Comparative study on the exhumation processes of high-pressure metamorphic belts, western Pacific

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To understand the tectonic processes of the high-pressure metamorphic belts in the western margins of Pacific Ocean. I studied the metamorphism and deformation of the rocks in the Sanbagawa metamorphic belt and the Haast schists of New Zealand, and compared them with those in the Kamuikotan and Franciscan belts. Predominant direction of stretching lineation is E-W in the Sanbagawa metamorphic belt at Nushima. An analysis of microfabrics of stretching lineation indicates that the pull-apart fabrics have been formed with increase of temperature at the final stage of metamorhism. I studies the Haast schists in term of the rock samples taken in 1997. The Haast schists along the Lake Hawea route can be divided into two zones ; the southern chlorite zone and the northern garnet zone. Plagioclase is albite (<AnィイD25ィエD2) in the chlorite zone, whereas that show two phases (AnィイD20-5ィエD2 and AnィイD220-30ィエD2) near the garnet isograd in the garner zone, and is AnィイD25-35ィエD2 in the further north … More higher-grade regions. Sudden decrease of modal compositions of epidote at the region where Ca-rich plagioclase appears suggests that the decomposition of epidote related to the Ca-rich plagioclase froming reaciton. The lack of biotite zone in the study area suggests that the metamorphic P-T trends of the Haast schist terrane is situated at the intermediate pressure condition between the Sanbagawa and the Scotich Highland just below the intersection of biotite- and garnet-forming curves.A large number of serpentinite seamounts in the western Pacific Izu-Ogasawara-Mariana forearcs indicate the serpentinite diapirism under the tensional stress field in the forearc regions. The presence of blueschist-facies rocks in the serpentinite seamounts provides evidence that the serpentinites generated from the hanging-wall peridotites have penetrated the high-pressure metamorphic region at depth in the accretionary prism. The high-pressure metamorphic terranes in the circum-Pacific regions, the Franciscan Complex, California, and the Kamuikotan metamorphic belt, Japan, are accompanied with serpentinite melange with deeper high-grade metamorphic rocks, whereas the Sanbagawa metamorphic belt, Japan, does not contain any serpentinite melange. It is highly probable that the former were formed under tensional stress regime, and the latter under compressional one. The rare occurrence of serpentinites and lack of serpentinite melange in the Sanbagawa belt may have been caused by the compressional stress environment in the trench-arc region during the metamorphism. Under such compressional stress environments, scraping off hanging-wall peridotites seems to be only a possibility to incorporate mantle materials into the accretionary prism which develops along the trench. The model can explain the limited occurrence of serpentinites to the high-grade pelitic schists in the Sanbagawa belt. Less