Geochronology and geochemistry of Late Jurassic to Early Cretaceous granitoids in the northern Great Xing'an Range, NE China: Petrogenesis and implications for late Mesozoic tectonic evolution

摘要

This study provides new zircon U-Pb geochronological and geochemical data for Late Jurassic to Early Cretaceous granitoids in the northern Great Xing'an Range (GXR), NE China, and uses these data to constrain the late Mesozoic tectonic evolution of the GXR. The zircons from these granitoids are magmatic, as indicated by their appearances during cathodoluminescence imaging and high Th/U ratios (0.21-235). Zircon U-Pb dating indicates that the granitoids formed during the Late Jurassic (similar to 161 Ma) and Early Cretaceous (similar to 132 Ma). The Late Jurassic granitoids (LJG) are dominantly monzogranites with high SiO2 and Na2O + K2O contents, and low MgO and FeOT contents. They are metaluminous, enriched in the light rare-earth elements (LREE) and the large-ion lithophile elements (LILE), depleted in the heavy REE (HREE) and high-field-strength elements (HFSE; e.g., Nb, Ta, Ti, and P), and have an adakitic affinity. The zircons from these monzogranites yield epsilon(Hf) (t) values of +5.7 to +8.2 and two-stage model ages (T-Dm2) of 749-618 Ma. These results indicate that the primary magma for the Late Jurassic granites was originated from partial melting of a thickened lower-crustal source. The Early Cretaceous granitoids are dominantly metaluminous to peraluminous porphyritic granites with higher HREE contents than the Late Jurassic monzogranites. They display negative Eu anomalies and have epsilon(Hf)(t) values of +3.1 to +9.0 with corresponding T-DM2 ages of 877-553 Ma, suggesting they formed from magmas generated by partial melting of juvenile lower-crustal material. Combining these data with the geochemistry of coeval volcanic rocks within the GXR, we infer that the northern GXR records a late Mesozoic transition from compression to extension. The compression was most likely related to the Middle Jurassic closure of the Mongol-Okhotsk Ocean, whereas the extension was related to delamination of a thickened region of the lithosphere and/or subduction of the Paleo-Pacific Plate beneath Eurasia. (C) 2018 Elsevier B.V. All rights reserved.