Tectonic Setting and Provenance of Sandstones from the Triassic Daheba Formation in the West Qinling Orogenic Belt

The Daheba formation is the latest strata exposed in the Xiahe-Luqu area of the West Qinling Orogenic Belt (WQOB), with recording the Indosinian regression event. In this study, based on the stratigraphic sequence characteristics, major and trace element compositions and zircon U-Pb dating of the sandstone, we traced the origin of the sandstone and explored the tectonic settings of the Daheba Formation. The Daheba Formation consists of feldspar sandstone, lithic feldspar sandstone, conglomerate clastic feldspar sandstone. The SiO₂ content of sandstone range from 64.11% to 68.72% with an average of 65.45%. The sandstones are also characterized by high Al₂O₃ (13.61%～15.19%), lower Fe₂O₃ (0.82%～1.72%), CaO (2.393%～3.32%) and MgO (0.06%～0.1%) contents. They have high rare earth elements (REE) range from 184.13×10⁻⁶ to 434.06×10⁻⁶, and show negative Eu anomaly in the chondrite normalized diagram. They also have similar Th, Zr, Ti, and Co, Ni compositions with that of active continental margin sediments. The detrital zircon U-Pb ages from the sandstones are mainly concentrated in 253.3～448.7 Ma, 1536～2067 Ma and 2204～2520 Ma, and show five age peaks at 278.2 Ma, 448.5 Ma, 1757.6 Ma, 1986.4 Ma and 2445 Ma, with the youngest ages at 253.3 Ma. These indicates that the Daheba Formation was deposited in an active continental margin setting, and the detritus originated from the upper crust igneous rocks. The main sources for the detritus is possibly of the basement of the northern Qinling, the eastern part of the Qilian, and the southern edge of the North China Plate in the northern parts of the study area with small parts from the magmatic rocks and polycyclic sediments in the West Qinling.