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

Quartz microstructures and c-axis fabrics formed during development of poly crystalline quartz ribbons in striped gneisses from the high-grade Alem Par aiba shear zone, in southeastern Brazil, are documented. Cluster analysis o f quartz grains in samples exhibiting different degrees of shear strain rev ealed that formation of ribbons was a mass conservative process, where isol ated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons. These ribbons are separated by feldspar-rich doma ins devoid of quartz. The stage at which individual, stretched quartz grain s 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 smoothnes s of the ribbon boundaries. This change is interpreted to represent a strai n-softening kink in the stress-strain-time path. Progressive ribboning is a ccompanied by strengthening of the c-axis fabric Z-maximum, indicative of c ontinued plastic Row by basal [ a ] glide. Operation of basal [a ] glide at these high-temperature conditions (680-700 degreesC) is interpreted to be a consequence of relatively dry deformation conditions. A model is then pro posed for development of straight quartz ribbons in high-grade striped gnei sses, where scattered quartz grains are continuously stretched and segregat ed by crystal-plastic processes. The small angle misorientation of the cont acting grains enables subsequent coalescence and resulting grain size enlar gement. Pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons. (C) 2001 Elsevi er Science Ltd. All rights reserved.