中国科学院地球化学研究所机构知识库

Zircon U-Pb dates, Sr-Nd-Hf isotopic compositions, and major element and trace element geochemistry were determined for the Baiyanghe granite porphyry in the Western Junggar, NW China. The porphyry, has a zircon U-Pb age of 313.4 +/- 23 Ma, is high in SiO2 (7634-78.30 wt.%) and K2O + Na2O (8.04-8.85 wt.%), relatively low in MgO (0.01-0.03 wt.%) and CaO (0.29-0.39 wt.%), and shows characteristics typical of A-type granites. Because its Y/Nb ratio (0.33-0.51) is lower than 1.2, it can be classified into A(1) group. Moreover, due to its positive values of epsilon Nd(t) (4.06-529) and epsilon Hf(t) (8.18-11.07), and high concentrations of incompatible elements, including Rb, Th. U and Nb, the Baiyanghe granite porphyry is suggested to have been derived from an oceanic island basalt-like mantle source. Their extremely high initial Sr-87/Sr-86 ratios (0.7143-0.7469) and high (La/Sm)(N) values (5.13-844) could be caused by the involvement of marine sediments. The low Ba, Sr, and Euconcentrations and high values of DI (973-98.0) of the granite porphyry suggest that its magma had undergone a high degree of fractional crystallization. Characterized by the features of within-plate granites, a back-arc basin tectonic setting is favored for the petrogenesis of the Baiyanghe granite porphyry. Such a setting indicates that the subduction of the lrtysh-Zaysan oceanic lithosphere beneath the Zharma-Saur arc was still active during the Late Carboniferous and the collision between the Siberia and Kazakhstan blocks probably occurred between ca. 316 Ma and 307 Ma.