DUCTILE DEFORMATION OF TONALITE IN THE SUOMUSJARVI SHEAR ZONE, SOUTH-WESTERN FINLAND

Different states of deformation of tonalite with a weak original conti guity of plagioclase were studied in the Suomusjarvi sheer zone, south -western Finland. Weakly deformed tonalite is characterised by strain concentration in a conjugate network of narrow shears. Quartz and biot ite deformed by crystal plas ticity while plagioclase shows extensive fracturing. An interconnected weak layer structure is typical for this stags of deformation, with a densely packed strong phase. a relativel y small proportion of weak matrix and a high competence contrast betwe en the strong and weak phases. A high concentration of stress and stra in rate in weak minerals produced continuous fracturing of relict plag ioclase grains. This deformation stage was not accompanied by changes in the bulk chemistry of the rock. Increasing deformation led to a maj or change in deformation mechanism, expressed by granular how in the d uctile matrix and rigid body rotation of ovoid feldspar crystals. The size of the plagioclase crystals decreased by a combination of dynamic s crystallisation and chemical dissolution. This strain stage was also associated with fluid infiltration into the progressively narrowing s hear 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 o void grains. (C) 1998 Elsevier Science Ltd. All rights reserved.