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 ( XRF) 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.
