40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex

Abstract

Rocks of the Palaeoproterozoic Transvaal Supergroup, situated between the Bushveld Igneous Complex and the Vredefort Impact Structure in the central parts of the Kaapvaal craton (South Africa), experienced a regional low-grade metamorphism synchronous with brittle–ductile deformation that formed small- to medium-scale folds, cleavages, monoclines and thrusts in the dolomites, slates, and phyllites. Single-grain 40Ar/39Ar step-heating laser probe dating of synkinematic white mica from the phyllites indicates ages of ~2.15 Ga and 2042.1 ± 2.9 Ma (plateaus and pseudo-plateaus, as well as cumulative probability statistics). These ages are clearly distinct from the emplacement age of the Bushveld Igneous Complex (~2.06 Ga) and from the Vredefort impact event (~2.02 Ga), both of which have previously been suggested as possible causes of the deformation. The new data suggest that at ~2.04 Ga, the rocks of the Transvaal Supergroup in the central craton were part of a more extensive fold-and-thrust belt, named here the Transvaalide fold-and-thrust belt. While it is difficult to explain the significance of the ~2.15 Ga ages, the ~2.04 Ga event may be related to orogenic activity along the margins of the Kaapvaal craton at this time.