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 kimb erlites and clinopyroxenes and garnets of the low-Cr megacryst suite from t he Gibeon Province, Namibia. Significant Sr and Pb isotope disequilibrium b etween the kimberlites and megacrysts establishes that the megacrysts are n ot cognate material. Calculated equilibrium melts for the megacrysts have r are earth element contents comparable with those of alkali basalts. Rb-Sr m ica ages, similar to 72 Ma, demonstrate that kimberlite volcanism occurred between 5 and 10 my after the inferred passage of the Discovery plume benea th 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 arg ued that the plume contributed little mass to the volcanism. The megacryst suite has a strong DUPAL Pb isotope signature. Two hypotheses can explain t he genesis of the kimberlite and megacryst suites. The first is that the DU PAL Pb isotope signature is derived from the lower mantle. The megacryst su ite 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. Alternative ly, fluid-rich melts derived from the Discovery plume migrated under the li thosphere and become concentrated in areas that were recently thermally per turbed asthenosphere, causing small degrees of melting and kimberlite magma tism. In this scenario the megacrysts represent polybaric fractionation pro ducts 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.