Geochronological and geochemical constraints on the heat source of thermal activity in the Rausu geothermal field, Shiretoko Peninsula, Hokkaido, Japan
The Rausu geothermal field is situated in the middle part of the Shiretoko Peninsula, east Hokkaido, Japan. It has been classified into “A-type” (characterized by high temperature/depth ratio) in terms of its Geothermal Activity Index. The geology of the area consists of Tertiary volcaniclastic rocks, a hornblende dacite intrusion and overlying Tertiary to Quaternary lavas. The dacite is 63 wt.% SiO2 and belongs to the calc-alkaline suite. The solidus and subsolidus temperatures of the dacite magma, calculated from mineral equilibrium, are 830°C and 741°C, respectively. The K-Ar age obtained from hornblende phenocryst is 3.0±0.8 Ma. The geological, geochronological and petrological data suggest that the heat source of the Rausu geothermal field is a long-lived, high-temperature granodioritic pluton existing beneath the intrusion. It is suggested that the thermal activity is caused by the upwelling of high-temperature fluids related to the pluton, through fracture networks within or around the intrusion.