Three types of subvertical leucogranites and pegmatites populate the Red Cross Lake pegmatite field, located in the Red Cross Lake greenstone belt of the Sachigo Subprovince, in the northwestern part of the Archean Superior Province of the Canadian Shield. The pegmatitic rocks are situated within, or closely adjacent to, the regional North Kenyon Shear Zone. Following consolidation and minor low-temperature alteration, the leucogranites and mainly the lepidolite pegmatites were subject to intensive shearing, which reduced the pegmatites to compositionally layered mylonite-gneisses. Predominant movement within the sheared pegmatite has been dextral strike-slip with minor normal dip-slip. The layered, lenticular and rodded units within the mylonites consist mainly of spodumene + quartz, albite + quartz + K-feldspar, and lepidolite + quartz + albite, and they represent deformed primary pegmatite assemblages. The mechanical response of individual mineral species grades from brittle to ductile; the sequence is from tourmaline through beryl, K-feldspar, amblygonite, pollucite, albite, spodumene, and quartz to micas. The most ductile minerals compose the groundmass of the sheared dikes, whereas the accessory minerals and K-feldspar are brittle and have undergone the least modification in grain size and shape.