Partial melting of high-grade metamorphic rocks in lower crustal part of the Hidaka Arc (Main Zone of the Hidaka metamorphic belt), northern Japan.

Abstract

Lower crustal part of the Hidaka Arc (Hidaka metamorphic belt) is composed of the granulite-facies unit (zone IV) and the main anatexis zone. The former can be observed on the surface, while the latter is considered to be existed beneath the Hidaka Main Thrust by the evidence of restitic Gr-Opx granulites as xenoliths included in intrusive anatectic tonalities. In pelitic granulites of zone IV, Bt-dehydration melting took place with forming leucosomes including euhedral Opx, An-rich Pl and Crd. Intercalated mafic granulites in zone IV also show the Hbl-dehydration melting and Opx-bearing leucosome formation. A small scale partial melting in zone IV is identified, where the produced melt would be effected to make plastic condition of lower crustal rocks and to form duplex structure in the lower crust of the Hidaka Arc. Partial molten melts derived from the main anatexis zone moved and assembled to form tonalitic magmas with peraluminous S-type and metaluminous I-type signatures, which intruded into the shallower crustal level along the floor-thrust and roof-thrust of duplex structure.