K-Ar ages for lava samples of Koganegahara volcano, Central Hokkaido, Japan

Koganegahara volcano is a large Quaternary composite volcano situated in the southwest of the Tomuraushi volcano group in Central Hokkaido, Japan. The lowermost eruptive units of the Koganegahara volcano have previously been dated at about 1.1 Ma. In this study, two lava samples from the uppermost eruptive units of the eastern and central parts of the volcano are dated, giving K-Ar ages of 0.70±0.02 Ma and 0.7±0.01 Ma. This indicates that the Koganegahara volcano was active for 400000 years from 1.1 to 0.7 Ma. Furthermore, based on previously reported and new age data, the K-Ar ages appear to be concentrated around 1.1-1.0 Ma and 0.75-0.7 Ma, suggesting that volcanic activity peaked in these periods. (author)