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Stable isotope study of the Archaean rocks of the Vredefort impact structure, central Kaapvaal Craton, South Africa

Fagereng, Ake, Harris, Chris, La Grange, Mandy and Stevens, Gary 2008. Stable isotope study of the Archaean rocks of the Vredefort impact structure, central Kaapvaal Craton, South Africa. Contributions to Mineralogy and Petrology 155 (1) , pp. 63-78. 10.1007/s00410-007-0224-6

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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 δ 18O value, with the majority being between 8 and 10‰, with a mean of 9.2‰ (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 δ 18O values that are on average 1.5‰ 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 δ 18O values than the core rocks with a mean value for metapelites of 7.7‰ (n = 45). Overall, through an ∼20 km thick section of crust, there is a general increase in whole-rock δ 18O value with increasing depth. This is the reverse of what is normal in the crust, largely because the collar rocks have δ 18O values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide. The collar rocks have δD values ranging from −35 to −115‰ (average −62‰, n = 29), which are consistent with interaction with water of meteoric origin, having a δD of about −25 to −45‰. We suggest that fluid movement through the collar rocks was enhanced by impact-induced secondary permeability in the dome structure.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Vredefort impact structure; Stable isotopes; Fluid-rock interaction; Witwatersrand Supergroup; Archaean basement
Publisher: Springer
ISSN: 0010-7999
Last Modified: 04 Jun 2017 06:06

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