A non-cognate origin for the Gibeon kimberlite megacryst suite, Namibia: Implications for the origin of Namibian kimberlites

Trace element and Sr-Nd Pb isotope analyses are presented on unaltered kimberlites and clinopyroxenes and garnets of the low-Cr megacryst suite from the Gibeon Province, Namibia. Significant Sr and Pb isotope disequilibrium between the kimberlites and megacrysts establishes that the megacrysts are not cognate material. Calculated equilibrium melts for the megacrysts have rare earth element contents comparable with those of alkali basalts. Rb-Sr mica ages, similar to 72 Ma, demonstrate that kimberlite volcanism occurred between 5 and 10 my after the inferred passage of the Discovery plume beneath the Gibeon region. Sr-Nd Pb isotope relationships of the kimberlite and megacrysts are distinct from that of the inferred plume and hence it is argued that the plume contributed little mass to the volcanism. The megacryst suite has a strong DUPAL Pb isotope signature. Two hypotheses can explain the genesis of the kimberlite and megacryst suites. The first is that the DUPAL Pb isotope signature is derived from the lower mantle. The megacryst suite therefore represents the high-pressure crystallization product of deep plume-related magmatism. This magmatism interacts with the sub-continental lithospheric mantle (SCLM) to produce the kimberlite magmatism. Alternatively, fluid-rich melts derived from the Discovery plume migrated under the lithosphere and become concentrated in areas that were recently thermally perturbed asthenosphere, causing small degrees of melting and kimberlite magmatism. In this scenario the megacrysts represent polybaric fractionation products from 'basaltic' asthenospheric-derived melts that ponded at the base of, but underwent interaction with, the subcontinental lithosphere. Storage of the megacrysts for an extended period (>10 and <100 my) is required to explain the homogeneous major and trace element compositions of individual megacrysts. Currently the latter explanation is favoured, on the assumption that the DUPAL geochemical signal is derived from the SCLM.