Cretaceous A-type volcanic-intrusive rocks and simultaneous mafic rocks along the Gan-Hang Tectonic Belt, Southeast China: petrogenesis and implications for the transition of crust-mantle interaction

摘要

Abundant evidence points to the Cretaceous crust-mantle interaction and plate subduction in the Gan-Hang Tectonic Belt (GHTB), southeastern China, but the evolutionary process remains poorly constrained. Here we conduct a comprehensive study on Daqiaowu granitic porphyry and diabase dikes in the eastern GHTB, in conjunction with previous studies on simultaneous felsic and mafic rocks along the GHTB, to demonstrate their petrogenesis and geodynamic evolutionary process. The Daqiaowu granitic porphyry (125Ma), as well as the coeval granitic rocks, exhibits high zircon saturation temperatures, alkalis, 10(4)*Ga/Al ratios, and Zr + Nb + Ce + Y contents, concluding a distinctive belt of the Early Cretaceous (similar to 137-125Ma) A-type volcanic-intrusive rocks in the GHTB. Their epsilon(Nd)(t) and zircon epsilon(Hf)(t) values gradually increased through time from approximately -9.0 to -1.0 and -10.0 to +4.0, respectively, implying increasing contribution of mantle-derived components to their formation, and hence progressively intensified crust-mantle interaction in an intra-arc rift environment (a geodynamic transition stage from continental arc to back-arc) during the Early Cretaceous. This plausibility is further supported by the Early Cretaceous Daqiaowu diabase dikes and coeval mafic rocks which exhibit arc-like magmatic signatures and were derived from mantle wedge. In contrast, the Late Cretaceous mafic rocks show ocean island basalt-like geochemical characteristics, reflecting a depleted asthenosphere mantle source. This discrepancy of mantle sources concludes that the geodynamic setting in the GHTB may have basically transferred to back-arc regime in the Late Cretaceous. Thus, the Cretaceous geodynamic evolutionary process in the GHTB can be defined as the Early Cretaceous gradually intensified crust-mantle interaction in a geodynamic transition stage (from continental arc to back-arc extension) and the Late Cretaceous back-arc extensional setting.