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Studies concerning metamorphic rocks and metamorphic belts in Japan since 1970, particularly during the last decade, are reviewed. These studies re-examined the constituents and metamorphic conditions of paired metamorphic belts and revealed the occurrences of various kinds of metamorphic rocks in terms of their ages and metamorphic facies. Above all, clarified were occurrences of comparatively small isolated tectonic zones, which were originally derived from regional metamorphic terranes and do not compose presently paired metamorphic belts. Serpentinite melange units were recognized in the Hida Marginal Belt, the Kurosegawa Tectonic Zone and the Kamuikotan Tectonic Belt. The former two contain rocks ranging from early Paleozoic to Jurassic. The Kamuikotan rocks are restricted to Jurassic and Cretaceous in their ages. Poly-metamorphic episodes were clarified in the Hida and Abukuma metamorphic terranes which mainly suffered low P/T type metamorphism. Evidence of poly-metamorphism in the Abukuma terrane is the occurrences of kyanite-bearing rocks in some localities and kyanite rivers and grains in many places. The concept of paired metamorphic belts for the Hidaka and Kamuikotani terranes was denied and it is revealed that the Hidaka terrane is an up-thrust middle-lower crust of the continent or island arc formed by collision in Tertiary time. In both sides of the N-S trending Hidaka terrane, high P/T metamorphic rocks of Mesozoic time are distributed, i.e. the Kamuikotan terrane in the west and the newly found Tokoro terrane in the east. As far as paired metamorphic belts are concerned, studies on the Sanbagawa and Ryoke terranes made progress together with study on mylonites along the Median Tectonic Line which is located between them. Discoveries of Triassic conodonts in the Sanbagawa calcareous schists and Jurassic radiolarians in cherts of the Mikabu greenstones zone confirm that the age of the Sanbagawa metamorphism is early Cretaceous in consistence with the radiometric ages. These ages mean that the Sanbagawa and Ryoke were formed almost simultaneously. Metamorphic zoning of the Sanbagawa Belt was established in central Shikoku for pelitic rocks and subsequently for basic rocks. This is a result of detailed mineralogical study by EPMA. Three zones of chlorite, garnet and biotite were discriminated for the pelitic rocks and the highest grade is the oligoclase-biotite zone. Lawsonite occurs in the pelitic schists of the lower part of the chlorite zone. Winchite and crossite ubiquitously occur in the basics cists with hematite, and pumpellyite-actinolite assemblages occur in the basic schists without hematite. Tectonic melange units were recognized in some places in the Sanbagawa Belt, particularly near various kinds of ultramafic-mafic bodies. Retrograde and prograde ultramafic-mafic bodies are distributed in the tectonic melange units. Higashi-Akaishi mass and Nikubuchi mass were originally cumulated igneous rocks formed under the conditions of garnet lherzolite and spinel lherzolite facies, respectively, which subsequently suffered retrogressive metamorphism. Some of the ultramafic bodies suffered progressive Sanbagawa motamorphism after serpentinization. These ultramafic bodies which have not experienced the stage of plagioclase lherzolite are considered to have been derived from the lower crustor upper mantle of the continent. The emplacement of these bodies may have been related to strike-slip movement along a plate boundary. In the Ryoke Belt, staurolite in pelitic rocks was found in the Hazu-Hongusan district where the pressure was high as compared with those of other Ryoke terrane. Takato district, central Japan, also may have suffered higher pressure metamorphism though it is still open to discussion. Granulite xenolithes were found in the Ryoke terrane and they may be basement rocks for the Ryoke metamorphic rocks. Radiometric ages and some characteristic lithofacies in each metamorphic terrane or zone are summ
日本地質学会の論文
1985-03-30

変成岩と変成帯

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