ACCRETIONARY OROGEN AND EVOLUTION OF THE JAPANESE ISLANDS - IMPLICATIONS FROM A SR-ND ISOTOPIC STUDY OF THE PHANEROZOIC GRANITOIDS FROM 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 a standard model for the accretionary orogeny. According to Maruyama (1997), the most important cause of the orogeny is the subduction of 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 "Nipponides", consisted predominantly of Permian to Recent subduction-accretion complexes with very 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 available Sr-Nd isotopic data to test the statements made by these authors. The isotopic data indicate that the granitoid generation in SW Japan was in strong contrast with that in two celebrated accretionary orogens: the Central Asian Orogenic Belt and Arabian-Nubian Shield. In fact, the Japanese isotopic signatures are more comparable with those observed in SE China and Taiwan, or in classical collisional orogens in the European Hercynides and Caledonides. A large proportion of the granitoids of SW Japan have Proterozoic Sm-Nd model ages, high initial ⁸⁷Sr/⁸⁶Sr ratios and negative ε_Nd(T) values. This raises questions about the types 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 ages. The scenario is comparable with that in Taiwan. The present study argues that accretionary orogens could be distinguished by the nature of the accreted lithological assemblages. Orogens with dominantly island arc assemblage would witness generation of granitoids with juvenile characters. This is best exemplified by the granitoids of ANS and many parts of CAOB. By contrast, orogens with accretionary complexes with a large proportion of recycled Precambrian crust relative to mantle-derived rocks would produce granitic rocks with more crustal signature. This is represented by the Japanese Islands. The Sr-Nd isotopic compositions of Japanese granitoids indicate that the source regions vary from dominantly mantle-derived rocks to an assemblage with a large proportion of recycled Precambrian crust. This implies that the proportion of the eroded Precambrian crust in the trench to the ocean plate stratigraphy was very variable in the subducted accretionary complexes during the Mesozoic. Alternatively, the accreted fragments, including MORB, basalts of seamounts and ocean plateaux, have not significantly participated in the generation of granitic magmas.