Petrography and geochemistry of the Lower Silurian sandstones from the Angzanggou Formation in the North Qilian Belt, China: implications for provenance, weathering and tectonic setting

The North Qilian Orogenic Belt is on the northeastern margin of the Tibetan Plateau. It is connected with the Alxa Block in the north and the Tethyan orogenic assemblage in the south. The Lower Silurian Angzanggou Formation, deposited in the northern area of the North Qilian Orogenic Belt, is the most important for revealing the architecture and orogenic processes of the North Qilian Belt. Provenance analysis of the Angzanggou Formation can reveal not only the tectonic evolution of the central orogenic belt of China but also Palaeozoic Asia plate reconstructions. Petrographic analysis indicated that the compositional and textural maturity of the sandstones was low. The detrital composition of the Angzanggou Formation samples consists of quartz (8-14 %), feldspar (6-29 %) and lithic fragments (56-86 %). The sandstones could be classified as litharenites or feldspathic litharenites. The detrital modal composition suggests that these sandstones were probably deposited in a fore-arc basin. The element ratios and some discrimination diagrams based on geochemistry indicate that felsic and intermediate rocks were the main source rocks. The SiO2/Al2O3 ratio, the index of chemical variability and the Th/Sc versus Zr/Sc discrimination diagram suggest that the compositional maturity and degree of recycling were moderate to low. The index of alteration (CIA) and the A-CN-K diagram indicate the intensity of weathering was moderate. The discrimination diagrams based on major and trace elements and petrographic discrimination diagrams imply that the Angzanggou Formation rocks were derived from a continental island arc, and a sedimentary cover probably overlaid the volcanic arc. Therefore, we infer that during Early Silurian time the North Qilian Belt sediments accumulated in a fore-arc basin.