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. Metamorphic facies are sets of mineral assemblages that are stable under specific pressure-temperature (P-T) conditions. These facies represent the range of metamorphic conditions under which a particular rock can exist in equilibrium. Metamorphic series, on the other hand, describe the progressive changes in mineral assemblages as the intensity of metamorphism increases along a specific P-T path. Understanding these concepts is crucial for deciphering the geological history of a region and reconstructing the conditions under which rocks were formed. This answer will discuss metamorphic facies and series, and then distinguish between the greenschist and prehnite-pumpellyite facies, focusing on their mafic mineral assemblages.
Metamorphic Facies and Series
Metamorphic Facies: A metamorphic facies is defined as a set of metamorphic mineral assemblages that were formed under similar P-T conditions. Each facies is named after a characteristic rock type that exemplifies the conditions of that facies. Common facies include zeolite, prehnite-pumpellyite, greenschist, amphibolite, granulite, and eclogite. The facies represent a range of P-T conditions, and rocks can transition from one facies to another as they are subjected to changing conditions.
Metamorphic Series: A metamorphic series represents a sequence of facies that a rock experiences as the intensity of metamorphism increases. This sequence is often observed in regional metamorphic terrains where rocks are subjected to progressively higher P-T conditions with increasing depth of burial. For example, a basaltic rock might progress through the zeolite, prehnite-pumpellyite, greenschist, amphibolite, and granulite facies as it is buried deeper and subjected to higher temperatures and pressures.
Mafic Assemblages of Low-Grade Metamorphism
Low-grade metamorphism typically occurs at relatively low temperatures (below 300°C) and pressures. Mafic rocks (rocks rich in magnesium and iron) undergoing low-grade metamorphism exhibit characteristic mineral assemblages that reflect these conditions. These assemblages often include minerals like chlorite, serpentine, epidote, albite, and actinolite. The specific assemblage present depends on the original composition of the rock and the precise P-T conditions.
Distinguishing Greenschist Facies from Prehnite-Pumpellyite Facies
Both the greenschist and prehnite-pumpellyite facies represent low-grade metamorphic conditions, but they differ in their specific P-T ranges and characteristic mineral assemblages. The prehnite-pumpellyite facies represents even lower temperatures and pressures than the greenschist facies.
The prehnite-pumpellyite facies is characterized by the presence of prehnite, pumpellyite, actinolite, albite, and epidote. It typically forms in rocks that have experienced low-temperature, low-pressure metamorphism, often associated with oceanic crust or accretionary wedges. The presence of pumpellyite, a calcium aluminum silicate, is a key indicator of this facies.
The greenschist facies, on the other hand, is characterized by the presence of chlorite, actinolite, epidote, albite, and sometimes muscovite. It forms at slightly higher temperatures and pressures than the prehnite-pumpellyite facies. The absence of pumpellyite and the presence of chlorite are key indicators of the greenschist facies.
The following table summarizes the key differences between the two facies:
| Feature | Prehnite-Pumpellyite Facies | Greenschist Facies |
|---|---|---|
| Temperature (°C) | 200-300 | 300-400 |
| Pressure (kbar) | 1-4 | 4-7 |
| Key Minerals | Prehnite, Pumpellyite, Actinolite, Epidote, Albite | Chlorite, Actinolite, Epidote, Albite, Muscovite |
| Typical Rock Type | Low-grade metavolcanic rocks, serpentinites | Metabasalts, low-grade metagraywackes |
In mafic assemblages, the presence of pumpellyite strongly suggests the prehnite-pumpellyite facies, while the presence of chlorite, along with actinolite and epidote, indicates the greenschist facies. The relative abundance of these minerals can also provide insights into the specific P-T conditions within each facies.
Conclusion
In conclusion, metamorphic facies and series are fundamental concepts in understanding the processes of metamorphism. The greenschist and prehnite-pumpellyite facies represent distinct low-grade metamorphic conditions, differentiated by their specific P-T ranges and characteristic mineral assemblages. Identifying these facies through the analysis of mineral assemblages, particularly in mafic rocks, allows geologists to reconstruct the metamorphic history of a region and gain insights into the tectonic processes that have shaped the Earth's crust. Further research into the fluid interactions during these metamorphic events will continue to refine our understanding of these crucial geological processes.
Answer Length
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