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 AE 8 Ma and formed from melting of an older Archaean crust, which was derived from a depleted mantle source at around 3Á65 Ga. The hafnium model age (T DM Hf) is significantly older than those obtained from zircons from numerous Neoarchaean granitoids of the Beit Bridge Complex, comprising the Singelele gneiss (2647 AE12 Ma), the Bulai granite (2612 AE 7 Ma), the Regina gneiss (2649 AE 9 Ma) and two samples of the Zanzibar gneiss (2613 AE 6 Ma). These granitoids show initial eHf(t) values between þ 0Á5 and À7Á1, which correspond to initial T DM Hf between 3Á46 and 3Á01 Ga. These variable T DM Hf initial and eHf(t) initial values are interpreted to be the result of different mixtures of reworked 3Á65 Ga Palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the Neoarchaean at $2Á65 Ga. This conclusion is supported by results obtained from the Mahalapye Complex, which was affected by migmatization and granite intrusions during the Palaeoproterozoic at 2Á02^2Á06 Ga. The Mokgware granite (2019 AE 9 Ma) contains zircon xenocrysts with Pb^Pb ages of 2Á52^2Á65 Ga and 2Á93 Ga and hafnium model ages of 3Á0^3Á4 Ga, indicating that this granite is derived from remelting of Archaean crust. In contrast, uniform T DM Hf initial