K/Ar and⁴⁰Ar/³⁹Ar analyses of meltrock from the Acraman impact structure, Gawler Ranges, South Australia

The Acraman structure, located in the Gawler Range Volcanics, Gawler Craton, South Australia, is the largest probable meteorite impact structure known in Australia; it is notable also as the likely source of volcaniclastic ejecta within the Late Proterozoic Bunyeroo Formation located ˜ 300 km east of the impact site and within the correlative Rodda Beds in the Officer Basin ˜ 450 km to the northwest of the impact site. Conventional K/Ar analyses and⁴⁰Ar/³⁹Ar total fusion and step heat experiments were performed on two samples of meltrock from the centre of the Acraman structure in an attempt to directly constrain the age of the impact event. The two samples of devitrified meltrock consist primarily of albite microlites in a hematitic devitrified matrix of cloudy K-feldspar and quartz. Electron microprobe analyses of the samples reveal that the albite + potassium feldspar have nearly pure end-member compositions (Ab₉₉ and Or₉₇) indicating they are likely low-temperature authigenic phases. K/Ar,⁴⁰Ar/³⁹Ar total fusion and integrated apparent ages for both samples agree within analytical uncertainty. Results of step heat experiments show relatively complex patterns likely due to the degassing of multiple phases. Based on apparent ages that are least affected by diffusional loss, a minimum estimate for the age of the meltrock is ˜450 Ma, Late Ordovician. Because the feldspar in the meltrock may have formed as a result of authigenic replacement and devitrification, the apparent age is regarded as a minimum age for the impact event. The K/Ar and⁴⁰Ar/³⁹Ar data presented here indicate the Acraman impact event occurred at least ˜ 450 Ma ago; however, these results do not provide an unequivocal test of the hypothesis that the ejecta blanket in the Bunyeroo Formation and Rodda Beds is uniquely linked to the Acraman impact structure.