What is a leptynite in geology ?

Leptynite - Metamorphism

Metamorphic facies diagram
A leptynite is a metamorphic rock close to gneisses (and sometimes considered a type of this rock), light in color, mainly composed of quartz and alkaline feldspars, poor in mica and amphibole, but sometimes with garnet and / or cordierite.

Leptynites derive from the metamorphism of arkosic sandstone (paraleptynites) or granite (ortholeptynites). The appearance of ultrazone (zone of very high degree of metamorphism just before anatexis) is sometimes defined by the formation of cordierite and / or garnet leptynite.
Leptynite de St-Yrieix-la-Perche, Haute-Vienne - © Populaire du Centre