Zircon U-Pb ages,geochemistry and its petrogenesis of Jurassic granites in northeastern part of the Da Hinggan-Mts

The U-Pb dating results determined by LA-ICPMS show that quite a large of Jurassic granites were still distributed inside the Proterozoic,Early and Late Paleozoic granite,which were recognized and categorized by predecessors.The intrusion sequence of Jurassic granite in the studied area could be classified into two stages,i.e.early and late stages on the basis of event occurring time, whose U-Pb dating in zircon are 188～190Ma and 171～181 Ma,respectively.The dating data of Jurassic granite are fully comparable to those of Jurassic granite existing in the other part of Northeast China.Geochemically,these granitics could be distinguished by two types-low-Sr and high-Yb granitics and high-Sr and low-Yb granitics,they possess the same or similar source rock compositions but originated at different depth.The early intruded granitics,characterized by the low-Sr and high Yb,originated at the low pressure in the mid crust,however,the late intruded ones,characterized by high Sr and Low Yb,similarly to C-type adakites and it originated at the relatively high pressure in lower part of the crust.Characteristics of zircon Hf isotope indicated that the source materials of Jurassic granitics came from newly accreted crustal materials and their mixture in the Neoproterozoic and phanerozoic.The Jurassic granitics in the studied area are dominantly composed of granodiorite and adameUite,which belong to aluminum saturated or oversaturated,high potassium calc-alkaline series-the granite in type I,with similar feature of the granite associations at the active continental margin.It is a part of stripped or belted Jurassic granitics in Northeast China distributed alone NNE direction,its origin might be related to the subduction of paleo-Pacific tectonic plate.