Petrogenetic studies of Permian pegmatites in the Chinese Altay: implications for a two-stage post-collisional magmatism model

Understanding the petrogenesis of rare-metal pegmatites is important for understanding ore-forming processes and their tectonic settings. In this study, we performed zircon U-Pb geochronological and Hf-O isotopic analyses of the Xiaokalasu (XKLS), Dakalasu (DKLS) and Yelaman (YLM) pegmatites in the Chinese Altay orogen. These pegmatites have low εHf(t) values (-0.6 ~ +4.3), two-stage model ages of 989 ~ 1293 Ma, and high δ18O values (+6.52 ~ +11.31), indicating that they may have been derived from the anatexis of mature sedimentary rocks in the deep crust, with a small amount of mantle-derived or juvenile material. Geochronological and Hf-O isotopic data for granitic intrusions in the Chinese Altay Mountains indicate that the εHf(t) values decreased from the Permian to the Triassic, which implies that two-stage post-collisional magmatism occurred in this region. During the Permian, the thin lower crust was cold; thus, magmatism likely originated in the deep crust close to the Moho surface and involved intense mantle-crust interactions. During the Triassic, asthenospheric upwelling provided heat to the lower crust, which increased the geothermal gradient and led to the anatexis of shallow crustal material.