Model Answer
0 min readIntroduction
Metamorphism, the transformation of existing rocks by heat, pressure, or chemically active fluids, results in the formation of new minerals and textures. Amphibolite facies represents a medium-to-high grade metamorphic environment, typically occurring at temperatures between 450-700°C and pressures of 3-8 kbar. Pelitic rocks, characterized by their high aluminum content and abundance of clay minerals, undergo significant mineralogical changes during amphibolite facies metamorphism. These changes are governed by a series of chemical reactions that aim to achieve equilibrium under the prevailing P-T conditions. Understanding these reactions is fundamental to deciphering the metamorphic history of a region.
Mineral Reactions in Pelitic Rocks during Amphibolite Facies Metamorphism
Pelitic rocks, initially composed of clay minerals (like kaolinite, illite, and chlorite), quartz, and minor amounts of other minerals, transform into characteristic amphibolite facies assemblages. The key reactions involve the breakdown of hydrous minerals and the formation of anhydrous phases. The following reactions are crucial:
1. Breakdown of Muscovite (White Mica)
Muscovite, a common mineral in lower-grade metamorphic rocks, becomes unstable in the amphibolite facies. It reacts to form sillimanite, potassium feldspar, and quartz. This reaction is a key indicator of increasing temperature and is often used to define the sillimanite zone.
Reaction: Muscovite + Quartz → Sillimanite + K-feldspar + H2O
2. Formation of Garnet
Garnet, specifically almandine garnet, is a common product of amphibolite facies metamorphism in pelitic rocks. It forms through reactions involving biotite, quartz, and iron-rich phases.
Reaction: Biotite + Quartz + FeO → Almandine Garnet + H2O + CO2
3. Formation of Staurolite (if conditions are suitable)
Staurolite is an aluminum-rich iron silicate that forms under specific P-T conditions. Its formation is often an intermediate step between garnet and sillimanite formation. The presence of staurolite indicates a particular metamorphic path.
Reaction: Garnet + Biotite + Quartz + H2O → Staurolite + K-feldspar + FeO
4. Breakdown of Chlorite
Chlorite, present in lower-grade pelitic rocks, breaks down to form biotite, quartz, and water.
Reaction: Chlorite + Quartz → Biotite + H2O
5. Formation of Andalusite/Sillimanite/Kyanite (depending on P-T conditions)
These aluminum silicate polymorphs are crucial indicators of metamorphic grade. While andalusite typically forms at lower pressures, sillimanite is favored at higher temperatures, and kyanite at higher pressures. In amphibolite facies, sillimanite is the most common polymorph.
Reaction (Sillimanite formation): Muscovite + Quartz → Sillimanite + K-feldspar + H2O
6. Reactions involving Hematite and Magnetite
Iron oxides like hematite (Fe2O3) can be reduced to magnetite (Fe3O4) under reducing conditions, common in metamorphic environments.
Reaction: 2 Hematite + 3 CO2 → Magnetite + 3 CO2
Table Summarizing Key Reactions
| Reaction | Reactants | Products | Metamorphic Significance |
|---|---|---|---|
| Muscovite Breakdown | Muscovite, Quartz | Sillimanite, K-feldspar, H2O | Indicates increasing temperature; Sillimanite zone |
| Garnet Formation | Biotite, Quartz, FeO | Almandine Garnet, H2O, CO2 | Common in amphibolite facies; indicates iron enrichment |
| Staurolite Formation | Garnet, Biotite, Quartz, H2O | Staurolite, K-feldspar, FeO | Intermediate stage between garnet and sillimanite |
| Chlorite Breakdown | Chlorite, Quartz | Biotite, H2O | Indicates increasing temperature |
These reactions are not isolated events but occur in a complex interplay, influenced by the bulk composition of the rock, fluid availability, and the overall P-T path. The resulting mineral assemblage provides valuable information about the metamorphic conditions experienced by the rock.
Conclusion
In conclusion, amphibolite facies metamorphism of pelitic rocks involves a series of characteristic mineral reactions driven by increasing temperature and pressure. The breakdown of hydrous minerals like muscovite and chlorite, coupled with the formation of anhydrous phases like sillimanite and garnet, defines the metamorphic assemblage. Understanding these reactions is crucial for interpreting the metamorphic history of rocks and reconstructing the tectonic processes that shaped the Earth’s crust. Further research into fluid-rock interactions and reaction kinetics will continue to refine our understanding of these complex metamorphic processes.
Answer Length
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