Major, trace and rare earth element geochemistry of the Permian Lucaogou oil shales, eastern Junggar Basin, NW China: implications for weathering, provenance and tectonic setting

Abstract

The Permian Lucaogou oil shales in the eastern Junggar Basin have long been regarded as important source rocks and reservoirs for tight oil exploration. The oil shales can also be used to assess the provenance and tectonic setting of the East Junggar region using major, trace and rare earth element geochemistry. The low chemical index of alteration ratios (37.36–64.18, 45.54 on average), low plagioclase index of alteration ratios (26.99–62.95, 42.99 on average), and high index of compositional variability ratios (0.64–1.66, 1.10 on average) suggest that the Permian Lucaogou oil shales mostly consist of immature sediments that have experienced a weak weathering intensity. The samples have low TiO2/Al2O3 and TiO2/Zr ratios that indicate a felsic origin with minor intermediate igneous rocks. The high Th/Sc and Zr/Sc ratios suggest a felsic origin without sediment recycling. The samples show fractionated light rare earth elements and relatively flat heavy rare earth elements patterns with weak negative Eu anomalies, which implies that the main provenance is lower Carboniferous intermediate–acid volcanic rocks. In addition, multiple major- and trace-element‐based discrimination diagrams show that the parent rock of clastic sediments formed mainly in a continental island arc and active continental margin environment, consistent with previous studies on the tectonic background of the East Junggar region during the late early Carboniferous. Key Points Immature sediments are present in the Permian Lucaogou oil shales and have a weak weathering intensity. The provenance of the clastic sediments of the Permian Lucaogou oil shales was predominantly early Carboniferous intermediate–acid volcanic rocks. The clastic sediments of the Permian Lucaogou oil shales were developed in a continental island arc and active continental margin environments. The Permian Lucaogou oil shales provide crucial information for tectonic setting and evolution of the northern Xinjiang during the Carboniferous–Permian.