Crystal growth in cataclasites: Diagnostic microstructures and implications

Abstract

Abstract Microstructures in cataclasites from low- and high-angle fault zones in granitic basement rocks of the Scandinavian Caledonides indicate the operation of several processes. Intensive formation of microcracks filled with fine-grained mineral fragments indicates brittle deformation. The formation of quartz crystals with euhedral terminations suggests crystal growth. Occurrence of undulose extinction, deformation lamellae and grain boundary recrystallization in other quartz grains illustrates plastic deformation. Crystal growth causes (micro)cracks to heal and thereby changes the behaviour of rocks undergoing brittle deformation. The crystallization products vary in size from c 10 μm to c. 5 mm. Narrow banding parallel to rhomb and prism planes is indicative of crystal growth. The growth bands show slightly different crystallographic misorientation. In some cases the boundaries of these bands could be identified as dislocation arrays decorated by bubbles. The occurrence of fibrous chalcedony aggregates and transitions from extremely fine-grained quartz to large grains with euhedral terminations suggests the presence of colloidal silica-bearing fluids during growth. The crystallization process leads to the formation of vein-like structures. After crystallization plastic deformation may take place, followed by brittle deformation and new crystal growth. It is concluded that during the formation of the cataclasites a cyclic operation of brittle failure, crystal growth followed by plastic deformation took place, and this cycle was repeated several times.