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UPSC MainsGEOLOGY-PAPER-II201110 Marks
Q3.

Mineral assemblages for granulite facies of metamorphism of metabasites (using ACF and AKF diagrams)

How to Approach

This question requires a detailed understanding of metamorphic petrology, specifically focusing on granulite facies metamorphism in metabasites. The answer should demonstrate knowledge of mineral assemblages characteristic of this facies and how they are represented using ACF and AKF diagrams. A systematic approach involves defining granulite facies, outlining typical metabasite compositions, detailing the mineral assemblages, and then illustrating their positioning on the diagrams. Emphasis should be placed on the pressure-temperature conditions and the implications for geodynamic settings.

Model Answer

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Introduction

Metamorphism, the transformation of pre-existing rocks under conditions of elevated temperature and/or pressure, results in the formation of new minerals and textures. Granulite facies represents the highest grade of regional metamorphism, typically occurring at temperatures exceeding 700°C and pressures ranging from 8-12 kbar. Metabasites, rocks derived from basaltic protoliths, are commonly subjected to granulite facies conditions in continental collision zones and areas of crustal thickening. Understanding the mineral assemblages formed during this process, and their graphical representation using ACF and AKF diagrams, is crucial for deciphering the metamorphic history of these rocks and the tectonic processes involved.

Metabasites and Granulite Facies

Metabasites, originating from basaltic magmas, are typically composed of plagioclase feldspar, pyroxenes (augite, orthopyroxene), and iron-titanium oxides. During prograde metamorphism, these minerals react to form new assemblages stable at higher temperatures and pressures. Granulite facies metamorphism is characterized by the breakdown of hydrous minerals and the formation of anhydrous mineral assemblages.

Mineral Assemblages in Granulite Facies Metabasites

The specific mineral assemblage formed in a granulite facies metabasite depends on the bulk composition of the protolith and the precise P-T conditions. Common mineral assemblages include:

  • Orthopyroxene + Plagioclase + Clinopyroxene + Garnet + Quartz: This is a typical assemblage formed under moderate to high pressure granulite conditions.
  • Orthopyroxene + Plagioclase + Spinel + Quartz: Indicates higher temperature and lower pressure conditions within the granulite facies.
  • Orthopyroxene + Plagioclase + Ilmenite + Quartz: Common in iron-rich metabasites.
  • Garnet + Clinopyroxene + Plagioclase + Quartz: Found in more alumina-rich metabasites.

ACF Diagram (Albite-Calcium Feldspar-Feldspar)

The ACF diagram is a ternary diagram used to represent the feldspar composition in metamorphic rocks. It plots the relative proportions of Albite (NaAlSi3O8), Calcium Feldspar (Anorthite - CaAl2Si2O8), and Alkali Feldspar (Orthoclase/Sanidine - KAlSi3O8). In granulite facies metabasites, the ACF diagram typically shows a field dominated by plagioclase, with limited or no alkali feldspar. The position within the plagioclase field reflects the Ca-content of the plagioclase, which is influenced by the bulk rock composition and metamorphic conditions. Higher temperatures generally favor more sodic plagioclase.

AKF Diagram (Alkali Feldspar-Kyanite-Feldspar)

The AKF diagram is used to represent the composition of metamorphic rocks, particularly those containing aluminum-rich minerals. It plots Alkali Feldspar (KAlSi3O8), Kyanite (Al2SiO5), and Feldspar (typically plagioclase). This diagram is particularly useful for understanding the metamorphic grade and the role of aluminum in the system. In granulite facies metabasites, the AKF diagram will generally show a field dominated by plagioclase and alkali feldspar, with kyanite absent or present only in very alumina-rich compositions. The relative proportions of plagioclase and alkali feldspar are sensitive to temperature and pressure. Higher temperatures favor alkali feldspar, while higher pressures favor plagioclase.

Graphical Representation on ACF and AKF Diagrams

The mineral assemblages described above can be projected onto the ACF and AKF diagrams. For example:

  • An assemblage of Orthopyroxene + Plagioclase (An50) + Quartz would plot near the Albite corner of the ACF diagram, reflecting the plagioclase composition.
  • An assemblage of Orthopyroxene + Plagioclase (An25) + Alkali Feldspar + Quartz would plot closer to the Alkali Feldspar corner of the ACF diagram, indicating a more potassium-rich feldspar component.
  • On the AKF diagram, the absence of kyanite indicates conditions beyond the kyanite stability field, consistent with granulite facies.

Geodynamic Implications

The formation of granulite facies metabasites is often associated with continental collision zones, such as the Himalayas, where crustal thickening leads to high temperatures and pressures. The mineral assemblages and their positions on the ACF and AKF diagrams can provide valuable insights into the P-T-t path followed by the rocks during metamorphism, helping to reconstruct the tectonic evolution of the region.

Conclusion

In conclusion, granulite facies metamorphism of metabasites results in characteristic mineral assemblages that reflect the high-temperature, moderate-to-high pressure conditions. The ACF and AKF diagrams are powerful tools for visualizing and interpreting these assemblages, providing crucial information about the metamorphic history and geodynamic setting of the rocks. Understanding these relationships is fundamental to deciphering the complex processes that shape the Earth's crust.

Answer Length

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Additional Resources

Key Definitions

Metabasite
An igneous rock of basaltic composition that has undergone metamorphism. Typically dark-colored and medium- to coarse-grained.
Prograde Metamorphism
The stage of metamorphism where rocks are subjected to increasing temperature and/or pressure, leading to the formation of new, higher-grade mineral assemblages.

Key Statistics

Granulite facies rocks constitute approximately 10-15% of the exposed Precambrian crust globally.

Source: Brown, M. (2007). Precambrian Geology. Cambridge University Press.

Temperatures in granulite facies typically range from 700°C to over 1000°C.

Source: Spear, F.S. (1993). Metamorphic Textures. Prentice Hall.

Examples

Eastern Ghats Granulites

The Eastern Ghats of India contain extensive exposures of granulite facies rocks, providing a well-studied example of high-grade metamorphism in a Precambrian terrain. These rocks reveal a complex history of multiple metamorphic events.

Frequently Asked Questions

▶What is the difference between granulite and amphibolite facies?

Granulite facies represents higher temperatures and pressures than amphibolite facies. Granulite facies is anhydrous, meaning it lacks water-bearing minerals, while amphibolite facies commonly contains minerals like hornblende and biotite.

Topics Covered

GeologyMetamorphic PetrologyMetamorphic FaciesMineral AssemblagesPhase Diagrams
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