Studies on the "Green Tuff" alteration and magnetization of Miocene Series

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The Late Cenozoic volcanic rocks are distributed in the western part of Gunma prefecture, Central Japan. but, it had been hard to determine the geologocal ages of the volcanic rocks which are poor in fossils. In this project, the writers determined the isotopic ages and measured the magnetization of the rocks. Number of samples for K-Ar ages is 65. Since then, the lower formations of the volcanic rocks have generally been believed to be Upper Miocene in age. Main results of this studies are as follows.1. The first volcanism in this area took place in early Upper Miocene age at the northwest end of Kanto mountains. the region of the most viloent volcanic eruptions had moved to northeast in Late Pliocene.2. Present topography on the east side of the Usui Pass had been formed by erosion during Brunhes Normal Epoch.3. K-Ar ages of altered rocks which were collected from same stratigraphic horizons do not indicate similar values. For instance, K-Ar ages, 6.2 - 5.6 Ma of the Do^^-zen Member of Kirizumi Formation show lerger values than those, 5.2 - 5.0 Ma of Yunosawa Member which is overlain by Do^^-zen Member.4. Volcanic rocks of the Myogi Mountains had formed during comparatively short time in the ages of 6 - 5 Ma. The lower half of the mountains is magnetized reversed and the upper is normal.5. Basement rocks of Haruna Volcano are exposed in areas of 910 meters above the sea. The K-Ar age of the rocks is 8.14 Ma.