Retrograde Metamorphism

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Retrogression of Metamorphic Rocks

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The changes in structuring and gathering of minerals which occur during burial and heating are known as prograde metamorphism, whereas those that form during uplift and cooling of a rock indicate retrograde metamorphism.

The Metamorphic rocks are evolved due to the transformation of other rocks under high heat and high pressure. The process of physical and chemical change of rocks is known as metamorphism. The word metamorphism is taken from the Greek for “change of form.

Below is an overview of retrograde metamorphism of rocks

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Formation of Metamorphic Rocks

Before metamorphism; the original rock or protolith may have been formed by solidification of a melt (and igneous protolith) or the lithification of discrete grains derived from weathering (a sedimentary protolith). Metamorphic rocks are formed in 3 ways. Three types of metamorphism are:

1. Contact Metamorphism – It takes place when magma intrudes the cooler upper body of the crust and makes a contact to create a metamorphic rock.

2. Regional Metamorphism – It occurs over broad areas of the crust, which have already undergone deformation over a period of time due to some event; resulting in mountain belts that have been exposed to extreme atmospheric conditions. Hydrostatic pressure, stress, temperature coupled with chemical activities.

3.Dynamic Metamorphism – Also known as cataclasis occurs from mechanical deformation with little long-term temperature change. Dynamic Metamorphism also occurs because of mountain-building.

Types of Metamorphic Rocks

Generally, the metamorphic rocks are considered to be the hardest as they undergo tremendous environmental changes. Metamorphic rocks can basically be found in two forms ie; foliated and nonfoliated. A few examples of foliated metamorphic rocks include Slate, schist, migmatite, phyllite, and gneiss. Marble and quartzite are nonfoliated metamorphic rocks.

The metamorphic grade is a general term we used to describe the temperature at which metamorphism occurs. Owing to the different temperature and pressure, different kinds of metamorphic rocks are formed with varied characteristics and would be significantly different from each other.

Sub-Division of Metamorphism

Metamorphism is divided into prograde and retrograde metamorphism. Prograde metamorphism is a change of mineral composition with increased heat and pressure. Whereas, Retrograde metamorphism is a change in mineral assemblage and its composition that occurs during uplift (releasing of pressure) and cooling (decreasing temperature) to reconstitute a rock, which is a rare process.

Factors That Prevent Retrograde Metamorphism

If retrograde metamorphism were a common process, then upon uplift and exposing metamorphic rocks would progressively return to mineral components stable at lower pressures and temperatures. Only 3 factors prevent retrograde metamorphism, two of which involve the fluid phase.

1. Faster chemical reactions due to high temperature

2. During the process of prograde metamorphism, a fluid phase vanishes as an outcome of the devolatilization reactions.

3. The fluid phase helps to catalyse chemical reactions.

Theory of Retrograde Metamorphism

The theory of Retrograde Metamorphism was first conceptualised by Becke in 1909 and later elaborated by Harker. It was Backe’s contention that these phyllites had formed from gneisses of the deep-seated zones of the earth’s crust as a result of a reversal of normal, progressive, regional metamorphism. To these rocks, he applied the term; diaphthorities.

As per Harker, “Retrograde Metamorphism”, “Retrogressive Metamorphism”, “Regressive Metamorphosis” and “Diaphthorities” are the terms which are not appropriately used as per their original connotations and confuse the readers as the precise definition of “Retrograde Metamorphism” has not been explained by most of the writers.

Process of Retrograde Metamorphism

Retrograde metamorphism in many ways is a reverse of prograde metamorphism. Retrograde reactions are usually very slow and may not impact only some parts of the rock and not the complete rock.

There are two factors that mitigate against complete retrogression of metamorphic rocks during their return.

a. Efficient removal of the water and carbon dioxide released.

b. Metamorphic reactions do not typically operate in reverse during cooling and reaction rates are increased by rising temperatures.

All the metamorphic rocks would eventually undergo a change in the mineral composition (Prograde metamorphism or retrograde) under the atmospheric conditions present near the earth. This process of change is called weathering.

FAQ (Frequently Asked Questions)

1. What Do We Mean By Metamorphism?

Answer: The geological term "Metamorphism" is derived from the Greek:  Meta = after, morph = form, so metamorphism reflects upon the after form. Geologically, this implies changes in mineral assemblage and textures that are led from exposing a rock to pressures and temperatures distinct from those under which the rock originally formed.

The original rock that experienced metamorphism is referred to as the protolith.   Protolith can be any kind of rock and often the alterations in mineralogy and texture are so dramatic that it is difficult to differentiate what the protolith was.

Remember that weathering and diagenesis are also changes in form that takes place in rocks. In geology, however, the diagenetic processes are limited to those which take place at temperatures below 200oC and pressures below 300 MPa (MPa = MegaPascals), which equals to about 3,000 atmospheric pressures.

Metamorphism thus takes place at temperatures and pressures above 200C and 300 MPa. Rocks can be submitted to these higher pressures and temperatures as they become buried deeper in the Earth. Such burial generally materializes as a result of tectonic processes like subduction or continental collisions.

2. What Happens During Prograde Metamorphism?

Answer: During the prograde metamorphism, a fluid phase is driven off as an outcome of the devolatilization reactions.  As the pressure rises, porosity of rocks also lessens, and therefore this fluid phase will possibly be driven out of the rock body. In the absence of the fluid phase it is not feasible to form carbonates as well as hydrous minerals, seeing that CO2 and H2O are two of the key components required in such reactions, may not be present.

The fluid phase also allows catalyzing the chemical reactions. Although the net reactions may seem to be solid-solid reactions, actually there may be more involved.

Moreover, given sufficient time, all metamorphic rocks will ultimately change to a deposition of minerals stable under conditions existing near the Earth’s surface.