Geochemistry of Late Carboniferous sedimentary rocks from the Zongwulong structural belt and adjacent areas, Qaidam Basin, China: Implications for provenance and tectonic setting

摘要

This paper documents the geochemistry of clastic sedimentary rocks from the Upper Carboniferous Zongwulong Group in the Zongwulong structural belt of the Qaidam Basin, NW China, in order to trace the sources of the sediments and to understand their tectonic settings at the time of deposition of the Zongwulong Group. Although the major elements differ among different samples, their low mean chemical index of alteration (mean CIA = 51.04-64.79), plagioclase index of alteration (mean PIA = 51.41-69.54), and relatively high mean index of compositional variability (mean ICV = 1.18-1.56) indicate relatively low maturity and weak to moderate chemical weathering of their source. The Zr/Sc-Th/Sc diagram and relatively high mean ICV suggest that all sediments of Zongwulong Group have undergone no significant sediment recycling. All samples yield relatively high Al2O3/TiO2 ratios (16-27), strong LREE/HREE differentiation (mean La-N/Yb-N = 10.83-12.66), and obvious negative Eu anomalies (mean Eu/Eu* = 0.61-0.79). Combined with characteristic ratios (La/Sc, Th/Sc, Cr/Th, and Th/Co) and the provenance discriminant diagrams (Al2O3-(CaO* + Na2O)-K2O and Ni-TiO2), these results indicate felsic source rocks. Combined with the findings of previous studies, the representative values (La, Ce, I REE pound, V, Rb/Sr, Eu/Eu*, La/Sc, Th/Sc, and Zr/Th) and tectonic setting discriminants (SiO2-K2O/Na2O, Fe2O3 + MgO-Al2O3/SiO2, La/Sc-Ti/Zr, and Th-Sc-Zr/10) indicate that the Zongwulong Group sediments were deposited in a back-arc extensional basin behind the active continental margin-continental island arc tectonic setting.