Sillimanite-bearing shear zones in syntectonic leucogranite: fluid-assisted brittle-ductile deformation under amphibolite facies conditions

In the Imja Kola valley (eastern Nepal), Miocene peraluminous leucogranite dykes were emplaced within transtensional shear zones synchronously with regional amphibolite–facies metamorphism in the metamorphic host rocks. Within the dykes the magmatic foliations, parallel to the metamorphic schistosity, are cross-cut by thin (millimetre to centimetre size) shear zones, which are made up of prismatic and/or fibrolitic sillimanite with a minor amount of white mica. These structures have a heterogeneous distribution and form networks of strongly deformed and foliated layers wrapping around poorly deformed granitic pods. The textural and strain features are consistent with development of sillimanite shear zones as a result of heterogeneous deformation and fluid action, which allowed the replacement of strained magmatic minerals such as feldspar and biotite by means of incongruent pressure solution. The described example illustrates that nucleation and development of heterogeneous shear zones under amphibolite facies conditions, is also strongly favoured by brittle fracturing and fluid action at the grain-scale along pre-existing planes of weakness. The growth of cracks and fractures can be regarded as a sequence of short lived brittle events, due to work hardening, which punctuate a history of plastic deformation.