In the western Blue Ridge province of the southern Appalachians, North Carolina, granite gneisses are converted to phyllitic mylonites (phyllonites) along the base of the Rector Branch thrust sheet. The mylonites show evidence for fluid infiltration and large volume losses. With decreasing distance from the thrust and local shear zones, feldspars undergo grain-size reduction by cracking followed by chemical breakdown to muscovite, sericite, and zoisite. The following reaction was important during mylonitization: 3 K-feldspar + 2 H + = muscovite + 2 K + + 6 SiO 2 Quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300° C and 500° C in the presence of a fluid. The mylonites are enriched by a factor of at least 2.5 in elements such as TiO 2, P 2O 5, Zr, Y, V, La, Ce and depleted in SiO 2 in comparison to their protolith gneisses. The immobile element enrichments are attributed to enrichment in residual phases such as ilmenite, zircon, apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60%. Mass balance calculations, based on major element chemistry, indicate the mylonites lost large amounts of SiO 2 and smaller amounts of alkalies and were infiltrated by fluid/rock weight ratios of between 10 2 and 10 3. Phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas. Reaction-enhanced softening was apparently important in strain localization.