Insights into the geochemical evolution and origin of Permian to Eocene (263-49 Ma) volcanism in Namibia
The South Atlantic and the adjacent voluminous Paraná-Etendeka igneous provinces in Brazil and Namibia, respectively, are considered a classic example of the succession of flood basalt volcanism, continental breakup and adjacent age-progressive hotspot track. The well-exposed Etendeka volcanism in western Namibia provide the opportunity to examine the classic plume head – plume tail concept. The different rock-types associated with the Etendeka event (carbonatite, silica-undersaturated rocks and basalt) and their varying isotopic compositions and ages can be used to gain insights into the spatial and temporal evolution of the plume and, by inference, into the supposably deep plume source region in the lower mantle at the edge of the seismically imaged African large low-shear-velocity province (LLSVP). In my PhD thesis I present a comprehensive data set of major and trace element and C-O-Sr-Nd-Pb-Hf isotopic compositions, and discuss the geochemical implications in context with the mantle plume model. The obtained results confirm that the Tristan-Gough plume was responsible for creating the Etendeka flood basalts and those contemporaneous igneous complexes. The high-Ti basalts, mainly found in northern Etendeka, have “Gough-type” composition as the Walvis Ridge basement and the Gough seamount track lavas, indicating that the Gough-type component dominated the Tristan-Gough plume from the initial plume head stage to the still persisting plume tail stage. The low-Ti basalts, exclusively occurring in southern Etendeka show a distinct isotopic composition which I term Doros-type (after a type locality). Considering published high 3He/4He ratios and high inferred potential temperatures for lavas belonging to the low-Ti group, the Doros source should also have derived from the lower mantle and ascended within the Tristan-Gough plume head. Accordingly, the spatial division of the high-Ti/Gough and low-Ti/Doros groups in the Etendeka province point to a geochemical zonation within the plume head, a model that is here proposed for the first time. Unfortunately, widespread crustal contamination of the siliceous plume magmas hampers the recognition of geochemical zonation in southern Etendeka. Carbonatites and silica-undersaturated rocks, however, are relatively insensitive to crustal contamination due to their natural enrichment in incompatible elements. By combining published data with my new findings from analyzed carbonatitic and silica-undersaturated intrusions, the occurrence and distribution of the Doros-type (low-Ti) magmas could be further constrained: In southwestern Etendeka, Doros-type magma only created the eponymous Doros intrusion, Messum intrusion and the Tafelkop and Horingbaai formations. In contrast, the eastern/southeastern Etendeka intrusions (Okorusu, Paresis, Etaneno and Osongombo) show Gough-type composition. Together with the northern Etendeka lavas, the Gough-type magmatism seems to form a horseshoe-shaped belt that encloses the Doros-type (low-Ti) magmatism. I interpret this pattern to reflect a heterogeneous plume head with potentially denser, less buoyant material in the center (Doros) and less dense (more buoyant) in the periphery. Such distribution would be consistent with fluid-dynamic models, postulating that the denser (Doros) component can only be entrained and carried to the upper mantle within the voluminous plume head, which provides sufficient buoyancy. This model would explain why rocks with Doros composition were only produced during the plume head stage but could not be found in any of the later (plume tail) eruption products of the Tristan/Gough mantle plume/hotspot. Several carbonatitic and silica-undersaturated intrusions were emplaced after the Etendeka event. In contrast to the Etendeka plume-associated Doros and Gough compositions, which both have an overall enriched mantle I (EM I) isotopic signature, these later magmatism shows a uniform HIMU-like (high time-integrated 238U/204Pb) composition. Such temporal succession was recently also found at Walvis Ridge. My findings imply that the same process is also responsible for creating the younger, HIMU volcanism in Namibia. Accordingly, the existence of several age-progressive, but transient hotspot tracks of HIMU composition are proposed that extent from the SW border of the African continent into the SE Atlantic. By combining the geochemical data with plate reconstructions and seismic tomographic mantle models, plumes of EM I-type composition (Tristan-Gough, Discovery and Shona) are proposed to ascend from the outer margin of the LLSVP, whereas subsequent (smaller) plumes of HIMU composition appear to detach from a steep topographic step on the inner surface of the LLSVP, recognizable at about 900-1200 km east of the outer margin. In conclusion, my results support the hypothesis that the EM I and HIMU components are derived from distinct reservoirs within a compositionally layered LLSVP.