Comparative study of pseudotachylite microstructures in felsic and mafic rocks from the Vredefort impact structure, South Africa. Implications for the experimental studies

Abstract

Abstract While the formation of tectonic PTs is widely accepted as related to friction melting for all rock types, the mechanism of formation of impact-related pseudotachylites (PTs) is strongly debated. Moreover, the influence of the target lithology on the initial impact PT formation, development and deformation has not been considered in detail, even though impact PTs in the Vredefort structure are found in various lithologies. Here, we investigate PTs that occur in felsic (granite) and mafic (dolerite) rocks that are in contact with one another in the core of the Vredefort impact structure. In this unique outcrop, multiple granitic clasts are observed inside the PTs hosted by dolerite, which indicates material exchange between PTs in the two lithologies, pointing to simultaneous shock deformation in the adjacent rock phases. This outcrop thus provides an unusual opportunity for studying the effect of host lithology on the formation and deformation of PT. Our investigations demonstrate that microstructures and textures in PTs and their chemical composition largely depend on the host rock, suggesting particular rheological syn- and post-formation behaviour of the melt veins of different composition. In granite, melt initiates due to preferential decomposition of mafic and hydrous minerals, and subsequent ductile deformation of quartz and feldspars. In dolerite, melt initiates as a result of comminution of the bulk rock. Due to the different mineral composition of rocks, and corresponding various fracture toughness of different minerals, granitic PTs contain abundant monomineralic clasts composed of more refractory phases, most often quartz, while PT veins in dolerite contain fewer, usually lithic clasts, representing fragments of the host rock. PTs in granite are chemically zoned, which has not been observed for PTs in dolerite. In felsic rocks, PTs are often framed by an ultramylonite and are associated with post-shock ductile features, such as sigmoidal-shaped clasts and dragged edges of the veins, whereas in mafic rocks, PTs are found to be associated with brittle features only, such as (ultra)cataclasites and micro-faults. These differences in initiation and development of PTs based on rock type are consistent with previous experimental studies and must be considered when discussing the formation mechanisms of impact-generated PTs.