Petrogenesis of the Qijianfeng granite in southern Tongbai-Dabie orogenic belt:Constraints from geochemistry, zircon U-Pb age and Hf isotope

Based on comprehensive geochronological, elemental and isotopic geochemical studies of the Qijianfeng granite in southern Tongbai-Dabie orogenic belt, the authors investigated the petrogenesis as well as the geodynamic settings and Mo mineralization of southern Tongbai-Dabie orogenic belt during Early Cretaceous. LA-ICP-MS zircon U-Pb dating yielded an age of (140±1) Ma for the Qijianfeng granite. The rocks exhibit adakitic characteristics, such as high SiO₂ (72.88%-73.60%), low MgO (0.33%-0.36%), high Sr (620×10^-6-642×10^-6) content and high Sr/Y(107-111) ratios, low Y (5.65×10^-6-5.95×10^-6) and Yb (0.52×10^-6-0.59×10^-6) content, with negligible Eu anomalies. The rocks are enriched in K₂O and depleted in Cr and Ni, and show enriched Sr-Nd isotopic compositions with initial ⁸⁷Sr/⁸⁶Sr and epsilon Nd(t) ranging from 0.7075 to 0.7078 and from -15.5 to -15.1, respectively. The rocks have zircon ε_Hf(t) values of -19.9--13.2 with corresponding two-stage Nd and Hf model ages from 2.15 to 2.19 Ga and from 2.00 to 2.43 Ga respectively, suggesting that the magmas were derived from partial melting of ancient garnet-bearing amphibolites from the thickened lower crust. Combined with the previous studies, the authors propose two-stage magmatic activities and accompanying Mo mineralization during Early Cretaceous in southern Tongbai-Dabie orogenic belt. The first stage Mo-related rocks (>130 Ma) were derived from the partial melting of the thickened lower crust, while the second stage Mo-related rocks (< 130 Ma) were generated in a normal crustal environment. It is suggested that the southern Tongbai-Dabie orogenic belt experienced a tectonic transition from collisional compression to post-collisional extension, i.e., collapse, delamination and thinning of over-thickened orogenic crust and lithosphere during early Cretaceous (ca. 130 Ma).