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Original Article |
1 1Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia (e-mail: pk@lithos.curtin.edu.au)
2 2Geology (BMS), Oxford Brookes University, Headington, Oxford OX3 OBP, UK
3 3Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 OQF, UK
4 4Department of Earth Sciences, University of Bristol, Queen's Road, Bristol BS8 1RJ, UK
Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during D2 have been dated using SHRIMP U–Pb geochronology. Zircon ages of 424 ± 8 Ma (Vagastie Bridge granite), 420 ± 6 Ma (Klibreck granite) and 429 ± 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D2 deformation, during mid- to late Silurian time. Titanite ages of 413 ± 3 Ma (Vagastie Bridge granite) and 416 ± 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550–500 °C. A mid- to late Silurian age for D2 deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of foreland-propagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arc–continent orogeny. The mid- to late Silurian D2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia.
Key Words: Scotland • Moine Supergroup • Caledonian orogeny • zircons • titanites • metagranites
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