Petrography and Geochemistry of Upper Carboniferous-Early Permian Sandstones from Zhanjin Formation in Qiwu Area, South Qiangtang Basin, Tibet: Implications for Provenance, Source Weathering and Tectonic Setting

The petrography, major, and trace elements concentration of the sandstones from the Zhanjin Formation in Qiwu area, south Qiangtang Basin, have been studied to determine their provenance, intensity of paleo-weathering and depositional tectonic setting. Petrography indicates that the composition and texture maturity of the sandstones are moderate. The detrital composition of Zhanjin Formation samples are dominated by quartz (70.1-80.3, average 74.1%) and feldspar (18.3-26.5, average 21.9%), but is low in lithic fragment (1.4-8.6, average 3.9%). The sandstone can be classified as arkose and litharenite arkose. The detrital model composition reflects that these sandstones are probably derived from a passive continental margin. The index of chemical variability (ICV), and Th/Sc vs. Zr/Sc discrimination diagram suggests that the compositional maturity and recycling were moderate to low. The index of alteration (CIA) and the A-CN-K diagram indicated that the intensity of weathering in the source area was low. The Al2O3/TiO2 vs. SiO2, Th/Sc, Co/Th, La/Sc, Cr/Th ratio values of the Zhanjin sandstones indicated that the sandstones were mainly derived from felsic source rocks. The Gondwana passive continental margin belt is probably the primary provenance area as evidenced by petrography and geochemistry features of the Zhanjin Formation. All kinds of tectonic discrimination diagrams based on major elements showed a rift setting for Zhanjin sandstones, which is consistent with the general views of study areas.