Characteristics and origin of the magma generated in the fore-arc and accretionary regions

点击翻译按钮获取中文摘要

Sixty samples of granitoids, sedimentary rocks and xenoliths from Kii, Shikoku, Kyushu and Yakushima in the Outer zone of Southwest Japan and twenty-two samples of rocks of the olivine basalt-dolerite-olivine gabbro-granophyre-rhyolite association from Shionomisaki in the fore-arc region have been analyzed for majro elements, Rb-, Sr-, Nd, Sm- concentrations, and oxygen-, Sr- and Nd-isotope rations. Specific features of the mantle-derived magma and its behaviors in the crust and the genesis of per-aluminous granitoids were quantitatively discussed.(1) The basaltic magma generated in the mantle beneath the fore-arc region has characteristic MORB-type chemical and isotopic features that are distinctly different from common arc magmas'. This depends to the specific tectonic positio of the fore-arc region where the slab-derived component cannot incorporate to the genesis of original baslt magmas. (2) The Sr-Nd systematic of the basalt-rhyolite association of Shionomisaki well fits to the b … More ulk-mixing model of the olivine basalt-average Shimanto sedimentary rock end-members, indicating that the MORB-type magma uprised to the accretionary part could derived isotopically much evolved acid magma by reaction with sedimentary rocks which proceeded simultaneously to the crystallization of the magma.(3) Detailed studies of the chemical and isotopic change in each granite body and suite clearly discriminate the process of changing granitoid magma mass during uprise, crystallization and intrusion in the upper crust and the process of generation of varying parental granodioritic magmas in the depth. (4) The isotopic variation in granitoids is principally due to the local variation and internal heterogeneity of the parent granite magma mass, and its Sr-Nd isotopic systematics perfectly fits to the two-step two-component mixing model. This includes that the parental granite magma has been formed by 'mixing' of the mantle-derived arc magma and sedimentary rocks of the Shimanto Super Group.(5) The parental granitoid magmas should be generated by the reaction, including melting and assismilation, of mantle-derived magma with lower crustal rocks, and their chemical and isotopic features should principally controlled by the degree of contribution and nature of lower-crust substances. The per-aluminous granitoids like those of the Outer zone of Southwest Japan can only generated in the young accretionary regions where only the sedimentary rocks can be incorporated in the process of generation of parental granite magma. Less