Geologic Mapping and U-Pb Zircon Ages of Early Paleozoic Plutonic Rocks in the Yadong and Kangmar Areas, Southern Tibet

High-grade metamorphic rocks with pre-Ordovician protoliths are exposed in the Yadong and Kangmar areas on the west side of the Yadong-Gulu rift in southern Tibet. In order to understand how the rocks have evolved, we conducted geologic mapping and U-Pb zircon dating in the two areas. In the Yadong area, the high-grade metamorphic rocks are exposed in the footwall of the South Tibet Detachment (STD) and thus parts of the Greater Himalayan Crystalline Complex. The metamorphic rocks are intruded by Tertiary deformed leucogranites that together with the STD are cut by younger undeformed Tertiary leucogranitic bodies. The metamorphic complex contains orthogneiss from which we collected a sample for age analysis. Among 12 dated zircon grains, 10 yield a concordia U-Pb SHRIMP age of 513 ± 10 Ma while the other give a ~900 Ma and ~ 400 Ma, which could result from inheritance and late thermal disturbance. The same orthogneiss sample also yields an Ar40/Ar39 biotite cooling age of 13 Ma. In the Kangmar area, we obtained U-Pb SHRIMP zircon ages of 478-461 Ma and 339 Ma from deformed plutonic bodies in the high-grade metamorphic complex. We obtained an Ar40/Ar39 cooling age of hornblende at ~ 43 Ma and biotite cooling ages between 18 and 13 Ma from the Kangmar metamorphic complex.