Zimbabwe craton

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The Zimbabwe craton is an example of Early Archaean lithology dating back to 3.46 Gigaannum (Ga.) in the southern African nation of Zimbabwe. Late Archean metamorphism joined the Southern Marginal Zone of the Kaapvaal craton to the Northern Marginal Zone of the Zimbabwe craton ca. 2.8-2.5 Ga. The 250 km wide Limpopo belt is an east-northeast trending zone of granulite facies tectonites that separates the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons.

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[edit] Ca. 3.4 Sebakwe proto-craton

According to a research paper by Horstwood et al. (1999): A single Early Archaean continental crustal segment existed in Zimbabwe, extending for over 450 km from the south-central Tokwe region, through the Midlands and up to the northern Shamva region of the present day craton. This ca. 3.4 crustal block, the Sebakwe Proto-craton, stabilised around 3.35 Ga with a later granitoid emplacement event around 3.2 Ga and provided the basement for the 3.0-2.6 Ga Late Archaean granite-greenstone magmatism. This second stage of evolution therefore resulted in the present predominance of these rocks, and occurred ca. 400 Ma after the initial stabilisation of the craton.

U-Pb dating single zircon data provide the oldest and most precise dates so far obtained from the Zimbabwe craton and indicate a much more widespread outcrop of Early Archaean lithologies. Granitoids of the same and greater age as the 3.46 Ga Tokwe granitoid gneisses in the south central portion of the Zimbabwe craton are recognised in the central Midlands region, strongly suggesting synchronous formation of the two areas. An undeformed and relatively unmetamorphosed leucosome of the Tokwe gneisses constrains the timing of deformation and metamorphism in this region at c.3.35 Ga. This coincides with published detrital zircon and granitoid data which indicate the abundance of this phase in the south-central region. Other zircon results indicate ca. 3.2 Ga granitoids in the northern, Shamva region of Zimbabwe, a phase also recognised around Tokwe. The synchroneity and extent of the Sebakwe Proto-craton is considered strong evidence supporting a predominantly intra-cratonic origin for the Late Archaean greenstone belts of Zimbabwe and refuting an arc accretion origin for the craton.

[edit] Ca. 3.46 Tokwe granitoid gneisses

The Tokwe ca. 3.46 Ga granitoid gneisses in the central and south central portion of the Zimbabwe craton coincide with a single geochronological event. Zircon crystal dating results indicate ca. 3.2 Ga granitoids in the central Midlands region and the northern Shamva region and suggest that "a single Early Archaean continental crustal segment [prolith] existed in Zimbabwe, extending for over 450 km from the south-central Tokwe region, through the Midlands and up to the northern Shamva region of the present day craton. This ca. 3.4 Ga crustal block, which we propose to be called the Sebakwe Proto-craton, stabilised around 3.35 Ga with a later granitoid emplacement event around 3.2 Ga and provided the basement for the 3.0-2.6 Ga Late Archaean granite-greenstone magmatism." (Horstwood et al., 1999).

[edit] Ca. 3.2-2.9 Ga Limpopo Central Zone

The 250 km wide Limpopo belt of southern Africa is an east-northeast trending zone of granulite facies tectonites separating the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons. Large scale ductile shear zones are an integral part of Limpopo belt architecture. They define the boundaries between the belt and the adjacent cratons and separate internal zones within the belt. The shear zones forming the external (northern, southern and western) margins of the belt are interpreted as uplift structures of the overthickened crust.

According to Chavagnac et al. (1999): The crustal evolution of the Limpopo Central Zone can be summarized into three main periods: 3.2-2.9 Ga, ~2.6 Ga, and ~2.0 Ga. The two first periods are mainly characterized by magmatic activity leading to the formation of Archaean Tonalite-Trondhjemite-Granodiorite (TTG) such as the Sand River Gneisses or the Bulai Granite intrusion. The Early Proterozoic event took place under high-grade metamorphic conditions during which partial melting formed large amount of granitic melt.

