K-Ar age of an andesite lava in the eastward of the Utsukushigahara Plateau, Nagano Prefecture, central Japan

Abstract

The Miocene Uchimura Formation, which consists of the Miocene submarine volcanic rocks and the Lower Pleistocene volcanic rocks in the Chushin Highland are widely distributed in the northeastern part of the Utsukushigahara Plateau, Chushin Highland, central Japan. An andesite lava, which effused on land surface, was found out in about 3 km eastward of the Utsukushigahara Plateau. The K-Ar age for whole rock sample of the andesite lava taken from basal part of the lava flow deposits indicates 2.11±0.05 Ma in this study. According to the stratigraphical, petrographical and whole rock chemistry data, it is considered that the andesite lava belongs to the volcanic rocks in the Chushin Highland. The result is suggested that the volcanic activity in the Chushin Highland was initiated around 2.1 Ma. Consequently, it is clear that the volcanic activity in the Chushin Highland had took place both in the Upper Pliocene and Pleistocene.