The origin of deformation lamellae in quartz

Abstract

Fabrics of 4 quartzite specimens containing numerous quartz grains with deformation lamellae are described in detail. Patterns of preferred orientation of deformation lamellae in all 4 specimens are similar in that the poles of the lamellae define a small-circle girdle (about an axis designated A). The orientations and strengths of maxima within the girdle, however, are not consistent in different specimens. [0001]-axes of grains containing deformation lamellae also define a small-circle girdle about the same axis (A). In each specimen the great-circles containing [0001] and the pole of the deformation lamellae for individual grains pass through, or close to, the axis A of the small-circle girdles. The deformation lamellae are shown to be late structures unrelated to the deformations which induced the preferred orientation of the quartz grains in the rocks. The lamellae are not parallel to rational crystallographic planes and they are con- sidered to represent kink-bands resulting from shearing parallel to [0001] on irregular planes in the zone of [0001]. The shearing is probably controlled by imperfections in the crystal structure, which commonly exist parallel to [0001] in quartz. According to this hypothesis the axis (A) of the small-circle girdle defined by poles of lamellae is the axis of maximum compressive stress during the deformation which produced the lamellae. This relationship may be used to obtain a dynamic interpretation of deformation lamellae in quartzose sedimentary and metamorphic rocks. The hypothesis is tested using data from the Baraboo quartzite and it is demonstrated that the deformation lamellae in the rocks of this formation may be related to the folding of the Baraboo syncline.