Zirzon U-Pb age and petrogenisis of plagiogranite porphyry in Changlingzi, Hexigten Banner, Inner Mongolia and its collision orogeny

The Changlingzi plagiogranite porphyry is located in the Xilinhot Late Paleozoic Accretion Complex within the Da Hinggan Mountains. This study is focused on the zircon U -Pb isotopic geochronology, Hf isotopic composition analysis and geochemistry of the Changlingzi plagiogranite porphyry. Zircon crystals from the plagiogranite porphyry yielded weighted average ²⁰⁶Pb/²³⁸U age of (248.1±4.7)Ma. The zircons from this porphyry have positive ε_Hf(t) values from 5.78 to 12.41, with T_DM2(Hf) ages from 914 to 488Ma. The plagiogranite porphyry has high content of SiO₂, Na₂O and Al₂O₃, and low content of TiO₂, MgO and CaO, showing a metaluminous-peraluminouslow-K to calc-alkaline affinity, with LREE enrichment and HREE depletion, suggesting I-type granite. In addition, the plagiogranite porphyry has high Sr and low Y values as well as high Sr/Y ratios (74.5~103.4), indicating that the plagiogranite porphyry belongs to adakite. It is suggested that the plagiogranite porphyry was formed by partial melting of the thickened newborn lower crust. And in Early Triassic, Xing' an-Mongolian orogenic belt was in a collisionorogeny tectonic setting, indicating that the Paleo-Asian Ocean had been closed in this period.