According to a study by Holzer et al. (1999): The Limpopo Central Zone shows relics of late Archean high grade metamorphism. In the Northern (NMZ) and Southern Marginal Zones (SMZ) that adjoin the Zimbabwe and Kaapvaal cratons, respectively, the last high grade metamorphic episodes were late Archean. The relics in the Central Zone are characterised by an anticlockwise p-T-evolution, at ca. 2.55 Ga. In the NMZ repeated crustal remelting and intrusion of charnoenderbitic magmas continued to 2.58 Ga, producing counterclockwise p-T paths. In contrast the SMZ consists of medium to high pressure granulites, which underwent a clockwise p-T-evolution at ca. 2.69 Ga, followed by decompression and isobaric cooling. In the Northern Kaapvaal Craton (Renosterkoppies Greenstone Belt, Pietersburg area) tectonism took place under amphibolite facies conditions at ca. 2.75 Ga and can therefore not be related to any events in the Limpopo belt. Thus the different tectonic units have different late Archean tectonometamorphic histories. Trace element geochemistry as well as Pb + Nd isotope characteristics of the SMZ are similar to those of the Kaapvaal Craton, with low Th and U concentrations around 2 and 0.7 ppm. Low U concentrations in the SMZ are not a consequence of high grade metamorphism. The NMZ, with high Th, U concentrations (10.8 and 2.5 ppm) and radiogenic Pb, resembles the adjoining Zimbabwe craton more. The differences in late Archean tectonic styles between NMZ and SMZ+KC is a possible consequence of the differences in Th and U content of these provinces.

"The Mahalapye granite in the extreme western part of the Central Zone is a post-tectonic intrusion that crystallized at 2.023 Ga. This suggests that mineral ages of ca 2.0 Ga from the eastern part of the Central Zone date metamorphism during reworking of Archaean age shear zones rather than a collision between the Kaapvaal and Zimbabwe cratons as has been earlier suggested (Armstrong and McCourt, 1999).

[edit] Ca. 2.7 Ngezi Group

The Ngezi Group, ca. 2.7 Ga, of the Bulawayan Supergroup is found throughout the Zimbabwean Province. In the Belingwe Greenstone Belt of south central Zimbabwe, the Ngezi Group has been uplifted and eroded. Sedimentation and volcanism probably occurred here as a result of intracontinental extension associated with late Archaean plume.

According to a study by Bickle et al. (1999): The Ngezi Group consists of a basal sedimentary sequence (Manjeri Formation), overlain by komatiitic and tholeiitic volcanic rocks (Reliance and Zeedebergs Formations), and a second sedimentary sequence (Cheshire Formation). The late Archaean (~2.7 Ga) Ngezi Group greenstones in the Belingwe Greenstone Belt, south central Zimbabwe, show uplift and increased erosion during deposition of alluvial fans, local derivation of sedimentary material containing no arc derived component and eruption of komatiitic lava. Sedimentation and volcanism probably occurred as a result of intracontinental extension associated with an active, late Archaean plume.

The Manjeri Formation was deposited in a fluviatile and shallow marine setting, with subsequent alluvial fans and fan-deltas during active tectonism. Changes in the degree of chemical weathering of the provenance area during deposition of the formation, as measured by CIA values, reflect uplift and increased rates of erosion. Detrital mineralogy and rare-earth element patterns, are consistent with derivation from very local sources. Palæogeographic variation in the measured Sm-Nd depleted-mantle model ages between 2.9 and 3.7 Ga are consistent with deposition over basement varying in age from ca. 2.9 Ga - 3.5 Ga, again suggesting local derivation. The facies and geochemical association imply sedimentation in an extensional continental setting.

In a study by Bickel et al. (1999a): Samples by the NERCMAR drill hole through the 2.7 Ga Manjeri Formation in the Belingwe Greenstone Belt compared to data on the metamorphosed and deformed iron formations from the 3.7 Ga Isua Greenstone belt. Carbon and sulphur isotopic fractionations in the Belingwe samples may be interpreted in terms of a complex bacteria/archaea eclogical community. REE and Nd-isotopic variations may be modelled by contributions from a reduced hydrothermal component and a component surprisingly similar in REE pattern to modern seawater. Isua rocks are less well preserved but the overall similarity of the REE compositions implies deposition from a broadly similar ocean to that in the late Archaean

[edit] Ca. 2.5 Ga Wedza and Chilimanzi suites, south-eastern Zimbabwe craton

The final stage of the cratonization process of the Zimbabwe craton is marked by emplacement of large volumes of monzogranitic and granodioritc material at 2.6-2.4 Ga. According to a paper by Kreissig and Schmidt (1999): Monzogranitic and granodioritic granitoids and gneisses of the Wedza and Chilimanzi suites form large intrusive complexes in the south-eastern part of the Zimbabwe craton, between the Mutare-Odzi greenstone belt and the Northern Marginal Zone of the Limpopo Belt. The older units of the Wedza suite are of syn-kinematic origin, while the younger Chilimanzi suite was emplaced late- to post-kinematically. Internal differentiation of the major elements indicates similar paths of magmatic evolution for both suites. Comparison with the major element distribution of older granodioritic/tonalitic intrusives within the Mutare greenstone belt suggests an overall magmatic evolution from primitive, tonalitic towards monzogranitic compositions.

