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

Abstract 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.