Barrovian Facies

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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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 a typical geothermal gradient of about 25 C per km. The typical pressures under which these rocks 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 22 Sept 1997

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