Set of metamorphic mineral assemblages, repeatedly associated with each other in space and time.
There is a predictable and common correspondence between the mineralogy of each rock and its bulk chemical composition.
The mineral assemblage defining the metamorphic facies indicates that a state of stable equilibrium has been attained over a restricted P-T condition.
Several different facies have been formally recognized and have names referring to distinct aspects of the facies.
e.g. green schist facies = characterized by schists in which green chlorite, talc, serpentine, epidote and actinolite are predominant.
Mineralogical characteristics of the metamorphic facies corresponding to those on the
Handout - Facies vs. Grade are:
Zeolites are groups of white to colourless hydrous alumina silicates, analogous in composition to feldspars.
Laumontite, heulandite and analcine are common, the assemblage Quartz + Laumontite + Chlorite is diagnostic.
Prehnite - Pumpellyite
Prehnite +/- Pumpellyite + Quartz is typical.
Prehnite - Ca Al 2 Si 2 O 3 (OH) 10 Pumpellyite - Ca 2 MgAl 2 (SiO 2 )(Si 4 O 2 )(OH) 7 H 2 O
2 Glaucophane - Lawsonite Schist or Blueschist
High P Low T
Glaucophane (Na MgAl 2 Si 2 O 8 (OH) 22 ) (alkali-rich amphibole) + Lawsonite (CaAl 2 Si 2 O 2 (OH) 7 H 2 O) are characteristic 2
Jadiete (NaAlSi O 2 ) + Qtz + Aragonite
Albite + Epidote + Actinolite + Chlorite + Calcite in mafic rocks
pyrophyllite in pelitic rocks
Hornblende + Plagioclase (An > 20)
Kyanite (Al SiO 2 ) in pelites
lower crustal lithology
cpx + opx + plag + Fe - Mg garnet
lower crust or upper mantle lithology
Feldspar-free assemblages, with jadiete-rich cpx + pyrope (Mg-rich garnet)
The pressure and temperature range for each facies given in
Handout - Facies vs. Grade. The assemblages listed above are mainly for mafic (Metabasite) lithologies. Boundaries between individual facies are not sharp, marked by reactions which occur over a P-T range depending on composition.
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