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

本文分析了日本西南外带喜井、国、九州和房久岛的60个花岗岩、沉积岩和捕虏体样品，以及弧前地区盐onomisaki的22个橄榄玄武岩-白云岩-橄榄辉长岩-花岗岩-流纹岩组合岩石样品的主要元素、Rb-、Sr-、Nd、Sm-浓度以及氧、Sr-和Nd同位素定量。定量地讨论了幔源岩浆的具体特征及其在地壳中的行为，并对含铝花岗岩类的成因进行了探讨。(1)弧前地幔中产生的玄武岩岩浆具有明显不同于普通弧岩浆的morb型化学和同位素特征。这取决于弧前地区的特定构造位置，在弧前地区，板块衍生成分不能与原始玄武岩岩浆的成因相结合。(2)盐onomisaki玄武岩-流纹岩组合的Sr-Nd系统与石满东橄榄玄武岩-平均沉积岩端段的More体混合模式吻合较好，表明上升到增生部分的morb型岩浆可以通过与沉积岩的反应衍生出同位素演化程度较高的酸性岩浆，并与之同时进行岩浆的结晶。(3)通过对各花岗岩体和花岗套化学同位素变化的详细研究，清晰地识别了上地壳隆起、结晶和侵入过程中花岗岩浆块的变化过程以及深部不同母花岗闪长岩岩浆的生成过程。(4)花岗岩类的同位素变化主要是由于母花岗岩岩浆块的局部变化和内部非均质性所致，其Sr-Nd同位素系统完全符合两步双组分混合模式。这包括母岩浆花岗岩是由幔源弧岩浆与石曼托超级群沉积岩“混合”形成的。(5)母岩浆应为幔源岩浆与下地壳岩石发生熔融和同化反应而产生，其化学和同位素特征主要受下地壳物质的贡献程度和性质控制。像日本西南外带这样的全铝花岗岩类只能在母花岗岩岩浆生成过程中只有沉积岩才能被纳入的年轻增生区中产生。少

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