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1 Department of Geology, University of Georgia, Athens, GA 30602, USA
2 Geology (BMS), Oxford Brookes University, Oxford OX3 0BP, UK (e-mail: rastrachan{at}brookes.ac.uk)
3 Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK
4 Present address: Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6001, Australia
Scientific editing by Ray Burgess.
40Ar/39Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c. 428, c. 421 and c. 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40Ar/39Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40Ar/39Ar hornblende ages and Rb–Sr and 40Ar/39Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c. 440 Ma and c. 410 Ma. These ages are interpreted to date cooling during and following ‘D2’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c. 470–460 Ma. Syn-D2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40Ar/39Ar isotopic systems in muscovite and hornblende, even in areas of low D2 strain.
Keywords: Caledonides, Sutherland, deformation, cooling, absolute age.
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