Continent-continent collision zones of various ages are characterized by low magnetic susceptibilities and hence weak intensities of induced magnetization; hydration reactions in deep-crustal rocks from the Grenville province of the Canadian Shield may offer an explanation for the weak induced magnetization of these zones. Troctolite with an oxide assemblage of Cr-bearing magnetite + Cr-Mg-bearing hercynite + ilmenite and magnetic susceptibilities of ∼10-3 to 10-2 (SI units) has been converted by hydration and deformation to a rutile-bearing plagioclase-amphibole-garnet gneiss with susceptibilities of ∼5 x 10-4. The complete transformation of troctolite to gneiss is essentially isochemical except for H2O content and occurs over 1-2 m in the field. The order-of-magnitude change in magnetic susceptibility is even more abrupt. This susceptibility contrast corresponds to decomposition of the oxides of the troctolite (particularly Cr-bearing magnetite), liberating Fe, Al, Cr, and Ti to be incorporated in amphibole and rutile. Pervasive deformation of the lower crust in continental collisions may be accompanied by widespread hydration and the destruction of magnetic minerals, explaining why many continental collision zones have weak intensities of induced magnetization.