Magmatic-hydrothermal evolution and tectonic implications of granitic pegmatites from the Ailao Shan-Red River shear zone in Southeast Asia

Granitic pegmatites are abundantly developed along the Ailao Shan-Red River shear zone (ASRR-SZ) in Southeast Asia, which provides vital insight into the Cenozoic tectonic record and magmatic-hydrothermal evolution for the ASRR-SZ. The zircon UPb ages reveal that the pegmatites formed at 28–23 Ma, broadly synchronous with left-lateral shearing. The major-element compositions show that the pegmatites have high SiO2 (65.6%–76.2%), moderate K2O (2.6%–7.4%), low MgO (0.1%–0.3%), and possessed a high differentiation index (DI = 82.4–95.3). The low Nb/Ta (4.0–14.1) and Zr/Hf (13.2–36.1), as well as the εNd(t) (−12 to −2) and εHf values (−17 to −0.2) of zircons for the high-fractionated pegmatites imply predominant contributions from the partial melting of crustal sources. Zircons CL images and Th/U ratio, and the zircons from the pegmatites fall in a transitional zone of magmatic to hydrothermal evolution in Ce/Ce*-(Sm/La)N discrimination diagram. The main range of Ti-in-zircon temperature for the zircon rims is ∼621 °C–860 °C. The homogenization temperatures of fluid inclusions are mainly within 180 °C–350 °C and represent the minimum trapping temperatures. Paleostress analysis indicates that the pegmatites were formed during the transpressional deformation of the ASRR-SZ. All these results reveal that the pegmatites petrogenesis and the magmatic-hydrothermal transition conditions during transpressional deformation of the ASRR-SZ.