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 i n South Africa, metapelites and leucocratic granitoids from two different p rovinces, the,granulite facies Southern Marginal Zone of the Limpopo Belt a nd the northern part of the adjacent,greenschist to amphibolite facies Kaap vaal Craton, have been studied. Both provinces resemble each other in terms of isotope and trace element characteristics. They show a general depletio n of incompatible elements independent from the grade of metamorphism, This observation conflicts with theories involving mobilisation during granulit e facies metamorphism. A further common fingerprint is the unusually high C r 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 crust al age of their cratonic source region. The,granitoid gneisses show somewha t larger variations of Nd model ages indicating some age heterogenity withi n 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 oth er Southern African provinces. These results support the view that the Sout hern Marginal Zone represents a part of the Kaapvaal Craton exposed at a de eper crustal level; it rules out a hypothesis that both provinces are separ ate amalgamated terranes. Furthermore, there is very little difference betw een granitoids and metapelites which indicates that the latter largely deri ved from the former. The exposure of the granulite facies metapelites of th e Southern Marginal Zone at the same level as the lower grade metamorphic r ocks of the Kaapvaal Craton indicates uplift along a sharp tectonic boundar y. (C) 2000 Elsevier Science B.V. All rights reserved.