Mineralogical and geochemical composition of Middle Permian Lucaogou Formation in the southern Junggar Basin, China: implications for paleoenvironment, provenance, and tectonic setting

Abstract

The mineralogy and geochemistry of the Middle Permian Lucaogou shale provide information regarding tectonic setting, depositional environment, sedimentary provenance, and chemical properties. The Lucaogou shale consists predominately of quartz and clay minerals with subsidiary feldspar. The mineralogical composition is enriched in SiO2, P2O5, CaO, and Na2O relative to post-Archean Australian shales (PAAS). The abundant free SiO2 and elevated CaO relative to PAAS have led to depletions of most of trace elements due to dilution effects. The chemical index of alteration (CIA) indicates that sediment-source region has undergone low to moderate chemical weathering. The index of compositional variability (ICV) suggests that the shales have low compositional maturity and are most likely dominated by first-cycle deposits. The dominance of plagioclase over K-feldspar in amounts and extrapolation of the analyzed shales back to the plagioclase-alkali feldspar line in the A-CN-K diagram indicate that the shales could have been derived from plagioclase-rich source rocks. The Al2O3/TiO2 ratio and chondrite-normalized rare earth element (REE) patterns display a derivation from felsic rocks. Tectonic setting discriminant diagrams and the chondrite-normalized REE patterns infer a continental island arc setting for sediment-source region. The redox-sensitive elemental ratios and paleosalinity indices reflect an anoxic mildly brackish-influenced to brackish-influenced paleoenvironment.