Continental basalts in the accretionary complexes of the South-west Japan Arc: Constraints from geochemical and Sr and Nd isotopic data of metadiabase

The Ryoke Belt is one of the important terranes in the South-west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta-sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian-early Jurassic). Initial ε(Sr) (+25-+59) and ε(Nd) (-2.1--5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB-signature, rather than those of either IAB or ACMB. The Sr-Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental-type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr-Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr-Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental-type lithosphere is widely represented beneath the western part of the Japanese Islands.