Metamorphism of arenaceous, argillaceous, and basic rocks | Geology Optional Notes for UPSC PDF Download

Understanding Metamorphic Processes

• Metamorphic processes are gradual and occur over geological timescales, often spanning millions of years.
• Direct observation and practical demonstrations of these processes are not feasible.
• Conclusions about metamorphic processes are drawn by studying their effects on rocks.

Diversity in Rock Behavior

• Different rock types exhibit varied responses under similar metamorphic conditions.
• Petrologists have extensively studied the effects of metamorphism on various rock types worldwide.

Trends in Metamorphic Changes

• Metamorphic effects on common rock types demonstrate specific trends of changes.
• These trends serve as illustrations of the alterations that occur during metamorphism.

Igneous Rocks:

• Acid Igneous Rocks:
  • Rich in free silica in the form of quartz (SiO2).
  • Example: Granite - a hard and resistant rock.
  • Effects of metamorphism on granite:
    • Under dynamic metamorphism, granite can form crush breccia.
    • Hard minerals like quartz and feldspars may flatten and elongate under loads, forming flaser granites.
    • Complete crushing under loads can lead to the formation of mylonites.
  • Contact Metamorphism:
    • Commonly involves simple recrystallization of minerals.
  • Regional Metamorphism:
    • Associated with heat, pressure, and chemically active fluids.
    • Granites can transform into gneiss through directed stress and recrystallization.

Basic Igneous Rocks

• Basic igneous rocks undergo significant changes during metamorphism, including the development of new minerals and textures.
• During contact metamorphism, only mineral composition changes while the original textures remain. Examples include meta-gabbros and meta-dolerites.
• In dynamothermal metamorphism, complex mineral changes occur along with the formation of foliation and granulation, producing rocks like gneisses and schists.
• Notable examples under dynamothermal metamorphism include hornblende schists and amphibolites.

Sedimentary Rocks

Argillaceous Rocks

• Argillaceous rocks consist of fine particles of clays, feldspars, quartz, and micas.
• Shales undergoing contact metamorphism can form new minerals like corundum and cordierite, leading to the creation of hornfels.
• Under optical metamorphism, shales harden into hornstone without significant recrystallization.
• Highly siliceous shales can transform into novaculite through complete recrystallization.
• Dynamic metamorphism in shales results in rearrangement of constituents, leading to slaty cleavage formation.
• Intense pressure and heating produce slate from shales with pronounced parallelism at the grain-axis level.
• Dynamothermal metamorphism advances shales beyond the slate stage, resulting in schist formation with new mineral growth.
• The transitional stage between schist and slate is known as phyllite, characterized by micaceous minerals.

Arenaceous Rocks

• These are types of sedimentary rocks primarily made up of siliceous grains with sizes ranging from 2 mm to 1/16 mm in diameter.
• Sandstone stands as a prevalent example of an arenaceous rock.

Formation under Metamorphic Conditions

• When exposed to contact metamorphism, sandstones typically undergo recrystallization to form Quartzite.
• Quartzite is differentiated from sedimentary rocks of similar composition by the term para-quartzite.
• Impure sandstones, especially those with argillaceous or calcareous cement, can metamorphose into Schistose-grits and similar schistose rocks.

Effects of Dynamic Metamorphism

• During dynamic metamorphism, sandstones may deform or be crushed based on the rock quality and stress intensity.
• The resulting rocks could be crush-breccia, micro-breccia, or mylonite.

Metamorphism of Carbonate Rocks

• Metamorphism of carbonate rocks, such as limestones, holds significant importance and interest in geological processes.
• From the metamorphism of these rocks, several key conclusions can be derived:
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  • Transformation into marble: Limestones subjected to metamorphic conditions can result in the formation of marble, a metamorphic rock with distinct properties.
  • Development of foliation: In some cases, metamorphism of carbonate rocks leads to the development of foliation, a layering structure that indicates the direction of pressure during metamorphic processes.
  • Mineral reorganization: The mineral composition of carbonate rocks can undergo reorganization during metamorphism, leading to the formation of new minerals such as calcite, dolomite, and others.

Recrystallization of Limestone in Contact Metamorphism

Key Concept:

• Under conditions of contact metamorphism, limestone undergoes recrystallization without new mineral formation.

Transformation into Marble:

• During this process, limestone transforms into the metamorphic rock known as marble.

Changes in Texture:

• The recrystallization results in a distinct alteration in grain size, leading to a granular texture in marble.

Example:

• For instance, when limestone is subjected to intense heat and pressure near a magma intrusion, it reorganizes its crystals, forming marble with a smooth, granular appearance.

Notes on Limestone Recrystallization

Recrystallization Process

• Recrystallization of limestone can lead to the formation of new minerals in the rock, influenced by impurities, temperature, and pressure.
• The primary chemical transformation involves the conversion of calcium carbonate (CaCO3) into calcium oxide (CaO) due to heat.
• Subsequently, calcium oxide reacts with impurities present, resulting in the creation of various new minerals.

Effect of Impurities

• Presence of silica as the sole impurity can yield wollastonite (calcium silicate) as a metamorphic product.
• Impurities like alumina (Al2O3) and magnesia (MgO) can lead to the formation of minerals such as anorthite and amphibole through recrystallization.

Chemical Reactions

• Chemical reactions during recrystallization illustrate the transformation process.
• For instance, the interaction between calcium oxide and specific impurities showcases the creation of new minerals.

Understanding Dedolomitization Process

• Definition: Dedolomitization is a metamorphic process that occurs when limestone originally contains dolomite. This process involves the breakdown of dolomite with the simultaneous reformation of calcite.
• Equation: The changes in the dedolomitization process can be represented by the following equation: Dolomite → Calcite
• Types of Metamorphic Changes:
  • Dependent on Impurities: The type of metamorphic changes that occur during dedolomitization can vary based on the impurities present in the dolomite.
  • Examples:
    • Example 1: If the dolomite contains magnesium, it may result in the formation of magnesite during the dedolomitization process.
    • Example 2: Presence of iron in dolomite can lead to the formation of siderite as a product of dedolomitization.