December 2, 2020

Metamorphic facies, Zones and Metamorphic Grades.

What are the metamorphic grades?

Metamorphic grades are defined as the intensity of the metamorphism that has affected a rock, which is called the “grade of the metamorphism”. It usually varies with the heat and pressure to which the rocks have been subjected. Parent rocks of the same composition from the different minerals in different grades of the metamorphism.

A Convenient way to indicate the range of possible metamorphic rocks in a particular setting is to group those
possibilities into metamorphic facies. In other words, a given metamorphic facies groups together
metamorphic rocks that form under the same pressure and temperature conditions, but which have different Protoliths.

The metamorphic grades are classified into three types.

1.Low Grade

It Prevails within a temperature range of 200-400 degrees celsius and a large pressure range. Laumonite and Prehentite are the best examples of low grades.

  2.Medium Grade

This grade prevails up to a temperature ranges from the 650-degree Celsius and indicated by the index minerals like staurolite and cordierite. Pressure variations play an important role in determining the stability of the various minerals that formed this grade of the minerals.

3.High-Grade

This grade is usually available at the temperature ranges of the 580-degree Celsius and pressure of the 3.5 kb and continuous up to the temperature of 800 degrees Celsius and above. A typical example indicates the high-grade metamorphism is provided by the breakdown of the muscovite mica in the presence of the quartz and granulites are the common resulting metamorphic rocks.

 

What is the metamorphic facies?

Metamorphic rocks can be classified depending upon the metamorphic environment they passed from. An association of metamorphic rocks that have been formed under physical factors of metamorphism is referred to as the metamorphic facies. The parent rock which is metamorphosed under the same conditions then the new mineral that created is also the same in the mineralogical composition. It is proved worldwide now and this theory of the metamorphic facies is the reason why it was accepted worldwide.

Another definition we can elaborate on the metamorphic facies is ” The groups of metamorphic rocks that have been formed under the set of physical and chemical conditions and that is characterized by the definite set of minerals.”

The concept of the metamorphic facies is to just classify the metamorphic rocks on the basis of the nature of the parent rock and their geological structures. This was derived by the P.Eskola in 1915 and this geologist had found various types of metamorphic facies in the metamorphic rocks. Mashiro in 1973 had added some more facies in the previous one and these facies are considered as the standard and sufficient.

What are the types of metamorphic facies?

Here are some important types of metamorphic facies that have been addressed which are most commonly studied under geology.

  1. Amphibolite Facies
  •       These facies usually found in the medium to high-grade metamorphic terrains. These minerals include the palgisocle, almandite. This facies represents those metamorphic conditions which occur in staurolite and sillimanite grade of metamorphism.

2. Granulite Facies

  •  This facies represents the maximum temperature conditions of the regional metamorphism, such as those found in Archaean terrains. The characteristics of the minerals of these facies are plagioclase, garnet, and diopside.

3. Ecologite facies

  •       These facies usually represents the most deep-seated conditions of metamorphism. The characteristic mineral is pyrope garnet and omphacite. Such minerals assemblages are commonly founded in Kimberlite pipes, may of which contain diamonds.

4. Green Schist Facies.

  • This facies represents the low grade of the metamorphism that found in the many regionally meta morphed areas. These minerals assemblages of green-schist facies include chlorite, epidote, Muscovite, and quartz.

 Metamorphic Zones

The grade metamorphism generally increases with the depth for the simple reason that both temperature and pressure factors become strong and stronger at deeper levels within the crust of the earth. This fact has given birth to the concept of metamorphic zones that signify the ranges of metamorphic effects at different depths below the surface. In other words, Zones indicate a depth-wise extension of particular grades of metamorphism in a general way.

There are main 3 types of metamorphic zones available.

1.The Epizone

It is the near-surface zone and is characterized by a low temperature generally less than 300 degrees celsius and strong shear stress. Rocks in this zone are, therefore, metamorphized chiefly under the influence of dynamic metamorphism. The common rocks resulting in this zone are slates and Mica Schists.

You can also read: Thermal Metamorphism

2. The Mesozone

It is the middle zone in ich the temperature factor becomes rather moderate (300-500 degrees Celcius) and the pressure factor becomes is both types: Shear, as well as high-grade schists like biotite-garnet schists, are chief rocks formed.

3. The Ketazone

It is a high temperature and great depth type metamorphic zone where hydrostatic stresses are quite dominant. Plutonic metamorphism is the representative kind and rocks formed in this zone include a great variety of Genesis.

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