Formation of the Jiangnan Orogen: Constraints from the geochronological and geochemical compositions of the Neoproterozoic granitoids

The Jiangnan Orogen (JO) is widely recognized as the collisional suture between the Yangtze and Cathaysia Blocks in South China, marked by intense Neoproterozoic magmatism, however, the precise geochronological constraint of the collision event remains controversial. This study focuses on geochronological and geochemical analyses of granitic rocks from the Guibei region of the western JO. The Sanfang, Yuanbaoshan, and Pingying plutons, composed of granites, exhibit zircon U-Pb ages ranging from 825 to 833 Ma. Granodiorites of the Caigun pluton and gabbro intrusion near the Sanfang pluton yield zircon U-Pb ages of approximately 833 Ma and 856 ± 8.0 Ma, respectively. These ages collectively indicate that these rocks represent products of the Neoproterozoic magmatic event. Neoproterozoic granites are characterized by high A/CNK ratios, low Ga/Al ratios, and Zr + Nb + Ce + Y contents, which are consistent with the classification of S-type granites. The Neoproterozoic granites display similar ε_Nd(t) values ranging from -6.24 to -5.09 and predominantly negative ε_Hf(t) values (< 0). Their linear geochemical variations suggest an origin from partial melting of crustal basement rocks followed by extensive fractional crystallization. The Neoproterozoic granodiorites exhibit high SiO₂ and Al₂O₃ contents, elevated K₂O/Na₂O ratios, low Mg# values, enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs), suggesting an arc-related origin for these granodiorites. They are also characterized by decoupled Nd and Hf isotopes, with negative ε_Nd(t) values (-5.68 to -5.50) and positive ε_Hf(t) values (> 0), suggesting a likely origin from the partial melting of a fluid-modified lithospheric mantle, with the assimilation of crustal components. The geochemical variations observed in these Neoproterozoic granites and granodiorites indicate their formation within collision- and arc-related settings, respectively, suggesting that the transition from a subduction-dominated regime to a collisional setting within the JO likely occurred around 830 Ma.