GEOCHEMISTRY AND IMPACT ORIGIN OF THE VREDEFORT GRANOPHYRE

The Vredefort impact structure contains a suite of granophyric dykes, referred to as the Vredefort Granophyre, occurring within and at the e dge of the Archaean basement core. New whole-rock chemical analyses, t ogether with previous data, represent a complete suite of the Granophy re occurrences. These data show that the Vredefort Granophyre has a re markable chemical homogeneity, within and between dykes, on a regional scale, and a unique composition (-67 wt.% SiO2, similar to 1 wt.% TiO 2, similar to 13 wt.% Al2O3, similar to 7 wt.% Fe2O3, similar to 3 wt. % MgO, similar to 4 wt.% CaO, similar to 3 wt.% Na2O, similar to 2 wt. % K2O). Five volumetrically abundant regional lithologies are: Transva al carbonate, Ventersdorp lava, Witwatersrand quartzite, Witwatersrand shale, and Outer Granite Gneiss. These lithologies are used as compon ents in both harmonic and least-squares mixing calculations to reprodu ce the Vredefort Granophyre composition. The best-fit mixture is made up of the five target rocks used and corresponds to: similar to 40% la va, similar to 30% quartzite, similar to 25% gneiss, similar to 3% sha le, and similar to 2% carbonate. These results are geologically reason able, given our knowledge of the pre-impact regional stratigraphic suc cession at Vredefort, but they do not conform with the clast populatio n in the dykes. This is easily reconcilable, however, since previous s tudies demonstrated that the clast population in impact melt rocks is not necessarily representative of the components, or their proportions , melted to form the melt. Characteristics of the Vredefort Granophyre are similar to those of terrestrial impact melt rocks. Our previous i nterpretation that the Vredefort Granophyre dykes are injections, and the only remaining evidence of the existence, of the impact melt rocks produced during the formation of the Vredefort impact structure about 2 Ga ago is confirmed by this geochemical study.