[Abstract]

The Late Pliocene to Early Pleistocene volcanic rocks in the Suwa area, central Japan, comprising an enormous quantity of volcanic rocks, are named the Enrei Volcanic Rocks. The authors will report two K-Ar ages of volcanic rocks from the Kirigamine Volcano, which composes the uppermost part of the Enrei Volcanic Rocks. Based on the newly obtained ages, previously reported radiometric ages, and volcanostratigraphic data, we identify activity periods of 1.3 to 0.75Ma for Kirigamine Volcano. We point out the possibility that the Oiwake Volcanic Graben partly covered by Kirigamine Volcano was mainly formed at ca. 0.85Ma. From the previous studies and this study, the duration of the activity of the Enrei Volcanic Rocks is mainly from ca. 1.5 to 0.75 Ma. This duration of activity of the Enrei Volcanic Rocks almost corresponds to the active period in the Early Pleistocene of the Yatsugatake Volcanoes that adjoined Suwa area. Thus, in the Suwa and Yatsugatake areas in the Early Pleistocene, volcanic activity occurred with enormous volumes, covering a large area (800km2).

Key words:

K-Ar dating, Kirigamine Volcano, Enrei Volcanic Rocks, Yatsugatake Volcano, Early Pleistocene