Reaction softening in mylonites from the Arnaboll thrust, Sutherland

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Abstract
Synopsis: Mylonites from the Arnaboll thrust, NW Scotland show considerable evidence for having been produced as a result of reaction softening or reaction-enhanced ductility. The effect has been to degrade an original, coarse-grained, granitic Lewisian gneiss to an extremely fine grained mylonite. Grain size reduction which relates to increasing strain has been accompanied by both a mineralogical and chemical change. Notably, feldspar disappears to be replaced by a fine-grained muscovite plus quartz assemblage at the highest strain. Chemical analyses and mass balancing suggest that this change is related to the addition and removal of material during shear deformation along this ductile shear zone. This has aided the reaction softening process and, possibly, superplastic behaviour in the rocks which characterises the highest strain ultramylonites found at the Arnaboll thrust.