Palaeomagnetism of the Karoo igneous rocks in Southern Africa

Princeton University, Department of Geosciences, Princeton, NJ, United States

Virtual geomagnetic poles have been derived from over 2000 samples from 691 sites in Karoo igneous rocks, ranging from basalts and dolerites through rhyolites, and at localities spread from Namibia through the central Karoo basin, Lesotho to the Lebombo. After appropriate analysis, poles derived from Lesotho lavas, Karoo dolerite dykes and sills, Sabie River basalts, Mashikiri and Letaba lavas, and Jozini rhyolites, when averaged, yielded a palaeomagnetic (south) pole at: N = 6, latitude = -69.2, longitude = 98.3, alpha = 3.3. The sites in Lesotho come from thirteen sections sampled through the entire basalt section, the single reversal encountered being traced throughout this country. This appears to correlate both palaeomagnetically and geochemically with the major reversal found in the Sabie River basalt in all three of the sections sampled through the Lebombo stratigraphic pile. At least four reversals were found in the Lebombo, but paucity of outcrops and sampling precludes precise correlation. Together with new radiometric age data, the similarity of pole positions and the paucity of reversals are consistent with the hypothesis that all these Karoo magmatic products were emplaced between approximately 190 and approximately 175 Ma, with the bulk of the basalts erupted rapidly and penecontemporaneously throughout the country around 183+ or -2 Ma. The Mbuluzi rhyolites in Swaziland, which are stratigraphically younger than the Jozini rhyolites, yielded a pole: N = 9 sites, latitude = -45.6, longitude = 98.6, alpha = 10.2, which is distinct from that of the main Karoo magmatism, suggesting a significant difference in age. Four sites from the Bumbeni syenite complex in KwaZulu-Natal, dated at approximately 145 Ma, yielded bipolar results and a pole almost identical to that obtained previously from the coeval Main and Male kimberlite fissures at Swartruggens in the North West Province (also bipolar). The mean pole from these two studies: N = 6, latitude = -31.7, longitude = 104.3, alpha = 6.3, is considered to be another reasonably robust result for that vintage. Two sites in the Peninsula dolerites near Cape Town dated at 132 Ma, plus one from further east, yielded a mean VGP at latitude = -42.2, longitude = 105.2, alpha = 9.1.

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