U-Pb zircon SHRIMP age of metamorphic rocks from the basement of the Qiangtang basin, northern Tibet, and its geological significance
藏北羌塘盆地基底变质岩的锆石SHRIMP年龄及其地质意义

Whether exist crystalline basement or not? This is crucial to the petroleum preservation in the Qiangtang basin, where had undergone intense tectonism due to the Qinghai-Xizang plateau uplift. There has been much debate, for a long time, concerning the existence of pre-paleozoic crystalline basement. Recently, the authors found gneiss rocks containing sillimanite and kyanite in Ejiumai to the north of the central uplift belt in the Qiangtang basin. Combined with very low metamorphic Ordovician strata being found in this area, we conclude that there exist metamorphic crystalline basement in the Qiangtang basin. The zircon grains from the gneiss rocks yield a wide range of ages, and fall into seven groups: 2498~2374Ma, 1780 ~1666Ma, 645~522Ma, 465~420Ma, 402~369Ma, 270~233Ma and 223~198Ma. Zircon cathodoluminescence (CL) image analyses, in conjunction with regional geological characteristics, show that the main metamorphism of the gneiss rocks occurred from 1780Ma to 1666Ma, supporting the existence of Precambrian crystalline basement in the Qiangtang basin. This paper also discusses the other age groups and their geological implications.