Geochronological, geochemical, and Sr–Nd–Hf isotopic studies of the Baiyanghe A-type granite porphyry in the Western Junggar: Implications for its petrogenesis and tectonic setting

摘要

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 ± 2.3 Ma, is high in SiO_2 (76.34–78.30 wt.%) and K_2O + Na_2O (8.04–8.85 wt.%), relatively low inMgO (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 A1 group. Moreover, due to its positive values of εNd(t) (4.06–5.29) and εHf(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 ~(87)Sr/~(86)Sr ratios (0.7143–0.7469) and high (La/Sm)N values (5.13–8.44) could be caused by the involvement of marine sediments. The low Ba, Sr, and Eu concentrations and high values of DI (97.3–98.0) of the granite porphyry suggest that itsmagma had undergone a high degree of fractional crystallization. Characterized by the features ofwithin-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 Irtysh–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.