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.
