Provenance and Tectonic Setting of the Middle Permian Zhesi Formation in Eastern Inner Mongolia, China: Implications from Petrology, Mineralogy, and Element Geochemistry

Four sets of high-quality source rocks are found in the Permian Zhesi Formation in eastern Inner Mongolia, China, which has good oil and gas resource potential. However, the research on the provenance and source area characteristics of the sediments is relatively weak. Petrographic, mineralogical, and geochemical analyses were conducted on mudstones and siltstones and fine sandstones from the eastern Inner Mongolia to investigate the provenance of the sediments, as well as the weathering intensity and tectonic setting of the source region. Petrographic data indicate that average quartz–feldspar–lithic (Q–F–L) proportions in the sandstones are Q = 43%, F = 3%, and L = 54%; the lithic fraction mainly contains volcanic clasts. The chemical index of alteration in mudstones varies from 73 to 80 (average 76), while the index of chemical variability ranges from 0.39 to 0.79 (average 0.75), and the average Th/U ratio is 3.66. Chondrite-normalized rare-earth element (REE) distributions show light REEs that are enriched relative to heavy REEs and a prominent negative Eu anomaly. These data indicate that the sediments are compositionally immature and have a weak-medium degree of weathering in the source region. The petrographic and mineralogical characteristics, combined with Zr/Sc–Th/Sc, Hf–La/Th, and Co/Th–La/Sc discrimination diagrams, reveal that these sediments are mainly derived from the rhyolite, basaltic andesite, and andesitic tuff in the early Permian Dashizhai Formation. Multidimensional tectonic discrimination diagrams, including geochemical data and Dickinson triangular charts, indicate that the tectonic setting of the provenance area is a continental island arc related to the active continental margin. These conclusions further confirm tectonic and magmatic events of the early Permian in eastern Inner Mongolia, China.