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, Dakalasu, and Yelaman pegmatites in the Chinese Altay orogen. These pegmatites have low εHf(t) values (−0.6 ~ +4.3), two‐stage model ages of 989 ~ 1,293 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. Two‐stage post‐collisional magmatism model. (1) At the beginning of asthenosphere upwelling, the thinned lower crust would have been in a low‐temperature condition, and thus the magmatism might be originated at deep crust and close to the Moho surface. Because of the intensive mantle–crust interactions, more mantle materials have contributed into the initial magma. (2) The upwelling of asthenosphere kept providing heat into the lower crust, and, as a result, the geothermal gradient increased. These processes make the partial melting of shallower crust materials possible. If implemented, there could generate felsic melts without mantle material contribution.