Glossary: Blueschist Facies
|Blueschist facies is a low to medium-grade metamorphic facies typified by low temperatures and associated with subduction. Blueschist facies rocks, however, are often present with orogenic zones as exhumed terrains sometimes including ophiolites and often mark sutures between continental regimes. Blueschist facies metamorphism occurs at temperatures of approximately 200 to 500 degrees celsius and pressures in excess of 4 to 12 kbar. |
Changes in mineralogy depend very much on protolith, however, production of glaucophane and jadeite are most characteristic, but are most common in rocks with basaltic protoliths. Chlorite, epidote and kyanite are common. Jadeite forms at higher pressures by the reaction of albite and quartz. Typical assemblages for different protoliths include:
mafic - glaucophane +/- lawsonite +/- jadeite (high-P) +/- actinolite or hornblende +/- albite (low-P) +/- titanite +/- pumpellyite +/- stilpnomelane +/- epidote +/- garnet.
pelites - glaucophane + chlorite + phengite +/- paragonite +/- lawsonite +/- epidote +/- kyanite +/- quartz (or coesite at very high P) +/- chloritoid +/- titanite.
quartz-feldspathic - jadeite (high-P) + lawsonite + muscovite +/- chlorite +/- titanite +/- glaucophane.
calc-silicate - calcite/aragonite.
Blueschist facies rocks have frequently experienced some retrograde metamorphism during exhumation to the surface.
Schists dominated by glaucophane, with basaltic protoliths are often termed blueschists. Blueschist facies rocks can be highly variable and include slates, schists, and blueschists.
Related Termsmetamorphic grade, metamorphic facies, zeolite facies, prehnite pumpellyite facies, greenschist facies, blueschist facies, amphibolite facies, granulite facies, eclogite facies, sanidinite facies, albite epidote hornfels facies, pyroxene hornfels facies, hornblende hornfels facies