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What is metamorphic zone and its types? What is Metamorphic facies? What is grades of metamorphism? What are the texture of metamorphic rocks?


 

Metamorphic Zone and Metamorphic facies: Metamorphic zone is an area where the same combination of minerals occurs in the bedrock during the metamorphism process. Metamorphic zones occur because metamorphic minerals have a unique range or interval of temperature and pressure at which they are stable. There are three types of metamorphic zones-

1)      Epizone: This zone of metamorphism occurs near the earth’s surface. In this zone, generally Cataclastic metamorphism conditions are observed.

2)      Mesozone: It is the intermediate zone of metamorphism, which is found below the Epizone. The mesozone condition that is the temperature and pressure promotes the regional metamorphism.

3)      Katazone: The bottommost metamorphic zones are known as Katazone. In this zone, the plutonic metamorphism is taken place.

 


Grades of metamorphism: The degree or intensity of metamorphism that has affected a rock is called the grade of metamorphism. It varies directly with the amount of heat and pressures that act or are subjected to a rock. The grade of metamorphism is expressed as very low, low, medium, and high. For example, the slate and phyllites are formed away from the intrusive igneous body, hence they contain low-grade metamorphism while the high-grade metamorphic rocks such as gneiss formed close to its margin. The increase in grade of metamorphism is accompanied by an increase in the grain size of rocks. For example, the phyllites and the slates are fine-grained rocks whereas the schist and gneiss are coarse-grained rocks.

 


Metamorphic facies: Metamorphic facies is a set of mineral assemblages in metamorphic rocks, which are formed under a similar set of Physico-chemical conditions.  The various metamorphic facies are as follows-

1)      Zeolite facies

2)      Green schist facies

3)      Amphibole facies

4)      Granulite facies

5)      Eclogite facies etc.

The texture of metamorphic rocks:

1)      Crystalloblastic texture: The Holocrystalline texture of a metamorphic is called Crystalloblastic texture. This texture is developed in rocks when minerals grains are recrystallized in a solid medium. The crystals which are showing well-developed crystal shapes are called idioblasts and those crystals which do not contain any definite shape such crystals are called xenoblasts.

 

2)      Porphyroblastic texture: When idioblasts occur as a large crystal embedded in the fine-grained groundmass, the texture is called Porphyroblastic. The large well-shaped crystals of this texture are called porphyroblasts.

 

 

3)      Granoblastic texture: In metamorphic rocks, if the major constituent grains are granular or equidimensional, the texture is called Granoblastic texture. 

 

4)      Palimpsest texture: The remnant texture of parent rock is found preserved in metamorphic rocks then the rock is said to have Palimpest texture. 

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