K-Ar ages of the lavas from Goshikigahara volcano, Central Hokkaido, Japan

The Goshikigahara volcano, situated at the northern part of the Tomuraushi volcano group in Central Hokkaido, Japan, is a large Quaternary composite volcano consisting of a basal volcanic edifice (Kaundake volcano) and three overlying volcanic edifices (Ponkaundake, Chubetsudake, and Goshikidake volcanoes). Based on the K-Ar ages determined previously for the Kaundake volcano (NEDO, 1990) and the K-Ar ages determined in this study for three volcanic rocks collected from the uppermost (or near uppermost) lavas of the Ponkaundake, Chubetsudake and Goshikidake volcanoes, the formation history of the Goshikigahara volcano can be summarized as follows: (1) ca. 1.0 Ma-0.95 Ma, voluminous lava and pyroclastic eruptions to form the Kaundake stratovolcano, (2) <0.95 Ma-0.8 Ma, volcanic activity on the western flank of the Kaundake volcano to build the main part of the Ponkaundake stratovolcano, and (3) ca. 0.75 Ma, volcanic activity on the eastern flank of Kaundake, forming the two lava plateaus of Chubetudake and Goshikidake.