METAMORPHISM AND GRANITE GENESIS IN THE HIDAKA METAMORPHIC BELT, HOKKAIDO, JAPAN

The Main Zone of the Hidaka Metamorphic Belt is an uplifted crustal section of island-arc type. The crust was formed during early Tertiary time, as a result of collision between two arc-trench systems of Cretaceous age. The crustal metamorphic sequence is divided into four metamorphic zones (I-IV), in which zone IV is in the granulite facies. A detailed study of the evolution of the Hidaka Belt, based on a revised P-T-t analysis of the metamorphic rocks, notably a newly found staurolite-bearing granulite, confirms a prograde isobaric heating path, after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks. During the peak metamorphic event (c. 53 Ma), the regional geothermal gradient attained 33-40-degrees-C km-1, and the highest P-T condition obtained from the lowest part of the granulite unit is 830-degrees-C, 7 kbar. In this part, X(H2O) of Gt-Opx-Cd gneiss is about 0.15 and that of Gt-Cd-Bt geneiss is 0.4. The P-T-X(H2O) condition of the granulite unit is well within a field where fluid-present partial melting of pelitic and greywacke metamorphic rocks takes place. This is in harmony with the restitic nature of the Gt-Opx-Cd gneiss in the lowest part of the granulite unit. The possibility that partial melting took place in the Main Zone is significant for the genesis of the peraluminous (S-type) granitic rocks within it. The S-type granitic rocks in this zone are Opx-Gt-Bt tonalite in the granulite zone, Gt-Cd-Bt tonalite in the amphibolite zone, and Cd-Bt-Mus tonalite in the Bt-Mus gneiss zone. The mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from S-type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust.