Metamorphism: Rock Transformations Under Pressure and Heat

Metamorphism: Rock Transformations

Metamorphism refers to the set of transformations that a rock undergoes when subjected to pressure and temperature conditions different from those that prevailed during its genesis.

Factors Influencing Metamorphism

Changes in temperature, pressure, and fluid flow alter the stability of minerals.

• Temperature Increase: This can be due to the confinement of rocks within the Earth's crust and the movement of fault blocks, which release heat.
• Pressure Increase: Pressure increases can result from:
  • Confinement: The accumulation of sediments exerts a uniform pressure inside, known as lithostatic pressure (Pl).
  • Folding: This introduces a horizontal pressure component called tectonic pressure (Pt).
  • Presence of a Fluid Phase: Fluids can also be under pressure due to lithostatic pressure (metasomatism).

Types of Metamorphism

• Pyrometamorphism: Characterized by high temperatures.
• Thermal or Contact Metamorphism: Occurs near high-temperature magma intrusions.
• Deep or Load Metamorphism: Involves the recrystallization of minerals caused by lithostatic pressure.
• Dynamic Metamorphism or Dynamometamorphism: Occurs when tectonic pressure (Pt) is added to lithostatic pressure (Pl).
• Regional or General Metamorphism: A combination of metamorphic actions, including the effects of temperature, lithostatic pressure, directed tectonic pressure, and fluid pressure.

Anatexia

Anatexia is the partial or complete melting of pre-existing metamorphic rocks, forming a new magma mass upon cooling. This results in grains of anatexia. It can occur in active continental margins, geosyncline bottoms, and even in deep areas of the Earth's crust.

Structure of the Country Rock

• Schistosity: The rock appears to be cut into sheets.
• Foliation: Found in micacites, gneiss, and migmatites. It involves the oriented distribution of recrystallized minerals.
• Lineation: The linear orientation of minerals.
• Microplicae: Observed in the lower structural levels.

Metamorphic Minerals

• Regional Metamorphism: Kyanite.
• Regional and Thermal Metamorphism: Quartz, muscovite, talc, calcite, biotite.
• Thermal Metamorphism: Andalusite.

Metamorphic Rocks

• Contact Metamorphism: Spotted schist, marble, quartzite.
• Regional or General Metamorphism: Clay schists, gneiss (composed of quartz, feldspar, and mica), serpentine, amphibolite, and eclogite.

Metamorphic Rates and Plate Tectonics

Metamorphism is associated with different plate tectonic settings:

• Constructive plate boundaries
• Destructive plate boundaries
• Passive plate boundaries
• Intraplate areas