Middle Jurassic-Early Cretaceous tectonic evolution of the Bayanhushuo area, southern Great Xing'an Range, NE China: constraints from zircon U-Pb geochronological and geochemical data of volcanic and subvolcanic rocks

摘要

This study presents new zircon U-Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing'an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol-Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early-Middle Jurassic (176-173 Ma) and Late Jurassic-Early Cretaceous (151-138 Ma). The Early-Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon epsilon(Hf)(t) values from +4.06 to +11.62 that yield two-stage model ages (T-DM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic-Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A(2)-type affinities. These volcanics have epsilon(Hf)(t) and T-DM2 values from +5.00 to +8.93 and from 879 to 627Ma, respectively. The geochemistry of these Late Jurassic-Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early-Middle Jurassic and the Late Jurassic-Early Cretaceous, respectively. This indicates that the Mongol-Okhotsk oceanic plate was undergoing subduction during the Early-Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic-Early Late Jurassic.