migmatite

Migmatite is a coarse-grained metamorphic rock that displays both metamorphic and igneous characteristics. It forms when a rock in the crust is heated to high temperatures and begins to partially melt, producing a granitic or granodioritic melt that separates from the unmelted residue. The resulting rock typically consists of two main constituents: a light-colored leucosome representing crystallized felsic melt, and a darker melanosome composed of the unmelted, mafic-rich residue. The two components can be interbanded or interwoven, with leucosome forming sheets or irregular patches within the darker matrix. The term migmatite reflects its mixed igneous‑metamorphic character.

Formation and textures are governed by high-temperature metamorphism often at granulite to amphibolite facies. Partial melting commonly involves quartzo‑feldspathic components in the presence of fluids, producing a melt enriched in silica and alkalis. The remaining solid phase, known as restite, may include feldspar, biotite, garnet, and other minerals. In some rocks the leucosome forms stromatic or curvilinear bands, giving a distinctive layered appearance.

Occurrence and significance: Migmatites are widespread in continental crust and are typical of zones that have experienced extreme crustal heating, such as major orogenic belts and deep crustal levels. They record the onset of partial melting during metamorphism and help constrain the thermal and tectonic history of their terranes. Dating of the melt and associated minerals provides insights into the timing of crustal differentiation and metamorphic events.