The relationship between mineralogy and bulk composition was examined in mylonites developed in a variety of rock types from the Grenville Front zone of eastern Labrador. Mylonites developed from olivine-, pyroxene-, and (or) amphibole-bearing protoliths are the most altered, typically being enriched in K2O and volatiles and depleted in CaO compared with the protolith. Bulk-compositional modification was accompanied by the formation of almandine-rich garnet and (or) relatively ferruginous hornblende in medium-grade (epidote–amphibolite to lower amphibolite facies) mylonites derived from both mafic and quartzo-feldspathic rocks. Protoliths containing biotite as the principal or sole Fe–Mg phase were the least altered during deformation. Apart from variations in Ti content, biotite in mylonitic assemblages derived from these rocks is compositionally similar to that in the low-strain precursor.Bulk-compositional changes recognized in narrow mylonite zones appear to have occurred in Grenvillian tectonites developed on a regional scale.