LA-ICP-MS ZIRCON U-Pb DATING AND GEOCHEMISTRY OF THE GRANITES IN GANNAN AREA, DAXINGANLING MOUNTAINS: Geological Implication

The formation age and petrogenesis of the two-mica monzogranite drilled from HGD-1 well in Gannan area of central-northern Daxinganling Mountains are determined and the regional tectonic setting is revealed on the basis of LA-ICP-MS zircon U-Pb chronology and petrogeochemistry studies. The zircons in granite are subidiomorphic-idiomorphic, with obvious oscillatory growth zone. The dating results show that the two-mica monzogranite was formed at 161 Ma, which is the product of magmatic event in early Late Jurassic. Geochemical analysis results indicate that the granite is characterized by high Si (SiO₂ 69.81%-74.3%), rich Al (Al₂O₃ 13.5%-16.8%), and poor Fe (Fe₂O₃ 0.26%-0.6%), belonging to quasi aluminous-weak peraluminous series in terms of chemical composition, with enriched LILEs (Rb, Ba and K) and HFSE (Hf), and depleted HFSEs (Nb, Ta and Ti), and relatively enriched LREEs and depleted HREEs, which is of I-type granite. The magma originates from the partial melting of newly thickened continental crust material. The study results reveal that the formation of the early Late Jurassic two-mica monzogranite is related to the continent-continent collision environment during the closure progress of Mongol-Okhotsk suture zone, and its formation age restricts the closing time of Mongol-Okhotsk Ocean to the early Late Jurassic in Gannan area of central-northern Daxinganling Mountains.