Deformation mechanisms in a high-temperature quartz-feldspar mylonite: evidence for superplastic flow in the lower continental crust

Abstract

Microstructures and crystallographic preferred orientations in a fine-grained banded quartz-feldspar mylonite were studied by optical microscopy, SEM, and TEM. Mylonite formation occurred in retrograde amphibolite facies metamorphism. Interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity. Strain incompatibilities during grain sliding in the superplastic quartz-feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar, the rate of which probably controls the rate of superplastic deformation. There is evidence for equal flow stress levels in the superplastic and crystal-plastic domains. In this case mechanism partitioning results in strain-rate partitioning. Fast deformation in the superplastic bands therefore dominates flow, and deformation is probably best modelled by a superplastic law. If this deformational behaviour is typical, shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress, or at low differential stresses in case of fixed strain rates.