Rio Grande Rise: A new type of extreme hotspot track, a propagating shear zone, or both?

南大西洋拥有与大陆洪水玄武岩相连的地幔柱轨迹的罕见例子之一，它提供了132 Myr的大陆裂谷和新海洋盆地发展期间地幔融化的记录。与该羽流系统的非洲一侧相比，南美洲一侧的火山岩及其与大陆裂谷和南大西洋打开的关系的现代地质年代学和地球化学资料很少。裂谷作用与巴西边缘丰富的火山活动有关，以岩脉和向海倾斜的反射层为代表，而让沙科海山链（JCSC）和里约热内卢大隆起（RGR）在海洋岩石圈上形成火山构造。这些熔岩中的许多可能与特里斯坦-高夫羽流有关，但广泛存在的长期火山活动和地幔断层扫描表明，另一个热点产生了一条不同的火山轨迹，将RGR与巴西的侏罗纪洪水玄武岩连接起来。我们建议通过高精度的地质年代学和地球化学方法来绘制南美板块火山活动的空间、时间和地球化学演化图，来测试板内岩浆活动和洪水玄武岩起源的竞争模型。我们的目标是首次使用下一代多收集器质谱仪对来自Resende-Ilha Grande Dike Swarm和巴西边缘、JCSC和RGR的向海倾斜反射序列的86个样品进行高精度年龄测定。此外，我们将分析巴西脉状和向海倾斜反射玄武岩的主微量元素和Sr、Nd、Hf、Pb同位素组成。我们将利用这些新数据：(1)确定RGR是否与巴西东南部的侏罗纪洪水玄武岩有关；(2)确定RGR与沃尔维斯岭的时空发展是同步的还是独立的；(3)利用这些信息在不同的羽流和非羽流模型之间进行测试，特别是我们最近提出的关于南大西洋盆地过去演化的相干地球物理假设。

The South Atlantic Ocean hosts one of the rare examples of a mantle plume track connected to continental flood basalts, which provides a 132 Myr record of mantle melting during continental rifting and development of a new ocean basin. In contrast to the African side of this plume system, very little modern geochronological and geochemical data exist for volcanic rocks on the South American side and its relation with continental rifting and the opening of the South Atlantic. The rifting processes are associated with abundant volcanism represented by dikes and seaward-dipping reflector sequences at the Brazil margin, whereas the Jean Charcot Seamount Chain (JCSC) and of the Rio Grande Rise (RGR) form volcanic structures on the oceanic lithosphere. Many of these lavas may be related to the Tristan-Gough plume, but widespread long-lived volcanism and mantle tomography suggest that another hotspot produced a different volcanic track connecting the RGR to Jurassic flood basalts in Brazil. We propose testing competing models for the origin of intraplate magmatism and flood basalts by using high-precision geochronology and geochemistry to map the spatial, temporal, and geochemical evolution of the volcanism on the South American Plate. We aim to determine for the first time high-precision ages using the next generation of multi-collector mass spectrometer for 86 samples from the Resende-Ilha Grande Dike Swarm and seaward-dipping reflector sequences at the Brazil margin, the JCSC, and the RGR. Additionally, we will analyse the major and trace element and Sr, Nd, Hf, and Pb isotope compositions of the Brazilian dike and seaward-dipping reflector basalts. We will use these new data to (1) establish whether the RGR is connected to Jurassic flood basalts in southeast Brazil, (2) determine if and when the temporal and spatial development of the RGR and Walvis Ridge is synchronous or independent, (3) use this information to test between different plume and non-plume models, in particular our recently proposed coherent geophysical hypothesis about the past evolution of the South Atlantic basin.