Facies of Regional and Contact Metamorphism | Geology Optional Notes for UPSC PDF Download

Concept of Metamorphic Facies

• Definition of Metamorphic Facies:
  • Eskola (1915) introduced the concept of metamorphic facies, defining it as a grouping of rocks likely metamorphosed under similar conditions.
  • According to Eskola, rocks within the same facies, if sharing identical chemical composition, are made up of the same minerals.
• Refinement of the Definition:
  • Turner, Fyfe, and Verhoogen (1958) enhanced this concept, identifying 11 distinct metamorphic facies.
  • They described metamorphic facies as sets of mineral assemblages consistently found together in both space and time.
  • There exists a dependable relationship between mineral composition and chemical composition within a given facies.
• Classification Based on Pressure/Temperature Gradients:
  • Facies of low pressure: These are characterized by specific mineral assemblages formed under lower pressure conditions.
  • Facies of medium to high pressure: These facies exhibit mineral compositions indicative of moderate to high pressure environments.
  • Facies of very high pressure: This category includes mineral assemblages formed under extremely high-pressure conditions.
• Illustrative Example:
  • In a Sillimanite zone, specific minerals indicative of a particular pressure and temperature range are observed, representing a distinct metamorphic facies.

Metamorphic Facies Overview

Pressure-Temperature Fields

• Pressure ranges across different facies:
  • Unattainable pressure
  • PT condition
  • Subgreenschist
  • Blueschist
  • Greenschist
  • Sanidinite
  • Ky Sil
  • And more

Low Pressure Facies

• Granulite and low-P contact metamorphism:
  • Temperature ranges for different facies
• Facies of Low Pressure:
  • Albite-Epidote-Hornfels Facies:
    • Occurs at low pressure and temperatures of 300-400°C
    • Characterized by minerals like quartz, muscovite, biotite, andalusite, and more
  • Hornblende-Hornfels Facies:
    • Defined by hornblende-plagioclase assemblage
    • Pressure ranges up to 3kbar
  • Pyroxene-Hornfels Facies:
    • Develops in the inner aureoles of contact metamorphism
    • Pressure ranges up to 3kbar
  • Sanidinite Facies:
    • Occurs in xenoliths in basic lavas and dykes
    • Characterized by minerals like mullite, cordierite, tridymite, and more

Unit 13: Facies of Medium to High Pressure

• Zeolite Facies

  Zeolite Facies represents the lowest-grade regional metamorphic facies. It is characterized by a temperature range of 200-400°C and pressure range of 4 to 8 kbar. Key minerals defining this facies include analcime, heulandite, laumontite, prehnite, pumpellyite, lawsonite, and albite.

• Prehnite-Pumpellyite Facies

  Prehnite-Pumpellyite Facies acts as a transitional zone between greenschist and zeolite facies. It features minerals such as quartz, albite, prehnite, pumpellyite, chlorite, stilpnomelane, muscovite, and actinolite. This facies is prominent in greywacke-type sediments.

• Greenschist Facies

  Greenschist Facies is a significant facies in low-grade regional metamorphism. Characterized by temperatures ranging from 400-500/550°C and pressures of 4 to 8 kbar, this facies is known for minerals like chlorite, chloritoid, stilpnomelane, and pyrophyllite, often found alongside muscovite and calcite.

  • Staurolite Zone
  • Kyanite Zone
  • Sillimanite Zone

• Amphibolite Facies

  Amphibolite Facies is characterized by the presence of hornblende-plagioclase mineral assemblage in metabasic rocks that have undergone regional metamorphism. Found in Precambrian terrains and eroded parts of orogenic belts, this facies typically experiences temperatures ranging from 540°C to 675°C with pressures exceeding 4 kbar.

  • Basic rocks: hornblende-plagioclase
  • Pelitic schists: biotite, muscovite, almandine garnet, staurolite, kyanite, sillimanite

Block 4: Metamorphic Petrology

Granulite Facies

• Granulite Facies is characterized by high temperatures ranging from 700/750°C to 800/850°C and pressures ranging from 3 to 13 kbar.
• Key minerals in granulite facies include pleochroic hypersthene, pyrope-almandine rich garnets, microperthitic K-feldspars, antiperthitic plagioclase, and granoblastic-mosaic or granulitic texture.
• These rocks are best exemplified in the Archaean crystalline complexes like the Southern Granulite Terrain of the Indian shield.

Facies of Very High Pressure

• Glaucophane-Lawsonite (Blueschist) Facies: Represents low-temperature, very high-pressure rocks found in subduction complexes like the Circum-Pacific belt and the Himalayas.
• Temperature range: 200-400°C; Pressure range: 6 to 8 kbar.
• Eclogite Facies: Characterized by Omphacite Garnet mineral pair in gabbroic or basaltic rocks under extreme pressure and temperature conditions.
• Temperature range: 800-1000°C; Pressure: >15 kbar.

Exercise: Short Answer Questions (SAQ 2)

1. a) Grade of metamorphism refers to the degree of metamorphic change a rock has undergone.
2. b) Prograde metamorphism involves changes occurring during increasing pressure and temperature, while retrograde metamorphism involves changes during decreasing pressure and temperature.
3. c) Metamorphic zones are based on the presence of key index minerals that indicate specific pressure and temperature conditions.
4. d) Mineral zone boundaries delineate the limits of specific mineral assemblages indicative of certain metamorphic conditions.
5. e) Metamorphic facies represent specific mineral assemblages that reflect a set of pressure and temperature conditions during metamorphism.
6. f) Three zones of greenschist facies are the chlorite zone, biotite zone, and garnet zone.

Take time to review and understand these concepts. Feel free to ask questions if you need further clarification.

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