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Single zircon ages for granitoid gneisses in the Central Zone of the Limpopo Belt, Southern Africa and geodynamic significance

Abstract:

Ion microprobe, evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the Central Zone (CZ) of the Limpopo mobile belt, South Africa define three broad age groups, namely at 3188-3314, 2637-2734 and ca 2500 Ma, respectively. All these granitoids, including the youngest group, display polydeformational features suggesting that much of the tectonic history of the CZ occurred in the late Archaean and early Proterozoic. The oldest group of predominantly tonalitic-trondhjemitic-granodioritic gneisses was found in relatively small domains from the extreme W to the E of the CZ and probably represents remnants of a once more widespread early Archaean terrain. The most abundant granitoid gneisses belong to the 2.6-2.7 Ga age group and were emplaced into already ductilely deformed gneisses of the oldest group. Several of these rocks were derived from intracrustal melting. Rare metamorphic zircons with ages of ~2560-2575 Ma reflect a high-grade metamorphic event in the CZ that has also been postulated by other workers. A strongly deformed orthogneiss with a protolith emplacement age of ~2510 Ma proves beyond doubt that at least part of the structural history in the CZ occurred in the early Proterozoic. An anatectic granite in the extreme W of the CZ intruded at ~2022 Ma and is probably related to the peak of granulite-facies metamorphism previously dated at 2027±6 Ma in the Messina area. The second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite-facies metamorphism in the Northern and Southern Marginal Zones of the Limpopo belt. Broad contemporaneity of extensive granitoid magmatism in all three Zones raises doubts about the exotic nature of the CZ and suggests a revised evolutionary model for the Limpopo belt.