RE-OS ISOTOPE AND GEOCHEMICAL STUDY OF THE VREDEFORT GRANOPHYRE - CLUES TO THE ORIGIN OF THE VREDEFORT STRUCTURE, SOUTH-AFRICA

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. Mo st Vredefort Granophyre samples have considerably higher Os contents t han the country rocks and Re-187/Os-188 and Os-187/Os-188 ratios that scatter about a 2 Ga isochron. Their initial Os-187/Os-188 ratios (at 2 Ga) overlap the meteoritic data range, indicating that all the grano phyre samples contain some meteoritic Os. The Re-Os isotopic compositi on of the granophyre is significantly different from that of local pre cursor rocks and indicates that up to 0.2% of a chondritic component c ould be contained in the granophyre. Our results confirm the interpret ation that the Vredefort Granophyre represents an impact melt rock.