Dislocation microstructure and phase distribution in a lower crustal shear zone – an example from the Ivrea-Zone, Italy

The analysis of fabric and microstructure across an amphibolite facies shear zone of mafic composition reveals that the strain-dependent change from grain size insensitive to grain size sensitive creep is associated with a fundamental reorganization of the mylonitic fabric. At moderate strain a banded mylonite evolves from a metagabbro, which displays a mechanically-induced compositional layering. Strain is concentrated in monomineralic layers of dynamically recrystallized plagioclase. At higher strain and decreasing grain size (10–30 µm) the phase segregation is progressively destroyed and replaced by a phase mixture of amphibole and plagioclase. Phase mixing in these ultramylonites is developed and stabilized by heterogeneous nucleation processes of amphibole and plagioclase within unlike phases and at dilatant sites. Nucleation appears to be controlled by grain-scale gradients in stress. A dispersed phase distribution in fine-grained ultramylonites indicates (water-assisted) diffusion processes that accommodate grain boundary sliding. Although diffusion-controlled creep plays a dominant role in these ultramylonites, the dislocation densities remain high (2.0–4.0×109 cm–2) and indicate that two competing mechanisms (dislocation and diffusion creep) accommodate grain boundary sliding. Commonly accepted criteria for superplastic or granular flow derived from monomineralic aggregates must be applied with caution to polymineralic rocks of mafic composition.