Age、Geochemistry And Tectonic Implications Of The No. Ⅰ Xindi Bimodal Intrusive Complex In Quruqtagh

In this study, field observations, SIMS zircon U-Pb age and Hf isotope compositions, whole-rock major and trace elements and Sr-Nd isotope compositions were carried on the No. I Xindi bimodal intrusive complex (mafic-granite complex) in order to decipher its petrogenesis and tectonic implications. Three zircon samples collected from gabbro, diorite and granite yield consistent concordant age of～735Ma, indicating that the mafic and silicic end members are coevally emplaced. The ultramafic-mafic end member contains high LILE and LREE and low HREE and HFSE, leading to low HFSE/LREE ratios (e.g., Nb/La <0.4). Isotopically, the mafic samples have low and variable initial Nd values (-2.4～-10.8) and high (87Sr/86Sr)i (0.70615-0.70908). Moreover, their zircon εHf(t) values ranging from-1.5to0.9. According to their elemental and isotope features, we suggest that the primitive magma of the ultramafic-mafic end member of the No.I bimodal intrusive complex in Xindi area derived from partial melting of a lithospheric mantle sources recently metasomatized by subduction-related fluid/melts. As for the granite end member, they show some characteristics of adakitic rocks such as high LREE and LILE and very high Sr/Y and (La/Yb)N ratios. On the other hands, the granites have very low Cr, Ni and Co contents and low εNd(t)(-13.6～-11.3) and zircon εHf(t) values (-9.8～-7.9). All these features argue that the granites were sourced from partial melting of a thickened mafic lower crust. Combing with the data obtained and previous studies, we suggest that No.I bimodal intrusive complex in Xindi area were formed in a extensional background and possibly related to the breakup of the Rodinia supercontinent.