Write on the metamorphism of argillaceous rocks.

Types of Metamorphism

Metamorphism is broadly classified into several types, each with distinct characteristics:

• Regional Metamorphism: Occurs over large areas, typically associated with mountain building processes and plate tectonic activity. It involves directed pressure and high temperatures.
• Contact Metamorphism: Happens when rocks are heated by nearby magma intrusions. It’s characterized by non-directed pressure and increasing temperature.
• Dynamic Metamorphism: Results from intense shear stress, often along fault lines.
• Burial Metamorphism: Occurs due to the increasing pressure and temperature as rocks are buried deeper within the Earth’s crust.
• Hydrothermal Metamorphism: Involves chemical alterations caused by hot, ion-rich fluids.

Metamorphism of Argillaceous Rocks: A Progressive Transformation

Argillaceous rocks, primarily composed of clay minerals (like kaolinite, illite, and smectite), undergo a predictable sequence of changes with increasing metamorphic grade. This progression is often illustrated using the Barrovian metamorphic facies.

Low-Grade Metamorphism (Slate Facies)

At relatively low temperatures and pressures, argillaceous rocks transform into slate. The clay minerals break down and re-crystallize into microscopic platy minerals like chlorite, muscovite, and sericite. This results in a characteristic slaty cleavage – the ability to split easily into thin, flat sheets. The rock remains fine-grained.

Intermediate-Grade Metamorphism (Phyllite Facies)

With increasing temperature and pressure, slate transitions into phyllite. The platy minerals grow larger, giving the rock a silky or sheen-like luster. The cleavage remains prominent, but is less well-defined than in slate. Small amounts of quartz and feldspar may begin to appear.

High-Grade Metamorphism (Schist Facies)

Further increases in temperature and pressure lead to the formation of schist. Larger, visible platy minerals, such as muscovite and biotite, dominate the rock’s texture. These minerals are aligned perpendicular to the direction of maximum stress, creating a distinct schistosity. Other minerals like garnet, staurolite, and kyanite may also be present, depending on the rock’s composition.

Highest-Grade Metamorphism (Gneiss Facies)

At the highest metamorphic grades, argillaceous rocks transform into gneiss. The platy minerals are segregated into bands, alternating with bands of quartz and feldspar. This creates a characteristic gneissic banding. The original sedimentary layering may be completely obliterated. Minerals like sillimanite are commonly found in gneisses formed from argillaceous protoliths.

Mineral Assemblages and Metamorphic Facies

The specific mineral assemblage present in a metamorphic rock is a function of both the temperature, pressure, and the original composition of the rock. The concept of metamorphic facies helps to categorize metamorphic rocks based on their mineral assemblages and the P-T conditions under which they formed.

Metamorphic Grade	Rock Type	Key Minerals	P-T Conditions (approx.)
Low	Slate	Chlorite, Sericite, Muscovite	Low T, Low P
Intermediate	Phyllite	Muscovite, Chlorite, Quartz	Moderate T, Moderate P
High	Schist	Muscovite, Biotite, Garnet, Staurolite, Kyanite	High T, Moderate-High P
Highest	Gneiss	Feldspar, Quartz, Biotite, Sillimanite	Very High T, High P

Structural Changes during Metamorphism

Besides mineralogical changes, argillaceous rocks also undergo significant structural changes during metamorphism. These include:

• Recrystallization: Minerals change in size and shape without changing their chemical composition.
• Pressure Solution: Minerals dissolve at points of high stress and precipitate in areas of low stress.
• Plastic Deformation: Minerals deform without fracturing, especially at high temperatures and pressures.
• Foliation Development: The alignment of platy minerals creates a layered or banded texture.