Lecture SIX Metamorphic Grade and Facies

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Lecture SIXMetamorphic 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

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Example Wadi Tweiba (S. Sinai) mineral zones
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Gabal Yelleq (Sinai) contact aureole mineral zones
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Metapelites metamorphic zones (Scotland)
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Distribution of minerals within metamorphic zones
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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.

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Metamorphic facies
Metamorphic facies is defined as a set of
metamorphic mineral assemblage, repeatedly
associated in space and time.
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Metamorphic grades and facies
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Metamorphic grades and facies
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Metamorphic series
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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.
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ACF diagram
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AKF diagram
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Plotting of mineral assemblage and rock
categories in ACF and AKF
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AFM plotting
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AFM diagram
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SCM diagram