Garnet amphibolite lenses around the Callander Bay alkaline carbonatite complex are transformed by fenitization into alkali feldspar – hastingsite rocks, partly by ionic exchange reactions, and partly by replacement of mafic minerals. The chemical composition of the mafic rocks converges with fenitization towards that of salic fenites. Literature data suggest that these observations are typical of mafic fenites. The fenitizing solutions at Callander Bay were alkali chloride brines 10–25 molar in strength with K/Na + K ratios of 0.23, and a K/H+ ratio of 30. These brines arose from the silicate portion of the complex, and the close association of carbonatite and fenite stems from a factor common to both carbonatite and alkaline silicate rocks, probably a high water content in the parental magma.