metamorphic facies
Quick Summary
Metamorphic facies are sets of mineral assemblages in rocks that form under specific pressure and temperature conditions during metamorphism. The concept, introduced by Pentti Eskola in 1914, helps geologists correlate metamorphic events. Different rock compositions within the same metamorphic event will exhibit distinct mineral assemblages, but these assemblages consistently relate to each other within a given facies.12
Common facies are named after the mineralogy of basaltic rocks under varying conditions:
* Blueschist facies: High pressure, low temperature.13
* Greenschist facies: Moderate pressure and temperature, characterized by green minerals like chlorite.14
* Amphibolite facies: Higher temperatures, with amphibole minerals like hornblende.1
* Granulite facies: Very high temperatures.1
Contact metamorphism, occurring near igneous intrusions, can create facies like sanidinite, characterized by very high temperatures and low pressures, often lacking water-bearing minerals. The specific pressure-temperature-time (P-T-t) path a rock follows during metamorphism dictates the preserved mineral assemblage and thus its facies.152
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