Brittle-ductile microfabrics in naturally deformed cordierite: Evidence for significant short-term strain-rate variations

Brittle and ductile microfabrics in cordierite from a migmatite were studied by universal-stage method and electron probe micro-analysis. The microfabrics record crystal fracturing and ductile deformation of one deformation event at ∼570 °C and ∼380 MPa, but at variable strain rates. The cordierite microfabrics evolve in four stages. (1) At strain rates of ≥∼10^-7 [s^-1] brittle micro shear zones form, producing rotated fine-grained cordierite crystal fragments. (2) At decreasing strain rates (001), (010), and (100) subgrain boundaries develop and fluid circulation leads to partial cordierite breakdown and oriented growth of sillimanite, staurolite, magnetite, and quartz along shear planes. (3) Subsequent to deformation and during continuous infiltration of fluids, the same minerals grow randomly along shear planes and cordierite recrystallizes statically. (4) During continuous annealing and fluid infiltration the recrystallized cordierite grains coarsen and sillimanite, staurolite, magnetite, and quartz form along the grain boundaries. Brittle high-temperature and high-strain-rate microfabrics, followed by ductile high-temperature deformation at lower strain rates and/or by recovery, as recorded in the cordierite studied, may be more common in naturally deformed rocks than typically assumed. This is evidence for and may be used as an analytical tool to detect paleo-seismic events.