Mesozoic Bimodal Volcanic Suite in Zhalantun of the Ba Hinggan Range and Its Geological Significance: Zircon U-Pb Age and Hf Isotopic Constraints

摘要

Mesozolc bimodal volcanic rocks of basaltic andeslte and rhyollte are widely distributed in the Da Hinggan Range, but their petrogenetic relationships and geodynamic implications are rarely constrained. Detailed studies on doleritic and porphyry dikes in the Zhalantun area indicate that they display features of magma mixing, suggesting their coeval formation. In situ zircon U-Pb dating shows that the porphyry was emplaced in the Early Cretaceous with a ~(206)Pb/~(238)U age of 130+ -1 Ma. Zircons from the iolerite also yield an Early Cretaceous emplacement age of 124+ -2 Ma although some inherited zircons have been Identified. These age results indicate that the Early Cretaceous was an important period of magmatism in the Da Hinggan Range. Zircons from porphyry are characterized by positive value of epsilon_(Hf)t) as high as 10.3+ -0.5 with Hf depleted mantle model age of 349-568 Ma, whereas magmatic zircons from the dolerite have epsilon_(Hf)(t) value of 11.0 + -1.4 with Hf depleted mantel model age of 342-657 Ma, consistent with those from the porphyry. Considering other data on the geological evolution of this area, it is concluded that the mafic magma originated from the partial melting of Paleozoic enriched lithospheric mantle, whereas the felsic magma came from recycling of juvenile crust formed during the Paleozoic. Both of the protoliths are closely related to the subduction of the Paleo-Asian Ocean during the Paleozoic, indicating that the Paleozoic is an important period of large-scale crustal growth in the area.