A deep mantle source for carbonatite magmatism: evidence from the nephelinites and carbonatites of the Buhera district, SE Zimbabwe

In the Buhera district of SE Zimbabwe the Dorowa and Shawa complexes are composed of dolomitic carbonatites with MgO and Mg# values comparable to those anticipated for mantle melts. These carbonatites are associated with olivine-bearing nephelinitic plugs and dykes which are correlatives of the picritic and nephelinitic lavas of the basal Karoo volcanic succession in the Nuanetsi–North Lebombo (NNL) region. The NNL volcanics are isotopically anomalous. Previous detailed Sr, Nd, Pb and Os isotopic studies indicate that the anomalous signatures result from mixing between enriched lithospheric mantle and deep mantle plume material. New geochemical and isotopic data are presented which reveal that the Buhera nephelinites have signatures similar to, but more enriched, than those of the NNL attaining εNd values of −22 to −15 at εSr of +8 to +20. By contrast the Shawa carbonatites are not isotopically anomalous with εNd (0 to +1) and εSr (+3 to +6) comparable to the `mildly depleted mantleʹ signature typical of carbonatites globally. A single analysis of a Dorowa carbonatite has an intermediate signature (εSr+7; εNd−10). These relationships provide unequivocal evidence that the Buhera nephelinites and carbonatites were derived from discrete, unrelated parental magmas derived from different sub-cratonic mantle segments. Clearly, the source of the Shawa carbonatites must lie below the zone of ancient LREE enriched lithosphere sampled by the Buhera and NNL silicate magmas and their isotopic signatures more closely approximate the asthenospheric component identified in the NNL picrites. As such these results require the existence of discrete carbonatite magmas at mantle depths in excess of those producing nephelinitic magmatism.