TERMS

When examining an individual thin section of a metamorphic rock, two groupings of minerals are present


The mineral assemblage or paragenesis consists of those minerals which grew at the same time in response to the metamorphism, i.e., they are in equilibrium, and exhibit a stable or metastable coexistence during the metamorphic event of interest. The minerals are in contact with each other.

In contrast the mineral association is a list of all minerals observed within a single thin section, whether they exist in equilibrium with each other or not. Minerals in the association may exist metastably in the rock being examined. Any relict grains from the original protolith would fall into the association.

A given mineral paragenesis forms under specific pressure and temperature conditions which reflect the metamorphic grade.

Metamorphic Grade - is used to signify the degree or state of metamorphism, which approximately corresponds to a progressive increase in temperature. Four Divisions of Grade are recognized:

The boundaries between the four divisions of grade are marked by significant changes of mineral assemblages in common rocks, corresponding with specific mineral reactions.

e.g., The boundary between medium grade and high grade, is based on two specific reactions:

  1. Takes place at P< 3.5 kbar
    2. s The reaction that defines this boundary is:
    Muscovite + Quartz = Orthoclase + Al2SiO5 + H2O vapour

  2. At P > 3.5 kbars
    3. This boundary is defined by the reaction:
    Albite + Orthoclase + Quartz = Granitic Liquid
    corresponds to the water saturated granite solids