Kaapvaal Craton

The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Ga (billion years ago) crust on Earth. Similarities of rock records from both these cratons, especially of the overlying late Archean sequences, suggest that they were once part of the Vaalbara supercontinent.[1]

This map shows the outlines of the southern African nations of Namibia, Botswana, Zimbabwe and South Africa. Kaapvaal's outline is superimposed on the countries.
The bold outline shows the parameters of the present-day Kaapvaal Craton.

Description

The Kaapvaal Craton covers an area of approximately 1,200,000 km2 (460,000 sq mi) 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.[2]

The Kaapvaal Craton formed and stabilised between 3.7 and 2.6 Ga[2] by the emplacement of major granitoid batholiths that thickened and stabilised the continental crust during the early stages of an arc-related magmatism and sedimentation cycle. The craton is a mixture of early Archean (3.0–3.5 Ga) granite greenstone terranes and older tonalitic gneisses (ca. 3.6–3.7 Ga), intruded by a variety of granitic plutons (3.3–3.0 Ga). Subsequent evolution of the Kaapvaal Craton (3.0–2.7 Ga) is thought to be associated with continent–arc collision that caused an overlaying succession of basins filled with thick sequences of both volcanic and sedimentary rocks. This was then followed by episodic extension and rifting when the Gaborone–Kanye and Ventersdorp sequences were developed. Early Archean crust is well exposed only on the east side of the craton and comprises a collage of subdomains and crustal blocks characterised by distinctive igneous rocks and deformations.

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 250 kilometres (160 mi) 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.

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.[3]

There is no indication that the Neoarchean to early Paleoproterozoic succession 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.[4] Evidence of the horizontal layering and riverine erosion can be found throughout the Waterberg Massif within the Limpopo Central Zone.

Barberton greenstone belt
Main article: 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. Only the rocks found in the Isua Greenstone Belt of Western Greenland are older.

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.[2] Studies using U-Pb single zircon dating for granitoid samples yield an age of 3340 +/- 3 Ma and represents the oldest granitoid phase recognised so far. "Following the trondhjemite-tonalite gneiss emplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses (3340–3200 Ma) and later, crosscutting, potassic granitoids.

These rocks consisting mainly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. The southern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodiorites dated at 3121 +/- 5 Ma....The data, combined with that from other parts of the Kaapvaal craton, further supports the view that the evolution of the craton was long-lived and episodic, and that it grew by accretionary processes, becoming generally younger to the north and west of the ca. 3.5 Ga Barberton- Eswatini granite-greenstone terrane situated in the southeastern part of the craton."[5]

See also

References
  1. Zegers, T.E., de Wit, M.J., Dann, J. and White, S.H. (1998) "Vaalbara, Earth's oldest assembled continent? A combined, structural, geochronological, and palaeomagnetic test", Terra Nova, 10, 250–259.
  2. 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.
  3. 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.
  4. 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" Archived 13 November 2007 at the Wayback Machine, Geological Society of America Abstracts with Programs, 36 (5), 255.
  5. 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, doi:10.1016/S0301-9268(00)00161-3.

Bibliography
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