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?

摘要

Eleven new baddeleyite U–Pb crystallisation ages and associated whole-rock geochemistryon NE–NNE-trending tholeiitic dykes cutting across the north-eastern corner of the Archaean KaapvaalCraton, the overlying Transvaal basin and the Bushveld and Phalaborwa igneous complexes collectivelydefine a 1875–1835 Ma Black Hills Dyke Swarm (BHDS). Dyke ages do not discriminate betweendyke trends or geographic location, but subdivide the BHDS into an older set of four more primitivedykes (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 compositionsare 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 assimilationof 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 derivedfrom a spinel-bearing ambient primordial and asthenospheric mantle source, anomalously low Nb andhigh Pb values for the more primitive older dykes may also have been inherited from a sub-continentallithospheric mantle source. The ages for the BHDS bridge a gap between c. 1889 and 1867 Ma mafic sillsand 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 areconsistent with all sills, volcanic rocks and BHDS feeders collectively belonging to a very voluminousand coherent igneous province, which arguably formed behind active Magondi and Okwa-Kheis arcs,along the western margin of the proto-Kalahari Craton.