Abstract:

      LA-ICP-MS zircon U-Pb dating data of the Mesozoic granitoids from Lingquan, Baogedewula and Shanghulin basins
in Manzhouli and Erguna area are obtained to constrain the regional tectonic evolution. Zircons from 12 representative granitoids are euhedral-subhedral in shape and display typical oscillatory zoning in CL images as well as high Th/U ratios of 0.31~3.63, implying their magmatic origin. The dating results indicate that the Mesozoic granitic magmatism in the region can be divided into three stages, i.e., the Middle Triassic, Early-Middle Jurassic, and Early Cretaceous magmatism. The Middle Triassic granitoids can be further subdivided into two stages, 241 Ma biotite syenogranite and 229 Ma syenogranite, which could be formed under an extensional environment after the final closure of the Paleo-Asian ocean in the late Paleozoic or early Mesozoic. The Early-Middle Jurassic granitoids can be further subdivided into two stages, 180±5 Ma biotite monzogranite and 171±2 Ma syenogranite. Combined with the existence of the coeval porphyry-type Mo deposit, our data suggest that the Early-Middle Jurassic magmatism could be formed under an active continental margin which was related to the subduction of the Mongol-Okhotsk oceanic plate beneath the Erguna Massif. The Early Cretaceous magmatic rocks consist of 140-150 Ma granite porphyry and 134±2 Ma quartz-monzonite porphyry. The former is similar to the Jixiangfeng Formation in time, the latter occurs as crater fills and could belong to the late evolutionary products of this magmatism. Combined with the lack of coeval volcanic rocks in the eastern Jilin- Heilongjiang provinces, the data suggest that these volcanic rocks could be formed under an extensional setting related to the evolution of the Mongol-Okhotsk orogen.

Key words: Manzhouli, Erguna, Zircon U-Pb chronology, Mesozoic, granitoid, tectonic setting