Evolution of the Northern High-Grade Margin of the Kaapvaal Craton, South Africa

Abstract

The northern Kaapvaal craton is divided into three distinctive crustal zones: (1) the high-grade Southern Marginal Zone (SMZ) of the Limpopo belt in the north; (2) a transitional zone of technically stacked metamorphic rocks which includes the area from the Sutherland belt in the north to the Pietersburg and Murchison belts in the south; and (3) a low-grade granite-greenstone terrane farther to the south. The change across the SMZ boundary is expressed by an overall increase in the grade of metamorphism, a strip of retrogression related to a zone of thrusting, abrupt changes in intensity of deformation, rock densities, magnetic lineament patterns, and isotopic ages in compositionally similar rocks. Regional metamorphism in the northern Kaapvaal craton is demonstrably not a continuum in the sense that the highest-grade rocks underwent all earlier stages of metamorphism; rather, that rapid local tectonic processes must have occurred. A proposed evolutionary path for the SMZ was initiated with crustal thickening during the collision of the two continents in the Late Archean. Tectonic burial and superimposition of granulite facies metamorphism was followed by upward movement of deeper crustal rocks and thrusting onto the adjacent transition zone. The original collisional record in the granulite terrane was, however, obliterated by the later mainly upward response of the thickened crust. Response related zones of thrusting acted as channelways for the migration of $CO_{2}$-rich fluids, which were responsible for the establishment of a retrograde orthoam-phibole isograd at lower temperatures. These fluids were also responsible for the local transformation of quartzo-feldspathic gneisses to charaockites in the hanging wall blocks of the thrust faults at higher grade. Late-tectonic granitic bodies intruded along some of these structures. The present reconstruction of the evolution of the northern margin of the Kaapvaal craton supports suggestions that plate tectonic processes were active in the Archean. The transition zone is made up of relatively smaller zones of higher-grade rocks thrust onto lower-grade rocks during the collision of the Zimbabwe and Kaapvaal cratons. It is suggested that the former possibly moved southwards over the latter and depressed the Limpopo belt to granulite conditions of metamorphism. The Limpopo belt then responded to this burial by subsequent regional uplift.