Abstract

The origin of granophyric rock dikes (the Vredefort Granophyre) at the 2 Ga Vredefort structure in South Africa has been controversial. They have been interpreted as either impact melt or igneous intrusions. Most Vredefort Granophyre samples have considerably higher Os contents than the country rocks and 187Re/188Os and 187Os/188Os ratios that scatter about a 2 Ga isochron. Their initial 187Os/188Os ratios (at 2 Ga) overlap the meteoritic data range, indicating that all the granophyre samples contain some meteoritic Os. The Re-Os isotopic composition of the granophyre is significantly different from that of local precursor rocks and indicates that up to 0.2% of a chondritic component could be contained in the granophyre. Our results confirm the interpretation that the Vredefort Granophyre represents an impact melt rock.

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