Geochronological and Geochemical Constraints on the Granitic Gneisses of South China Affinity Within the Sulu Ultrahigh-Pressure Belt, Eastern China: Early Response to the Breakup of Rodinia Supercontinent

Abstract

The Rongcheng area is situated along the northeastern tectonic margin of the Sulu orogenic belt, where extensive exposures of Neoproterozoic granitic gneisses are present. These rocks are intimately associated with the breakup of the Rodinia supercontinent and serve as an optimal geological archive for investigating the mechanisms underlying the assembly of the Rodinia supercontinent. However, considerable controversy persists in research on rock genesis and tectonic setting. This study systematically investigates the petrography, whole-rock geochemistry, zircon U–Pb geochronology and Lu-Hf isotopic compositions of Neoproterozoic granitic gneisses from the Rongcheng area to constrain their diagenetic age, petrogenesis, tectonic setting and geological implications. The obtained ²⁰⁶Pb/²³⁸U weighted mean ages ranging from 750 to 798 Ma demonstrate that the protoliths of these granitic gneisses were emplaced during the Neoproterozoic period. Geochemically, the granitic gneisses constitute an I-A type peraluminous granite-granodiorite suite within the high-K calc-alkaline series, characterised by high SiO₂ contents (66.09–76.91 wt%), alkali enrichment (Na₂O + K₂O = 6.56–9.15 wt%) and CaO depletion (0.35–3.60 wt%). These rocks exhibit significant enrichment in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs; e.g., Ba, K and U), accompanied by marked depletion in heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs; e.g., Nb, Ta, Y and Ti). Lu-Hf isotopic analyses reveal ε_Hf (t) values ranging from −21.0 to −1.37 and two-stage Hf model ages (T_DM2Hf) spanning 1.77 to 2.88 Ga, indicative of Paleoproterozoic to Archean crustal residence. Petrogenetic constraints indicate the Rongcheng granitic gneisses primarily originated from the partial melting of crustal materials with minor contributions from mantle sources, as evidenced by their negative εHf(t) values (−21.0 to −1.37). These rocks formed within a Neoproterozoic post-collisional extensional environment, representing the initial response to the breakup of the Rodinia supercontinent in the northeastern Sulu orogenic belt.