Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40Ar/39Ar and Rb-Sr chronology

40Ar/39Ar and Rb-SI 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 bate 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 'D-2' 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. D, structures were superimposed on structures and metamorphic fabrics formed Juring a regional Mid-Ordovician tectonothermal event dated previously, at c. 470-460 hla. Syn-D-2 temperatures were generally >600 degreesC 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 D-2 strain.