blueschistfacies

Blueschist facies is a set of metamorphic minerals and textures that form under conditions of high pressure and relatively low temperature. These conditions are typically found in the subduction zones of convergent plate boundaries. The characteristic minerals of blueschist facies include glaucophane, lawsonite, epidote, and aragonite. The presence of these minerals indicates that the rocks have been subjected to pressures exceeding 0.5 GPa and temperatures generally below 400°C.

Rocks that have undergone blueschist-facies metamorphism often exhibit a distinctive blue or greenish-blue color due to the presence of glaucophane, a sodium-rich amphibole. The textures can vary but often include fine-grained, foliated fabrics. Blueschists are important indicators of past subduction events and provide valuable insights into the tectonic processes occurring deep within the Earth's crust and upper mantle. They are found in many orogenic belts around the world, often associated with ophiolite complexes and fragments of oceanic crust that have been tectonically exhumed. The low-temperature, high-pressure conditions are crucial for the stability of the index minerals, differentiating blueschist facies from other metamorphic facies like greenschist or eclogite.