GEOCHEMISTRY AND GEOCHRONOLOGY OF HARIAMA ISLAND ARC GRANITOIDS IN BEISHAN AREA OF INNER MONGOLIA: Geological Implication

The Hariama island arc granitoids are outcropped in Gongpoquan island arc belt of Beishan orogenic belt, including diorite, quartz diorite, plagiogranite, tonalite and so on, associated with the arc volcanic rocks of Ordovician-Silurian Gongpoquan Formation. The LA-ICP-MS zircon U-Pb dating results indicate that the diorite (430.9±4.7 Ma) and tonalite(452.5±3.2 Ma) were formed in the Late Ordovician-Early Silurian. The petrogeochemical analysis results show that the granitoids belong to calc-alkaline, quasialuminous-weakly peraluminous I-type, with the Mg^# value of 44.58-51.85, Rb/Sr ratio of 0.025-0.282, Nb/Ta ratio of 0.068-14.01 and Zr/Hf ratio of 13.05-34.00. The ΣREE+Y is 59.87×10^-6-118.56×10^-6. The right-dipping chondrite-normalized REE distribution curves reflect the LREE is enriched, with (La/Yb)_N of 3.11-23.86 and unobvious Eu anomaly. The primitive mantle-normalized trace element spidergram reveals the enrichment of LILEs (Rb, Ba, Th, U and K) and medium incompatible elements (Ce, La and Hf), and depletion of HFSEs(Nb, Ta, P and Ti), with the characteristics of island arc granite, indicating the crust magma is mixed with magma from deep depleted mantle. Combined with other geological and geochemical evidences, it is considered that Hariama granitoids were formed in island arc setting, which is the result of northward subduction of Hongliuhe-Niujuanzi-Xichangjing paleo-ocean basin. The subduction began in the late Late Ordovician at the latest, and the ocean basin should be closed later than the Early Silurian.