Quartz plastic segregation and ribbon development in high-grade striped gneisses

Quartz microstructures and c-axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high-grade Além Paraı́ba shear zone, in southeastern Brazil, are documented. Cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process, where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons. These ribbons are separated by feldspar-rich domains devoid of quartz. The stage at which individual, stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode, which is reflected by an abrupt increase in the smoothness of the ribbon boundaries. This change is interpreted to represent a strain-softening kink in the stress-strain-time path. Progressive ribboning is accompanied by strengthening of the c-axis fabric Z-maximum, indicative of continued plastic flow by basal<a> glide. Operation of basal <a> glide at these high-temperature conditions (680–700°C) is interpreted to be a consequence of relatively dry deformation conditions. A model is then proposed for development of straight quartz ribbons in high-grade striped gneisses, where scattered quartz grains are continuously stretched and segregated by crystal–plastic processes. The small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement. Pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons.