gnaisses

Gnaisses, the French plural form for gneiss, designate a class of metamorphic rocks defined by pronounced foliation and compositional banding that results from high-grade metamorphism of pre-existing rocks.

They form during regional metamorphism at convergent plate boundaries, where elevated temperatures and pressures promote mineral segregation into alternating light bands rich in feldspar and quartz and dark bands rich in mafic minerals, producing the characteristic gneissic texture.

The mineralogy typically includes quartz and feldspar in the light bands, with mafic minerals such as biotite, hornblende, and sometimes garnet or pyroxene in the dark bands; accessory minerals vary with metamorphic conditions and the nature of the original rock (the protolith).

Gneiss can be derived from igneous rocks (orthogneiss) or sedimentary rocks (paragneiss), depending on the protolith before metamorphism. Variants occur across a range of metamorphic conditions, commonly associated with amphibolite to granulite facies in higher-grade terrains.

Gneisses are widespread in continental crust and form major parts of shield areas and metamorphic belts. They are important for understanding regional tectonic histories in regions such as the Canadian and Baltic shields, the Alps, and the Himalayas, and are sometimes used as construction or decorative stone in local contexts.