lecture six metamorphic grade and facies n.
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Lecture SIX Metamorphic Grade and Facies

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Metamorphic grade and Facies

Systematic spatial distribution in mineral assemblages that formed during metamorphism in metamorphic terrains allow to delineate mineral zonation, using index minerals (e.g. chlorite, biotite, garnet, staurolite, cordierite, sillimanite, andalusite, wollastonite, diopside… etc.)

  • - Mineral zone: Zones in the field, which mark the first appearance of an index mineral, such as chlorite zone, garnet zone.
  • Mineral isograde: boundary marked between two mineral zones, which include:
  • Mineral-in: first app
  • Mineral-out: the last appearance of

Metamorphic grades

Metamorphic grades is a general term for describing the relative P-T conditions under which the metamorphic rocks form. The grades could subdivided into:

  • Very low grade
  • Low grade
  • Medium grade
  • High grade
  • Very high grade

The boundaries between the grades are chosen to correspond to important discontinuous reactions (could recognized as major isograde), and they correlate with the scheme of metamorphic facies.


Metamorphic facies

Metamorphic facies is defined as a set of metamorphic mineral assemblage, repeatedly associated in space and time.


Graphical representation of metamorphic mineral assemblage

1- ACF and AKF diagrams: Suitable for mafic rocks and calc-silicates.

2- AFM diagram: Useful for pelitic rocks.

3- CAS (CaO - Al2O3 - SiO2) diagram: Useful for marly rocks (calcareous mudstones).

4- MCS (MgO - CaO - SiO2) diagram: Useful for ultramafic rocks.