Phase mixtures in shear zones

Abstract

Deformation of polymineralic rocks at elevated temperatures usually results in grain size refinement but also in a grain scale mixture of mineral phases. Phase mixtures may either be homogeneous or exhibit a compositional and microstructural layering. Depending on the host rock stability, mixtures may consist of combinations of redistributed or newly, synkinematically formed phases. In general either neighbour-switching or heterogeneous nucleation are envisaged as processes responsible for mixing during diffusion creep s.l. including grain boundary sliding and grain-scale transport processes. In order to quantify phase mixtures, neighbourhood relations can be analysed to differentiate random, clustered or anti-clustered distributions. Heterogeneous nucleation is usually considered to allow for anti-clustered distributions, while neighbour switching during grain boundary sliding potentially produces random distributions. Here, phase mixing is explored based on contact densities as well as on centre-to-centre distances. In particular, the effect of directionality that is neighbour relations as a function of the relative position in a 2D section is considered. The direction of neighbours is considered by the normal of the boundary trace as well as alternatively, by the direction of the centre-to-centre join. Given sufficiently large datasets and non-extreme mixtures (e.g. with 0.2 < phase proportion < 0.8) a confidence interval of the results can be defined. Large datasets of ultramylonites of different metamorphic grade, phase proportions, compositions (ultramafic, mafic, quartzo-feldspatic) and microstructures (layered, isotropic) are tested. It is found that phase anti-clustering is generally more pronounced in a direction close to the stretching direction in either layered or homogeneous ultramylonites. In layered mylonites, layer-normal relations are frequently found to be random while intralayer relations are often anti-clustered. In the different rock types, specific anti-clustered phases can be discriminated, e.g., orthopyroxene with respect to olivine, k-feldspar with respect to plagioclase and quartz, and hornblende with respect to plagioclase. Other phase assemblages e.g. quartz-plagioclase are frequently found to be distributed randomly, hinting at mineral specific roles during diffusion creep s.l. and generally at element mobilities in deforming metamorphic rocks.