Petrogenesis and tectonic implications of the Jiudaowan Pluton in Jinning, western margin of the Yangtze Block: Evidence from geochemistry and U–Pb geochronology

Abstract

The western margin of the Yangtze Block hosts diverse Neoproterozoic igneous rocks, with exposed S-type granites serving as key indicators for deciphering regional geological evolution. This study focuses on the Jiudaowan granite pluton, located on the western margin of the Yangtze Block, through systematic petrographic, whole-rock geochemical, zircon and monazite U–Pb geochronology, and whole-rock Nd isotopic analyses aiming to elucidate its petrogenesis and tectonic significance. The Jiudaowan granite pluton is a composite body, consisting of the Luotaijiu, Jiudaowan, and Daheishan units, characterized by biotite monzogranites, muscovite–plagioclase granites, and two-mica monzogranites, respectively. LA-ICP-MS zircon and monazite U–Pb dating reveals crystallization ages between 832 and 798 Ma. The three units are peraluminous, containing minerals such as muscovite, garnet, and tourmaline, and exhibiting high SiO₂ (72.99–77.83 wt%), Al₂O₃ (12.36–15.02 wt%), and A/CNK values (1.06–1.43), confirming their classification as peraluminous S-type granites. Compositional variations within the Jiudaowan granite pluton are primarily controlled by protolith composition and melting mechanisms. The pluton is distinguished by low CaO/Na₂O ratios (0.02–0.18), high Rb/Sr (0.83–113) and Rb/Ba (0.33–15.2) ratios, and negative ε_Nd(t) values (−13.6 to −9.1), indicating derivation from partial melting of heterogeneous metasedimentary sources. MgO, TiO₂, Rb/Sr, and whole-rock Zr saturation temperatures suggest that the Luotaijiu and Daheishan units formed via biotite dehydration melting, whereas the Jiudaowan unit resulted from muscovite dehydration melting. Additionally, the Jiudaowan granite pluton displays a clear negative correlation between Al₂O₃, CaO, Fe₂O₃T, MgO, TiO₂, and SiO₂, along with pronounced Eu negative anomalies and depletions in Sr and Ti, suggesting fractional crystallization of feldspar, mica, and Fe-Ti oxides during magma emplacement. Similarly, variable incompatible element ratios of Nb/U (1.07–18.97) and Nb/La (0.24–26.88) further indicate minor crustal assimilation and contamination during magma evolution. Integrating regional geological data, we propose that the Jiudaowan pluton formed during crustal thickening associated with post-collisional extension, likely related to the breakup of the Rodinia supercontinent.