Neoproterozoic backarc S-type granite in the Jiangnan fold belt, South China: Implication for initiation of modern-style plate tectonics

Phanerozoic backarc S-type granites within accretionary orogens typify modern-style plate tectonics, whereas their Precambrian counterparts remain enigmatic, often attributed to collisional orogeny or mantle plume−related processes. This study investigated the Shexian granitoids from the Jiangnan fold belt, South China, through integrated geochronological and geochemical analyses. Zircon U-Pb geochronology constrains the emplacement age of the Shexian granitoids to ca. 832−830 Ma. The petrography (e.g., muscovite and cordierite) and geochemical signatures (e.g., molar Al2O3/[CaO + Na2O + K2O] = 1.19−1.90, magmatic zircon δ18O = 7.2‰−10.8‰, whole-rock εNd[t] = −2.6 to −1.0, and the presence of normative-CIPW corundum) of the Shexian granitoids indicate an S-type affinity. High CaO/Na2O and low Al2O3/TiO2, Rb/Ba, and Rb/Sr ratios collectively point to magma derivation dominated by metagraywacke-psammite sources, which contained 20%−50% juvenile (Hf-O isotopic calculation) crustal material, potentially sourced from the Shuangxiwu arc system. Heterogeneous and depleted zircon Hf (εHf[t] = −5.7 to +9.6) and whole-rock initial Sr (87Sr/86Sri = 0.703259−0.712012) isotopic signatures of the Shexian granitoids likely reflect disequilibrium partial melting of heterogeneous metasedimentary sources. Pressure-temperature (P-T) parameters, geochemical trends (e.g., negative correlation between Rb/Sr versus Sr and Ba, and low CaO/Al2O3 and Al2O3/[Fe2O3T + MgO + TiO2] ratios), and coexisting arc magmatism in the Jiangnan fold belt suggest the Shexian granitoids were formed by fluid-absent muscovite dehydration melting at low P-T conditions (<5 kbar; <850 °C), indicative of rapid exhumation of thickened backarc sequences during slab rollback. The spatiotemporal association with 860−820 Ma arc magmatism and diagnostic rock assemblages (backarc S-type granites, suprasubduction zone ophiolites, arc-related volcanics, turbidites) collectively identify the Jiangnan fold belt as a Neoproterozoic accretionary orogen and trench-arc-basin system, demonstrating that modern-style plate tectonics had become established by the Neoproterozoic. Furthermore, remarkable similarities in zircon geochemistry (high P contents and low fO2) between Neoproterozoic and Phanerozoic S-type granitoids imply comparable surficial conditions linked to modern-style plate tectonics from the Neoproterozoic to present.