The evolution of the lithosphere in southern Africa: A perspective on the basic granulite xenoliths from kimberlites in South Africa

Abstract

Major, trace element, and strontium, neodymium, and lead isotope data are presented on mafic granulite xenoliths from the margins of the Kaapvaal Craton in southern Africa. Those from the Markt pipe in the central Cape province preserve good correlations between Cr and Mg#, negative correlations between Co, Sc, Y, and V with Al 2O 3, and positive correlations between Sr, Rb Sr , and Eu Eu ∗ with Al 2O 3 Such trends reflect magmatic differentiation and crystal cumulation, and they have not been affected significantly by subsequent metamorphism. Present-day strontium and neodymium isotope ratios range from 0.70411–0.71179 and from 0.51149–0.51294. The lead isotope data from the Markt granulites yield a bulk rock PbPb age of 3.19 ± 0.07 Ga and those from the other Cape province granulites have similar lead isotope ages (3.2 ∼ 3.4 Ga). These ages are supported by two PbPb mineral-whole ages of ∼2.9 Ga, indicating their Archaean origin, even though both SmNd whole rock and mineral errorchrons yield Proterozoic ages. A PbPb errorchron for the granulites from northern Lesotho yields an age of 1.49 ± 0.36 which is indistinguishable from their SmNd errorchron age of 1.24 ± 0.17 Ga. Thus, it is inferred that the granulite xenoliths from northern Lesotho are from Proterozoic lower crust, whereas the granulite xenoliths from the central Cape province are from Archaean lower crust. Most lead isotope data from the granulite xenoliths plot to the left of the Geochron and indicate relatively low U Pb ratios. Lead isotope ratios in the granulites from Markt correlate with A1 2O 3, and such correlations are considered to be largely due to magmatic processes. The lead isotope trends displayed by the Archaean granulite xenoliths represent μ ( 238U 204Pb ) fractionation during extraction of the Archaean crust from the mantle, as do those preserved in greenstone belt rocks from the southern Kaapvaal Craton. Group 2 kimberlites, mantle xenoliths from Kimberley, and Nuanetsi picritic basalts plot on similar Archaean fractionation trends. It is argued that segments of both the crust and uppermost mantle beneath southern Africa stabilised in the Archaean, and that both were remobilised in subsequent magmatic and orogenic events.