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. The sequence of metamorphic minerals appearing in rocks subjected to increasing metamorphic grade is a fundamental concept in understanding regional metamorphism. This sequence, often referred to as a metamorphic series or facies, reflects changes in pressure and temperature conditions. The given sequence – Chlorite → Biotite → Garnet → Staurolite → Kyanite → Sillimanite – represents a typical progression observed in pelitic (clay-rich) rocks undergoing regional metamorphism, indicating increasing metamorphic grade. Understanding this sequence is vital for deciphering the tectonic history of a region.
Metamorphic Mineral Assemblage Progression
The sequence Chlorite → Biotite → Garnet → Staurolite → Kyanite → Sillimanite represents a progressive increase in metamorphic grade, reflecting higher temperatures and pressures. Each mineral assemblage is stable under a specific range of P-T conditions.
1. Chlorite Facies (Low Grade Metamorphism)
Chlorite is the first mineral to appear in the sequence, indicating low-grade metamorphism. This facies typically occurs at temperatures between 200-300°C and low pressures. Chlorite forms through the alteration of ferromagnesian minerals like olivine and pyroxene, or from the hydration of biotite. It’s commonly found in chlorite schists.
2. Biotite Facies (Low to Medium Grade Metamorphism)
As temperature and pressure increase, Biotite becomes stable. This facies generally occurs at temperatures between 300-400°C and relatively low pressures. Biotite forms through the breakdown of chlorite and other hydrous minerals. It’s a common constituent of phyllites and schists. The appearance of biotite signifies a transition towards higher metamorphic conditions.
3. Garnet Facies (Medium Grade Metamorphism)
With further increases in temperature (400-550°C) and pressure, Garnet appears. Garnet formation indicates a more significant metamorphic event. Almandine garnet, a common variety, forms from the reaction of iron-rich minerals like biotite and chlorite with silica and alumina. Garnet porphyroblasts are often visible in schists and gneisses. The presence of garnet suggests a regional metamorphic event.
4. Staurolite Facies (Medium to High Grade Metamorphism)
The appearance of Staurolite marks a transition to higher metamorphic grades (500-650°C and moderate pressures). Staurolite is a distinctive mineral known for its cruciform (cross-shaped) twins. It forms through complex reactions involving garnet, biotite, and other minerals. Staurolite is a key indicator mineral for medium-to-high grade regional metamorphism.
5. Kyanite Facies (High Grade Metamorphism)
Kyanite forms under high-pressure, medium-to-high temperature conditions (600-750°C and high pressure). It is an aluminum silicate mineral and is stable at high pressures due to its unique crystal structure. Kyanite is often found in schists and gneisses. Its presence indicates significant regional metamorphism and potentially deep burial or tectonic compression.
6. Sillimanite Facies (High Grade Metamorphism)
Finally, Sillimanite appears at the highest metamorphic grades (700-850°C and moderate to high pressure). Like kyanite, sillimanite is an aluminum silicate, but it is stable at lower pressures than kyanite. Sillimanite is commonly found in granulites and high-grade gneisses. Its presence indicates intense regional metamorphism and often represents the peak metamorphic conditions.
The sequence is not always complete, and the exact mineral assemblage present depends on the bulk composition of the protolith (parent rock). For example, a mafic protolith will produce different metamorphic minerals than a pelitic protolith.
| Mineral | Temperature (°C) | Pressure (kbar) | Typical Rock Type |
|---|---|---|---|
| Chlorite | 200-300 | 1-2 | Chlorite Schist |
| Biotite | 300-400 | 2-3 | Phyllite, Schist |
| Garnet | 400-550 | 3-5 | Schist, Gneiss |
| Staurolite | 500-650 | 4-7 | Schist, Gneiss |
| Kyanite | 600-750 | 7-10 | Schist, Gneiss |
| Sillimanite | 700-850 | 4-8 | Granulite, Gneiss |
Conclusion
The sequence Chlorite → Biotite → Garnet → Staurolite → Kyanite → Sillimanite provides a valuable framework for understanding the progression of regional metamorphism. Each mineral assemblage represents a specific set of P-T conditions, allowing geologists to reconstruct the metamorphic history of a region. The presence or absence of certain minerals can indicate the intensity of metamorphism and the tectonic processes involved. Further research into metamorphic petrology continues to refine our understanding of these complex processes and their implications for Earth’s dynamic history.
Answer Length
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