Isotope and Trace Element Geochemistry of Cretaceous Damaraland Lamprophyres and Carbonatites, Northwestern Namibia: Evidence for Plume--Lithosphere Interactions

Major and trace element and radiogenic and stable isotope data are reported for Cretaceous lamprophyres and carbonatites associated with alkaline igneous complexes in Damaraland, northwestern Namibia. Lamprophyres, emplaced as late-stage dykes, stocks and diatremes, include both alkaline and ultramafic varieties. Many of the latter contain abundant carbonate-rich ocelli. The carbonatites are primarily sövites and Fe-rich beforsites emplaced as plugs and dykes. Both lamprophyres and carbonatites are characterized by strong, but variable, enrichment in incompatible trace elements (e.g. La 45−7232 ppm; Nb 89–1961 ppm; Sr 700–43 000 ppm), and variable but coherent initial 87Sr/86Sr (0.70351–0.70468), 143Nd/144Nd (0.51244–0.51266) and Pb isotope ratios (e.g. 206Pb/204Pb = 17.77–19.99). Individual lamprophyre types show distinct, but restricted, ranges in isotopes and incompatible element ratios (Zr/Nb = 0.9–2.0; Y/Nb = 0.15–0.34; La/Ybn = 20–58), indicating derivation from a trace element enriched, heterogeneous mantle source. Lamprophyres and carbonatites found within the same complexes have similar isotope ratios, indicating derivation from a common source. Isotope and trace element variations, coupled with the temporal and spatial association with the palaeoposition of the Tristan plume, are thought to indicate that the Damaraland lamprophyre and carbonatite magmatism formed as a consequence of melting of metasomatic vein material introduced into an isotopically depleted subcontinental lithospheric mantle by alkaline melts or fluids derived from the upwelling Tristan mantle plume at the time of continental break-up.