Spatial distribution of zircon with shock microtwins in pseudotachylite-bearing granite gneisses, Vredefort impact structure, South Africa

Abstract

Mechanical microtwins in zircon is a characteristic shock feature and a potential tool for estimating shock pressure and/or shear stress. However, so far, zircon with microtwins has rarely been described in situ. The present study of pseudotachylite-bearing granitic gneisses from the Vredefort impact structure focuses on the spatial distribution of zircon with microtwins and other deformation features.Zircon hosted by granitic gneiss does not reveal extensive shock deformation. With decreasing distance to pseudotachylite, zircon progressively develops (sub)planar fractures, planar deformation bands, low-angle boundaries, rarely thin {112} microtwin lamellae and, closest to the vein, it is fragmented. In contrast, zircon hosted by pseudotachylites contains thick and closely-spaced microtwin lamellae, which form up to three sets within individual grains. Microtwin lamellae are associated with planar {112} low-angle boundaries, and with overprinting crystal-plastic and brittle deformation. We suggest that planar {112} low-angle boundaries in zircon are indicative of shock deformation; and that microtwins in zircon within pseudotachylite veins are an indicator of the impact origin for the latter, as opposed to endogenic, seismically-induced friction melts. Locally-elevated shock pressure and shear stress induce the formation of pseudotachylites in the target rock as well as shock microtwins in zircon.