Abstract: The Xingdong Group distributed in the northern Xiao Hinggan Mountains and the Mashan Group in the Jiamusi area, Northeast China, are always considered to be Early Proterozoic metamorphic rocks. However, the confirmation of approximate 500 Ma metamorphic age of the Mashan Group raised a question on the metamorphic age of the Xingdong Group. According to the research of the paper, the “migmatite” of the Sizishan Formation, Xingdong Group distributed in northern Xiao Hinggan Mountains should be a suit of gneissic quartz monzonitegranite intrusions. Their whole rockmineral (hornblende) Rb-Sr isochronic dating is (482±19) Ma, indicating that an Caledonian magmatic events existed in the giant granitic belt of the Xiao Hinggan Mountains. The metamorphism of the Xingdong Group may also have relation to the Early Caledonian tectonothermal events. The identification of the Early Caledonian granites is of important significance in studying the geological tectonic evolution of the area.

Key words: Xiao Hinggan Mountains, Xingdong Group, Early Paleozoic, Rb-Sr isotopic dating