Granular zircon from Vredefort granophyre (South Africa) confirms the deep injection model for impact melt in large impact structures

Abstract

The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report “former reidite in granular neoblastic” (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO4) and reversion to zircon resulted in zircon grains composed of fine neoblasts (∼0.5–3 µm) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8–10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (≥30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.

Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

“…In the present sample, shocked granular zircon within and around pseudotachylitic vein in a lithic clast in granophyre is observed [19]. The only granular zircon grain with fine granules (≤3 µm) reported from Vredefort (besides the zircons from a present sample) was found in heavy mineral separates from granophyre [39].…”

“…Farther away from the PT vein, at a distance greater than 3-6 mm, granular zircon is not found, but zircon with planar fractures (PFs) is abundant. At a distance greater than 12 mm, no distinct shock deformation features in zircon are observed [19].…”

“…Zircon grains with shock microtwins (four in total) were found within the PT vein and at a distance of 6 and 7.5 mm from the PT vein. Within the PT vein and in the foliated granite around the vein, granular zircons are present, being spatially associated with the PT vein in thin sections (see also [19]). Farther away from the PT vein, at a distance greater than 3-6 mm, granular zircon is not found, but zircon with planar fractures (PFs) is abundant.…”

“…The small sample size and the high degree of alteration make it difficult to perform isotopic dating on the PT vein in question. Later, Kovaleva et al [19] found shocked granular zircons, which are spatially associated with the PT vein, and suggested that this discovery is an indirect proof for the impact origin of the PT vein. Based on zircon deformation microstructures, the authors have further suggested that the clast was derived from the uppermost levels of the impact structure.…”

“…Therefore, the Vredefort granophyre dykes are a library, which provides an opportunity to study otherwise eroded impactites. By investigating these lithic clasts, we can learn about the impact processes occurring closer to the impacted surface [19]. French and Nielsen [18] emphasized the importance of studying shock features in lithic clasts enclosed in impact melts.…”