METAMORPHIC ROCKS

Metamorphic Facies

Eskola in 1915 developed the concept of metamorphic facies as “ A group of rocks of varying chemical composition, characterised by a definite set of minerals which have arrived approximately at equilibrium under a given combination of pressure – temperature conditions”.

The different metamorphic facies are defined by mineralogical composition of a rock. When the temperature and pressure in a rock body change, the rock can cross into a definite facies and some minerals remain stable while others become unstable or metastable.

INDEX MINERAL : Every Metamorphic Facies has some Index Mineral by which it can be recognized.

TYPES OF METAMORPHIC FACIES

  1. Zeolite Facies

  • This Metamorphic Facies has lowest Metamorphic grade.
  • It is named Zeolite, because of the presence of strongly hydrated tectosilicate.
  • MINERALOGICAL CHARACTERSTICS- It shows the assemblage of Quartz + Laumonite + Chlorite (which are easily diagnostic).
  • Facies represent a range of conditions involving:

– Low Pressure (1-4kb)

– Low Temperature (30-500°C)

  • This facies usually develop in :
  1. Thick sedimentary sequence of Continental Margins.
  2. Rock sequence is subjected to hot water conducting system.
  3. Areas of tectonic burial at the leading edge of thrust sheets.
  • This Facies is most often experienced by pelitic sedimentary rocks rich in Aluminium, silica, Potassium, Sodium but generally low in Iron, Magnesium and Calcium.
Zeolite Facies
Zeolite Facies

 

  1. Prehnite- Pumpellyite Facies:

  • It has a little higher temperature and pressure conditions than the Zeolite Facies.
  • It is named so because of the presence of two minerals:

Prehnite (Ca-Al Phyllosilicate), Pumpellyite (Sorosilicate)

MINERALOGICAL CHARACTERSTICS:

  • Prehnite, Pumpellyite and Quartz is typically (without Zeolite or Glaucophane or Lawsonite).
  • Facies represent a range of conditions involving –
  • Low-Moderate Pressure (2 to 7 kb)
  • Low Temperature (250 to 350°)
  • This Facies usually develop during burial Metamorphism in thick sedimentary Sequence on Continental margin and in Intra Continental Basins.
  • This metamorphic facies is typical of Seafloor alteration of Oceanic crust around MOR Spreading Centre.
  • It grades between Zeolite and Greenshist Facies.
Prehnite- Pumpellyite Facies
Prehnite- Pumpellyite Facies

  1. Blue Schist Facies:

  • Also known as Glaucophane –Lawsonite Schist.
  • This facies is constructed to form under:
  • Pressure (-0.6 GPa) or > 5
  • Temperature (200 to 500°C).
  • Name Blueschist derives from the fact that at this Metamorphic Grade, rocks of ordinary Bulk Composition are often bluish because they contain Na bearing blue Amphibole- Glaucophane.

MINERALOGICAL CHARACTERSTICS:

Glaucophane and Lawsonite also Jadiete, Quartz and Aragonite.

  • Rocks subjected to this facies gain heat by conduction with hotter lower crustal rocks if they remain at the 15-18 km depth.

Blue Schist Facies

  1. Amphibole Facies:

  • It is a high grade Metamorphic Facies of Regional Metamorphism.
  • Rocks under this Facies have medium pressure and moderate to high temperature.
  • It is named so because of the presence of Amphibole rich in rocks.

MINERALOGICAL CHARACTERSTICS

  • Hornblende + Plagioclase (Anorthite> 20) in mafic rocks and Kyanite in politic (in the presence of Plagioclase Feldspar and Garnet).
  • Disappearance of Epidote and increase in Calcium in plagioclase are characterstic chemical change as Metamorphic Intensity increases through this Facies.
  • Amphibole Facies rocks are widely distributed in Orogenic Belts.

Amphibole Facies

  1. Granulite facies:

  • This Facies is a result of High Grade Metamorphism at medium Pressure.
  • This Facies is associated with Orogenic Regional Metamorphism.

MINERALOGICAL CHARACTERSTICS

  • Augite + Orthopyroxene +Plagioclase, also Fe-Mg Garnet.
  • Temperature Ranges from 500 to 700°C whereas Pressure ranges from 8 to 70 kb.
  • Amphibole and Mica both Disappear from Granulite Facies.
  • Changes in mineralogy depends on different Protolith, however production of pyroxene is characterstic.
  • Less Intense Temperature and Pressure from rocks of Amphibolite Facies and moreIntense temperature and pressure form rocks of Granulite facies.

Granulite facies

  1. Eclogite Facies:

  • Facies of highest Pressure and Temperature and high depth.
  • It is named so for Metabasic rock Eclogite.

MINERALOGICAL CHARACTERSTICS

  • Feldspar free assemblage typified by Jadiete rich Clinopyroxene and Pyrope rich Garnet.
  • Continued subduction of Blueshcist facies oceanic Crust will produce Eclogite facies assemblage in Metamorphic Basalt.
  • These Facies are essentially recognized in rocks of basaltic composition, which are transformed into Spectacular red and green rocks composed of Hydrous mineral assemblage Garnet plus Omphacite.

 Eclogite Facies

 

 

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