Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally molten material, and are not generally the result of solid-state reactions. Migmatites are composed of a leucosome, new material crystallized from incipient melting, and a mesosome, old material that resisted melting. Commonly, migmatites occur within extremely deformed rocks that represent the base of eroded, mountain chains, typically within Precambrian cratonic blocks.
Migmatites often appear as tightly, incoherently folded (ptygmatic folds) dikelets, veins and segregations of light colored granitic composition called leucosome, within dark colored amphibole and biotite rich material called the melanosome. The light colored material has the appearance of having been mobilized or molten. Once enough leucosomes join up to form a network and granite is produced, the residual material is known as restite.
Ptygmatic folds are formed by highly plastic ductile deformation of the gneissic banding, and thus have little to no relationship to a defined foliation unlike most regular folds. Ptygmatic folds can occur restricted to compositional zones of the migmatite, for instance in fine-grained shale protoliths versus in coarse granoblastic sandy protolith.
Migmatised igneous or lower-crustal rocks which melt do so to form a similar granitic I-type granite melt, but with distinct geochemical signatures and typically plagioclase dominant mineralogy forming monzonite, tonalite and granodiorite compositions. Volcanic equivalents would be dacite, trachyte and trachydacite.
It is difficult to melt mafic metamorphic rocks except in the lower mantle, so it is rare to see migmatitic textures in such rocks. However, eclogite and granulite sensu stricto are roughly equivalent mafic rocks.