A HIMU volcanic belt along the SW African coast (~83–49 Ma): New geochemical clues to deep mantle dynamics from carbonatite and silica-undersaturated complexes in Namibia

The origin of carbonatitic and highly silica-undersaturated volcanism, common along the SW coast of Africa extending from Angola through Namibia to the tip of South Africa, is still poorly understood. Here we present new geochemical data (major and trace element and Sr-Nd-Pb-Hf-O-C isotopes) from the Agate Mountain calcio- to magnesio‑carbonatites (∼83 Ma), Dicker Willem calcio‑carbonatites (49 Ma) and Swakopmund basanitic plugs (76–72 Ma) along the coast of Namibia that were emplaced after the EMI (enriched mantle one) type Etendeka flood basalts. The trace element and isotopic composition of Agate Mountain carbonatites and Swakopmund basanites indicate that they were derived from a HIMU-type (high time-integrated 238U/204Pb with radiogenic Pb isotope ratios) magma source, similar to the St. Helena global HIMU endmember in the South Atlantic. The Agate Mountain carbonatites form part of the late-stage Walvis Ridge HIMU hotspot track overlying the EM1-type Walvis Ridge basement forming part of the Tristan-Gough hotspot track. The Dicker Willem carbonatites, however, extend to higher 206Pb/204Pb than St. Helena, but have similar 206Pb/204Pb to Mangaia HIMU lavas in the Pacific. Compared to Mangaia HIMU, the Dicker Willem carbonatites with mantle-type O and C isotopes have higher 207Pb/204Pb and 87Sr/86Sr but lower 143Nd/144Nd, suggesting it may represent a new HIMU endmember flavor. The HIMU carbonatitic and silica-undersaturated rocks form a belt of age-progressive volcanic tracks, including: 1) from the Walvis Ridge, through NW Namibia to central Angola, 2) from the Vema Seamount via Dicker Willem carbonatite to Gibeon kimberlites and carbonatites, 3) from the Namaqualand to Bushmanland and to Warmbad volcanic centers in northwestern South Africa, and 4) along the older end of the Shona EMI-type volcanic track extending into South Africa. Geochemical and seismic tomographic data suggest that the EMI and HIMU mantle plumes are generated from different geochemical domains at the base of the lower mantle. The Tristan-Gough, Discovery and Shona EM1 volcanic tracks are derived from a common low-velocity anomaly (superplume-like structure with three branching arms) ascending from the outer margin, possibly lower primoridal layer, of the African large low-shear-velocity province (LLSVP). Seismic low-velocity anomalies can be traced from beneath the belt of HIMU volcanism to an internal and shallower part of the LLSVP, located ∼900–1200 km east of the outer LLSVP margin and suggest that HIMU-type (possibly subducted oceanic lithospheric) material overlies EMI-type (possibly primordial) material in the internal part of the LLSVP.