Grain coarsening maps for polymineralic carbonate mylonites: A calibration based on data from different Helvetic nappes (Switzerland)

Abstract

Microstructures of carbonate mylonites with 0–40 vol.% second-phase particles from different Helvetic nappes (Switzerland) have been analyzed. While the mean calcite grain size D cc increases with temperature ( T), second phases pin calcite grain boundaries. Two types of second phases can be distinguished: small second phases included in calcite grains and larger second phases at calcite grain boundaries, both coarsening with volume fraction and T. These trends are controlled by an interaction of different processes during deformation, including diffusion, dynamic recrystallization, grain growth and pinning. In terms of D cc− T dependencies, the microstructures of the investigated nappes are similar. The relationship between D cc and T differs for high T and low T. These observations can be explained by a change in the simultaneous activity of dominant mechanisms (e.g., diffusion and dislocation creep) as a function of T and second-phase content. In contrast, the relation log( d p) − 1 / T ( d p second-phase grain size) is linear with constant slope for all nappes indicating that growth processes of second phases are identical. Despite differences in fluid activity and strain rate between individual nappes, the microstructural relationships of D cc, second phases and T remain similar. Therefore, grain coarsening maps of second phase and recrystallization controlled calcite aggregates are a representative tool to predict deformation conditions and mechanisms for other shear zones developed under similar physical conditions.