Geology and Petrology of Musa volcano, east Hokkaido.

Musa volcano is situated at the central part of the Akan-Shiretoko volcanic chain, which belongs to the Kurile arc, eastern Hokkaido. In this region, Pliocene subaqueous volcanism (Yunosawa, 574m highland, and 626m-peak volcanoes) has changed to terrestrial one (Musa volcano) in early Pleistocene. Musa volcano is composed of a cluster of several stratovolcanoes and lava domes, and is topographically divided into two stages, older and younger. K-Ar age of the andesite from the younger stage has been obtained to be 0.48 ± 0.19 Ma. The eruptive rocks of Musa volcano ranges from basaltic andesite to dacite, and are defined as low-K series of Gill (1981). K2O content of the rocks increases from the volcanic front (Musa and Mashu volcanoes) to the back arc side (Shari, Shiretoko-Iwo volcanoes). The zonal distribution of lava chemistry (eg. K2O) in the southern part of the Kurile arc transverses the echelon arrangement of the volcanoes.