抄録
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.
