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

Department of Geology, Texas Christian University, Fort Worth, Texas 76129, USA email: r.hanson{at}tcu.edu

W.A. Gose

Department of Geosciences, University of Texas, Austin, Texas 78712, USA email: wulf{at}mail.utaxas.edu

J.L. Crowley, J. Ramezani and S.A. Bowring

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA email: jlcrowle{at}mit.edu ramezani{at}mit.edu sbowring{at}mit.edu

D.S. Bullen and R.P. Hall

School of Earth, Environmental and Physical Sciences, University of Portsmouth, Portsmouth, PO1 3QL, UK email: Dean.Bullen{at}port.ac.uk pete.hall{at}port.ac.uk

J.A. Pancake

EG&G Technical Services, Inc., Morgantown, West Virginia 26505, USA email: Pancake{at}eg.netl.doe.gov

J. Mukwakwami

Department of Geology, University of Zimbabwe, Harare, Zimbabwe email: jmukwakwami{at}yahoo.co.uk

We report U–Pb baddeleyite crystallization ages of ~1927 and ~1879 to ~1872 Ma for dolerite sills intruding the Waterberg Group in Botswana and South Africa. These data increase the known extent of ~1.9 Ga intraplate magmatism in southern Africa and place tighter age constraints on the Waterberg Group than previously available. In South Africa, ~1.88 Ga dolerite intrudes upper Waterberg strata, constraining most, if not all, of the succession to have accumulated between ~2.06 Ga (age of the underlying Bushveld Complex) and ~1.88 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 ~1872 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 15.6°north, 17.1°east, A₉₅ = 8.9°. These data are interpreted to indicate that the ~1879 to ~1872 Ma dolerites were intruded into the Waterberg Group during voluminous magmatism associated with development of the Soutpansberg rift basin. Older, ~1927 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.

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