An isotopic and geochemical study of the northern Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt:: are they juxtaposed terranes?

To test existing lateral accretion models for the Archean Kaapvaal Craton in South Africa, metapelites and leucocratic granitoids from two different provinces, the,granulite facies Southern Marginal Zone of the Limpopo Belt and the northern part of the adjacent,greenschist to amphibolite facies Kaapvaal Craton, have been studied. Both provinces resemble each other in terms of isotope and trace element characteristics. They show a general depletion of incompatible elements independent from the grade of metamorphism, This observation conflicts with theories involving mobilisation during granulite facies metamorphism. A further common fingerprint is the unusually high Cr and Ni concentrations in the metapelites. In addition, they define a very narrow range of Nd model ages (2.9-3.05 Ga) constraining the average crustal age of their cratonic source region. The,granitoid gneisses show somewhat larger variations of Nd model ages indicating some age heterogenity within the crust, However, the data from both provinces completely overlap. The Southern Marginal Zone and the Kaapvaal Craton also show uniform low Pb-207/Pb-204 relative to their Pb-206/Pb-204 ratios that differ clearly from other Southern African provinces. These results support the view that the Southern Marginal Zone represents a part of the Kaapvaal Craton exposed at a deeper crustal level; it rules out a hypothesis that both provinces are separate amalgamated terranes. Furthermore, there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former. The exposure of the granulite facies metapelites of the Southern Marginal Zone at the same level as the lower grade metamorphic rocks of the Kaapvaal Craton indicates uplift along a sharp tectonic boundary. (C) 2000 Elsevier Science B.V. All rights reserved.