Portal de Periódicos da CAPES

Geochemistry and impact origin of the Vredefort Granophyre

1997; Geological Society of South Africa; Volume: 100; Issue: 2 Linguagem: Inglês

1996-8590

A. M. Therriault, W. U. Reimold, A. M. Reid,

Geological and Geochemical Analysis

The Vredefort impact structure contains a suite of granophyric dykes, referred to as the Vredefort Granophyre, occurring within and at the edge of the Archaean basement core. New whole-rock chemical analyses, together with previous data, represent a complete suite of the Granophyre occurrences. These data show that the Vredefort Granophyre has a remarkable chemical homogeneity, within and between dykes, on a regional scale, and a unique composition ( approximately 67 wt.% SiO 2 , approximately 1 wt.% TiO 2 , approximately 13 wt.% Al 2 O 3 , approximately 7 wt.% Fe 2 O 3 , approximately 3 wt.% MgO, approximately 4 wt.% CaO, approximately 3 wt.% Na 2 O. approximately 2 wt.% K 2 O). Five volumetrically abundant regional lithologies are: Transvaal carbonate, Ventersdorp lava. Witwatersrand quartzite, Witwatersrand shale, and Outer Granite Gneiss. These lithologies are used as components in both harmonic and least-squares mixing calculations to reproduce the Vredefort Granophyre composition. The best-fit mixture is made up of the five target rocks used and corresponds to: approximately 40% lava, approximately 30% quartzite, approximately 25% gneiss, approximately 3% shale, and approximately 2% carbonate. These results are geologically reasonable, given our knowledge of the pre-impact regional stratigraphic succession at Vredefort, but they do not conform with the clast population in the dykes. This is easily reconcilable. however, since previous studies 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 interpretation 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.