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

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Abstract
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 ( ) at 3153 Ma. Those from the lower Witwatersrand sediments (West Rand Group) range from 3305-3044 Ma with , and zircons in the upper Witwatersrand sediments (Central Rand Group) are between 3207-2894 Ma old with . 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. The wide spread of zircon ages (411 Ma) evident in the data set indicates that granites formed virtually continuously between circa 3300-2900 Ma in the Witwatersrand source area. Of the zircon ages 45% fall within 30 m.y. of the geometric mean of the total data set, suggesting that a major crust-forming event occurred at . Granitoids in the source area can be divided into (i) pre-Dominion basement; (ii) Dominion granites, whose emplacement coincided with the extrusion of Dominion volcanics, and (iii) Randian granites, which were emplaced synchronously with Witwatersrand deposition. This sequence of events supports recent tectonic models that view the Witwatersrand sequence as having been deposited in a foreland basin. 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.