ZIRCON U-Pb AGE AND GEOCHEMISTRY OF THE XINLITUN INTRUSIVE ROCKS IN ZHALANTUN AREA, INNER MONGOLIA

Based on the study of geochemistry and zircon U-Pb isotopic chronology of the Xinlitun intrusive rocks in Zhalantun area, Inner Mongolia, the magma source and petrogenesis are discussed. The zircons from Xinlitun granodiorite, which yield a LA-ICP-MS U-Pb age of 135.6±1.3 Ma, show typical oscillatory growth rings and high Th/U ratios (0.63-1.07), indicating a magmatic origin. The rocks, with SiO₂ content of 63.49% to 66.68%, present high potassium calcalkaline and metaluminous characteristics. The ratios of LREE/HREE (10.07-13.42) and (La/Yb)_N (15.8-23.3) suggest enrichment of LREE and deficiency of HREE. The δEu values range from 0.61 to 0.80, showing weak deficiency. These features suggest that the Xinlitun rock body belong to metaluminous I-type granite. The Xinlitun granite, with high Sr and low Yb, was formed in the lower crust in high pressure. The rock association characteristics of Xinlitun rock body is similar to those of the active continental margin granite, suggesting that it be possibly formed in active continental margin and related to the subduction of Paleo-Pacific plate.