Zircon U-Pb age and geochemistry of the magmatic rocks in the Jiamusi massif, NE China and their tectonic implications.

The Jiamusi massif is located in the key parts of the superimposed and transformed area between circum Pacific tectonic regime and Central Asian Orogenic Belt (CAOB). The north and south sides of the massif are adjacent to the Bureya block of the Russian Far East and Khanka block, respectively, which were called Bureya-Jiamusi-Khanka block. These areas had undergone the evolution of both the Paleo-Asian and the Pacific tectonic systems, and were characterized by the Paleozoic-Mesozoic complex amalgamation, making them the focus areas for study about the superimposition and transformation of circum Pacific and Paleo-Asian tectonic regimes. The Paleozoic-Mesozoic magmatic rocks are distributed widely in Huanan lift within the Jiamusi massif, which is significant for constraints on the regional tectonic evolution. The results of LA-ICP-MS zircon U-Pb dating and geochemical analyses indicate that the gneissic granites formed in Middle Permian (267 2Ma) and have an affinity to I-type granite. They are relatively enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), suggesting that they were likely produced by the partial melting of subducted slab and formed in an island -arc environment. While the syenogranites formed in Middle Triassic (244 2Ma), and they have SiO2 = 74.29% 74.88%, Al2O3 = 12.99% 13.28%, (I\Ta2O + K2O) = 8. 61% 9. 17%, and Na2O/1(20 = 0.35 O. 42, indicating that the syenogranites are alkaline series. In addition, A/NK-A/CNK illustration shows that the granites are peraluminous series, and they are characterized by enrichments in LREEs and LILEs, and depletions in HREEs and HFSEs. These geochemical characteristics are similar to the syn-collisional granite. The primary magmas for the syenogranites were generated by the partial melting of the lower continental crust. These evidences suggest that the magmatic rocks were formed in an island -arc environment related to the subduction of the Paleo-Asian oceanic plate beneath the Jiamusi massif in the Middle Permian. The Middle Triassic syenogranite recorded a syncollisional environment related to the final closure of the Paleo-Asian Ocean. Therefore, Middle Triassic is an important geological turning period for CAOB.