GEOCHEMICAL CHARACTERISTICS AND TECTONIC ENVIRONMENT OF THE EARLY JURASSIC A-TYPE GRANITES IN YICHUN AREA, HEILONGJIANG PROVINCE

The Yichun area of Heilongjiang Province is tectonically located in the Xiaoxinganling-Zhangguangcailing arc-basin system in the eastern Xingan-Mongolian orogenic belt, with multiple-stage magmatic activities occurred in Mesozoic, which has recorded the tectonic evolution of arc-basin system. The paper studies in detail the petrology, geochemistry and chronology of the Mesozoic granites in Baihuaqingniandui area of northern Yichun. The results show that the Mesozoic granites exposed in the area mainly include fine-medium-grained syenogranite and porphyritic monzogranite. The LA-ICP-MS zircon U-Pb dating yields the age of 190.6±1.7 Ma, indicating the fine-medium-grained syenogranite was formed in the Early Jurassic. Lithogeochemically, the granites are characterized by high Si, rich in alkali, poor in Mg, Ca, P and Ti, with the A/CNK value of 1.05-1.15, belonging to peraluminous, high potassium calc-alkaline series, and depletion of HFSEs (Nb, Ti and P) and LILEs (Ba and Sr), and enrichment of elements such as Rb, K, Th and Hf. The REE patterns show asymmetrically right-dipping type, with obvious negative Eu anomaly. These geochemical characteristics reveal that the Early Jurassic granites in the area are typically of A₂-type, formed in the post-orogenic extensional tectonic setting. Combined with the magmatic activities and tectonic events in Xiaoxinganling Mountains, it is believed that the Early Jurassic magmatic emplacement in Yichun area may have occurred under the post-orogenic extensional tectonic background caused by the westward subduction of paleo-Pacific Plate.