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 U-Pb 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 epsilon Nd(t) (-12 to -2) and epsilon Hf 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 similar to 621 degrees C-860 degrees C. The homogenization temperatures of fluid inclusions are mainly within 180 degrees C-350 degrees 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.

Automated summary

Granitic pegmatites formed along the Ailao Shan-Red River shear zone (ASRR-SZ) in Southeast Asia at 28-23 Ma during left-lateral shearing. They are characterized by high SiO2 (65.6%-76.2%), moderate K2O (2.6%-7.4%), low MgO (0.1%-0.3%), and high differentiation index (DI = 82.4-95.3). Zircon geochemistry suggests a crustal origin, and the pegmatites formed during transpressional deformation with temperatures around 621-860℃ and fluid trapping temperatures of 180-350℃.