Kaapvaal Craton

The Kaapvaal Craton is an Archean cratonic block in southern Africa and one of the world's oldest well-preserved fragments of continental crust. Geochronological evidence indicates that it assembled episodically between about 3.5 and 2.5 billion years ago through magmatic-arc development, accretion and the amalgamation of several crustal domains.

Similarities between late-Archean successions on the Kaapvaal and Pilbara Cratons led to the hypothesis that they once formed the proposed Vaalbara supercontinent, although the reconstruction remains debated.

Description The Kaapvaal Craton covers an area of approximately and is joined to the Zimbabwe Craton to the north by the Limpopo Belt. To the south and west, the Kaapvaal Craton is flanked by Proterozoic orogens, and to the east by the Lebombo monocline that contains Jurassic igneous rocks associated with the break-up of Gondwana.

The exposed basement is a mosaic of granite–greenstone domains and older tonalitic gneisses. Crust-forming magmatism was episodic rather than uniform: different eastern, central, northern and western domains record distinct sequences of granitoid emplacement, greenstone-belt development and tectonic assembly. Much of the basement is concealed beneath younger Neoarchean and Paleoproterozoic volcanic and sedimentary successions.

Late Archean metamorphism joined the Southern Marginal Zone of the Kaapvaal Craton to the Northern Marginal Zone of the Zimbabwe Craton approximately 2.8–2.5 Ga by the wide orogenic Limpopo Belt. The belt is an east-northeast trending zone of granulite facies tectonites that separates the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons.

Post-Archean magmatism Stabilisation of the Archean crust did not end igneous activity. The craton was affected by several later events, including emplacement of the Bushveld Igneous Complex at about 2.05 billion years ago and Mesoproterozoic alkaline magmatism associated with the Pilanesberg Complex at about 1.4 billion years ago. The Pilanesberg Complex belongs to a wider province of smaller alkaline-silicate and carbonatite centres. Modern uranium–lead ages for many of its silicate rocks converge near 1.4 billion years, whereas the ages of some associated carbonatites are younger or less well constrained.

Limpopo Central Zone The crustal evolution of the Limpopo Central Zone can be summarised into three main periods: 3.2–2.9 Ga, 2.6 Ga, and 2.0 Ga. The first two periods are characterised by magmatic activity leading to the formation of Archaean Tonalite-Trondhjemite-Granodiorite (TTG) such as the Sand River Gneisses and the Bulai Granite intrusion. Early Proterozoic high-grade metamorphic conditions produced partial melting that formed large amounts of granitic melt.Chavagnac, V., Kramers, J. D. and Naegler, T. F. (1999) "Can we Still Trust Nd Model Ages on Migmatized Proterozoic Rocks?", Early Evolution of the Continental Crust, Journal of Conference Abstracts, 4 (1), A08:4A:13:G2.

There is no indication that the Neoarchean to early Paleoproterozoic successions on the craton were sourced from the 2.65–2.70 Ga orogenic event preserved in the Limpopo Metamorphic Complex. However, younger late-Paleoproterozoic red bed successions contain zircons of this time interval as well as many ~2.0 Ga detrital zircons. This implies that the Limpopo Complex together with the Zimbabwe Craton only became attached to the Kaapvaal Craton at approximately 2.0 Ga during formation of the Magondi Mobile Belt which in turn sourced the voluminous late Paleoproterozoic red beds of southern Africa.Beukes, N.J., Dorland, H.C., Gutzmer, J., Evans, D.A.D. and Armstrong, R.A. (2004) "Timing and Provenance of Neoarchean-Paleoproterozoic Unconformity Bounded Sequences on the Kaapval Craton" , Geological Society of America Abstracts with Programs, 36 (5), 255. Evidence of the horizontal layering and riverine erosion can be found throughout the Waterberg Massif within the Limpopo Central Zone.

Barberton greenstone belt

The Barberton greenstone belt, also known as the Makhonjwa Mountains, is situated on the eastern edge of Kaapvaal Craton. It is well known for its gold mineralisation and for its Komatiites, an unusual type of ultramafic volcanic rock named after the Komati River that flows through the belt. Some of the oldest exposed rocks on Earth (greater than 3.6 Ga) are located in the Barberton greenstone belt of the Eswatini–Barberton areas and these contain some of the oldest traces of life on earth.

Johannesburg Dome The Archaean Johannesburg Dome is located in the central part of the Kaapvaal Craton and consists of trondhjemitic and tonalitic granitic rocks intruded into mafic-ultramafic greenstone.Nguuri, T.K. et al. (2001) “Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons”, "Geophysical Research Letters", 28, 2501–2504. Uranium–lead dating of single zircon grains identifies an early granitoid phase at 3340 ± 3 Ma and a later granodioritic phase at 3121 ± 5 Ma. Together with ages from elsewhere on the craton, these results support long-lived, episodic crustal growth, with major granitoid domains generally becoming younger toward the north and west.Poujol, M. and Anhaeusser, C. R. (2001). “The Johannesburg Dome, South Africa: new single-zircon U–Pb isotopic evidence for early Archaean granite–greenstone development within the central Kaapvaal Craton”. Precambrian Research. 108 (1–2): 139–157. .

Kaapvaal Craton

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