greenschist

Greenschist is a metamorphic facies representing low-grade regional metamorphism. The name comes from the characteristic green minerals that commonly form in rocks of this facies, especially chlorite. Greenschist rocks typically exhibit a greenish to gray-green appearance due to the presence of chlorite, and may also contain actinolite, epidote, albite, quartz, and calcite.

The mineral assemblage of greenschist facies is modest and stable at relatively low temperatures and pressures. Common minerals include chlorite, actinolite or tremolite, epidote, albite, quartz, and often muscovite or calcite. The textures are generally foliated or schistose as platy minerals align during metamorphism, producing a green, slaty to schistose rock commonly called a greenschist.

Greenschist forms under low-grade metamorphic conditions during regional metamorphism, typically at modest burial depths with heating. It lies at a lower temperature and pressure than amphibolite facies and is higher than zeolite and prehnite-pumpellyite grades in the classic metamorphic sequence. The protoliths are varied and include basalts and other volcanic rocks, as well as pelitic and arenaceous sediments; upon metamorphism, these can become greenschist rocks such as greenstone.

In geology, greenschist facies are important for understanding early metamorphic histories and tectonic processes. Rocks of this facies are common in ancient and modern subduction-related settings and in greenstone belts, where metamorphosed volcanic and sedimentary sequences preserve evidence of low-grade thermal histories.