Zimbabwe craton

The Zimbabwe craton is an example of Early Archaean lithology dating back to 3.5 Gigaannum Ga. in the southern African nation of Zimbabwe^[1]. 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.

Ca. 3.4 Tokwe Segment

This is the larger of two Early Archaean continental crustal fragments that stabilised in Zimbabwe before 3.3 Ga, extending for over 450 km between Shurugwi, Zvishavane and Masvingo. The much smaller northern fragment is known as the Rhodesdale Gneiss ^[2]. 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. This crustal block, also known as the Sebakwe Proto-craton, was fully stabilised by 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. ^[3]. 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. The synchroneity and extent of the Tokwe Segment 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 ^[4].

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.

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 ^[5].

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.

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 ^[6].

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. 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 ^[7].

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.

See also

• Kaapvaal craton
• Limpopo Belt

References

1. ^ Wilson, J.F., Nesbitt, R.W., Fanning, C.M., 1995. Zircon geochronology of Archaean felsic sequences in the Zimbabwe craton: a revision of greenstone stratigraphy and a model for crustal growth. In: Coward, M.P., Ries, A.C. (Eds.), Early Precambrian processes, vol. 95. Geological Society Special Publications, pp. 109–126.
2. ^ Dirks, P.H.G.M. and Jesma, H.A. 2002. Crust–mantle decoupling and the growth of the Archaean Zimbabwe craton. Journal of African Earth Sciences, 34, 157–166
3. ^ 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. [1]
4. ^ Blenkinsop, T.G., Martin, A., Jelsma, H.A., Vinyu, M.L., 1997. The Zimbabwe Craton. In: de Wit, M.J., Ashwal, L.D. (Eds.), Greenstone Belts. Oxford University Press, Oxford, pp. 567–580
5. ^ 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]
6. ^ Bickle, Michael James, Hazel Joan Chapman, Mary R. Fowler, Nathalie Grassineau, Morag Hunter, Euan George Nisbet, and Tony Martin. (1999) "Geochemistry of the Early Oceans." Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust. Online: [3]
7. ^ Kreissig, Katharina and Andreas Schmidt. (1999) "Crustal Evolution in the SE-Zimbabwe Craton." [4]