FORMATIVE PROCESSES AND EXHUMATION TECTONICS OF HIGH-TEMPERATURE METAMORPHIC BELTS IN ISLAND ARCS

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1) Study of the Hidaka metamorphic belt : Thermal structure and effects of magma to metamorphism in the deep island arc crust was revealed based on the analyzes of conditions and processes of partial melting and generation of tonalitic magma of granulite facies metamophic rocks in the Hidaka belt. Related to the results, it is proposed that the depth of initiation of crustal anatexis constraints the depth of crustal scale decollement formed in the crust.2) Study of the Ryoke metamorphic belt : The following aspects were made clear according to the studies of original lithology, structures and tectono-metamorphic histories in major metamorphic fields of the Ryoke belt ; 1) the original lithology involves chert-clastic sequence, coherent strata of shale and sandstone, broken formation, and melange. 2) deformational stages are defined as D1 (flattening with S1 foliation), D2 (local shearing with intrafolial folds and shear planes S2 accompnied with prograde metamorphism), D3 (local sheari … More ng with retrograde metamorphism) and D4 (shortening with upright folds and crenulation cleavages).The thermal structure and heating process were analyzed and numerically sumilated to confirm that syn-metamorphic granitic intrusives playd as a heat source. CHIME monazite age determinations have been advanced and showed that the metamorphic age is 100Ma, and granitic intrusions were continued from 95Ma to 80Ma. There is no younging trend of intrusive age from west to east of the Ryoke belt, while the younging trend of K-Ar ages so far advocated implies a continuous exhumation taking place from west to east.3) Study of the Higo gneiss region : Saphilline-corundum bering peraluminous rocks were found to occur as blocks included in serpentinite. The Nd-Sm and SHRIMP zircon ages indicate that the metamorphic rocks were formed at 250Ma, and suffered polymetamorphic thermal effect during the Ryoke period.4) Studies on source rocks of high temperature metamorphic rocks in these belts have been advanced to make clear the nature of sedimentary rocks and their source regions ; Early to late Proterozoic clastics are included in paragneisses in the Ryoke belt as in the sedimentary rocks consisting Jurassic accretionary complex. Less