Geochemistry and petrogenesis of mafic sills in the 1.1 Ga Umkondo large igneous province, southern Africa

Abstract

The detailed petrogenesis of mafic sills occurring throughout southern Africa provides strong support for the development of an Umkondo large igneous province on the eastern margin of the Kalahari craton at 1.1Ga. The sills are most extensively developed in the Waterberg and Middelburg basins in northern South Africa and south-eastern Botswana. They are typical fractionated continental tholeiites with subophitic to ophitic dolerites, gabbros and gabbro-norites, and largely basaltic andesite in composition. The vast majority of the sample set defines one major geochemical subgroup, here referred to as the Mesoproterozoic Post-Waterberg sills A (MPWA sills), which is characteristically LREE enriched with relatively unfractionated HREEs, and with normalised incompatible element profiles similar to modern island arc andesites. A small number from the sample set define a minor subgroup (MPWB sills), which has so far only been recognised in the Middelburg basin, South Africa and which is characterized by fractionated HREEs. Both the major and trace element geochemical signatures of the MPWA sills are indistinguishable from the type Umkondo sills and less common lavas documented from Eastern Zimbabwe and mafic sills on the Grunehogna craton in present day Eastern Antarctica. This provides strong supporting evidence for an Umkondo large igneous province developed on the Kalahari craton at 1.1Ga. Despite crustal-type Sr–Nd isotopic signatures in the MPWA sills, bulk contamination by the continental crust is ruled out in favour of derivation from a primitive mantle-like asthenospheric source with a contribution from the subcontinental lithospheric mantle modified by a previous subduction event. The smaller MPWB magma type could represent a smaller degree melt at greater depth from a modified MORB-like source, although the relationship between the two subgroups remains unclear.