The Glenelg-Attadale Inlier, NW Scotland, with emphasis on the Precambrian high-pressure metamorphic history and subsequent retrogression: an introduction and review

Abstract

Synopsis The Glenelg-Attadale Inlier of the NW Highlands of Scotland represents the largest area of exposed basement within the Moine Supergroup and, thus, affords the best opportunity of studying the pre-Moine history and the relationship of the basement to the Moine. The Inlier can be divided into Western and Eastern Units, based on lithological associations, and the boundary is marked by a major ductile shear zone with a dominantly reverse (top-to-the-west) sense of shear. The presence of eclogites and of other eclogite facies rocks within the inlier is the only unequivocal examples of crustal eclogites exposed within the British Isles. Eclogites in the Eastern Unit (P–T c. 20 kbar and 750–780°C) has been radiometrically dated (Sm–Nd garnet–omphacite–whole rock) to c. 1082 Ma and their retrogression to amphibolite facies (P–T conditions: minimum 14–16 kbar and c. 650°C) have been dated (U–Pb zircon) to c. 995 Ma. This demonstrates that the Eastern Unit underwent tectonic thickening to c. 70 km during the Grenvillian orogeny and was decompressed into the mid-crust by the end of the Grenvillian orogenic cycle. Rare eclogites in the Western Unit appear to have been formed earlier in the Palaeoproterozoic ( c . 1700–1750 Ma) and their juxtaposition against the Eastern Unit eclogites may be fortuitous or may reflect a fundamental tectonic manifestation of Wilson Cycle tectonics in orogenic belts. The basal Moine shows a deformed unconformable relationship against the Western Unit. However, the Eastern Unit is entirely enveloped by profound shear zones against the Western Unit and the Moine Supergroup, suggesting that the Eastern Unit is allochthonous. Direct dating of mylonites within the shear zone between the Western and Eastern Units, containing coaxially sheared Moine metasediments, suggests an age of c. 670 Ma and this may reflect the time at which the units were juxtaposed and the Moine infolded. Final deformation and metamorphism of the Glenelg-Attadale Inlier and the Moine occurred during the Caledonian orogeny, with major folding and extensional shearing operating sometime after c. 437 Ma, resulting in distinctive fold interference patterns. Terminal thrusting of the Moine and GAI westward over the Laurentian Caledonian foreland occurred at c. 435–430 Ma.