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

Abstract

The Vredefort impact structure in South Africa is deeply eroded to its lowermost levels. However, granophyre (impact melt) dykes in such structures preserve clasts of supracrustal rocks, transported down from the uppermost levels of the initial structure. Studying these clasts is the only way to understand the properties of already eroded impactites. One such lithic clast from the Vredefort impact structure contains a thin pseudotachylite vein and is shown to be derived from the near-surface environment of the impact crater. Traditionally, impact pseudotachylites are referred to as in situ melt rocks with the same chemical and isotopic composition as their host rocks. The composition of the sampled pseudotachylite vein is not identical to its host rock, as shown by the micro-X-ray fluorescence (µXRF) and energy-dispersive X-ray (EDX) spectrometry mapping. Mapping shows that the melt transfer and material mixing within pseudotachylites may have commonly occurred at the upper levels of the structure. The vein is spatially related to shocked zircon and monazite crystals in the sample. Granular zircons with small granules are concentrated within and around the vein (not farther than 6–7 mm from the vein). Zircons with planar fractures and shock microtwins occur farther from the vein (6–12 mm). Zircons with microtwins (65°/{112}) are also found inside the vein, and twinned monazite (180°/[101]) is found very close to the vein. These spatial relationships point to elevated shock pressure and shear stress, concentrated along the vein’s plane during impact.

Microstructural Deformation and the Age of Monazite-(Ce) from Diatectite Granite in the Jarva-Varaka Structure (Kola Region, Russia)

“…Deformation microstructure, including twins, plastically deformed domains, and recrystallized domains (neoblasts), is a common feature of monazites that have been deformed either in tectonic or shock metamorphic environments (for example, [17] and references therein). Impact-related microstructural deformation has been described in shock-deformed monazite from the well-known impact structures such as Vredefort in South Africa, Yarrabubba and Woodleigh in Australia, and Araguainha in Brazil [27][28][29][30][31][32][33]. Similar deformational twins and dynamically recrystallized neoblasts in monazite have been documented in tectonic environments [42].…”

“…Monazite has been reported as a shocked mineral at several impact structures, including Haughton in Canada [27], Araguainha in Brazil [28,29], Yarrabubba and Woodleigh in Australia [29,30], and Vredefort in South Africa [17,[29][30][31][32][33], and it was interesting to compare the obtained results. Figure 1.…”