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 crucial 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 essential for interpreting the P-T paths experienced by rocks during orogenic events and other metamorphic processes.
Concept of Facies and Grade in Metamorphism
Metamorphic Facies: A metamorphic facies is a set of mineral assemblages that are in equilibrium under specific P-T conditions. It’s a broad grouping based on the dominant mineralogy, reflecting the prevailing physical and chemical conditions during metamorphism. Rocks belonging to the same facies have experienced similar metamorphic conditions, even if their original compositions were different. The facies are named after a characteristic rock type that exemplifies the conditions.
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 form at specific P-T conditions. For example, the appearance of kyanite indicates a higher grade than the appearance of chlorite.
Important Facies Associations in Regionally Metamorphosed Rocks
Regionally metamorphosed rocks are formed over large areas, typically during orogenic events (mountain building). These rocks experience high pressures and temperatures due to burial and tectonic forces.
1. Zeolite Facies:
This is the lowest grade facies, formed at low temperatures (200-300°C) and low pressures. It’s commonly found in sedimentary rocks. Characteristic minerals include zeolites, clay minerals, and epidote. Example: Shallowly buried sedimentary basins.
2. Greenschist Facies:
Formed at moderate temperatures (300-500°C) and moderate pressures. It’s characterized by the presence of green minerals like chlorite, actinolite, epidote, and albite. Commonly found in areas of regional metamorphism and around igneous intrusions. Example: Scottish Highlands.
3. Amphibolite Facies:
Formed at moderate to high temperatures (500-700°C) and moderate to high pressures. Amphibole (hornblende) and plagioclase are key minerals. Often found in the cores of mountain belts. Example: Parts of the Himalayas.
4. Granulite Facies:
Represents high-grade metamorphism, formed at high temperatures (700-1000°C) and moderate to high pressures. Characterized by anhydrous minerals like pyroxene, garnet, and feldspar. Often associated with continental collision zones. Example: Eastern Ghats, India.
5. Blueschist Facies:
This facies is unique as it forms under high pressure and relatively low temperature conditions. It’s typically associated with subduction zones. Characteristic minerals include glaucophane, lawsonite, and jadeite. Example: Franciscan Complex, California.
Important Facies Associations in Thermally Metamorphosed Rocks
Thermally metamorphosed rocks are formed due to the heat from igneous intrusions. These rocks typically experience high temperatures but relatively low pressures.
1. Hornfels Facies:
Formed at moderate to high temperatures (400-700°C) and low pressures. It’s characterized by fine-grained, dense rocks with little or no foliation. Minerals include hornblende, pyroxene, and feldspar. Example: Rocks surrounding granite intrusions.
2. Pyroxene Hornfels Facies:
Represents higher temperature conditions than hornfels facies (600-800°C) and low pressures. Dominated by pyroxene minerals. Example: Contact aureoles around large gabbroic intrusions.
3. Sanidinite Facies:
This is the highest temperature facies, formed at very high temperatures (800-1000°C) and low pressures. It’s characterized by the presence of sanidine (a high-temperature form of potassium feldspar) and fayalite. Example: Contact aureoles around very large granitic intrusions.
| Facies | Temperature (°C) | Pressure (kbar) | Typical Minerals | Geological Setting |
|---|---|---|---|---|
| Zeolite | 200-300 | <1 | Zeolites, Clay Minerals | Shallowly buried sediments |
| Greenschist | 300-500 | 2-5 | Chlorite, Actinolite, Epidote | Regional metamorphism, around intrusions |
| Amphibolite | 500-700 | 5-10 | Hornblende, Plagioclase | Mountain belts |
| Hornfels | 400-700 | <2 | Hornblende, Pyroxene, Feldspar | Contact metamorphism |
Conclusion
In conclusion, the concepts of metamorphic facies and grade are fundamental to understanding the conditions and processes involved in metamorphism. Facies provide a framework for classifying metamorphic rocks based on their mineral assemblages and the P-T conditions under which they formed, while grade indicates the intensity of metamorphism. Distinguishing between regionally and thermally metamorphosed rocks based on their facies associations allows geologists to reconstruct the tectonic and thermal histories of different regions. Further research into the complexities of fluid interactions and reaction kinetics will continue to refine our understanding of these crucial metamorphic processes.
Answer Length
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