Impact Related Pseudotachylitic Breccia in the Schurwedraai and Baviaan- Krantz Alkali Granite Complexes in the Collar of the Vredefort Dome, South Africa

Introduction: The formation of impact related pseudotachylitic breccias and their occurrence in large impact structures such as Vredefort and Sudbury is still a matter of debate. The main formation processes that are currently considered – in the absence of large-scale fault/shear zones where massive friction melts could be generated-are shock melting, frictional melting and decompression melting, or combinations of these. Local formation and accumulation of allochthonous melt in dilatational zones are also debated. Hence the purpose of this work is to contribute to the understanding of the genesis of such pseudotachylitic breccia bodies in the central uplift of a very large impact structure. Macro-and microscopic structural, micropetrographic, and chemical analysis of pseudotachylitic breccia occurrences in the Schurwedraai and Baviaan-Krantz Alkali Granite intrusion in the northwestern part of the collar of the Vredefort Dome, South Africa, have been carried out. Structural observations: Detailed fieldwork determined that pseudotachylitic breccia veins and dikes exhibit preferred orientation trends. Relatively smaller pseudotachylitic breccia veins (< 1m width) show preferred ENE-WSW and NW-SE strike trends and larger pseudotachylitic breccias veins (> 1 m width) a preferred NW-SE strike trend. Pseudotachylitic breccia networks have prominent NW-SE and ENE-WSW strike trends. These results indicate that the majority of pseudotachylitic breccia bodies seemingly follow a concentric and radial pattern with respect to the center of the Vredefort Dome. Another notable observation was that thin shear zones (< 3 cm in width) with alternating zones with pronounced grain comminution and enhanced recrystallization and apparently melt-bearing (pseudotachylite like) bands are intimately related to thicker occurrences of pseudotachylitic breccias (e.g. Fig. 1). This scenario could be interpreted to suggests that friction in narrow zones led to initial melt formation followed by discharge of melt into adjacent open fractures and pockets (as discussed in [1]) or dilata-tional sites. Petrographic and chemical methods: Samples of host rock and pseudotachylitic breccias were analysed in the laboratory. Pseudotachylitic breccia occurences sampled for this study range from a few millimeters wide veinlets to large (up to 6 m wide) network zones. Polished thin sections of these samples were initially investigated by optical microscopy. Scanning electron microscopy was used to investigate in more detail textures and mineralogy of pseudotachylitic breccia matrices and adjacent host rock. X-ray fluorescence spec-trometry for major and trace elements was employed to compare bulk chemical compositions of host rock and pseudotachylitic breccias pairs. Electron microprobe analysis (EMPA) with operating conditions of 20 kV and …