U-PB AGES ON SINGLE DETRITAL ZIRCON GRAINS FROM THE WITWATERSRAND BASIN, SOUTH-AFRICA - CONSTRAINTS ON THE AGE OF SEDIMENTATION AND ON THE EVOLUTION OF GRANITES ADJACENT TO THE BASIN

U-Pb ages of single detrital zircon grains from various stratigraphic horizons in the Dominion and Witwatersrand sequences provide constraints on the maximum age of sedimentation as well as indicating the pattern of age distribution in the (granitoid) source area providing detritus into the basin. Zircon ages in the Dominion sediments range from 3191-3105 Ma with a geometric mean (X̄) at 3153 Ma. Those from the lower Witwatersrand sediments range from 3305-3044 Ma with X̄ = 3097 Ma, and zircons in the upper Witwatersrand sediments are between 3207-2894 Ma old with X̄ = 3053 Ma. Ages of detrital zircons generally decrease upward in the stratigraphic record, and <3000 Ma old zircons are only found in the Central Rand Group. This trend implies that younger granites may have formed at some time subsequent to lower Witwatersrand deposition, or that continued erosion of the hinterland resulted in the unroofing of successively younger granites. Unconformity-bound sedimentary packages in the Witwatersrand sequence are considered to represent the depositional responses to on-going tectono-magmatic events in the source area. -from Authors