Abstract

Integrated U-Pb geochronology and palaeomagnetic study of mafic to felsic volcanic rocks of the Derdepoort Belt of South Africa are employed to test the hypothesis that the Pilbara and Kaapvaal Cratons were joined as part of a Late Archaean "Vaalbara" supercontinent. An age of 2782+ or -5 Ma is deduced for eruption of the Derdepoort basalts, bracketed by a concordant SHRIMP zircon age of 2781+ or -5 Ma for overlying felsic volcanics and a concordant isotope dilution zircon age of 2783+ or -2 Ma for underlying granite of the Gabarone Complex. Based on the low (subgreenschist) metamorphic grade of the basalts, the presence of highly stable single domain magnetite, and a positive conglomerate test, the magnetization of the Derdepoort basalts is inferred to date from the time of their emplacement and cooling at 2782 Ma. Results yield a primary palaeopole at 005 degrees E, 40 degrees S (A 95 = 18 degrees ), and indicate a palaeolatitude of 64.5+ or -17.5 degrees for the Kaapvaal Craton at 2782 Ma. Published palaeomagnetic data for the Mount Roe Basalts of the Pilbara Craton indicate a palaeolatitude of 34.3+ or -6.4 degrees at 2772+ or -2 Ma. The latitudinal separation of 30 degrees implies that the cratons were not contiguous at 2.77 to 2.78 Ga, although the possibilities that the cratons could have been joined during other intervals of time, or that they were non-contiguous parts of a larger continent, are not ruled out.

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