LA-ICP-MS Zircon U-Pb Age of Monzonite Granite-Quartz DioritePluton in Xiaoheishan Area of BeishanOrogenic Beltand Its Geological Significance, Inner Mongolia

Petrology, geochemical data and LA-ICP-MS zircon U-Pb dating have been obtained from the Xiaoheishan monzogranite and quartz diorite, with the aim of constraining the tectonic nature of the Beishan area. LA-ICP-MS zircon U-Pb age data indicate that the monzonite granite was formed during Middle Ordovician and the quartz diorite was emplaced during Early Silurian. Furthermore, the monzonite granite belong to high-K cal-alkaline, metaluminous-weakly peraluminous with features of rich SiO₂ and Al₂O₃. They also show strong enrichment in LREEs and Rb, K, Th, U, depletion in HREEs and HFSEs (Nb, Ta, Ti). The quartz diorite belongs to cal-alkaline with metaluminous characteristics. In addition, they invariably exhibit relatively gentle light rare earth elements (LREE) enrichment with flat heavy rare earth elements (HREE) and weak negative Eu anomalies (δEu=0.69-0.92) in the chondrite-normalized REE patterns, depletion of Nb, Ta, P, Sr, Ti and enrichment of Rb, K. Based on the regional geology, the above characteristics reflect that the monzonite granite was formed under the tectonic setting of syn-collision, while the quartz diorite volcanic arc after collisional. This implies that the ocean basin represented by Hongliuhe-Niujuanzi-Xichangjing ophiolite mélange belt disappeared during 436.1-461.5 Ma.