Direct dating of mylonite evolution: a multi-disciplinary geochronological study from the Moine Thrust Zone, NW Scotland

Rb-Sr dating of sub-closure temperature, syn-deformationly crystallized white micas from the Moine mylonites (Knockan and Dundonnell), has yielded ages which vary from 437 Ma to 408 Ma. Morphological and major element analyses of the micas indicate that all the micas within the analysed samples were (re)crystallized during the Moine Thrust shearing. The ages yielded are therefore interpreted as marking the end of crystal plastic deformation associated with shearing on the Moine Thrust. The variation in ages between samples is significantly greater than the individual analytical errors and may be a product of strain localization within the shear zone. Alternatively it may indicate flow of strontium-rich fluids derived from outside the shear zone implying that feldspars may act as potentially unreliable initial isotopic reservoirs even in very high strain samples within the greenschist-facies shear zones. However, the Rb-Sr age are internally consistent and compatible broadly with existing geological and geochronological data for the region suggesting that even if the feldspars have not acted as perfect initial isotopic reservoirs the resulting errors were minimal. The general cessation of ductile deformation at c. 430 Ma probably dates the general initiation of imbrication within footwall to the Moine Thrust. The age of 430 Ma is therefore likely to be a time of significant brittle displacement on the underlying Ben More Thrust. K-Ar dating of the (re)crystallized Moine mylonite micas yielded anomalously old ages in comparison to Rb-Sr values as a result of incorporation of excess Ar during shearing. The degree of incorporated excess Ar decreases away from the base of the shear zone into the hanging wall, becoming indiscernible at c. 1.5 km structurally above the base of the shear zone. The distance (c. 1.5 km) may represent the distance that fluid can have flowed out of the shear zone during deformation.