DIRECT DATING OF MYLONITE EVOLUTION - A MULTIDISCIPLINARY GEOCHRONOLOGICAL STUDY FROM THE MOINE THRUST ZONE, NW SCOTLAND

Rb-Sr dating of sub-closure temperature, syn-deformationly crystallize d white micas from the Moine mylonites (Knockan and Dundonnell), has y ielded ages which vary from 437 Ma to 408 Ma. Morphological and major element analyses of the micas indicate that all the micas within the a nalysed samples were (re)crystallized during the Moine Thrust shearing . The ages yielded are therefore interpreted as marking the end of cry stal plastic deformation associated with shearing on the Moine Thrust. The variation in ages between samples is significantly greater than t he individual analytical errors and may be a product of strain localiz ation within the shear zone. Alternatively it may indicate flow of str ontium-rich fluids derived from outside the shear zone implying that f eldspars 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 re gion suggesting that even if the feldspars have not acted as perfect i nitial isotopic reservoirs the resulting errors were minimal. The gene ral cessation of ductile deformation at c. 430 Ma probably dates the g eneral initiation of imbrication within footwall to the Moine Thrust. The age of 430 Ma is therefore likely to be a time of significant brit tle 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 f rom the base of the shear zone into the hanging wall, becoming indisce rnible 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 f lowed out of the shear zone during deformation.