paragneisses

Paragneiss is a metamorphic rock of gneissic texture whose protolith is sedimentary or derived from sedimentary rocks. It is typically formed during regional metamorphism at high temperatures and pressures, producing pronounced foliation and coarse, compositional banding. The light-colored bands are mainly quartz and alkali feldspar; the dark bands are rich in mica (biotite or muscovite) and minor mafic minerals; garnet, sillimanite, or kyanite may occur as porphyroblasts, depending on metamorphic grade.

The protoliths include pelites (shales, mudstones), psammites (sandstones), and calcareous rocks; paragneiss can originate from carbonate rocks that produce calc-silicate minerals.

The term paragneiss contrasts with orthogneiss, which arises from a granitoid or other igneous rock. In practice, petrographic analysis and isotopic dating are used to determine provenance. If partial melt occurs, paragneiss may develop leucosomes and melanosomes, forming part of migmatites.

Paragneiss is common in continental crust and occurs in metamorphic belts associated with orogenic events. It displays a characteristic layering at multiple scales and can grade into granulite or amphibolite facies forms, depending on conditions.