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

Ar-40/Ar-39 and Rb-SI mineral ages have been determined from various lithol ogies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb-Sr muscovite ages of c. 428, c. 42 1 and c. 413 Ma obtained from Moine Thrust Zone mylonites are interpreted t o date closely regional thrusting during the bate Silurian to Early Devonia n. Ar-40/Ar-39 muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb-Sr analyses of muscovites from t he same samples; it is likely that this discrepancy results from a componen t of extraneous or 'excess' argon. Ar-40/Ar-39 hornblende ages and Rb-Sr an d Ar-40/Ar-39 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 Balt ica with Scottish segments of Laurentia resulting in the Scandian orogeny a nd broadly coeval Moine Thrust Zone. D, structures were superimposed on str uctures and metamorphic fabrics formed Juring a regional Mid-Ordovician tec tonothermal event dated previously, at c. 470-460 hla. Syn-D-2 temperatures were generally >600 degreesC and sufficient to achieve more or less comple te thermal rejuvenation of Rb-Sr and Ar-40/Ar-39 isotopic systems in muscov ite and hornblende, even in areas of low D-2 strain.