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Original Article |
1 1Department of Earth Sciences, La Trobe University, Bundoora, Vic. 3086, Australia (e-mail: I.Buick@latrobe.edu.au)
2 2Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 0200, Australia
3 3School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
4 4School of Geosciences, Monash University, Clayton, Vic. 3800, Australia
5 5Department of Geological Sciences, University of Cape Town, Rondebosch 7700, Cape Town, South Africa
Supposedly mid-Archaean (c. 3.3 Ga) granulite-grade marbles from the Gumbu Group, Central Zone, Limpopo Belt (South Africa) have anomalously high
13C values (+4.6 to +7
) interpreted as minimum values for their limestone protoliths. Such elevated 13C values are unknown in any Archaean carbonate worldwide, but occur in Palaeoproterozoic (c. 2.4–2.1 Ga) carbonates; for example, on the adjacent Zimbabwe and Kaapvaal cratons. This suggests that the Central Zone contains a previously unrecognized Palaeoproterozoic cover sequence, contrary to the prevailing view. This conclusion is supported by zircon U–Pb sensitive high-resolution ion microprobe ages from two metasediments, which, in addition to c. 2.02 Ga metamorphic zircon overgrowths, contain detrital zircons no older than c. 2.68 Ga, and some possibly as young as c. 2.2 Ga. The recognition of post-3.3 Ga metasediments in the Central Zone requires a major revision of models of the tectonic evolution of the Limpopo Belt.
Keywords: Precambrian, Limpopo Belt, carbon isotopes, age.
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