Evidence for deformation‐induced K‐feldspar replacement by myrmekite

Abstract

ABSTRACT Several examples of deformation‐induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths, namely a muscovite‐poor S‐C mylonite and a single foliation, muscovite‐poor mylonitic gneiss. Back‐scattered SEM and conventional optical microscopy show that in both rock types, syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of K‐feldspar grains that faced the local inferred incremental shortening direction for the mylonite. Myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction.The replacement of K‐feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains. We suggest that the ambient temperature, pressure and chemical activities were such that the replacement reaction was favoured, but the addition of extra strain energy along the high‐pressure sides of the grains localized the reaction at these sites. This energy could arise from elastic strain, or strain associated with tangled dislocations or twin boundaries. The relative roles of stress and strain energy concentrations in driving the replacement reaction are not known, but both were probably important.