Late Mesozoic Tectonic-Magmatic Evolution of the Tongbai Orogen, Central China: Implications for Granites Petrogenesis and Gold Mineralization

Abstract

Late Mesozoic granitoids are widely distributed throughout the East Qinling-Tongbai-Dabie orogenic belt and its periphery, while large-scale gold deposits during the same period were primarily found in the Xiaoqinling-Xiong'ershan area, west of the Tongbai orogen. The deep dynamic mechanisms behind this phenomenon are still poorly understood. In this study, we conducted a comprehensive analysis of geochronology, petrology, geochemistry and Sr-Nd-Hf isotopes on the late Mesozoic Yuhuangding (YHD) and Zhulindian (ZLD) plutons in the Tongbai orogen. Zircon U-Pb dating yields intrusion ages of ca. 140, 131, and 132 Ma for the YHD and ZLD monzogranites and mafic microgranular enclaves within the ZLD pluton, respectively. Both the monzogranites exhibit adakitic affinities with a thickened lower crust origin. Meanwhile, the Sr-Nd-Hf isotopic composition of the YHD monzogranites indicates an affinity with the northern Qingling (N-QL) crust, whereas the younger ZLD monzogranite shows a distinct linkage to the southern North China Craton (S-NCC) basement. Such isotopic dichotomy suggests that the Tongbai orogenic belt exhibits a “crocodile-mouth” crust architecture, characterized by the northward subduction of the N-QL crust and the southward wedged S-NCC crust. Research data on granitoids throughout the East Qinling-Tongbai-Dabie orogenic belt, including their geochemical and isotopic variations, along with the seismic velocity profiles, indicate that the East Qinling-Tongbai-Dabie orogenic belt underwent lower crustal delamination and thinning during the early Cretaceous, which resulted in the formation of the observable high-velocity zone (HVZ) within the upper mantle. The discontinuities of the HVZ suggest that the intense extensional environment following delamination probably created lithospheric weak zones along the west of Nanyang Basin, which in turn facilitated the contribution of mantle-derived materials and heat, promoting the formation of granitoids and gold mineralization. These processes may be triggered by subduction and/or roll-back of the Paleo-Pacific plate.