Pb and Nd Isotopic Compositions of Basement and Granitoid in the Qilianshan: Constraints on Tectonic Affinity

The northwestern boundary of the Yangtze (South China) block has not been efficiently constrained yet. Pb and Nd isotopic compositions of the Qilianshan Precambrian basements and granitoids in the western segment of the Central Oro-genie System (COS) of China provide constraints on the northwestern boundary of the Yangtze block. Most samples from the Qilianshan Precambrian basement have mantle-depleted Nd isotopic model ages (TDM)ranging from 0. 75 to 2. 5 Ga, with a peak value of -2. 1 Ga. The Qilianshan Paleozoic granitoids have TDM values ranging from 1. 07 to 2. 14 Ga. They show that crustal growth of the Qilianshan mainly occurred in the Proterozoic, without significant Archean information. The Precambrian basement rocks and the Paleozoic granitoids are characterized by high radiogenic Pb isotopic composition. Most samples have 206Pb/204Pb 18. 0, 207Pb/204Pb 15. 5 and 208Pb/204Pb 38. 0. The TDM values and Pb isotopic compositions of the Qilianshan basement and granitoids are similar to those of the Yangtze block but distinct from those of the North China block, indicating that the crustal basement of Qilianshan has an affinity with the Yangtze block. It is suggested that the northwestern boundary of the Yangtze block is located along the north side of the Qilianshan belt. Since the Proterozoic, the Qilianshan belt had experienced lithospheric rifting and subsequent Qilian ocean opening. These tectonic events had thus happened within the Yangtze block.