The model ages mark a discrete and well established Archean crust forming event in Africa. Small observed variations may indicate minor contribution of juvenile crust. U/Pb isotopic multigrain analyses of distinctly zoned zircons revealed highly discordant ages of 2.507-2.585 Ga for the Wedza and 2.402-2.448 Ga for the Chilimanzi suite. These ages confirm the intrusion age of about 2.6 Ga for the Wedza suite. However, the age of 2.4 Ga for the posttectonic Chilimanzi suite conflicts with the timing of the Great Dyke emplacement into an already consolidated crust. The geochemical and radiometric investigations suggest a dynamic crust forming process initialised at ca. 3.2-2.9 Ga with the formation of a crustal protolith. The final stage of this process is marked by emplacement of large volumes of monzogranitic/granodioritc material at 2.6 to 2.4 Ga.

[edit] Southern Witwatersrand Basin

The Witwatersrand is called the "Ridge of White Waters" because of the numerous world-famous waterfalls that occur along an east-west trending ridge. A stable basement is exposed to the north, called the Johannesburg Granite Dome. It is made up of predominantly medium to coarse-grained granite-gneiss dated over 3.1 Ga, and 3.5 Ga greenstone amphibolite and serpentinite. The Witwatersrand sequence is divided into two divisions, the lower dominantly marine, shale-rich West Rand Group and the upper dominantly fluvial, sandstone-rich Central Rand Group. Sedimentary deposition is about 7 km thick and ended around 2.7 Ga when widespread faulting resulted in extensive lavas of the Ventersdorp Supergroup being poured out over the landscape, covering the entire Witwatersrand basin.

The sediments were derived from ancient mountains to the north and north-west of a shallow inland sea. These mountains were made up largely of various quartz-rich granitic rocks, and earlier mafic and ultramafic rocks representing primitive crust. During this long time-span the nature of the depositional basin changed by tectonic evolution from a zone of thermal subsidence to an active foreland basin, as the Zimbabwe craton moved south. Quartz pebble conglomerates were formed from alluvial river gravels that were transported into the basin by numerous large rivers around 2.7 Ga and were reworked along the shore line. These conglomerates are generally thin, channelised and well sorted.

The large amounts of gold and uranium were probably deposited into the basin from the northern ancient mountains with early fluvial gravel deposits. These minerals are thought to have been concentrated and reworked to form the major goldfields of the Witwatersrand basin. However, an opposing view suggests that tectonic hydrothermal replacement was responsible for the gold deposits. The 2.0 Ga Vredefort meteorite impact resulted in down-warping of much of the basin, preserving the gold reefs from erosion.

[edit] Economic Geology

Nearly half of all gold ever mined has come from the Witwatersrand Basin area first found near Johannesburg in 1886. Over 48,000 tons of gold have been produced from seven major goldfields distributed over the 350 km long depositional basin. Uranium has also been an important product.

[edit] See also

[edit] References

  • Armstrong, Richard and Stephen McCourt. (1999) "SHRIMP U-Pb Zircon Geochronology of Granites from the Western Part of the Limpopo Belt Southern Africa: Implications for the Age of the Limpopo Orogeny." [1]
  • Bickle, Mike, Hazel Chapman, Morag Hunter and Euan Nisbet. (1999) "Continental Extensional Setting for the Archaean Belingwe Greenstone Belt, Zimbabwe." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust. Online: [2]
  • Bickle, Michael James, Hazel Joan Chapman, Mary R. Fowler, Nathalie Grassineau, Morag Hunter, Euan George Nisbet, and Tony Martin. (1999a) "Geochemistry of the Early Oceans." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust. Online: [3]
  • Chavagnac, Valerie, Jan D. Kramers, and Thomas F. Naegler. (1999) "Can we Still Trust Nd Model Ages on Migmatized Proterozoic Rocks?" Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust. Online: [4]
  • Holzer, Lorenz, Jan D. Kramers, Katharina Kreissig and Mathis Passeraub. (1999) "The Two Marginal Zones of the Limpopo Belt Show Contrasting Styles of Archean Tectonometamorphism." [5]
  • Horstwood, Matthew S.A., Robert W Nesbitt, Stephen N Noble, and James F. Wilson. (1999) "The Sebakwe Proto-Craton: An Extensive Early Archaean Crustal Block in Zimbabwe - A Reassessment of Craton Formation, Stabilisation and Growth." Journal of Conference Abstracts, Vol. 4, No.1, Symposium A08, Early Evolution of the Continental Crust. [6]
  • Kreissig, Katharina and Andreas Schmidt. (1999) "Crustal Evolution in the SE-Zimbabwe Craton." [7]