THE LATE JURASSIC I-TYPE GRANITE FROM BAOHETUN OF TUQUAN REGION IN EASTERN INNER MONGOLIA: Geochemistry, Petrogenesis and Geologic Implication

The Baohetun intrusive pluton in Tuquan region of eastern Inner Mongolia is outcropped in the eastern slope of the middle section of Daxinganling Mountains. The granite porphyry shows a zircon U-Pb age of 145.9±3 Ma, formed in Late Jurassic. The lithogeochemical data reveal that the intrusion is characterized by high Si (69.62%-77.43% of SiO₂, averagely 74.14%), rich alkali (6.93%-8.94% of K₂O+Na₂O, averagely 8.27%), rich Al (12.27%-14.46% of K₂O+Na₂O, averagely 13.35%) and poor Ti, Fe and Mg, with A/CNK>1.1, belonging to high-K calc-alkaline series. The trace elements feature the enrichment of Th, K, Hf and Rb, and depletion of Ba, Nb, Ta, Sr, P and Ti, while the REEs show enriched LREEs with (La/Yb)_N=2.2-13.8 and obvious negative Eu anomaly (δEu=0.1-0.63). These features above indicate that the granite porphyry belongs to peraluminous high fractioned I-type granitoids. The samples are plotted into the post-orogeny field in the tectonic discrimination diagram, which illustrates the Baohetun granite is classified as postorogenic granitoids. Combined with previous study, it is believed that the Baohetun granite porphyry is probably related with the extensional tectonics after the closure of Mongolia-Okhotsk Ocean.