Petrogenesis and tectonics of late Early Cretaceous Shoshon-itic volcanic rocks in Xilin Hot, Inner Mongolia

According to the research on zircon dating and cross section, the authors redefined Late Jurassic Manketou Formation as Early Cretaceous Meletu Formation in the southwestern segment of the Da Xinggan Mountains. Volcanic rocks are divided into two kinds of lithology, the lower segment is mainly trachy-andesite with minor trachyte, whereas the upper is trachyte and rhyolite. The zircon U-Pb age of trachyte is 118.1±1.3Ma, suggesting that its formation age is Early Cretaceous. Rocks have rich alkali, rich po-tassium, and high aluminum whose value varies in a wide range, high Fe₂O₃/FeO ratios (< 0.5), and relatively rich light rare earth ele-ments and LILE (such as Rb, Ba, Th and Pb) but poor high field strength elements (such as Nb, Ta, and P). Rocks have obvious iron-depletion evolution trend in the AFM diagram and "Barbed" evolution trend in the SiO₂-K₂O diagram, with conspicuous char-acteristics of shoshonites. Studies show that the intermediate acidic volcanic rocks of the Meletu Formation in the study area have magmatic evolution characteristics of the same source region. Magma originated from enrichment of mantle metasomatism and was formed in a stretching tectonic background and related to the retreat of subduction of the Pacific Plate.