LA-ICP-MS zircon U-Pb age of high-grade metamorphic rocks from the Qiangtang Basin, northern Tibet, and its geological implications

Located in the northern margin of the "Central Uplift" of the Qiangtang Basin, the Ejiumai high-grade metamorphic rocks composed of orthogneiss and paragneiss are only representative high-grade metamorphic rocks in the basement of the Qiangtang Basin. The orthogneiss is biotite monzonitic gneiss with magmatic features. The CL images of its zircons have the core-mantle-rim structure. LA-ICP-MS zircon U-Pb dating indicates that the ages of the core range from 242Ma to 2490Ma, which might record the information of the source. The mantle shows clearly magmatic oscillation zoning, with age of 207Ma±2Ma and corresponding Th/U values of 0.02~0.30. These ages imply the time of magmatic crystallization of the original rock, and are consistent with the ages of Late Triassic high-pressure metamorphism and arc-type magmatism in Qiangtang area. The rim displays ages of 161~197Ma with Th/U values between 0.02 and 0.15. These data might have recorded the peak metamorphic age, and probably indicated the response to the northward subduction of the Bangong-Nujiang Tethyan Ocean. Geochemical compositions of the orthogneiss show the characteristics of arc-type magmatic rocks. The protolith of the Ejiumai high-grade metamorphic rocks was related to the Late Triassic tectonic, magmatic and angular unconformity events in this area. It is thus suggested that the uniform basement of the Qiangtang Basin was formed in the Late Triassic period. The results obtained by the authors play a key role in understanding the timing and nature of the basement of the Qiangtang Basin as well as in evaluating resources of this oil-gas-bearing basin.