Zircon U-Pb age and geochemistry of the late ordovician monzonitic granite in the Xiaoxilin from the lesser Khingan

In this paper, we present new U-Pb zircon ages and major and trace elements for the Late Ordovician monzonitic granite rocks in Xiaoxilin area in the Eastern lesser khingan of Northeast China to elucidate the westward subduction of the Oceanic plate in Late Ordovician. U-Pb zircon dating results demonstrate that the monzonitic granites formed at 451±5Ma, belonging to the Late Ordovician. These monzonitic granites are characterized have SiO₂ of 66.45%~72.93%, Na₂O of 2.41%~3.93%, K₂O of 3.09%~5.58%, CaO of 2.02%∼3.76%, ALK of 6.56%∼7.99% and A/CNK=0.97~1.02, belonging to the high-K calc-alkaline series. The monzonitic granites is characterized by the enrichment of LILEs (e.g., Rb and K) and incompatible elements (e.g., Th and U), and depletion of HFSEs (e.g., Nb, Ta, Sr, P and Ti), as well as very lesser Eu anomalies (Eu/Eu*=0.49~1.06). The rock falls into the zone of Pre-Plate collision with the characteristics of active continental margin before the collision. Combine the mineral assemblage and geochemical characteristics, it is believed that the monzonitic granite in Xiaoxilin area was formed in the environment of oceanic crust subduction, which indicates that there was an westward subduction oceanic plate between Jiamusi Massif and Songnen-Zhangguangcailing Massif. The melt/fluid precipitated from the subducting oceanic plate confessed the overlying mantle wedge and caused the partial melting of it, which resulted in the arc magmatism. The Jiamusi Massif and Songnen-Zhangguangcailing Massif had not yet joined in the Late Ordovician.