ACCRETIONARY OROGEN AND EVOLUTION OF THE JAPANESE ISLANDS-IMPLICATIONS FROM A Sr-Nd ISOTOPIC STUDY OF THE PHANEROZOIC GRANITOIDS FROM SW JAPAN

The Japanese Islands represent a segment of a 450 Ma old subduction-related orogen developed along the western Pacific convergent margin, and most tectonic units are composed of late Paleozoic to Cenozoic accretionary complexes and their high P/T metamorphic equivalents. The formation of the Japanese Islands has been taken as the standard model for an accretionary orogeny. According to Maruyama (1997), the most important cause of the orogeny is the subduction of an oceanic ridge, by which the continental mass increases through the transfer of granitic melt from the subducting oceanic crust to the orogenic belt. Sengor and Natal'in (1996) named the orogenic complex the "Nipponides," consisting predominantly of Permian to Recent subduction-accretion complexes with veil, few fragments of older continental crust. These authors pointed out the resemblance in orogenic style between Japan and the Central Asian Orogenic Belt (CAOB). The present work uses new and published Sr-Nd isotopic data from the literature to test the statements made by these authors. A large proportion of the granitoids from SW Japan have high initial Sr-87/Sr-86 ratios, negative epsilon(Nd)(T) values and Proterozoic Sm-Nd model ages. The Japanese isotopic data are in strong contrast with those of two celebrated accretionary orogens, the Central Asian Orogenic Belt and Arabian-Nubian Shield, but are quite comparable with those observed in SE China and Taiwan, or in classical collisional orogens in the European Hercynides and Caledonides. This raises questions about the bulk composition of the continental crust in SW Japan, or the type of material accreted in accretionary complexes, and negates the hypothesis that the "Nipponides" contains very few fragments of older continental crust. The subduction-accretion complexes in Japan are composed mainly of recycled continental crust, probably of Proterozoic age. This study supports the idea that proto-Japan was initially developed along the southeastern margin of the South China Block.