Retrogression of Metamorphic Rocks
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.
FAQs on Retrograde Metamorphism
1. What exactly is retrograde metamorphism in geology?
Retrograde metamorphism is a geological process where high-grade metamorphic rocks are altered back to lower-grade mineral assemblages. This 'reversal' happens when there is a decrease in temperature and pressure, typically during the uplift and erosion of mountain belts. For this to occur, a catalyst, usually water (H₂O), must be reintroduced to the rock, facilitating the chemical reactions that form minerals stable at the new, less extreme conditions.
2. How does retrograde metamorphism differ from prograde metamorphism?
The primary difference lies in the direction of change in temperature and pressure. Prograde metamorphism occurs as rocks are buried deeper, leading to an increase in temperature and pressure, forming higher-grade minerals. In contrast, retrograde metamorphism happens as rocks are brought back towards the surface, involving a decrease in temperature and pressure. A key distinction is that prograde metamorphism releases water (dehydration), while retrograde metamorphism requires the addition of water (hydration) to proceed.
3. What are some common examples of rocks showing retrograde metamorphism?
A classic example is the formation of serpentinite from the hydration of ultramafic rocks like peridotite. Another common instance is a high-grade gneiss undergoing retrogressive change, where minerals like garnet or sillimanite are replaced by lower-grade minerals such as chlorite or muscovite along fractures or shear zones where water could infiltrate.
4. Why is retrograde metamorphism considered less common or complete than prograde metamorphism?
Retrograde metamorphism is less common primarily due to two factors:
Lack of Water: During prograde metamorphism, water is expelled from the rock through dehydration reactions. For the reverse reactions to occur during cooling, water must be reintroduced, which is often difficult deep within the Earth's crust.
Reaction Kinetics: Chemical reactions are significantly slower at lower temperatures. As the rock cools, there may not be enough thermal energy to overcome the activation energy required for new, lower-grade minerals to form, effectively 'freezing' the high-grade assemblage in place.
5. What specific role does water play in facilitating retrograde metamorphism?
Water acts as a crucial chemical reactant and a catalyst in retrograde metamorphism. It participates directly in hydration reactions, where anhydrous (water-lacking) high-grade minerals are converted into hydrous (water-bearing) low-grade minerals. For example, garnet (anhydrous) can react with water to form chlorite (hydrous). Water also acts as a fluid medium, enabling the transport of ions and significantly speeding up chemical reactions that would otherwise be extremely slow in a dry, solid state.
6. Under what tectonic conditions does retrograde metamorphism typically occur?
Retrograde metamorphism is strongly associated with tectonic uplift and exhumation. This process occurs when deeply buried rocks are brought towards the Earth's surface, common in the later stages of mountain-building (orogeny). As the overlying rock is eroded away, both pressure and temperature decrease. If fluids can infiltrate the rock mass through faults or fractures during this uplift, the conditions become suitable for retrograde reactions to take place.
7. How can a geologist identify evidence of retrograde metamorphism in a rock sample?
Geologists identify retrograde metamorphism by observing specific textures under a microscope. Key indicators include:
Reaction Rims: Where a high-grade mineral crystal is surrounded by a 'rim' of one or more lower-grade minerals.
Pseudomorphs: When a lower-grade mineral (like chlorite) completely replaces a higher-grade mineral (like garnet) but retains the original crystal shape of the garnet.
Overprinting: Seeing fine-grained, lower-grade minerals growing over or cutting across larger, higher-grade mineral grains, indicating they formed later in the rock's history.
