Abstract
The Northern Yatsugatake volcanoes occupy the northern part of the Yatsugatake volcanic chain, central Japan. According to the geological and the petrographical studies, the volcanic products of the Northern Yatsugatake volcanoes are grouped into the four volcanic units such as Kasuga, Nijinotaira, Fukinotaira and Tateshina volcanics in ascending order. The previously reported age and paleomagnetic data are re-evaluated, and it became clear that the lavas of Nijinotaira and Fukinotaira volcanics were formed in the Early Pleistocene age. The whole rock chemistry for sixty samples from the study area was newly analyzed. The results show that the rocks of the Kasuga volcanics are mainly basalt to basaltic andesite. On the other hand, the rocks of the Nijinotaira and Fukinotaira volcanics are andesite to dacite. The volcanic activity of the northern Yatsugatake started about 1.2Ma. The basalt to basaltic andesite magma erupted between 1.1 to 1.0Ma. The volcanism was characterized by eruption of thin lava flows, which formed stratovolcanoes. Then the andesite to dacite magma erupted between 1.0 to 0.9 Ma. The volcanism was characterized by eruption of thick lava flows. At the last stage, andesite to dacite lavas erupted during the Middle to Late Pleistocene. The petrochemical features of the magma of the northern Yatsugatake volcanoes are similar to those of the Enrei volcanic rocks, which located on the west of the northern Yatsugatake volcanoes. In the Northern Yatsugatake and Enrei areas, about 200 km3 of magma erupted during the Early Pleistocene. Such a voluminous magma eruption may have been performed under the tensional stress field, which formed by the horizontal movement of the Itoigawa-Shizuoka Tectonic Line.
