LASER PROBE AR-40 AR-39 DATING OF COESITE-BEARING AND STISHOVITE-BEARING PSEUDOTACHYLYTES AND THE AGE OF THE VREDEFORT IMPACT EVENT

Age determinations have been made on pseudotachylytic rocks from the c ontroversial Vredefort structure of South Africa using the laser micro probe Ar-40/Ar-39 dating technique. Coesite- and stishovite-bearing ve ins in a quartzite from the Central Rand Group of the collar rocks wer e dated using a 10-mu m diameter focused ultra-violet laser beam. Thes e yielded a weighted mean age of 2027 +/- 18 Ma (2 sigma). Six pseudot achylytes, sampled from four different locations within the Outer Gran ite Gneiss of the core, were dated using an 50-100-mu m diameter focus ed infrared laser beam. These pseudotachylytes exhibit altered vein ma rgins with apparent ages considerably younger than ages obtained from the fresher centres of veins. The best weighted mean pseudotachylyte m atrix age obtained was 2018 +/- 14 Ma (2 sigma). Most of the clasts wi thin the pseudotachylyte matrices retain significantly older (e.g., Ar chean) ages, indicative of their parent rock history. Our results show that five of the seven dated samples possess matrix ages of similar t o 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 o f the Vredefort structure. The results affirm that the Vredefort Dome is a meteorite impact structure and show that it formed at 2018 +/- 14 Ma (2 sigma).