Paleoproterozoic intraplate magmatism and basin development on the Kaapvaal Craton:: Age, paleomagnetism and geochemistry of ∼1.93 to ∼1.87 Ga post-Waterberg dolerites

We report U-Pb baddeleyite crystallization ages of similar to1927 and similar to1879 to similar to1872 Ma for dolerite sills intruding the 'Waterberg Group in Botswana and South Africa. These data increase the known extent of similar to1.9 Ga intraplate magmatism in southern Africa and place tighter age constraints on the Waterberg Group than previously available. In South Africa, similar to1.88 Ga dolerite intrudes upper Waterberg strata, constraining most, if not all, of the succession to have accumulated between similar to2.06 Ga (age of the underlying Bushveld Complex) and similar to1.88 Ga. This is consistent with derivation of much of the group from uplifted sources in reactivated segments of the Limpopo Belt. The dolerites are typical continental tholeiites, but their trace-element contents discriminate them from dolerite sills of the 1.1 Ga Umkondo Igneous Province, which occur in the same region. Paleomagnetic samples from dolerite intrusions in the Waterberg Group in South Africa (including one sill with a U-Pb baddeleyite age of similar to1872 Ma), and from dolerite sills and basalt flows in the Soutpansberg Group to the east-northeast, yield antipodal directions with a site mean pole at 15.6degreesnorth, 17.1degreeseast, A(95) = 8.9degrees. These data are interpreted to indicate that the similar to1879 to similar to1872 Ma dolerites were intruded into the Waterberg Group during voluminous magmatism associated with development of the Soutpansberg rift basin. Older, similar to1927 Ma dolerite in Botswana is similar in age and geochemistry to basalts in the craton-margin Olifantshoek Supergroup, suggesting that the mafic magmatism in those two regions is genetically related.