Model Answer
0 min readIntroduction
Metamorphism, the transformation of existing rocks (igneous, sedimentary, or even other metamorphic rocks) due to changes in physical or chemical conditions, results in a wide variety of metamorphic rocks. These rocks are classified based on the pressure-temperature (P-T) conditions under which they formed, and these conditions are represented by ‘metamorphic facies’. A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar P-T conditions. Understanding these facies is crucial for deciphering the geological history of a region and the tectonic processes that have affected it. This answer will illustrate the various metamorphic facies on a P-T diagram and briefly explain each one.
Metamorphic Facies and the P-T Diagram
A P-T diagram is a graphical representation of pressure and temperature conditions. Metamorphic facies are depicted as fields on this diagram, representing the range of P-T conditions under which a particular mineral assemblage is stable. The boundaries between these fields represent metamorphic reactions where one mineral assemblage transitions into another.
(Note: Since I cannot directly render images, I have provided a link to a standard metamorphic facies diagram. An actual answer would include a hand-drawn or digitally created diagram.)
Explanation of Metamorphic Facies
1. Zeolite Facies (Low T, Low P)
This is the lowest-grade metamorphic facies, occurring at temperatures between 200-350°C and pressures less than 2 kbar. It is typically found in sedimentary basins and is characterized by the presence of zeolite minerals, such as analcite, phillipsite, and wairakite. These minerals form due to the alteration of volcanic glass and other unstable materials.
2. Prehnite-Pumpellyite Facies (Low T, Low-Medium P)
Found at temperatures of 250-400°C and pressures of 2-4 kbar, this facies is common in regionally metamorphosed oceanic crust. Characteristic minerals include prehnite, pumpellyite, and actinolite. It often develops in areas that have experienced low-grade regional metamorphism.
3. Greenshist Facies (Low-Medium T, Low-Medium P)
Occurring at temperatures of 350-500°C and pressures of 2-7 kbar, the greenschist facies is characterized by the presence of green minerals like chlorite, epidote, actinolite, and albite. It is commonly found in regionally metamorphosed areas, such as mountain belts, and represents a moderate degree of metamorphism.
4. Amphibolite Facies (Medium T, Medium-High P)
This facies develops at temperatures of 500-700°C and pressures of 5-10 kbar. It is characterized by the presence of amphibole (hornblende), plagioclase, and garnet. Amphibolite facies rocks are common in regionally metamorphosed terrains and represent a higher degree of metamorphism than the greenschist facies.
5. Granulite Facies (High T, Medium-High P)
Found at temperatures above 700°C and pressures of 7-12 kbar, the granulite facies is characterized by anhydrous mineral assemblages like pyroxene, garnet, and feldspar. It represents high-grade metamorphism and is often associated with continental collision zones. The absence of hydrous minerals indicates relatively dry conditions.
6. Blueschist Facies (Low T, High P)
This facies is unique as it forms under relatively low temperatures (250-500°C) but very high pressures (7-15 kbar). It is typically found in subduction zones and is characterized by the presence of blue amphibole (glaucophane), lawsonite, and jadeite. The high pressure is due to the descending oceanic plate.
7. Eclogite Facies (Medium-High T, Very High P)
Representing the highest pressure metamorphic facies, eclogite facies rocks form at temperatures of 500-800°C and pressures exceeding 15 kbar. They are characterized by the presence of garnet and omphacite (a sodium-rich pyroxene). Eclogite facies rocks are found in deeply subducted oceanic crust and represent extreme metamorphic conditions.
Factors Influencing Facies Development
Besides P and T, other factors like fluid composition (presence of water or CO2) and bulk rock composition also influence the development of metamorphic facies. For example, the presence of water lowers the melting point of rocks and can promote the formation of hydrous minerals.
Conclusion
Metamorphic facies provide a powerful tool for understanding the thermal and pressure history of rocks and the tectonic processes that have shaped the Earth's crust. By analyzing the mineral assemblages present in metamorphic rocks and relating them to the P-T conditions represented by the facies diagram, geologists can reconstruct the conditions under which these rocks formed. The understanding of metamorphic facies is fundamental to deciphering the complex geological evolution of mountain belts, subduction zones, and other tectonically active regions.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.