The deformation of plagioclase investigated using electron backscatter diffraction crystallographic preferred orientation data

Extensive plagioclase grain orientation datasets have been collected using the rapid and accurate electron backscatter diffraction SEM technique. The crystallographic preferred orientation (CPO) of a porphyroclastic-rich plagioclase layer and an equigranular plagioclase layer in an amphibolite facies shear zone from Harris, Scotland, were investigated in order to study their CPOs and deformation mechanisms. The layers have random CPOs. Together with other microstructures and misorientation distributions, these suggest dynamic recrystallisation followed by dominant grain boundary diffusion creep. This is surprising given the coarse (100–250 μm) recrystallised grain-size of the layers. Plagioclase within tectonites of these grain-sizes in the amphibolite facies are generally inferred to deform by dislocation creep from microstructures and observed strong CPOs. Deformation mechanism maps show that this style of deformation is possible at very slow strain rates.