A fracture model for quartz ribbons in straight gneisses

Polycrystalline composite quartz ribbons in straight gneisses of the Grenville Province are explained as dilatant,quartz-filled fractures formed parallel to a strong preexisting gneissosity. The ribbons are thin (less than or equal to 3 mm), but long and continuous and have straight and truncating contacts with bordering grains of other minerals. They are similar in morphology to polycrystalline ribbons interpreted in the literature as resulting from high strains. In contrast, little strain is required if the ribbons form as quartz-filled fractures. A monoclinic symmetry of the shapes of ribbons and inclusions can be explained by fracture interactions as they propagate. The symmetry is consistent with a late extensional, south-side-down sense of shearing, which has been documented elsewhere in the Grenville Province. (C) 1997 Elsevier Science Ltd.