Metamorphic petrology is the study of metamorphic rocks, which are rocks that have been transformed from one rock type into another through the action of heat, pressure, and chemically active fluids. This field of geology is concerned with the composition, structure, and origin of metamorphic rocks, as well as the processes that form and alter them.
Metamorphic rocks can be classified based on their composition and the type of metamorphism that they have undergone. Some common types of metamorphic rocks include:
- Regional metamorphic rocks: these rocks have been subjected to regional metamorphism, which occurs over a large area due to the action of heat and pressure caused by the movement of tectonic plates. Examples of regional metamorphic rocks include gneiss and schist.
- Contact metamorphic rocks: these rocks have been subjected to contact metamorphism, which occurs when a rock is in contact with a body of molten magma. The heat from the magma can cause the rock to undergo changes in its mineralogy and texture. Examples of contact metamorphic rocks include hornfels and marble.
- Hydrothermal metamorphic rocks: these rocks have been subjected to hydrothermal metamorphism, which occurs when hot, chemically active fluids flow through the rock and alter its minerals. Examples of hydrothermal metamorphic rocks include quartzite and slate.
Metamorphic petrology is important for understanding the processes that take place within the Earth’s interior and the history of the Earth’s crust. It is also useful for identifying the sources of minerals and other resources that are found in metamorphic rocks.
Metamorphism
Metamorphism is the process by which a rock is transformed from one rock type into another through the action of heat, pressure, and chemically active fluids. This process can occur within the Earth’s crust or mantle, and can affect both igneous and sedimentary rocks.
There are several types of metamorphism, including:
- Regional metamorphism: this type of metamorphism occurs over a large area and is caused by the action of heat and pressure caused by the movement of tectonic plates. Regional metamorphism can result in the formation of metamorphic rocks such as gneiss and schist.
- Contact metamorphism: this type of metamorphism occurs when a rock is in contact with a body of molten magma. The heat from the magma can cause the rock to undergo changes in its mineralogy and texture. Contact metamorphism can result in the formation of metamorphic rocks such as hornfels and marble.
- Hydrothermal metamorphism: this type of metamorphism occurs when hot, chemically active fluids flow through the rock and alter its minerals. Hydrothermal metamorphism can result in the formation of metamorphic rocks such as quartzite and slate.
Metamorphism can result in the formation of a wide range of metamorphic rocks, which can be classified based on their composition and the type of metamorphism that they have undergone. These rocks can have a variety of textures and structures, depending on the conditions under which they formed.
Classification of Metamorphic Rocks
Metamorphic rocks can be classified based on several different criteria, including their composition, texture, and the type of metamorphism that they have undergone.
One common method of classification is based on the composition of the rock. Some common types of metamorphic rocks based on composition include:
- Foliated metamorphic rocks: these rocks have a layered or banded appearance due to the alignment of minerals along planes of weakness in the rock. Examples of foliated metamorphic rocks include gneiss, schist, and slate.
- Non-foliated metamorphic rocks: these rocks do not have a layered or banded appearance and do not have a preferred orientation of minerals. Examples of non-foliated metamorphic rocks include marble, quartzite, and hornfels.
Metamorphic Facies
Metamorphic facies are a series of rock assemblages that are characteristic of particular pressure-temperature conditions and that are indicative of the type of metamorphism that a rock has undergone. The concept of metamorphic facies was first proposed by the geologist Norman Bowen in the 1920s and is used to classify metamorphic rocks based on the conditions under which they formed.
There are several metamorphic facies that are commonly recognized, including:
- Greenschist facies: this facies is characterized by the presence of the minerals chlorite, epidote, and actinolite, and is indicative of low-grade metamorphism at temperatures of 250-400°C and pressures of 10-20 kilobars. Rocks of the greenschist facies are typically fine-grained and have a greenish color due to the presence of chlorite.
- Amphibolite facies: this facies is characterized by the presence of the minerals amphibole and plagioclase, and is indicative of high-grade metamorphism at temperatures of 500-700°C and pressures of 20-30 kilobars. Rocks of the amphibolite facies are typically medium- to coarse-grained and have a darker color due to the presence of amphibole.
- Granulite facies: this facies is characterized by the presence of the minerals orthoclase and plagioclase, and is indicative of very high-grade metamorphism at temperatures of 750-900°
Metamorphic Minerals
Metamorphic minerals are minerals that form during the process of metamorphism, which is the transformation of a rock from one rock type into another through the action of heat, pressure, and chemically active fluids. These minerals form as the rock is subjected to changing conditions that cause the minerals in the rock to recrystallize or rearrange themselves in new ways.
Some common metamorphic minerals include:
- Chlorite: a greenish mineral that forms under low-grade metamorphism and is indicative of the greenschist facies.
- Epidote: a greenish-yellow mineral that forms under low- to medium-grade metamorphism and is indicative of the greenschist and amphibolite facies.
- Actinolite: a greenish mineral that forms under low- to medium-grade metamorphism and is indicative of the greenschist and amphibolite facies.
- Amphibole: a dark-colored mineral that forms under high-grade metamorphism and is indicative of the amphibolite facies.
- Plagioclase: a white or gray mineral that forms under high-grade metamorphism and is indicative of the amphibolite and granulite facies.
- Orthoclase: a white or pink mineral that forms under very high-grade metamorphism and is indicative of the granulite facies.
The presence of specific metamorphic minerals can provide information about the conditions under which a metamorphic rock formed and the type of metamorphism that the rock has undergone.
