anatectic

Anatectic, in geology, refers to processes and textures produced by anatexis, the partial melting of solid rock during metamorphism. Partial melting occurs under high temperatures and often with elevated water activity, producing a melt that typically has a granitic composition and can migrate through the surrounding solid matrix.

Rocks affected by anatexis commonly form migmatites, which display a two-stage fabric: a light-colored leucosome consisting of crystallized melt, and a dark melanosome that represents residual, unmelted material. The coexistence of these components records the separation of melt from solid residue during metamorphism and the subsequent movement of melt through the rock.

Anatectic textures are identified in hand specimen and thin section by features such as interstitial or planar melt pockets, curved grain boundaries around melt films, and vein-like domains that separate melt-rich regions from the residuum. These textures indicate localized melting rather than complete rock melting and crystallization.

Significance of anatexis lies in crustal differentiation and metamorphic evolution. Partial melting generates granitic-like melts that can intrude or accumulate to form new crustal material, influencing the chemical and mineralogical makeup of crustal segments. Studying anatectic rocks helps constrain the pressure–temperature conditions of metamorphism and provides insights into the thermal history of crustal regions.

See also migmatite, partial melt, leucosome, melanosome.