Abstract

Shear zones are often characterized by the presence of mylonitic fabrics. The textural development of such fabrics is enhanced by the presence of a fluid phase. In a single specimen of rock from the Brevard fault zone in North Carolina, we can demonstrate the development of ultramylonite domain by focused fluid flow. The ultramylonite interface retains a sharp textural and chemical discontinuity; this suggests that solute transport was dominantly parallel to the tectonic layering. Major chemical changes between the ultramylonite zone and the protolith include losses of SiO2, Na2O, and K2O and gain of CaO, FeO, and H2O in the high-strain domain.

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