Shocked and thermally metamorphosed zircon from the Vredefort impact structure, South Africa: a transmission electron microscopic study

Abstract

Transmission electron microscopic (TEM) analysis of zircon crystals from a migmatitic pelitic granulite from the central part of the Vredefort Dome, the central uplift of the ca . 250–300 km diameter Vredefort impact structure in South Africa, reveals a nanometer- to micrometer-scale subplanar lamellar structure that is attributed to high-pressure (> 15 GPa) shock deformation during a 2.02 Ga mega-impact event. Two types of subplanar structures have been identified: (a) dislocation bands and (b) nm-scale lamellar features. The dislocation bands could represent the optically discernable planar features that have been described in naturally shocked zircon from a number of impact structures and Cretaceous/Tertiary boundary deposits. However, whether their nature is similar to that of planar deformation features (PDFs) known from other rock-forming minerals or whether they represent a form of shock-induced planar fracturing could not be determined. Annealing of these bands and localized formation of subgrains is consistent with the slow decay of shock heat from rocks beneath the very large (250–300 km diameter) Vredefort impact crater. The question of whether the nm-scale lamellar features represent an equivalent of the nm-scale PDFs identified previously in zircon could not be resolved. Following the shock event, amorphization (metamictization) has destroyed the zircon crystal structure in places.