Continental growth, preservation, and modification in Southern Africa

To understand the origin, modification, and preservation of continents on Earth, a multidisciplinary study is examining the crust and upper mantle of southern Africa. Xenoliths of the mantle brought to the surface by kimberlites show that the mantle beneath the Archean Kaapvaal craton is mostly melt-depleted peridotite with melt extraction accompanying crust formation in the Archean. Eclogitic xenoliths from the craton record subduction of altered oceanic crust beneath the craton at ca. 3 Ga. Proterozoic age peridotite found beneath the surrounding Proterozoic accretionary belts provides evidence for crust-mantle coupling and long-term stability of the upper 150 km of the lithosphere. Petrologic examination of Archean ultramafic magmas (komatiites) from South Africa indicates that some komatiitic magmas contain substantial quantities of water (>4 wt%). This finding strengthens the possibility that the cratonic lithosphere formed initially in a subduction zone setting, the demise of which led to accretion of the arc crust and thickening of the lithospheric mantle to create a stable, thick, continental lithosphere. Geochronologic studies of lower crustal xenoliths from the craton show a prolonged thermal evolution of the lower crust extending to 1 Ga. This thermal evolution is also reflected in ca. 1 Ga ages of some eclogitic diamond inclusions from the lithospheric mantle.