Preferred Orientation in Quartz Ribbon Mylonites

Abstract

Quartz mylonites composed of elongate ribbon quartz without appreciable recrystallization at grain boundaries were examined and contrasted. One was from a lower greenschist facies environment at Mount Isa, Australia, in which the c -axis preferred orientation of the ribbons is either a pronounced orthorhombic distribution or a small-circle distribution (with a small opening angle) about the normal to the foliation and lineation. The other was an upper greenschist or lower amphibolite facies mylonite from Risfjallet in the Swedish Caledonides, in which the c -axis preferred orientation of the ribbon is a maximum lying close to the foliation and normal to the lineation. Variation in preferred orientation can be accounted for by temperature and (or) strain-rate differences, with basal-slip mechanisms predominant at lower temperatures and prismatic slip (and possibly other slip systems), together with diffusion-controlled processes, predominant at higher temperatures.