A Study on the Provenance of Early to Late Triassic Clastic Rocks From the Northwestern Sichuan Basin, Southwestern China: Constraints on the Early Mesozoic Tectonic Evolution of the Western Yangtze Block

U-Pb ages of 637 new detrital zircons of Triassic sandstones from the Northwestern Sichuan Basin (NWSB), together with the petrology and paleocurrent data, are used to constrain the sediment provenance and tectonic-paleogeographic evolution of the western Yangtze Block. The U-Pb age data for the Lower Triassic detrital zircons generally show populations at similar to 1,850 Ma, 980-705 Ma, 680-510 Ma, and 290-230 Ma with a minor cluster at similar to 2.4 Ga. Such age spectrum features together with the eastward to northeastward paleocurrent direction implied that the Northwestern Sichuan Basin was dominantly fed by the uplifted Khamdian Paleoland. The Upper Triassic detrital zircons yield age populations at 855-730 Ma, 455-415 Ma, and 290-215 Ma and a prominent age group of 1.9-1.7 Ga. These age spectrum features together with paleocurrent data showed a provenance change with a major source of the Longmenshan thrust belt and Songpan-Ganzi terrane for the south area of NWSB and the Qinling orogenic belt for the north part of NWSB. Such transition of the provenance probably indicated the subsidence of the Khamdian Paleoland and uplift of the Songpan-Ganzi terrane and Qinling orogenic belt from the Early Triassic to the Late Triassic. This tectonic inversion most likely resulted from the gradually thermal subsidence of the Emeishan large igneous province (ELIP) and the gravitational loading triggered by the eastward shortening of the Songpan-Ganzi terrane as well as the ongoing convergence between Yangtze Block and North China Block.