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 εHf(t) values, ranging from +5.8 to +9.0, indicative of derivation from a depleted mantle source, while they show negative whole-rock εNd(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.新元古代（Neoproterozoic）时期扬子克拉通（Yangtze Craton）的构造背景，及其在罗迪尼亚超聚合体（Rodinia superassembly）中的古大陆配置，至今仍是当前研究与争论的热点议题。本研究聚焦于四川西部Simian地区新发现的大河坝辉长岩（Daheba gabbro）。锆石U-Pb定年（Zircon U–Pb dating）结果显示，其206Pb/238U年龄为794 ± 2 Ma，限定该岩浆侵位时代为新元古代早期。大河坝辉长岩具有较低的SiO₂（42.29–48.82 wt%）与K₂O + Na₂O（2.73–3.94 wt%）含量，但TiO₂含量较高（2.80–3.22 wt%）。其呈现类弧型微量元素地球化学特征，表现为大离子亲石元素（large ion lithophile element）富集、高场强元素（high-field-strength element）亏损。该辉长岩的锆石εHf(t)值整体偏高，范围为+5.8至+9.0，指示其源区为亏损地幔；而全岩εNd(t)值为负值，介于-6.1至-4.5之间。这种Nd-Hf同位素解耦特征，结合微量元素模拟结果，表明新元古代大河坝辉长岩起源于石榴石二辉橄榄岩（garnet lherzolite peridotite）源区，且可能受到沉积物衍生流体与熔体的交代作用影响。该辉长岩同时兼具弧相关岩浆作用的典型地球化学特征（大离子亲石元素富集、高场强元素亏损）与洋中脊玄武岩（mid-ocean ridge basalt）特征（高Ti含量与Ti/V比值），暗示大河坝辉长岩可能形成于新元古代时期与活跃洋壳板片俯冲相关的伸展型弧后构造环境。四川西部Simian地区的大河坝辉长岩形成于新元古代时期，其产出环境为弧后裂谷构造环境。