Cooling process of the basal tonalite magma, Hidaka metamorphic belt, northern Japan

Abstract

The low-pressure / high temperature type Hidaka Metamorphic Belt (HMB) in Hokkaido, northern Japan, represents a tilted crustal section of a magmatic arc of Tertiary age. Exposed crustal section forms duplex structure which was formed by the uplift tectonic process. Syn-tectonic tonalite magma intrudes along the floor thrust, ramp, and roof thrust of the crustal-scale duplex. The tonalite magma was generated by anatexis of the unexposed lowermost crust.
Pyroxene-bearing tonalites (basal tonalite body) is distributed in the Niikappu river region, northern part of the Hidaka Metamorphic Belt. The various evidences that show a cooling process can be observed within the tonalite body, such as the orthopyroxene pseudomorph and the aplite veins. Cooling process of the tonalite body has been revealed from these textures. The P-T-t paths of the syn-tectonic tonalite suite and the metamorphic layer show the uplift tectonics of the crust. A P-T-t path of the delaminated lowermost crust also can be presumed.