Neoproterozoic backarc rift in the western Yangtze Block: constraints from the Daheba gabbro, Sichuan, China

The tectonic setting of the Yangtze Craton during the Neoproterozoic, along with its paleocontinental configuration within the Rodinia superassembly, remains a topic of current research and debate. This study focuses on the newly identified Daheba gabbro within the Simian region of western Sichuan Province. Zircon U–Pb dating reveals a ²⁰⁶Pb/²³⁸U age of 794 ± 2 Ma, constraining the magmatic emplacement to the early Neoproterozoic period. The Daheba gabbro has low SiO₂ (42.29–48.82 wt%) and K₂O + Na₂O (2.73–3.94 wt%) contents, but high TiO₂ (2.80–3.22 wt%). It exhibits arc-like trace-element signatures, characterised by large ion lithophile element enrichment and high-field-strength element depletion. The gabbroic rocks exhibit elevated zircon ε_Hf(t) values, ranging from +5.8 to +9.0, indicative of derivation from a depleted mantle source, while they show negative whole-rock ε_Nd(t) values (−6.1 to −4.5). The decoupled Nd–Hf isotopic data, along with trace-element modelling, indicate that the Neoproterozoic Daheba gabbro originated from a garnet lherzolite peridotite source, likely influenced by metasomatism from sediment-derived fluids and melts. The gabbro exhibits geochemical characteristics typical of both arc-related magmatism—evidenced by large ion lithophile element enrichment and high-field-strength element depletion—and mid-ocean ridge basalt, as indicated by high Ti and Ti/V ratios. This suggests that the Daheba gabbro likely developed in an extensional backarc setting associated with active oceanic slab subduction during the Neoproterozoic. The Daheba gabbro in the Simian area was emplaced during the Neoproterozoic. The Daheba gabbro formed in a backarc rift setting.