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

点击翻译按钮获取中文摘要

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.