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

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 8Ma 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 (TDM) is significantly older than those obtained from zircons from numerous Neoarchaean granitoids of the Beit Bridge Complex, comprising the Singelele gneiss (2647 12Ma), the Bulai granite (2612 7Ma), the Regina gneiss (2649 9Ma) and two samples of the Zanzibar gneiss (2613 6Ma). These granitoids show initial eHf(t) values between þ 0 5 and 7 1, which correspond to initialTDM between 3 46 and 3 01Ga. These variable TDMinitial and eHf(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 9Ma) 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, uniformTDM Hf initial ages of 2 61^2 67 Ga obtained from a diorite gneiss (2061 6Ma) of the Mahalapye Complex indicate that its protolith may have been formed from remelting of a Neoarchaean juvenile crust. Variable eHf(t)initial values from 3 7 to þ6 3 of zircon cores (2711 11Ma) in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the Neoarchaean.