BIMODAL INTRUSIVE ROCKS IN XIAOHONGSHAN VANADIUM-TITANIUM MAGNETITE OREFIELD IN BEISHAN AREA OF INNER MONGOLIA: Petrology, Geochronology, Geochemistry and Geological Implication

Study on the petrology, zircon U-Pb chronology and whole-rock geochemistry of intrusive rocks in Xiaohongshan vanadium-titanium magnetite orefield of Beishan area in Inner Mongolia shows that the lithology of the intrusion is gabbro and granite with the diagenetic age of 431.1±2.4 Ma and 424.9±2.3 Ma respectively. The two types of rocks were formed at a similar time of the same tectono-magmatism event. The SiO₂ content in the intrusive rocks shows double peaks, and the characteristics of rare earth and trace elements indicate that they have the geochemical features of island-arc magmatic rocks, with the source area metasomatized by subduction-related fluid. The geochemical characteristics reflect Xiaohongshan granite is originated from the partial melting of lower crust materials at high temperature while the gabbros resulted from the partial melting of enriched mantle. Combined with the regional geological background and tectonic discrimination, it is concluded that the Middle and Late Silurian bimodal magmatic assemblage in Xiaohongshan area is the product of continental margin extension induced by the southward subduction of Beishan ocean.