THE OLDEST PART OF THE BARBERTON GRANITOID-GREENSTONE TERRAIN, SOUTH-AFRICA - EVIDENCE FOR CRUST FORMATION BETWEEN 3.5-GA AND 3.7-GA

Many stratigraphic and age relationships in the southern part of the B arberton greenstone belt (BGB) remain unresolved due to strong deforma tion including thrusting, nappe stacking and strike-slip faulting. A r elatively undisturbed sequence from the lower Onverwacht Group (simila r to 3.48-3.45 Ga), followed by the upper Onverwacht (similar to 3.42- 3.3 Ga), the Fig Tree Group (similar to 3.26-3.23 Ga) and the Moodies Group (> 3.22 Ga) was established by single zircon dating using variou s techniques, but the position of the Theespruit Formation is still un certain. Using the single zircon evaporation and the vapour digestion techniques we obtained remarkably uniform (207)pb/(206)pb and U-Pb age s of 3544 +/- 3 to 3547 +/- 3 Ma for felsic rocks mapped as Theespruit in the Steynsdorp Anticline of the southeastern BGB, some 100 Ma olde r than all other dated greenstone units. These rocks were intruded by the 3502-3511 Ma old Steynsdorp TTG pluton containing zircon xenocryst s as old as 3553 +/- 4 Ma. A 3.5 Ga granodiorite plug intrusive into m etavolcanics of the Komati Formation (lower Onverwacht Group) containe d two 3702 +/- 2 Ma zircon xenocrysts, the oldest so far measured in t he Barberton-Swaziland area and testifying to the presence of very anc ient crust in the region. The Theespruit felsic metavolcanics have eps ilon(Nd(t)) values between +1.1 and -1.1 and Nd T-DM model ages betwee n 3.5 and 3.7 Ga, suggesting variable contamination of their protolith s with older continental crust. The above zircon ages extend the histo ry of the BGB back to similar to 3.55 Ga. We suggest that the area of the Steynsdorp Anticline constitutes the oldest nucleus of the BGB ont o which successively younger units were tectonically and magmatically accreted. We also speculate that the mafic-felsic volcanic units of th e southern BGB may perhaps represent distinct oceanic plateaux, rather than ocean floor material, which amalgamated between 3.55 and 3.42 Ga ago. Our data support the concept that the BGB consists of a number o f discrete, fault-bounded terranes, and that large-scale lithological correlations are therefore not justified.