Oceanic crust subduction in the Zhurihe area, central Inner Mongolia during Middle Ordovician: Constrains from age and geochemistry of trondhjemites

This paper reports zircon U-Pb ages and Hf isotope data as well as geochemical data of trondhjemites which are latest discovered from the Zhurihe area in central Inner Mongolia, with the aim of constraining oceanic crust subduction during Middle Ordovician.The dating results show that the trondhjemites formed in Middle Ordovician (472±3 Ma and 466±5 Ma).The trondhjemites manifest as the geochemical characteristics of typical subducted-oceanic-crust-type adakites being rich in Si, Al, Na and poor in K, with high values of Sr and low values of Y and Yb, with Eu and Sr positive anomalies, with intense fractionation between light rare earth elements and heavy rare earth elements, being rich in large ion lithophile elements (LILEs) and loss of high field strength elements(HFSEs).The zircon ε_Hf(t) values of trondhjemites are between +8.8 and +15.1, so are T_DM1 values between 475 Ma and 728 Ma as well as T_DM2 values between 476 Ma and 885 Ma.According to the latest regional geological survey, there exist an Early Paleozoic oceanic subduction zone in the Zhurihe area what indicates at least during Middle Ordovician the Paleo-Asian Ocean began to subduct towards the North China plate in the Zhurihe area.The magmatic emplacement-crystallization activities which are represented by the trondhjemites with the characteristics of typical adakites are response to the oceanic crust subduction.