Prolonged magmatism and time constraints for sediment deposition in the early Archean Barberton greenstone belt: Evidence from the Upper Onverwacht and Fig Tree groups

The single zircon evaporation, SHRIMP ion-microprobe and conventional dissolution techniques were used to determine Pb-207/Pb-206 and U-Pb ages on samples from the Upper Onverwacht and Fig Tree groups of the early Archean Barberton greenstone belt, South Africa. Zircons from dacitic rocks of the upper Hooggenoeg Formation yield ages of similar to 3445-3452 Ma. A tuff in the basal Kromberg Formation has a mean age of 3416 +/- 5 Ma. A tuffaceous band, 5 cm thick, in the uppermost Kromberg Formation contains igneous zircons with a mean age of 3334 +/- 3 Ma. The 1700 m section of Kromberg Formation between these two samples is composed of basaltic lavas, minor komatiites and cherry metasediments. The overlying Mendon Formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m. A cherty, stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 +/- 3 Ma. The basal Fig Tree Group has units as old as 3259 +/- 3 Ma, and upper units in the Fig Tree are as young as 3225 +/- 3 Ma. Xenocrystic zircons in the Upper Onverwacht and overlying Fig Tree Group samples suggest that successive igneous units inherited zircons from underlying units and that, over several hundred million years, episodes of intermediate to felsic igneous activity took place at 20-40 Ma intervals. Structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution, but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the Barberton greenstone belt.