Nature of the archean midcrust in the core of the Vredefort dome, Central Kaapvaal Craton, South Africa

Extreme uplift associated with the formation of the 2.02 Ga Vredefort dome has exposed a substantial cross section through the crystalline early Archean basement complex rocks of the Kaapvaal craton. The rocks comprise polydeformed high-grade tonalite-trondhjemite-granodiorite (TTG) gneisses, migmatites and late-tectonic intrusive granitoids that straddle the upper amphibolite-to granulite-facies transition. Field, petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal-scale layering as has been previously proposed. No evidence has been found to support exposure of either a melt-depleted, refractory, lower crust or an upper crustal batholithic granite layer; however, the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones. Granitic leucosomes in the upper amphibolite-facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks, rather than of in situ derivation. Leucosome compositions in the granulite-facies migmatites are more variable, ranging from granitic and charnockitic to enderbitic, probably reflecting at least some local derivation. Leucosomes and small granitoid bodies show local-scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration, and fractionation of minerals such as K-feldspar within the melts.