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

日本中部群马县西部分布着晚新生代火山岩。但由于缺乏化石，火山岩的地质年龄一直难以确定。在这个项目中，作者确定了同位素年龄并测量了岩石的磁化强度。K-Ar年龄样品65个。从那时起，火山岩的下部地层通常被认为是上中新世。主要研究结果如下：1.该地区的第一次火山活动发生在晚中新世早期，位于关东山脉的西北端。上新世晚期火山喷发最强烈的区域向东北方向移动.在布鲁赫正常时期，由于侵蚀作用，形成了目前的地形。同一层位的蚀变岩K-Ar年龄值不一致。例如，桐泉组道前段的K-Ar年龄为6.2 - 5.6 Ma，比被道前段覆盖的汤泽段的K-Ar年龄5.2 - 5.0 Ma偏大。Myogi山火山岩的形成时间较短，为6 - 5 Ma。山的下半部分被磁化反转，而上半部分是正常的。Haruna火山的基岩暴露在910米以上的海面上。岩石的K-Ar年龄为8.14Ma。

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.