migmatization

Migmatization is the geological process by which metamorphic rocks reach partial melting, producing migmatites that combine metamorphic and igneous features. These rocks typically exhibit a two-fraction texture, with light-colored leucosomes representing the crystallized partial melt and darker melanosomes representing the remaining solid material. The contact between leucosome and melanosome can be sharp or gradational, and the texture may be interleaved on a variety of scales.

Formation occurs under high-temperature crustal conditions, usually within granulite to upper amphibolite facies. The presence of fluids and volatiles lowers melting temperatures, enabling partial melting in situ. The melt that forms can segregate and migrate, yielding leucosomes that often have granitic compositions, while the residuum remains as melanosome material that preserves unmelted minerals and restite components.

Geologic significance includes recording crustal reworking and regional anatexis during tectonic processes such as mountain building. The chemistry of leucosomes provides insights into the composition of partial melts, and isotopic dating of minerals within migmatites constrains the timing of melting and subsequent cooling. As such, migmatites are valuable for understanding crustal differentiation, thermal histories, and the evolution of continental interiors.

Identification relies on textural evidence of bimodal mineralogy and the presence of a melt-derived leucosome juxtaposed with metamorphic melanosome. Migmatites are widely reported in ancient continental crust and are interpreted as markers of extreme metamorphism and crustal melting in orogenic belts.