Radioisotopic age constraints of the Cambrian Ritland impact structure, Norway

Abstract Secondary ion mass spectrometry U‐Pb geochronology has been performed on zircon grains separated from impact melt rock from the 2.7 km‐in‐diameter Ritland impact structure, southwestern Norway. Scanning electron microscope‐based imaging techniques, including electron backscatter diffraction analysis, reveal various zircon grain microtextures, including shock‐recrystallization and high‐temperature zircon decomposition. Analyses from unshocked zircon grains yield two distinct concordant age populations at 1.5 and ~2.5 Ga, interpreted to represent igneous crystallization ages. The former aligns with Telemarkian magmatism (1.52–1.48 Ga) which dominates the local area of the Sveconorwegian orogeny and the target sequence at Ritland. The latter indicates a more ancient zircon population in Southern Norway, representing detrital grains in cover sediments present at the time of impact in the Cambrian. Collectively, the U‐Pb data form two distinct discordant arrays with poorly resolved lower intercept ages spanning the Cambro‐Ordovician boundary. The melt rock at Ritland is highly altered, and significant postimpact Pb loss is observed throughout the U‐Pb data, likely in response to burial‐induced thermal overprinting during the Caledonian orogeny. Post‐filtering and selection of the data to minimize the effects of nonimpact‐specific Pb loss, the two discordia produce indistinguishable lower intercept ages of 586 ± 73 Ma (MSWD 1.6, n = 15) and 545 ± 48 Ma (MSWD = 11, n = 9) which coincide in the Cambrian–Late Ediacaran. We therefore provide radioisotopic support for previous stratigraphic age constraints for the formation of the structure (500–542 Ma).