Barrovian Facies

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

The Barrovian facies series, named for George Barrow, who described them around 1900,is the "normal" metamorphic facies series. These are the rocks formed in atypical geothermal gradient of about 25 C per km. The typical pressures under which theserocks form are 5-10 kb (mid-crustal depths). All temperatures are approximate and dependent on pressure.

Greenschist Facies

400 C  - Quartz-Albite-Muscovite-Chlorite Subfacies

bar400.gif (2435 bytes) The phyllosilicate stilpnomelane is present:
Stilp = K1-x(Fe",Mg,Al)3-xSi4O10(OH)2.nH2O

Carbonate-rich rocks may contain dolomite or calcite along with chlorite (dashed lines). Magnesite may be present in magnesium rich rocks.

450 C  - Quartz-Albite-Epidote-Biotite Subfacies

bar450.gif (2194 bytes) Stilpnomelane breaks down:
Stilp + musc = Bi + Chl + Qz

K-feldspar no longer coexists with chlorite:
Mic + Chl = Bi + Musc + Qz + H2O

Chlorite and calcite are no longer compatible:
3Chl + 10C + 21Qz = 3Act + 2Ep +8H2O + 10CO2

500 C  - Quartz-Albite-Muscovite-Almandine Subfacies

bar500.gif (2341 bytes) Chlorite begins to break down, releasing iron to form almandine:
(Fe,Mg)-Chl + Qz = Alm + Mg-Chl

It also releases iron to form hornblende:
Chl + Tr/act = Ep + Qz + Hbl

Almandine-Amphibolite Facies

550 C  - Staurolite-Almandine Subfacies

bar550.gif (2424 bytes) The breakdown of chlorite is complete, chloritoid disappears and staurolite appears:
Fe-Chl + Musc = St + Bi + Alm + H2O
Ctd + Chl + Mus = St + Bi + Alm + H2O

If there are appreciable volatiles, scapolite may form in addition to plagioclase. The scapolites are a solid solution series of marialite (3albite + NaCl) and meionite (3anorthite + CaCO3 or CaSO4.

600 C  - Kyanite-Almandine-Muscovite Subfacies

bar600.gif (2312 bytes) Staurolite is no longer stable and disappears:
3St + 2Qz = Alm + 5Ky + H2O
St + Musc + (Mg,Fe)-Bi = Ky/Sil + Fe-Bi + H2O (Staurolite breaks down and the reaction yields more iron-rich biotite)

680 C  - Sillimanite-Almandine-Orthoclase Subfacies

bar680.gif (2137 bytes) Kyanite transforms to sillimanite
Microcline transforms to orthoclase

Muscovite becomes unstable:
Musc + Qz + Na-Plag =
       Sil + (K,Na)-Fsp + Ca-Plag + H2O
Musc + Bi + Qz = Alm + 2Ksp + H2O

Epidote is no longer stable:
Ep + Qz = An + Gr + Hematite + H2O

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Created 22 Sept 1997, Last Update