LA-ICP-MS U-Pb dating of zircon from granite-gneiss in the Amdo area, northern Tibet, China.

Amdo gneiss which exposed in south of Amdo County about 30km, in the northern Tibet constitutes the major lithologic of Nierong Group on Nierong microcontinent. Nierong was a large micro-lens gripping Gangdise-Nyainqentanglha plate of northern Tibet and Qiangtang-Sanjiang composite plate in the Bangong Co-Nujiang River suture zone. The CL images of biotite-monzonite-gneiss zircons show distinct oscillatory zoning. Th/U ratios range from 0.63 to 1.04 with an average of 0.85. Gneiss zircons LA-ICP-MS U-Pb in situ isotopic age is 488.0±4.2 Ma. Dating results show that the Amdo gneiss protolith was emplaced in the Pan-African period, and verifies that Nierong microcontinent was effected by the late Pan-African thermal motion. This study provides an important chronological evidence for basement properties and tectonic evolution of the Nierong microcontinent.