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

posted on 21.06.2016 by John Parnell, Darren Mark, Anthony E. Fallick, Adrian Boyce, Scott Thackrey

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.