Carbon and U-Pb evidence for a Palaeoproterozoic crustal component in the Central Zone of the Limpopo Belt, 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 delta(13)C values (+4.6 to +7%o) interpreted as minimum values for their limestone protoliths. Such elevated delta(13)C 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.