A chemical and multi-isotope study of the Western Cape olivine melilitite province, South Africa: Implications for the sources of kimberlites and the origin of the HIMU signature in Africa

We present major and trace element and Sr-Nd-Pb-Hf-Os isotopic data for the 76-58 Ma Western Cape melilitite province, an age-progressive magmatic lineation in which primitive olivine melilitite intrusives and alkali basalt lavas have been emplaced on the southwestern margin of South Africa. The magmas range from alkali basalts with strong HIMU isotopic and trace element affinities on the continental shelf to melilitites with kimberlite-like incompatible element compositions and EM 1 isotopic affinities on thick Proterozoic lithosphere (i.e. ⁸⁷Sr/⁸⁶Sr_i = 0.7029-0.7043, ε_Nd(t) = + 5 to + 2, ²⁰⁶Pb/²⁰⁴Pb_i = 20.5-18.1). The samples are characterized by radiogenic ¹⁸⁷Os/¹⁸⁸Os_i (0.15-0.21) and unradiogenic Hf isotope ratios, causing some of them to fall outside the compositional range of oceanic basalts. This suggests that they tap a mafic HIMU source component that is less dilute than or compositionally different from that tapped by HIMU ocean island basalts. Os-Pb and Nd-Hf isotopic co-variations suggest that the sources of the Western Cape melilitites were generated by a two-stage mixing process in which (1) mafic material (e.g. ancient oceanic crust) underwent solid-state mixing with a small proportion of asthenospheric peridotite and then (2) high-degree melts of this hybrid material mixed with low-degree melts of metasomatized continental lithospheric mantle. A similar process may help to explain the isotopic variations of Group 1 kimberlites, but the available data do not allow us to rule out a major role for convecting mantle components in kimberlite genesis. Using the Western Cape melilitites as an analog, we propose a model in which the strong HIMU isotopic signature present in most Late Cretaceous to Recent alkaline magmatic provinces across Africa is supplied not by conventional mantle plumes, but rather by pods of recycled oceanic crust brought up from the deep mantle within a long-lived, and laterally broad mantle upwelling feature located beneath the African plate.