyama Formation and 0.65 Ma from juvenile andesite volcanic block of the Takanomine Formation. Fission-Track dating resulted in ca. 1 Ma from the Otanigawa Tuff of the Kajiyashiki Formation. As the Kajiyashiki Formation includes abundant andesite volcanic blocks of the Takanomine Formation, the activity of the Takanomine Formation started before 1 Ma and at least sustained until 0.65 Ma. Petrological analyses revealed characteristics of magma in each formation especially shown in SiO₂–K₂O diagram which depicts discriminative variation of K-series in Pleistocene epoch.