Model Answer
0 min readIntroduction
Metamorphism, derived from the Greek words ‘meta’ (change) and ‘morph’ (form), refers to the alteration of pre-existing rocks (igneous, sedimentary, or even other metamorphic rocks) by heat, pressure, and chemically active fluids. These changes occur in the solid state, without melting. The intensity of these factors dictates the type and grade of metamorphism, leading to the development of distinct metamorphic zones. These zones are characterized by specific mineral assemblages indicative of the prevailing pressure-temperature (P-T) conditions. Understanding these zones is crucial for deciphering the tectonic history and geological evolution of a region. This answer will focus on the zones of metamorphism with specific reference to the Himalayas, India.
Zones of Metamorphism
Metamorphic zones are broadly classified based on the intensity of metamorphism, which is directly related to the depth and temperature conditions. The most commonly recognized zones, progressing with increasing metamorphic grade, are:
- Zeolite Zone: Lowest grade, characterized by the presence of zeolite minerals.
- Greenschist Zone: Characterized by green colored minerals like chlorite, epidote, and actinolite.
- Amphibolite Zone: Presence of amphibole minerals like hornblende, along with plagioclase feldspar.
- Granulite Zone: Highest grade, characterized by anhydrous minerals like pyroxene and garnet.
- Eclogite Zone: Very high pressure, low temperature conditions, resulting in the formation of eclogite minerals like omphacite and garnet.
Factors Controlling Metamorphic Zones
Several factors influence the development of metamorphic zones:
- Temperature: Increases with depth and proximity to magmatic intrusions.
- Pressure: Increases with depth due to the weight of overlying rocks (lithostatic pressure) and tectonic forces (directed pressure).
- Fluid Activity: Chemically active fluids (water, carbon dioxide) facilitate metamorphic reactions.
- Rock Composition: The original composition of the protolith influences the resulting metamorphic minerals.
Metamorphic Zones in the Himalayas
The Himalayas provide an excellent example of varying metamorphic zones due to the collision of the Indian and Eurasian plates. The collision resulted in intense deformation, thrusting, and metamorphism.
Lower Himalayan Zone (Lesser Himalayas)
This zone exhibits predominantly Greenschist to Amphibolite facies metamorphism. Protoliths include sedimentary and volcanic rocks. Minerals like chlorite, epidote, actinolite, hornblende, and plagioclase are common. The metamorphic grade increases towards the Main Central Thrust (MCT). This zone represents relatively lower pressure and temperature conditions compared to the higher Himalayan zones.
Higher Himalayan Zone (Greater Himalayas)
This zone displays a progression from Amphibolite to Granulite facies metamorphism. Rocks here have experienced higher temperatures and pressures due to deeper burial and tectonic activity. Garnet, pyroxene, and sillimanite are indicative minerals. The metamorphic grade is highest in the core of the Himalayas.
Tethys Himalayan Zone
This zone, representing the remnants of the Tethys Sea, exhibits the highest metamorphic grades, including Granulite and even localized Eclogite facies. The presence of eclogite suggests very high-pressure, low-temperature conditions, possibly related to subduction processes during the early stages of the India-Eurasia collision. The rocks are often intensely deformed and recrystallized.
Zonal Distribution - A Simplified Table
| Zone | Metamorphic Facies | Typical Minerals | Pressure (kbar) | Temperature (°C) |
|---|---|---|---|---|
| Zeolite | Zeolite | Zeolites, Albite | < 2 | < 200 |
| Greenschist | Greenschist | Chlorite, Epidote, Actinolite | 2-5 | 200-350 |
| Amphibolite | Amphibolite | Hornblende, Plagioclase | 5-10 | 350-500 |
| Granulite | Granulite | Pyroxene, Garnet | >10 | >500 |
| Eclogite | Eclogite | Omphacite, Garnet | >15 | < 600 |
Conclusion
In conclusion, zones of metamorphism represent a progressive change in mineral assemblages reflecting increasing temperature and pressure conditions. The Himalayas exemplify this concept, showcasing a clear zonal distribution from Greenschist to Granulite and even Eclogite facies, directly linked to the complex tectonic history of the region. Studying these zones provides valuable insights into the processes of mountain building and the evolution of continental crust. Further research utilizing advanced geochronological and geochemical techniques will continue to refine our understanding of metamorphic processes in the Himalayas 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.