STABLE ISOTOPE EVIDENCE FOR IMPACT-RELATED PSEUDOTACHYLITE FORMATION AT VREDEFORT BY LOCAL MELTING OF DRY ROCKS

Abstract

We have determined the major element composition, δ18O and δD values, and water content of impact-related granophyre, and pseudotachylite, from various Vredefort Dome localities, aiming to constrain the mechanism of melt formation and the relationship between pseudotachylite and granophyre. The granitoid gneisses and the pseudotachylites they host have almost identical average δD and δ18O values (−67 and 8.6‰, and −67 and 8.4‰, respectively). The water contents of the pseudotachylites are extremely low, consistent with the isolation of the pseudotachylites from free water during and since their formation. There is a bimodal distribution of water content in pseudotachylites, with one group averaging 0.28 ± 0.03 weight % (n = 9) and the other 0.59 ± 0.06 weight % (n = 9). The Vredefort granophyre, which has been interpreted as the pooled product of impact melting, has average δD and δ18O values of −69‰ and 7.6‰, respectively (n = 2) and also has a very low water content (0.23 weight %). Differences in major element and O-isotope composition between the granophyre and the pseudotachylites are not consistent with a simple relationship, but can be explained by a higher component of greenstone in the granophyre. A strong correlation between host and pseudotachylite δ18O values is consistent with a system where the melt composition is controlled by the immediate surroundings. The pseudotachylites with higher water content have slightly higher δ18O values (9.1 compared to 8.1‰). This is opposite to the relationship predicted if water content is related to the proportion of biotite entering the melt. It is possible, instead, that this relates to the incorporation of higher proportions of material altered at low temperature in the high-water group.