Orthogneisses

Orthogneisses are high-grade metamorphic rocks formed by the metamorphism of pre-existing igneous rocks, typically granitoids, under heat and pressure sufficient to develop a gneissic texture. The protoliths are usually granites, granodiorites, tonalites, or diorites; rock that originated as sedimentary material may form paragneiss. The term orthogneiss distinguishes igneous-derived gneisses from paragneisses derived from sedimentary rocks.

The defining feature is gneissic banding: alternating light- and dark-mineral-rich layers produced by recrystallization and differential mineral growth during metamorphism. The foliation aligns with these bands. Mineral assemblages depend on metamorphic grade and original composition: quartz and feldspars (K-feldspar and plagioclase) are common, with biotite and amphibole; higher-grade varieties may contain garnet, sillimanite, or kyanite.

Geologic settings: Orthogneisses are common in continental crust, especially in ancient shields and in slices within orogenic belts. They form under amphibolite to granulite facies metamorphism; some are migmatitic, with partial melting creating leucosome and melanosome regions that complicate the texture.

Age and interpretation: Zircon dating of orthogneiss protoliths yields the crystallization ages of the original igneous rock, while metamorphic ages constrain later tectonometamorphic events. Distinguishing orthogneiss from paragneiss involves petrographic and field evidence of igneous protoliths and the absence of primary sedimentary features or detrital components. Orthogneisses thus provide insights into crustal evolution, metamorphism, and tectonic processes in continental regions.