Ultramylonites and their significance for the understanding of the history of the Vredefort impact structure, South Africa

The Vredefort crater is well known in the geological literature, because it is situated in the Witwatersrand Group, the repository of almost half of the world's gold reserves, and was generated by an explosion process. One of the key problems in the history of the crater is the timing of formation of cordierite-orthopyroxene reaction structures, which develop extensively around relict garnet in the metapelites of the granulite core of the impact structure. In the middle 1970s, W. Schreyer and K. Abraham discovered a unique pseudotachylite (ultramylonite) cutting cordierite-orthopyroxene symplectites in the metapelites of the granulite core of the crater. They argued on the basis of that discovery that the symplectites were formed long before (∼3 Ga) the explosion (∼2 Ga) and only reequilibrated subsequently. This idea has served for many years as a starting point for many geodynamic models of the formation of the crater core. However, up to the present time, the ultramylonites cutting symplectites were never described in detail. This paper presents (1) the analysis of microscopic textural relationships between symplectites and ultramylonites, (2) the results of detailed microprobe investigations of minerals from these structures, and (3) the comprehensive mechanism of ultramylonite formation and proof for the continuous decrease of P-T parameters at the final stage of upheaval of the "central uplift" of the Vredefort crater. This evidence is aimed at completing the discussion of many years on the nature of the core of the Vredefort crater.