A model for the development of a domainal quartz c-axis fabric in a coarse-grained gneiss

Abstract

Quartz c-axis fabrics in high-grade coarse-grained layered gneisses reveal a distinctive pattern, with three perpendicular maxima, which can be interpreted in terms of syntectonic recrystallization and growth. Mapping of the distribution of c-axes within a single thin section demonstrates that the fabric is domainal. Fabric domains are on the scale of millimetres and are associated with individual quartz-rich patches. No single domain exhibits the overall fabric of the complete thin section. Within each domain, weighting of the c-axis fabric by the area of each quartz grain demonstrates that the largest grains have their c-axes preferentially at the maxima. A model is presented to account for the development of this fabric. Grains which can keep their stored strain energy to a minimum by the use of a single glide system, oriented approximately parallel to the plane and direction of maximum resolved shear stress, grow at the expense of other grains. The largest, most successful grains deform by glide on only one of either the basal ( a) or the prism ( a) systems, depending on the crystallographic orientation of the grain. Each domain develops its own glide direction by co-operation of the glide systems of its constituent grains. The successful grains in a domain are those which can accommodate that direction. The bulk strain of the rock is achieved by the addition of the strains distinct to each domain.