P-T conditions of granulite-facies rocks from the Hidaka metamorphic belt, Hokkaido, Japan.

The Hidaka metamorphic belt, running roughly N-S through the axial part of Hokkaido, is divided into the western zone of ophiolite sequence and the main zone to the E of regional metamorphic and plutonic rocks. The latter is further divided into the upper and lower sequences on the basis of the lithology. Rocks of basic composition predominate in the lower sequence, while the upper sequence is composed largely of pelitic and psammitic metamorphic rocks with minor basic rocks. Plutonic rocks with migmatitic features intrude at the boundary between the upper and lower sequences and in the middle of the upper sequence. The metamorphic zonal classification of the main zone gives the following five progressive zones: zones I, II, III, IV and V from E to W. Zone I is of the greenschist facies, zone III of the amphibolite facies, and zones IV and V of the granulite facies; zone II is transitional. In the granulite-facies area, two distinct rock types occur: garnet-hypersthene-cordierite granulite and garnet-cordierite-biotite granulite. The geothermometer applied to the garnet-biotite, garnet-cordierite and garnet-orthopyroxene pairs gave a T of approx 730oC, while the garnet-orthopyroxene-plagioclase-quartz geobarometer gave P of approx 6.5 kbar. The estimated conditions are considered to be of the lower crust at about 23 km depth. The inferred geothermal gradient at the time of metamorphism was approx 33o/km.-M.Ha