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
- 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 :
- Thick sedimentary sequence of Continental Margins.
- Rock sequence is subjected to hot water conducting system.
- 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.
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)
- 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.
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.
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.
- 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.
- 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.
- This Facies is a result of High Grade Metamorphism at medium Pressure.
- This Facies is associated with Orogenic Regional Metamorphism.
- 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.
- Facies of highest Pressure and Temperature and high depth.
- It is named so for Metabasic rock Eclogite.
- 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.