Baddeleyite U–Pb ages and gechemistry of the 1875–1835 Ma Black Hills Dyke Swarm across north-eastern South Africa : part of a trans-Kalahari Craton back-arc setting?

Abstract

Eleven new baddeleyite U–Pb crystallisation ages and associated whole-rock geochemistry on NE–NNE-trending tholeiitic dykes cutting across the north-eastern corner of the Archaean Kaapvaal Craton, the overlying Transvaal basin and the Bushveld and Phalaborwa igneous complexes collectively define a 1875–1835 Ma Black Hills Dyke Swarm (BHDS). Dyke ages do not discriminate between dyke trends or geographic location, but subdivide the BHDS into an older set of four more primitive dykes (MgO = 9.4–6.8 wt.%) and a younger set of seven dykes with more differentiated compositions (MgO = 5.6–4.2 wt.%). Despite being emplaced over a c. 40 Myr period, major element compositions are remarkably consistent with a single inversely modelled bulk fractionating assemblage of 57.5% plagioclase, 29.5% augite and 13.0% olivine. This fractionating assemblage requires an additional assimilation of bulk continental crust (at a low r-value of 0.2) for reversed modelling of parental rare earth elements. Even though this crustal assimilation indicates that primary magmas could potentially have been derived from a spinel-bearing ambient primordial and asthenospheric mantle source, anomalously low Nb and high Pb values for the more primitive older dykes may also have been inherited from a sub-continental lithospheric mantle source. The ages for the BHDS bridge a gap between c. 1889 and 1867 Ma mafic sills and c. 1830 Ma rhyodacitic pyroclasts, interbedded in the top of a ~3 km-thick Sibasa basalt sequence, which combine into a continuous c. 1.89–1.83 Ga igneous province. Similar geochemical signatures are consistent with all sills, volcanic rocks and BHDS feeders collectively belonging to a very voluminous and coherent igneous province, which arguably formed behind active Magondi and Okwa-Kheis arcs, along the western margin of the proto-Kalahari Craton.