Relationship between detrital zircon age-spectra and the tectonic evolution of the Late Archaean Witwatersrand Basin, South Africa

Abstract

SHRIMP U–Pb detrital zircon age-spectra for 14 samples from the West Rand and Central Rand Groups of the Witwatersrand Basin indicate that felsic source rocks mostly formed at ~3090–3060 and ~3000–2870 Ma, with a smaller proportion formed prior to 3100 Ma. Provenance evaluation indicates that source rocks have largely vanished through erosion, such that greenstone terrains extending from the Amalia–Kraaipan Belt in the southwest to the Murchison Belt in the northeast are models of, but not the actual, source areas. Analysis of detrital zircon age-modes reveals that the West Rand and Central Rand Groups record different basins. The West Rand Group is characterised by age-subpopulations with a much-smaller range and less complex distribution than those in the Central Rand Group. Furthermore, detrital zircon age-spectra for the West Rand Group generally decrease in complexity up-section, whereas they become increasingly more-complex in the Central Rand Group. It is concluded that the source area for the West Rand Group is consistent with a passive-margin setting, with the decrease in complexity up-section of zircon age-spectra being the consequence of increasing maturity of the hinterland. Conversely, the source area of the Central Rand Group was far-more dynamic, with an increasing age-range of source rocks being exposed through time. Pulsed tectonic rejuvenation of the source area is implied. Contemporaneous emplacement, uplift and erosion of granitoids supports a magmatic fold-thrust belt as the source, and a complementary retro arc basin setting. Detrital xenotime grains record the ages of high-U granitoids emplaced in the fold-thrust belt during deposition of the Central Rand Group, and are much closer to depositional ages than are detrital zircon grains. Change from a passive margin to a retro arc basin across the West Rand-Central Rand boundary is emphasised by change from simple to increasingly complex detrital zircon age-spectra. This study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in Archaean sandstones due to severe chemical weathering and authigenic alteration, thereby enabling distinction between passive-margin and retro arc-basin stages through systematic changes in the age complexity of the source area.