Lecture SIX Metamorphic Grade and Facies. Metamorphic grade and Facies.
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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.)
Metamorphic grades is a general term for describing the relative P-T conditions under which the metamorphic rocks form. The grades could subdivided into:
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 is defined as a set of metamorphic mineral assemblage, repeatedly associated in space and time.
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.