Metamorphic Facies

See http://plate-tectonic.narod.ru/metpetrographylinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html

A. Definition

-"In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, the mineral composition is controlled only by the chemical composition." (Eskola, 1915)

-Both descriptive and interpretive

-11 facies generally used (Eskola defined 5).
-Most common prograde sequence goes through middle of diagram: greenschist-granulite
-High-P sequence: blueschist-eclogite
-Low-P: sequence: various hornfels facies
-Low-grade facies: zeolite & prehnite-pumpellyite

II. Application to mafic rocks

A. This discussion is based on historical development of facies concept.

-Mafic = basalt/gabbro - andesite/diorite, or graywacke
-Many met. reactions involve hydrous minerals => water must be present for mafic rocks to be metamorphosed.

B. Low-grade facies

-Zeolite facies:

-First change is volcanic glass > heulandite (CaAl2Si7O18*7H2O)
-Crystalline minerals unchanged.
-As T increases (Heulandite > laumontite (CaAl2Si4O12*4H2O)/Ca-plag > Na-plag/Chlorite forms [extremely variable, ~ (Mg,Fe,Al)6(Si,Al)4O10(OH)8]).
-Prehnite-pumpellyite facies:

-Prehnite: sheet silicate [Ca2Al(AlSi3O10(OH)2], metamorphic or cavity-filling (Zeolites > epidote [Ca2(Al,Fe)Al2Si3O12(OH)] or zoisite [Ca2Al3Si3O12(OH)]/Actinolite at higher T

C. Medium P facies

-Greenschist: characterized by chlorite, epidote, actinolite [Ca2(Mg,Fe)5Si8O22(OH)2]: typical rocks in shaded area/ more Al-rich rocks may have chloritoid [(Mg,Fe)2Al4 Si2O10(OH)4]
-Amphibolite:

-Forms through 2 changes (Albite > oligoclase / Actinolite > hornblende/ Actual chemical reactions are complex)

-Granulite facies:

-Characterized by anhyrous minerals as Hb > Opx + Cpx.
-Garnet is common, Plag and Q also present.
- granulites represent high T, and since they don''t melt, must be dry.

D. Low-P facies

-In mafic rocks, these facies grade into higher-P facies
-Low-P facies more distinct in pelitic rocks.

E. High-P facies

-Blueschist facies:

-Characterized by blue amphibole: glaucophane [Na2Mg3Al2Si8O22(OH)2] or riebeckite
-Reactions involve breakdown of chlorite + albite to glaucophane.
-Also, albite > jadeite (sodic pyx - NaAlSi2O6) + Q

-Eclogite facies:

-Characterized by garnet + omphacite (green pyroxene)
-Reaction is Gln + Prg (paragonite = Ms polymorph) > Prp [pyrope garnet Mg3Al2Si3)12] + Omp [(Ca,Na)(Fe,Mg,Al)2)6].
-Also, An breaks down to Omp + Prp or to an assemblage with Ky