The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland

The Mesoproterozoic Stoer Group of the ‘Torridonian' has been dated at 1177 ± 5 Ma (2σ), based on ⁴⁰Ar/³⁹Ar dating of authigenic K-feldspars precipitated in degassing structures within the Stac Fada Member, interpreted previously as an impact ejecta deposit. Fluid inclusion analysis of the K-feldspar shows that it precipitated from hot fluids, and so must immediately post-date emplacement of the ejecta blanket. The oxygen isotopic composition of K-feldspar is remarkably constant at 9.7 ± 0.2‰ (V-SMOW) and is consistent with derivation from evaporated lake water. The new age supports previous determinations of 1199 ± 70 Ma from Pb–Pb dating of limestone and 1180 Ma from palaeomagnetic data, but provides much higher precision.