Late-Stage HIMU-Type Volcanism on the Walvis Ridge: Not just Part of an Age-Progressive Tristan-Gough Hotspot Track

Abstract:

The Walvis Ridge forms the NE portion of the Tristan-Gough hotspot track. It links the Etendeka large igneous province (LIP) in Africa, initially connected to the Parana LIP in South America, to the Guyot Province, that ends at the active volcanic islands of Tristan da Cunha and Gough. After the plume head stage, the hotspot changed from a ridge-centered plume tail, forming the Walvis Ridge and Rio Grande Rise (130-60 Ma), to an intraplate setting resulting in the geochemical distinct Tristan and Gough subtracks (Rohde et al. 2013; Geology 41). New major and trace element and radiogenic isotope data have been generated from 36 new dredge locations on the Walvis Ridge during R/V Sonne cruises SO233 and SO234. Based on the bathymetric data, we have identified tectonic structures and subsidence rates which indicate a complex geodynamic interplay of the Walvis Ridge formation and westward migration of the Mid Atlantic Ridge and the Rio Grande Rise. Our new results confirm that the age-progressive basement of the Walvis Ridge reflects only the enriched Gough component with no evidence of the Tristan component being present (Hoernle et al., 2015; Nat. Comm.). Superimposed large seamounts (including ridge- and guyot-like structures), especially in the SE portion of the Walvis Ridge, belong to a later-stage of alkalic volcanism with distinct HIMU incompatible element and Sr-Nd-Pb-Hf isotopic composition. The HIMU late-stage volcanism (206Pb/204Pb up to 20.8) is similar in composition to St. Helena and a late-stage (Eocene) sample from the Rio Grande Rise (Rohde et al., 2013; Tectonophysics 604). The new geochemical, bathymetric and existing age data indicate a magmatic reactivation c. 20-40 Ma after the formation of the Walvis Ridge basement, which may be related to passage of the Walvis Ridge over a batch of upwelling St. Helena type plume material. Our new results indicate a more complex formation of the Walvis Ridge than previously thought, which included two major phases of formation separated by a time gap of c. 20-40 Ma.