Stable isotope study of the archaean rocks of the vredefort impact structure, central kaapvaal craton, south africa

The Vredefort dome in the Kaapvaal Craton was formed as a result of the impact of a large meteorite at 2.02 Ga. The central core of Archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the Witwatersrand Supergroup, exposing a substantial depth section of the Archaean crust. Orthogneisses of the core show little variation in whole-rock delta(18) O value, with the majority being between 8 and 10 parts per thousand, with a mean of 9.2 parts per thousand (n=35). Quartz and feldspar have per mil differences that are consistent with O-isotope equilibrium at high temperatures, suggesting minimal interaction with fluids during subsequent cooling. These data refute previous suggestions that the Outer Granite Gneiss (OGG) and Inlandsee Leucogranofels (ILG) of the core represent middle and lower crust, respectively. Granulite-facies greenstone remnants from the ILG have d18 O values that are on average 1.5 parts per thousand higher than the ILG host rocks and are unlikely, therefore, to represent the residuum from the partial melting event that formed the host rock. Witwatersrand Supergroup sedimentary rocks of the collar, which were metamorphosed at greenschist-to amphibolite-facies conditions, generally have lower d18 O values than the core rocks with a mean value for metapelites of 7.7% (n = 45). Overall, through an similar to 20 km thick section of crust, there is a general increase in whole-rock d18 O value with increasing depth. This is the reverse of what is normal in the crust, largely because the collar rocks have d18 O values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide. The collar rocks have delta D values ranging from -35 to -115 parts per thousand ( average -62 parts per thousand, n = 29), which are consistent with interaction with water of meteoric origin, having a dD of about -25 to -45 parts per thousand. We suggest that fluid movement through the collar rocks was enhanced by impact-induced secondary permeability in the dome structure.