Stepped fibres in sillimanite-bearing veins: valid shear-sense indicators in high grade rocks?

Sillimanite-bearing veins with striking lineated and stepped fibre surfaces occur in migmatitic amphibolite- and granulite-facies sillimanite-bearing gneisses in the Betic Cordillera, southern Spain. The quartz-dominated and peraluminous leucogranite veins occupy brittle to semi-brittle hydraulic fractures and shear fractures that cross-cut the main foliation of the gneisses. The shear sense suggested by fibre steps concurs with independent criteria where observed (e.g. foliation deflection at vein margins). Petrographic study suggests that oriented quartz–sillimanite fibres were formed during vein dilation, and experienced later ductile and brittle strain, with microboudinage and microfracturing of sillimanite aggregates perpendicular to the long axes of the fibres due to their pronounced anisotropy. The geometry of the steps is thus controlled by the initial fibre orientations, reflecting the original sense of shear during vein formation.