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UPSC MainsGEOLOGY-PAPER-II201115 Marks
Q14.

Explain for the confinement of glaucophane at high pressure and hornblende at high temperature metamorphism of basic rocks.

How to Approach

This question requires a detailed understanding of metamorphic petrology, specifically the conditions under which certain metamorphic minerals – glaucophane and hornblende – are stable. The answer should focus on the pressure-temperature (P-T) conditions, the chemical environment, and the role of basic rock composition. Structure the answer by first defining the metamorphic facies, then explaining glaucophane stability, followed by hornblende stability, and finally comparing the two. Include relevant diagrams or tables if possible.

Model Answer

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Introduction

Metamorphism, the transformation of existing rocks by heat, pressure, or chemically active fluids, results in the formation of new minerals and textures. The stability of these minerals is governed by the prevailing pressure-temperature (P-T) conditions and the bulk chemical composition of the protolith. Basic rocks, rich in magnesium and iron, undergo distinct metamorphic pathways depending on these factors. Glaucophane, a blue-green amphibole, is a hallmark of high-pressure, low-temperature metamorphism, while hornblende, a common amphibole, is typically associated with high-temperature metamorphism. Understanding the confinement of these minerals within specific metamorphic environments is crucial for deciphering the tectonic history of a region.

Metamorphic Facies and P-T Conditions

Metamorphic facies are defined as a particular set of mineral assemblages that are stable under specific P-T conditions. The stability of minerals is often represented graphically using metamorphic phase diagrams. These diagrams delineate the P-T regions where specific minerals are stable. The formation of glaucophane and hornblende are linked to different facies.

Glaucophane Confinement in High-Pressure Metamorphism

Glaucophane (Na2Mg3Al2Si8O22(OH)2) is characteristic of the blueschist facies, which is defined by relatively low temperatures (200-400°C) and high pressures (greater than 4 kbar, typically 7-12 kbar). Several factors contribute to its stability:

  • High Pressure: Glaucophane’s structure is stabilized by high confining pressure. The Si-O bonds become more compact under pressure, favoring the formation of dense minerals like glaucophane.
  • Low Temperature: Lower temperatures inhibit the breakdown of glaucophane to other amphiboles or pyroxenes.
  • Water Activity: The presence of water (H2O) is crucial for glaucophane stability. Water acts as a flux, facilitating the necessary chemical reactions.
  • Alkali Content: The presence of sodium (Na) is essential for glaucophane formation, as it is a key component of its chemical formula. Basic rocks, often containing plagioclase feldspar rich in sodium, provide this necessary component.
  • Basic Rock Composition: Basic rocks, with their lower silica content and higher magnesium and iron content, are more prone to forming glaucophane-bearing assemblages. The presence of jadeite (NaAlSi2O6) is also common in glaucophane schists, further indicating high-pressure conditions.

Geologically, glaucophane-bearing rocks are commonly found in subduction zones, where rocks are subjected to high pressure due to the descending plate. Examples include the Franciscan Complex in California and the Sanbagawa belt in Japan.

Hornblende Confinement in High-Temperature Metamorphism

Hornblende ((Ca,Na)2-3Mg3-5Fe2+2-3AlSi6-8O22(OH,F)2) is stable under a wider range of P-T conditions, but is particularly favored in the amphibolite and granulite facies, which are characterized by higher temperatures (500-800°C or higher) and moderate to high pressures (3-10 kbar). The stability of hornblende is influenced by:

  • High Temperature: Elevated temperatures promote the formation of hornblende by facilitating the necessary reactions and increasing the solubility of components.
  • Moderate to High Pressure: While not as pressure-sensitive as glaucophane, hornblende requires sufficient pressure to remain stable.
  • Calcium and Iron Content: Hornblende is rich in calcium (Ca) and iron (Fe). Basic rocks, especially those with significant calcium-rich plagioclase and iron-bearing minerals, provide the necessary components.
  • Water and Fluorine: The presence of water (H2O) and fluorine (F) in the fluid phase is important for hornblende stability.

Hornblende is commonly found in metamorphic rocks formed during regional metamorphism associated with orogenic belts, or in contact metamorphic aureoles around igneous intrusions. The formation of hornblende often involves the breakdown of other minerals like pyroxenes and biotite at higher temperatures.

Comparison of Glaucophane and Hornblende Stability

Feature Glaucophane Hornblende
Metamorphic Facies Blueschist Amphibolite, Granulite
Pressure High (7-12 kbar) Moderate to High (3-10 kbar)
Temperature Low (200-400°C) High (500-800°C+)
Key Elements Na, Mg, Al, Si, OH Ca, Fe, Mg, Al, Si, OH, F
Geological Setting Subduction Zones Regional Metamorphism, Contact Metamorphism

Conclusion

In conclusion, the confinement of glaucophane and hornblende in basic rocks is dictated by distinct P-T conditions. Glaucophane’s stability is favored by high pressure and low temperature, typical of subduction zones, while hornblende thrives under high temperature and moderate to high pressure conditions associated with regional and contact metamorphism. The chemical composition of the protolith, particularly the availability of Na, Ca, Fe, and Mg, also plays a crucial role. Understanding these factors allows geologists to reconstruct the metamorphic history and tectonic evolution of a region.

Answer Length

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Additional Resources

Key Definitions

Metamorphic Grade
Metamorphic grade refers to the intensity of metamorphism, typically expressed in terms of temperature and pressure. Higher grade metamorphism indicates higher temperatures and/or pressures.
Eclogite
Eclogite is a high-pressure metamorphic rock composed primarily of garnet and omphacite (a sodium-rich pyroxene). It forms under extremely high pressure conditions, typically found in subduction zones or deeply buried continental crust.

Key Statistics

The Sanbagawa belt in Japan, a classic example of a high-pressure metamorphic terrane, extends over 300 km in length and contains extensive glaucophane-bearing schists.

Source: Isozaki, Y. (1997). Metamorphic Rocks of Japan. Elsevier.

The average heat flow in the Earth's crust is approximately 87 mW/m², which contributes to the geothermal gradient driving metamorphic processes.

Source: Pollack, H. N., Hurter, S., & Johnson, J. R. (1993). Heat flow near the continental margin. Reviews of Geophysics, 31(2), 183-202.

Examples

The Franciscan Complex

The Franciscan Complex in California is a highly deformed and metamorphosed assemblage of rocks, including serpentinites, greywackes, and blueschists containing abundant glaucophane. Its formation is linked to the subduction of the Farallon Plate beneath the North American Plate.

Frequently Asked Questions

▶What role do fluids play in metamorphic reactions?

Fluids, primarily water and carbon dioxide, act as catalysts in metamorphic reactions, increasing the rate at which minerals transform. They also transport elements, facilitating the formation of new mineral assemblages.

Topics Covered

GeologyMetamorphic PetrologyMetamorphismMineral StabilityBasic Rocks
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