Geochronology, Hf isotope and trace element of zircon and apatite for neoproterozoic granodiorites in the eastern Jiangnan Orogen: Implications for the neoproterozoic tectonic evolution

摘要

The Neoproterozoic tectonic evolution of the Jiangnan Orogen is crucial in understanding the amalgamation of the Yangtze and Cathaysia blocks. The Neoproterozoic granodiorites in South Anhui represent typical magmatism in the Jiangnan Orogen. A combination of mineralogy, zircon U-Pb geochronology, apatite trace elements, and zircon Lu-Hf isotopes were performed to identify the source nature, interpret the magmatic evolution, and determine the petrogenesis and tectonic attributes of the Neoproterozoic granodiorites in South Anhui. The LAI-CP-MS zircon U-Pb dating constrains that the Neoproterozoic granodiorites in South Anhui were formed between 850 Ma and 810 Ma, exhibiting the spatiotemporal characteristics of being younger to the southeast, which is consistent with the direction of the Neoproterozoic subduction and retreat of the Paleo-South China Ocean. The presence of Al-rich minerals and trace elements in the apatite reveal that the Neoproterozoic granodiorites are peraluminous S-type granitoids with characteristics of non-adakitic granitoids, low-oxygen fugacity and weak fluid activity. The Neoproterozoic S-type granodiorites originated from the partial melting of the heterogeneous metasedimentary sources, which were composed of abundant 0.86-1 Ga juvenile crust and minor recycled 1.5-2.2 Ga or even older mature continental crustal materials. The decoupling between Hf-Nd isotopic and whole-rock geochemical compositions indicates that these granodiorites were generated in a back-arc extension associated with accretionary orogeny during 860-810 Ma. In combination with regional geological data, it can be proposed that the growth of the Neoproterozoic continental crust in the southeast margin of the Yangtze Block was a result of the long-term subduction of the Paleo-South China Ocean and the development and extinction of multi-stage arc-basin systems. The subduction continued until similar to 780 Ma, and the final amalgamation of the Yangtze and Cathaysia blocks may have occurred in the Early Paleozoic.

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