Metasomatism and Granitization, Migmatites | Geology Optional for UPSC PDF Download

Metasomatism

• Definition: Metasomatism, a term coined by C.F. Naumann in 1826, refers to a metamorphic process characterized by the subsolidus alteration of chemical components in a rock due to the infiltration of aqueous fluids.
• Nature of the Process: Metasomatism occurs while the rock remains in a solid state, with its volume staying constant throughout the modification.
• Mechanism: The process involves the introduction and/or removal of chemical constituents within the rock through the influence of percolating fluids.
• Example: An illustration of metasomatism is the alteration of a basic igneous rock by silica-rich fluids, transforming it into a skarn deposit with new minerals like garnet and pyroxene.

The Lithospheric Mantle as a Historical Record

The lithospheric mantle acts as a historical archive, documenting various geological events that have impacted it since its formation. These events involve the infiltration of different fluids and materials, which have repeatedly altered the original mantle rocks. This constant alteration results in a highly varied and metasomatized lithospheric mantle, exhibiting heterogeneity at scales ranging from microns to large terranes.

Deciphering Geological Evolution

Understanding this complex record is crucial for reconstructing the evolution of the lithosphere and comprehending the sources and processes of volatile substances from deep within the Earth over time.

Effects of Metasomatism

Metasomatism, the alteration of rocks through the introduction of external fluids or melts, can lead to significant changes in rock composition. Even small amounts of fluids can influence the rate of metamorphic reactions in rocks, impacting the mineral assemblages present.

Metasomatic Signatures

One key outcome of metasomatism is the development of hydrous and element-rich phases within rocks, which visually replace original minerals.

• Geochemical Changes: This process often results in an enrichment of light rare earth elements (LREEs) and other incompatible trace elements within the affected rocks.
• Examples of Metasomatism: Instances of metasomatism can be observed in peridotite xenoliths found in basalts, as well as in highly alkaline mafic and ultramafic magmas and calcareous skarn formations. By studying these processes and their effects, geologists can gain valuable insights into the dynamic nature of Earth's lithospheric mantle and its evolution over time.

Metasomatic Processes Overview

Definition of Metasomatism: Metasomatism refers to the alteration of rocks through various processes when they are subjected to new physical conditions different from their original formation environment.

Examples of Metasomatic Processes:

• Contact metamorphism: Occurs when rocks change mineralogy due to chemical reactions, like in the contact aureole at Crestmore, California.
• Na-metasomatism: Associated with ocean floor metamorphism, such as sulfide precipitation in black smokers.
• Alkali metasomatism: Linked with carbonatites producing fenites.
• Mantle metasomatism: Infiltration and depletion of granulites are also part of metasomatic processes.

Common Examples of Metasomatism:

• Shallow plutons interacting with calcareous or ultramafic rocks.
• Layers, lenses, and pods with differing compositions in metamorphosing rocks.
• Veins where fluids move from one rock type to another along fractures.

Role of Metasomatic Processes: Metasomatism, in conjunction with igneous and tectonic activities, significantly contributes to the formation of Earth's crust and lithospheric mantle, shaping their ongoing evolutionary history and interrelationships.

Significance of Metasomatic Events:

• Metasomatic events can be dated through the alteration of minerals like zircon, monazite, and xenotime, providing insights into fluid chemistry.
• They lead to phenomena such as serpentinization, skarns, and greisens, indicating extensive fluid flow and rock alteration.

Mineral Zonation due to Metasomatism: Metasomatism involves the re-equilibration of mineral assemblages due to changes in pressure, temperature, and chemical environment. For instance, mineral zonation in ultramafic pods in pelites at the Shetland Islands illustrates the impact of metasomatic processes.

Granitization

Definition: The term "granitization" refers to the process of transforming crustal rocks into a granitic mineral assemblage without passing through the magmatic stage. This transformation occurs due to the influence of metasomatic fluids.

Mechanism: Granitization involves the introduction of essential chemical components of granite into the original solid parent rock. Concurrently, unnecessary elements are eliminated from the rock through the percolation of metasomatic fluids along grain boundaries and the diffusion of ions within crystals.

Historical Context: The concept of granitization was initially proposed as a method to produce granites without the need to inject large volumes of granitic magma into the Earth's crust. However, this idea is no longer widely accepted in contemporary geological studies.

Migmatite

Definition of Migmatite: Migmatite is a unique type of rock that consists of two distinct components:

• Leucosome: The leucosome is the light-colored, granitic or felsic part of migmatite. It is formed by the partial melting of the original rock, often reaching temperatures similar to those needed for creating granite.
• Melanosome: In contrast, the melanosome is the darker, more mafic or schistose part of migmatite. It remains largely unchanged and represents the solid-state, non-melted portion of the rock.

When migmatite forms, the leucosome and melanosome components combine to create a visually striking rock with distinct light and dark bands or layers.

Formation Process:

• Migmatite is typically formed under conditions of high temperature and pressure, leading to partial melting of the original rock.
• During this process, the rock reaches temperatures close to those required for the formation of granite.

Examples of Migmatite:

• One common example of migmatite is when gneiss, a metamorphic rock, partially melts to form migmatite, showcasing its characteristic leucosome and melanosome components.
• Another example is the occurrence of migmatite in regions with intense tectonic activity, where rocks experience high temperatures and pressures, leading to partial melting and the formation of migmatite.

Significance of Migmatites in Geology

Migmatites as Indicators of Metamorphic History

• Migmatites play a crucial role in indicating the metamorphic history of a region. When partial melting is present, it suggests that the rocks have undergone high-grade metamorphism with increased temperatures.
• Example: Imagine a scenario where migmatites are found in a region with evidence of partial melting. This indicates a history of intense heat and pressure affecting the rocks.

Insights into Crustal Differentiation

• Migmatites offer insights into the differentiation processes within the Earth's crust. The separation of leucosome and melanosome components signifies the division of melt from solid residue, contributing to the formation of various rock types.
• Example: Consider a situation where the leucosome and melanosome components of a migmatite are visibly distinct. This indicates the process of crustal differentiation that occurred in the region.

Association with Tectonic Processes

• Migmatites are commonly linked to tectonic activities like convergent plate boundaries and mountain-building events. The intense pressure and heat generated during these events can trigger partial melting and the creation of migmatites.
• Example: Visualize a scenario where migmatites are found in a region with evidence of past tectonic activity. This suggests that the rocks underwent significant pressure and heat due to geological movements.

Mineral Resource Potential

• Migmatites, particularly those with substantial granitic components, hold economic significance due to the possible presence of valuable minerals. The granitic leucosome may contain economically important elements such as quartz, feldspar, and occasionally minerals like mica.
• Example: Suppose a migmatite is discovered with a high concentration of quartz and feldspar. This indicates the potential for valuable mineral extraction in the area.

In summary, migmatites are rocks of great geological importance that offer insights into the intricate processes that have influenced the Earth's crust. Studying migmatites enhances our understanding of metamorphism, tectonics, and the geological evolution of specific regions.