The gneisses of the Chapleau portion of the Kapuskasing Structural Zone (KSZ) and of the Wawa Gneiss Terrane (WGT) can be partitioned into subregions on the basis of magnetic polarity of 2.45 Ga Matachewan diabase dykes emplaced within them. West of the Ivanhoe Lake fault zone (ILFZ) the polarity sequence across the trend of the dyke swarm is normal–reverse–normal (N–R–N) over a traverse distance >100 km. Highly correlated with dyke N polarity is the presence of tea-coloured groundmass plagioclase and hornblende with high Al, Na, and Ti content. Reversely magnetized dykes within the WGT and in dykes of either polarity in regions far removed from the KSZ have groundmass plagioclase with hydrous alteration and relatively Al-poor amphiboles. Although N and R magnetized dykes have groundmass plagioclase with comparable Fe contents, the plagioclase of N dykes and Kapuskasing dykes in the highest grade country rock contain discrete, micrometre-sized, Ti-poor magnetite particles. The mineralogical variations are independent of whole-rock bulk chemistry and are ascribed to greater crystallization depth for N magnetized dykes. The magnitude of the amphibole compositional change in dykes at the western N–R boundary within the WGT is comparable to that across the ILFZ. The western N–R boundary is wholly within the WGT, whereas the ILFZ juxtaposes upper and lower crustal levels. As a consequence, only some of the differential uplift between the KSZ and the Abitibi belt can be accounted for in post-Matachewan dyke time. Limited data from the amphiboles in the 2.04 Ga east-northeast-trending Kapuskasing dykes suggest that this phase of faulting along these block boundaries postdates Kapuskasing dyke emplacement.