Archaean to Proterozoic Crustal Evolution in the Central Zone of the Limpopo Belt (South Africa-Botswana): Constraints from Combined U-Pb and Lu-Hf Isotope Analyses of Zircon

Abstract

A combined set of U–Pb and Lu–Hf in situ laser ablation ICP-(MC)-MS zircon analyses were obtained from orthogneisses and granitoids in the Central Zone of the Limpopo Belt, which comprises the Beit Bridge and Mahalapye complexes. The results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events, even if the distinct zircon domains underwent multiple Pb loss and the texture–age relationships, as obtained by cathodoluminescence images and U–Pb analyses, are ambiguous. Furthermore, the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination. The combined U–Pb and Lu–Hf data reveal that the Sand River gneiss suite of the Beit Bridge Complex was emplaced at 3283 ± 8 Ma and formed from melting of an older Archaean crust, which was derived from a depleted mantle source at around 3·65 Ga. The hafnium model age (TDMHf) is significantly older than those obtained from zircons from numerous Neoarchaean granitoids of the Beit Bridge Complex, comprising the Singelele gneiss (2647 ± 12 Ma), the Bulai granite (2612 ± 7 Ma), the Regina gneiss (2649 ± 9 Ma) and two samples of the Zanzibar gneiss (2613 ± 6 Ma). These granitoids show initial εHf(t) values between + 0·5 and −7·1, which correspond to initial TDMHf between 3·46 and 3·01 Ga. These variable TDMHfinitial and εHf(t)initial values are interpreted to be the result of different mixtures of reworked 3·65 Ga Palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the Neoarchaean at ∼2·65 Ga. This conclusion is supported by results obtained from the Mahalapye Complex, which was affected by migmatization and granite intrusions during the Palaeoproterozoic at 2·02–2·06 Ga. The Mokgware granite (2019 ± 9 Ma) contains zircon xenocrysts with Pb–Pb ages of 2·52–2·65 Ga and 2·93 Ga and hafnium model ages of 3·0–3·4 Ga, indicating that this granite is derived from remelting of Archaean crust. In contrast, uniform TDMHfinitial ages of 2·61–2·67 Ga obtained from a diorite gneiss (2061 ± 6 Ma) of the Mahalapye Complex indicate that its protolith may have been formed from remelting of a Neoarchaean juvenile crust. Variable εHf(t)initial values from −3·7 to +6·3 of zircon cores (2711 ± 11 Ma) in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the Neoarchaean.