The approximate extent or intensity to which metamorphism has changed an original rock is termed as metamorphic grade. The metamorphic grade varies directly with the temperature and pressure to which the rock have been subjected. Different mineral assemblages are formed under different grades of metamorphism even if original rock have same composition. The grade of metamorphism have been divided into 3 categories Low Grade, Medium Grade and High Grade. These grades are indicated by the presence of a set of minerals that are called the index minerals, these minerals are stable only at particular temperature and pressure range which is characteristic of that particular grade.
- Low Grade- For low grade the temperature ranges from 200 to 400 degrees and a large pressure range. Important minerals associated with this grade are Laumonite, Prehnite and lawsonite.
- Medium Grade- The temperature range for this grade is 400 to 580 degrees, pressure plays important role in determining the stability of various minerals formed in this range. Important minerals associated with this grade are Staurolite and Cordierite.
- High Grade- The temperature ranges from 580 degrees to 800 degrees and pressure of 3.5kb and above. Index mineral for this grade are Hypersthene and Granulites.
Isograd is defined as a line of geological map of metamorphic rock joining the points of same grade of metamorphism as indicated by the presence of same type of index minerals, an assemblage of index minerals rather an individual mineral is used for drawing isograds.
The grade of metamorphism generally increases with depth, as the temperature and pressure increases as we go downwards from the surface of the earth. The metamorphic zones signify the range of metamorphic effect at different depths below the surface. Three metamorphic zone are:
- Epizone – It is the near surface zone and is characterized by a low temperature and strong shear stress. Rocks in this zone are, therefore metamorphosed chiefly under the influence of dynamic metamorphism. The common rocks resulting in this zone are slate and mica schists.
- Mesozone – It is the middle zone in which the temperature ranges from 300 to 500 degrees and pressure factor is of both: shear as well as hydrostatic type. Dynamothermal metamorphism is the typical process of this zone and high-grade schists like biotite- garnet schists are chief rocks formed.
- Ketazone – It is the high temperature and greater depth type metamorphic zone where hydrostatic stress are quite dominant. Plutonic metamorphism is the representative kind and rocks formed in this zone include great variety of Gneisses.
Metamorphic rocks are often classified on the bases of metamorphic environment through which these rocks have passed. A metamorphic facies is a set of metamorphic mineral assemblages that have formed under a closely related metamorphic environment (within a range of temperature- pressure- chemically active fluid conditions) is termed as metamorphic facies. A metamorphic facies may be characterized by a set of minerals or typical rocks. Myashiro in 1973 proposed ten metamorphic facies which are named in table below.