中国科学院地球化学研究所机构知识库

The Jiangjunshan and Dakalasu alkali-feldspar granites are located in the central part of the Chinese Altay orogen. In this paper, we present detailed geochemical, zircon U-Pb, and Hf isotopic data of these granites. The Jiangjunshan and Dakalasu alkali-feldspar granites show a high content of SiO2 (72.05-73.27 and 69.55-71.04wt%, respectively), total alkalis (Na2O+K2O=8.41-8.71 and 7.24-8.66wt%, respectively), and high-field strength elements (Zr+Nb+Ce+Y=400.3-482.9 and 156.7-339.3ppm, respectively), as well as high Ga/Al ratios (10,000xGa/Al=3.46-4.19 and 2.62-3.28, respectively) and depletion in Ba, Nb, Sr, and Ti, showing geochemical characteristics similar to those of A-type granites. Zircon U-Pb dating of the Jiangjunshan and Dakalasu alkali-feldspar granites yielded weighted mean dating Pb-206/U-238 ages of 268.3 +/- 1.9 and 270.4 +/- 1.9Ma, respectively, indicating that these granites intruded during the Permian. The Jiangjunshan and Dakalasu alkali-feldspar granites show highly variable zircon epsilon(Hf)(t) values ranging from -7.0 to +5.6, implying that these granites originated from a mixing of mantle-derived magma with crustal materials. Our data on the Jiangjunshan and Dakalasu alkali-feldspar granites, coupled with previous studies of Permian magmatism and metamorphism, suggest that the tectonic regime was in a postcollisional extensional environment in the Chinese Altay orogen during the Permian. Therefore, the change in stress from compression to extension and asthenospheric upwelling triggered by slab break-off plays a significant role in the generation of Jiangjunshan and Dakalasu alkali-feldspar granites.