The Mesoproterozoic Stoer Group of the 'Torridonian' has been dated at 1177 +/- 5 Ma (2 sigma), based on Ar-40/Ar-39 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 parts per thousand (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.
- moine thrust zone
- Northwest Scotland