GRANITIZATION

formation of granite or closely related rocks by metamorphic processes, as opposed to igneous processes in which such rocks form from a melt, or magma, of granitic composition. In granitization, sediments are transformed in the solid state or in a partially molten state. The solid-state process requires the addition and removal of chemical components by solid-state diffusion, vapour transport, or the movement of certain fluids such as aqueous solutions. Solid-state granitization may occur on a small, localized scale, as in the formation of migmatite in which igneous rocks of granitic composition are intermixed with high-grade metamorphic rocks. Where more massive granites are formed, partial melting usually has occurred. The most convincing evidence for such large-scale granitization is the common sequence of relations where high-grade metamorphic rocks merge into migmatite gneiss and, in turn, into foliated granites and finally into massive granites (as in the vast rock complexes in fold-mountain zones). In recent years geochemical criteria have been developed to help sort out the various types of granites, and both granitization and crystal-liquid differentiation from magmas are invoked to explain differences between granites from the same geological region.