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The Kalahari craton is a craton, an old and stable part of the continental lithosphere with thick crust and deep lithospheric roots extending up to a few hundred kilometers into the Earth's mantle, that occupies a large portion of South Africa and consists of the Kaapvaal, the Zimbabwe craton, the Limpopo belt, and the Namaqua Belt.

Contents

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. To the north, the Damara-Lufilian-Zambezi Fold Belt separates the Congo craton to the north with 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[1].

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 Limppoo 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.

Kalahari craton with Laurentia, Rodinia and Gondwana

Early paleomagnetic studies showed that the Umkondo sills in eastern Zimbabwe correlated with similar mafic intrusions in Botswana and South Africa. It was therefore suggested that the Umkondo igneous province is present over a large part of the Kalahari craton, and also a detached fragment now located in East Antarctica. New paleomagnetic data are in excellent agreement with this. The great majority of the sites have the same polarity suggesting that the dolerites were emplaced during a limited time span, consistent with the geochronology. The pole position places the Kalahari craton off southeast Laurentia within the Rodinia supercontinent. In Laurentia, there is widespread within-plate magmatism coeval with the Umkondo event, raising the possibility that the two igneous provinces were linked within Rodinia[2].

The Kalahari craton probably converged with southwestern Laurentia between 1060 and 1030 Ma to become part of the Rodinia supercontinent by 1000 Ma. In Rodinia, the Kalahari craton lay near East Antarctica with the Namaqua–Natal orogenic belt facing outboard and away from the Laurentian craton[3]. The Kalahari and Congo cratons probably collided during the assembly of Rodinia, however they could have also been just juxtaposed during the assembly of Gondwana (550-500 Ma) at the end of the Neoproterozoic. In the former case, the transcontinental, ~550 Ma Damaran-Lufilian-Zambezi orogen separating the two cratons would represent a Himalayan-style collisional belt formed by the consumption of a wide ocean. In the later case it may have been a closure of narrow Neoproterozoic basins that developed across previously assembled parts of Rodinia[4]. A third view is that there was a convergence between the Kalahari craton and a composite Congo-Laurentia craton during the assembly of Rodinia, generating the Kibaran-Grenvillian-Llano belts. This orogenic belt system extends for over 3000 km and is over 400 km wide in Africa and is the result of the convergence of Paleoproterozoic/Archaean cratonic blocks forming the Congo craton to the north and a mosaic including the Kalahari, Bangweulu, Tanzania and West-Nilian cratons (hereafter called the Kalahari craton) to the south[5].

The arc on the flank of Madagascar collided with the African margin at ~640 Ma, and remained attached to the African margin when Madagascar rifted during Gondwana breakup. This ~640 event has been considered to date the collision of India, Madagascar, parts of eastern Antarctica and Kalahari cratons (IMSLEK terranes) with the Congo-Tanzania Craton and Arabian Nubian Shield[6].

Economic Geology

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Metals

The Witwatersrand system is the world's most productive source of gold. Gold ores occur in fracture systems in the greenstone belts. The Transvaal system contains important iron ores, and the Bushveld igneous complex contains platinum, chromium, titanium-iron, and tin ores. Manganese of high-grade is mined in the large Kalahari manganese field from hydro-thermal enrichments in stratabound deposits that formed approximately 2.1 Ga.[7][8]

Diamonds

Diamond mining from the Kalahari craton has been extensive. Based on current annual output the Southern African Kalahari craton (approximately 48 million carats (9,600 kg)) produces more than the combined Central African cratons (37 million carats (7400 kg)) and the West African craton (2.1 million carats (420 kg)). The difference is even more pronounced in value terms where the Kalahari craton (approximately US$5.1 billion) is clearly ahead of the Central African (US$2.2 billion) and West African cratons (US$0.34 billion). The dominance of the Southern African Craton is largely due to the excellent infrastructure, favourable climate and stable political and socio-economic regime, which have ensured continuous exploration and mining activities for the past century [9]. The Kasai craton extends over the border into Angola where 77 diamond kimberlites have been discovered. Eleven others have been found in the less explored DRC section of the craton[10].

See also

References

  1. ^ * Timm, John and Volkar Schenk. (2003) “Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration.” Contrib Mineral Petrol (2003) 146: 174–191[1]
  2. ^ Blenkinsop, T., S. Bowring, J. Crowley, I. Dalziell, W. Gose, R. Hanson, J. Mukwakwami, J. Pancake, and J. Ramezani. (2003) “New Paleomagnetic and geochronical Data from the Meso-Proterozoic Data Umkondo Dolerites, South Africa.” Geophysical Research Abstracts, Vol. 5, 07201[2]
  3. ^ Jone, D.L., S. Pisarevsky, C. McA. Powell, and M.T.D. Wingate. “Paleomagnetic Constraints on the Position of the Kalahari Craton in Rodinia.”[3]
  4. ^ Hanson, R.E. (2004) “Mesoproterozoic Tectonic Evolution of the Kalahari Craton: Implications for Rodinia Reconstructions.” Geological Society of America , 2003 Seattle Annual Meeting, Paper No. 110-4[4]
  5. ^ Deschamps, Y., A.B. Kampunzu, and J.P. Milesi. (2003) “Africa within Rodinia Supercontinent: Evidence from the Kibaran Orogenic System.” Geological Society of America, 2003 Seattle Annual Meeting, Paper No. 110-3[5]
  6. ^ Cutten, Huntley M.C. (2006) “Ph.D. Mozambique Belt
  7. ^ Armbruster, Thomas, Edwin Gnos and Igor M. Villa. (2003) “Norrishite, K(Mn23+Li)Si4O10(O) 2,an oxymica associated with sugilite from the Wessels Mine, South Africa: Crystal chemistry and 40Ar-39Ar dating.” American Mineralogist, Volume 88, pages 189-194[6]
  8. ^ Beukes, N.J., D.A.D. Evans, J. Gutzmer, and J.L. Kirschvink. (2001) “Paleomagnetic Constraints on Ages of Mineralization in the Kalahari Manganese Field, South Africa.” Economic Geology, Vol. 96, 2001, pp. 621-631[7]
  9. ^ MSA Geoservices (2006) “Africa’s Diamond Exploration”
  10. ^ Gravity Diamond Limited. (2006) "Discovery and Development from Ground-Breaking Exploration Technology"[8]

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