Grain size stabilisation by dispersed graphite in a high-grade quartz mylonite: an example from Naxos (Greece)

High grade quartz mylonites from Naxos, Greece, consist of alternating thin layers of pure quartz and quartz layers with 0.3–3 vol.% finely dispersed graphite particles. Graphite-free layers are coarse grained (100–300 μm), show undulose extinction, subgrains, lobate grain boundaries and have a strongly developed crystallographic preferred orientation. In these layers, dislocation flow is interpreted to be the dominant deformation mechanism. In contrast, graphite-rich layers are fine-grained (30–70 μm), have equant quartz grain shapes and have a crystallographic preferred orientation that becomes progressively weaker with decreasing quartz grain size. Given the high temperature of deformation and the need for a c-axis fabric destroying mechanism, grain boundary sliding is interpreted to be important in these layers. Analysis shows an inverse relationship between quartz grain size and the graphite dispersion, suggesting stabilization of quartz grain size by graphite particles. Graphite particles larger than 5 μm are concentrated along quartz boundaries, suggesting that stabilisation only operates above a certain critical graphite particle size. This study shows that a dispersed second phase such as graphite in a naturally deforming rock can inhibit grain boundary migration, stabilise the grain size and enhance grain boundary sliding at the expense of dislocation flow.