Zircon U-Pb chronology and geochemical characteristics of Late Si-lurian monzogranite in Ongniud Bannar, Inner Mongolia

The granite widely distributed in the Xiaoyingzi lead-zinc ore deposit of Ongniud Bannar, Inner Mongolia, consists of monzogranite as revealed by field geological survey and thin section identification. Zircon LA-ICP-MS U-Pb dating result shows that the monzogranite was formed in late Silurian (419.3±9.2Ma) rather than in Mesozoic as previously held. The values of SiO₂, K₂O, K₂O/Na₂O, Al₂O₃ are 74.41%~75.05%, 3.05%~4.42%, 0.70~1.11, 13.22%~14.02% respectively, the aluminum saturation index-es (A/CNK) range from 0.99 to 1.1, and the oxides of Fe, Mg, Ti are relatively low, which implies that the monzogranite is Ⅰ-type granite. The ratios of light and heavy rare elements (LREE/HREE) are between 10.61 and 16.61, and the (La/Yb)_N ratios are be-tween 12.52 and 27.44, suggesting obvious fractionation between light and heavy rare elements. The values of δEu are 0.48~0.81 with moderate negative europium anomaly. It is enriched in large ion lithophile elements (LILEs) Rb, Ba, U, K, depleted relatively in high field strength elements (HFSEs) Nb, Zr, Hf, and depleted relatively in Sr, Yb. All these geochemical characteristics show that the monzogranite is a product of volcanic arc on an active continental margin. The regional geological data reveal that the monzogranite was formed in the process of extinction of Bainaimiao island arc in the setting of subduction of the Paleo Asian Ocean to the North China craton.