Kalahari Craton - Geology

Geology

Kalahari craton has formed a stable unit for the past 2.3 billion years (2.3 Ga). As such, it contains some of the oldest known rocks and microfossils in the world. The oldest rocks are in the Gauteng, Limpopo, Mpumalanga, and North West provinces, previously known as the Transvaal, that consists of granites, gneisses, and migmatites that are ~3.4 Ga. Within this granitic basement are a number of greenstone belts where the rocks have been less highly metamorphosed and contain many primary features. These rocks comprise the Swaziland system. They form a thick pile of volcanics and cherts, which pass up into turbidites and sandstones. The ancient basement became stabilised around 3.0 Ga, and was then covered by a thick sequence of shallow-water sediments, lavas, and igneous intrusions.

Additional crust was formed and reworked along all of the craton’s margins at 1.4–1.0 Ga. The Damaran-Lufilian-Zambezi Fold Belt separates the Congo craton to the north from the Kalahari. Central Zambia exhibits the largest number of Precambrian eclogite occurrences in Africa. These eclogites and the associated gabbros are interpreted as relics of a fossil subducted slab that marks a suture zone between the Congo craton to the north and the Kalahari.

The Zambezi Belt and the Lufilian Arc are part of the Pan-African orogenic system that crosscuts southern Africa, separating the Congo and Kalahari cratons and their respective Paleo- and Mesoproterozoic units. These Pan-African belts were formed during the assembly of the Gondwana supercontinent. Eclogite facies metamorphism occurred at ca. 600 Ma, whereas peak metamorphism during the subsequent continental collision occurred around 530 Ma ago, with metamorphic P–T conditions reaching the high pressure amphibolite facies. The Zambezi Belt crosscuts and thus divides the Meso-proterozoic parts of Zambia into the Irumide Belt and the Choma-Kalomo Block. The junction of the Neoproterozoic belts and the Mesoproterozoic parts is marked by a 200 km long and up to 40 km wide zone containing lenses of eclogite, metagabbro, gabbro and rare ultramafic rocks. The eclogite-bearing zone is located in the interior of the Zambezi Belt, and runs parallel to strike. The eclogites and associated mafic rocks form isolated hills of 10 to 100 metres in diameter.

Pan-African belts along the northern and eastern craton margins are associated in one area with a 1.4 Ga ophiolite/arc terrane. The orthogneisses form part of an extensive region north of the Kalahari craton and east of the Congo craton that includes large amounts of 1.15–1.0 Ga arc-type rocks containing juvenile crustal components. This region may represent one of the main convergent zones active during Rodinia assembly, although its original relations are obscured by intense Pan-African overprinting. Along the southern margin, the Namaqua–Natal–Maud belt underwent arc magmatism, terrane accretion, polyphase amphibolite- to granulite-grade contractional/transpressional deformation, and late-syntectonic granite intrusion at 1.38–1.0 Ga. A largely buried orogen along the western margin records amphibolite-grade deformation and granitoid plutonism at 1.35–1.2 Ga and is inferred to connect with the Namaqua belt to the south. 1.1–1.0 Ga granitoid orthogneisses within Widespread intraplate magmatism affected much of the Kalahari craton at 1.1 Ga and is inferred to record impact of a mantle plume inboard of the Namaqua–Natal–Maud belt.

The Limpopo Belt separates the Rhodesian province to the north from the Transvaal province to the south. The Limpopo Belt also joins the Zimbabwe craton to the north with the Kaapvaal craton to the south. The belt is composed of a granitic basement with narrow greenstone belts. Within the greenstone belts, the sequence consists of basic volcanics, covered by greywackes, shales, and conglomerates.