Ductile deformation of tonalite in the Suomusjärvi shear zone,south-western Finland

Abstract

Different states of deformation of tonalite with a weak original contiguity of plagioclase were studied in the Suomusjärvi shear zone, south-western Finland. Weakly deformed tonalite is characterised by strain concentration in a conjugate network of narrow shears. Quartz and biotite deformed by crystal plasticity while plagioclase shows extensive fracturing. An interconnected weak layer structure is typical for this stage of deformation, with a densely packed strong phase, a relatively small proportion of weak matrix and a high competence contrast between the strong and weak phases. A high concentration of stress and strain rate in weak minerals produced continuous fracturing of relict plagioclase grains. This deformation stage was not accompanied by changes in the bulk chemistry of the rock. Increasing deformation led to a major change in deformation mechanism, expressed by granular flow in the ductile matrix and rigid body rotation of ovoid feldspar crystals. The size of the plagioclase crystals decreased by a combination of dynamic recrystallisation and chemical dissolution. This strain stage was also associated with fluid infiltration into the progressively narrowing shear zone. A steady-state fabric was probably attained in the core of the shear zone, which is characterised by a decrease in grain size and plagioclase aspect ratio, and a loss of shape-preferred orientation. A low stress concentration in a weak matrix in dynamic equilibrium is consistent with the proportion of relict plagioclase and size of the ovoid grains.