Laser probe argon‐40/argon‐39 dating of coesite‐ and stishovite‐bearing pseudotachylytes and the age of the Vredefort impact event

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Abstract
Abstract— Age determinations have been made on pseudotachylytic rocks from the controversial Vredefort structure of South Africa using the laser microprobe 40Ar/39Ar dating technique. Coesite‐ and stishovitebearing veins in a quartzite from the Central Rand Group of the collar rocks were dated using a 10‐μm diameter focused ultra‐violet laser beam. These yielded a weighted mean age of 2027 ± 18 Ma (2σ). Six pseudotachylytes, sampled from four different locations within the Outer Granite Gneiss of the core, were dated using an 50–100‐μm diameter focused infrared laser beam. These pseudotachylytes exhibit altered vein margins with apparent ages considerably younger than ages obtained from the fresher centres of veins. The best weighted mean pseudotachylyte matrix age obtained was 2018 ± 14 Ma (2σ). Most of the clasts within the pseudotachylyte matrices retain significantly older (e.g., Archean) ages, indicative of their parent rock history. Our results show that five of the seven dated samples possess matrix ages of ∼2000 Ma, similar to the age of the Granophyre (Walraven et al., 1990), a supposed impact melt rock (French and Nielsen, 1990). The dating of coesite‐ and stishovite‐bearing veins equates the shock event with pseudotachylyte formation, generation of the Granophyre and creation of the Vredefort structure. The results affirm that the Vredefort Dome is a meteorite impact structure and show that it formed at 2018 ± 14 Ma (2σ).
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