Revisiting the ca. 845–820-Ma S-type granitic magmatism in the Jiangnan Orogen: new insights on the Neoproterozoic tectono-magmatic evolution of South China

The Neoproterozoic tectonic evolution of the Jiangnan Orogen is controversial, with one of the issues being whether the ca. 850–820-Ma granitoids were generated by mantle plumes or the collision between the Yangtze and Cathaysia blocks. This paper tackles this problem by examining the age and petrogenesis of one of the granitoids, the Getengling pluton in the central Jiangnan Orogen, and through comparison with a regional geochronological–geochemical database compiled from previous studies. The Getengling pluton is characterized by high A/CNK values (~1.5), slight negative whole-rock <i>ε</i>_Nd(<i>t</i>) values (−2.8 to −3.4), and positive zircon <i>ε</i>_Hf(<i>t</i>) values (0.7 ± 1.1), suggesting S-type granite affinities with juvenile contributions. Rb/Sr, Rb/Ba, and high CaO/Na₂O ratios indicate psammitic sources with both clay-rich and clay-poor characters. These geochemical characteristics are distinct from those of the granitoids (typically of A type) associated with mantle plumes. The zircon laser ablation-inductively coupled plasma-mass spectrometry U–Pb age of 845 ± 4 Ma obtained in this study, together with other ca. 835–820 Ma ages of S-type granites in the Jiangnan Orogen, indicates that the felsic magmatism in the Jiangnan Orogen lasted for ca. 25 Ma, which is longer than typical plume-related felsic magmatism. In addition, the mafic rocks in the Jiangnan Orogen and elsewhere in the South China Block are geochemically distinct from the coeval mantle plume-related ones in Australia and west Laurentia. In geochemical diagrams diagnostic of tectonic settings, the Getengling pluton and other ca. 850–820 Ma intrusions plot in the syn- and post-collisional fields, whereas the pre-850 and post-820-Ma igneous rocks plot in the arc and within-plate settings, respectively. This sequential tectonic evolution from plate subduction through collision to within-plate environments further supports the hypothesis that the ca. 850–820-Ma granitoids in the Jiangnan Orogen resulted from the Yangtze–Cathaysia collision rather than from mantle pluming.江南造山带的新元古代构造演化长期存在争议，其中一个核心议题是：约850~820 Ma的花岗质岩石究竟形成于地幔柱作用，还是扬子地块与华夏地块的碰撞过程。本文针对该问题展开研究，以江南造山带中部的葛藤岭岩体（Getengling pluton）为研究对象，剖析其形成时代与岩石成因，并结合前人研究构建的区域年代学-地球化学数据库开展对比分析。葛藤岭岩体具有高A/CNK值（约1.5）、全岩ε_Nd(t)值轻微负值（-2.8~-3.4）以及锆石ε_Hf(t)值为正值（0.7±1.1）的特征，指示其属于带有新生地壳贡献的S型花岗岩。Rb/Sr、Rb/Ba比值以及高CaO/Na₂O比值表明，该岩体的源区为兼具富黏土与贫黏土特征的砂质岩系。上述地球化学特征与地幔柱相关的花岗质岩石（多为A型花岗岩）存在显著差异。本次研究获得的锆石激光剥蚀-电感耦合等离子体质谱U-Pb年龄为845±4 Ma，结合江南造山带内其他约835~820 Ma的S型花岗岩年龄数据，显示该造山带的长英质岩浆作用持续了约25 Ma，这一时长显著长于典型地幔柱相关的长英质岩浆活动。此外，江南造山带及华南地块其他区域的镁铁质岩石，在地球化学特征上与澳大利亚、劳伦西亚西部同期地幔柱相关岩石存在明显区别。在构造环境判别图解中，葛藤岭岩体及其他约850~820 Ma的侵入岩均落入同碰撞与后碰撞构造环境，而850 Ma之前及820 Ma之后的火成岩则分别落入弧环境与板内环境。这种从板块俯冲、碰撞到板内环境的连续构造演化序列，进一步支持了江南造山带内约850~820 Ma的花岗质岩石形成于扬子-华夏地块碰撞而非地幔柱作用的假说。