Granulite facies metamorphism and anatexis in the Hidaka metamorphic belt, Hokkaido, northern Japan

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1) P-T analysis of granulite pacies gneisses in the Hidaka metamorphic belt show 800-850ﾟC, 7-7.2kb at the peak condition (Dl-stage), 700-750ﾟC, 5kb during horizontal displacement forming duplex structure (D2) and 600-400ﾟC, 3kb during the final uplifting movement accompanying tilting with dextral transcurrent slip due to transpression.2)During the peak metamorphic condition, granitic to tonalitic magmas are thought to have form through anatexis of pelitic and psammitic gneisses. Chemical and mineralogical analysis indicate that Opx-Gt-Cd granulites are restites left after partial melting of Gt-Cd gneisses to form granitic melts which occur as veins or pods. In deeper part, larger extents of partial melting might have occurred to give rise large tonalitic plutons in the Hidaka belt.3)Petrological and experimental studies on the genesis and crystallization of tonalitic magma indicate that the magma might have formed at higher temperature than 950ﾟC and deeper than 8kb, with 4-5% water c … More ontained in the melted rocks, and that the crystallization took place at 5-6kb when the temperature decreased from 900 to 700ﾟC. The crystallising orger of minerals is Gt-Pl-Opx-Bt-Qz-Cd-Kfs. The stability of Opx is largely controlled with Or contents of the magma.4)Metamorphic enclaves in Miocene Shimanto granites were studied to compare the metamorphic and deformational processes in the Hidaka belt. The high temperature-low pressure regional metamorphism took place in the Shimanto accretionary prism prior to generation of the granites. The regional thermo-dynamic process in accretionary prisms is rather common in the Japanese high temperature terranes such as Hidaka mentioned above and Ryoke metamorphic belt.5)The highest grade gneisses in the Ryoke belt attained at 700ﾟC at 14km deep. In deeper part, granitic magmas are expected to have been formed through anatexis of gneisses. REE analysis of migmatite and gneisses confirmed the possibility.6)Actual melting processes in natural environments are only observed for rock enclaves within high temperature magmas such as tonalite and basalt. Their examinations argue that rocks melt very heterogeneously, and that the melts are hardly mixed each other owing to high viscosity, . Less