Chronology, Geochemistry and Hf Isotope of Granite Porphyry in Wenduermiao-Jining Area, Inner Mongolia and Its Geological Significance

In order to explore the closure time and evolution process of the Paleo Asian ocean in Wenduermiao-Jining area, it analyzes the petrology, geochemistry, geochronology and zircon Hf isotope composition. Zircon U-Pb dating shows that, Kuaziliang granite porphyry formed in the Middle Permian (270±3 Ma), while Wulannaoer granite porphyry formed in the Middle Triassic (241±2 Ma) that is the first Triassic granite discovered in the region. These rocks characterize with high contents of SiO₂, Al₂O₃, very low Mg^# values. They also display enrichment in LREEs and LILEs, depletion in HREEs and HFSEs, with obviously negative Eu anomalies. Kuaziliang granite porphyry exhibits negative ε_Hf(t) values (0.25 to -12.33), old Hf isotopic t_DM2 from 1 278 Ma to 2 071 Ma, indicating that it was derived from partial melting of ancient crustal material, while Wulannaoer granite porphyry exhibits positive ε_Hf(t) values (2.03-5.94), relatively new Hf isotopic t_DM2 from 892 Ma to 1 144 Ma, indicating that it was derived from partial melting of juvenile crustal material. Analyzed by synthesis, it speculates that before the Middle Permian, the Paleo-Asian ocean was closed in the Wenduertemple-Jining area; during the Middle Permian, the region is in the post-collision stage, forming the Kuaziliang granite porphyry. During Late Permian through Early Triassic, continuous collision orogeny in the region; the Middle Triassic, the region is in the post-orogenic extension stage, forming the Wulannaoer granite porphyry.