Ultramafic rocks in the centre of the Vredefort structure (South Africa): Possible exposure of the upper mantle?

Abstract

The rocks in the central region of the Vredefort structure are interpreted as lower-crust that has undergone uplift of > 36 km during the Vredefort cryptoexplosion event 2.0 Ga ago. The outcrops in this central region are mainly high-grade leucogranulites interbanded with mafic granulites and supracrustals. Rocks from a borehole located near the centre of the structure indicate that the core region is underlain by ultramafic rocks which probably represent the upper mantle beneath this region of the Kaapvaal craton. The main lithologies sampled by the borehole are serpentinized hornblende-bearing harzburgite with minor bands of pyroxenite and glimmerite. The major- and trace-element chemistry of these ultramafic rocks is similar to that of fertile oceanic lithosphere. Mineral chemistry and petrographic data suggest that the Vredefort ultramafics are metamorphic rocks which have undergone a complex tectonic and metamorphic history. SmNd and RbSr isotopic data (together with the field relationships) provide some evidence that these rocks are Archaean but also that they have been chemically and isotopically altered. Similar hydrated and chemically altered ultramafic rocks occur in the Jamestown ophiolite complex in the Barberton greenstone belt. Accordingly, we speculate that the crust-mantle transition zone in the Kaapvaal craton consists of a zone of hydrated Archaean oceanic lithosphere.