Model Answer
0 min readIntroduction
Metamorphism, the transformation of existing rocks by heat, pressure, or chemically active fluids, results in a wide variety of new rocks with altered mineralogy and texture. Understanding the conditions under which these changes occur is paramount in deciphering the geological history of a region. Two fundamental concepts in this regard are ‘metamorphic facies’ and ‘metamorphic grade’. Metamorphic facies represent a set of metamorphic mineral assemblages that are stable under specific pressure-temperature (P-T) conditions, while metamorphic grade refers to the intensity of metamorphism, indicating the degree to which a rock has been altered. These concepts are crucial for interpreting the P-T paths experienced by rocks during orogenic events and other metamorphic processes.
Defining Facies and Grade
Metamorphic Facies: A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar P-T conditions. It’s a broad grouping based on the mineralogy, reflecting the prevailing physical and chemical environment during metamorphism. Rocks belonging to the same facies have experienced similar metamorphic conditions, even if they started with different protoliths (parent rocks). The facies are named after a characteristic rock type or locality where they were first described.
Metamorphic Grade: Metamorphic grade describes the intensity of metamorphism. It’s a relative scale, ranging from low-grade (mild conditions) to high-grade (intense conditions). Increasing grade is generally associated with increasing temperature and/or pressure. The grade is often indicated by the presence of index minerals – minerals that are stable only within a specific range of P-T conditions. For example, the appearance of sillimanite indicates a higher grade than the appearance of chlorite.
Facies Associations in Regionally Metamorphosed Rocks
Regional metamorphism occurs over large areas, typically associated with convergent plate boundaries and orogenic belts. It is characterized by directed pressure and high temperatures. Here are some important facies associations:
- Zeolite Facies: Lowest grade, typically found in sedimentary basins. Characterized by zeolite minerals, formed at low temperatures (100-200°C) and low pressures.
- Greenschist Facies: Intermediate grade, common in many orogenic belts. Characterized by minerals like chlorite, actinolite, epidote, and albite. Temperatures range from 200-400°C and pressures are relatively low to moderate. Often found in areas that have undergone mild deformation.
- Amphibolite Facies: Higher grade than greenschist. Characterized by amphibole (hornblende), plagioclase, and garnet. Temperatures range from 400-600°C and pressures are moderate to high. Commonly found in the cores of fold belts.
- Granulite Facies: Highest grade of regional metamorphism. Characterized by anhydrous minerals like pyroxene, garnet, and feldspar. Temperatures exceed 600°C and pressures are high to very high. Often associated with continental collision zones.
- Blueschist Facies: Unique facies associated with high-pressure, low-temperature metamorphism, typically found in subduction zones. Characterized by blue amphibole (glaucophane), lawsonite, and jadeite.
Facies Associations in Thermally Metamorphosed Rocks
Thermal metamorphism, also known as contact metamorphism, occurs when rocks are heated by the intrusion of magma. It is characterized by high temperatures and relatively low pressures. The metamorphic grade decreases with distance from the intrusion.
- Hornfels Facies: Most common contact metamorphic facies. Characterized by fine-grained, dense rocks (hornfels) with minerals like quartz, feldspar, and pyroxene. Temperatures range from 300-700°C.
- Pyroxene Hornfels Facies: Higher temperature variant of the hornfels facies, characterized by the presence of pyroxene minerals.
- Sanidinite Facies: Highest temperature contact metamorphic facies, formed near very hot intrusions. Characterized by minerals like sanidine (a high-temperature feldspar) and fayalite.
- Albite-Epidote Hornfels Facies: Formed in contact aureoles around granitic intrusions in relatively hydrous rocks. Characterized by albite and epidote.
| Facies | Temperature (°C) | Pressure (kbar) | Typical Mineral Assemblage | Metamorphic Setting |
|---|---|---|---|---|
| Greenschist | 200-400 | 1-4 | Chlorite, Actinolite, Epidote | Regional (low to moderate grade) |
| Amphibolite | 400-600 | 4-8 | Hornblende, Plagioclase, Garnet | Regional (intermediate to high grade) |
| Hornfels | 300-700 | 1-3 | Quartz, Feldspar, Pyroxene | Thermal (contact) |
| Granulite | >600 | >8 | Pyroxene, Garnet, Feldspar | Regional (high grade) |
Conclusion
The concepts of metamorphic facies and grade are fundamental to understanding the processes that shape the Earth’s crust. By analyzing the mineral assemblages present in metamorphic rocks, geologists can reconstruct the P-T history of a region and gain insights into its tectonic evolution. Distinguishing between regionally and thermally metamorphosed rocks, and recognizing the characteristic facies associations within each, is crucial for accurate geological interpretation. Further research utilizing advanced geochronological and geochemical techniques continues to refine our understanding of metamorphic processes and their role in Earth’s dynamic systems.
Answer Length
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