A metamorphic facies series is a sequence of facies that occurs across a metamorphic terrane due to differences in pressure and temperature (P/T) conditions. Each facies series is characterized by the development of a particular sequence of individual facies, with each facies stable at a specific range of temperature and pressure conditions
There are five metamorphic facies series :-
- Contact facies series (very low P)
- Buchan or Abukuma facies series (low P regional)
- Barrovian facies series (medium P regional)
- Sanbagwa facies series (high P, moderate T)
- Franciscan facies series (high P, low T)
High P/T series :- These series typically occurs in the subduction zones where normal isotherms are depresses by the subduction of cool lithosphere faster than it can equilibrate thermally.
The facies sequence is (Zeolite) – (Prehnite-pumpellyite) – blueschist facies – eclogite facies.
Medium P/T series :- It is characterized by common orogenic belts (Barrovian type) with the sequence (zeolite facies) – (prehnite-pumpellyite facies) – greenschist – amphibolite facies – (granulite facies)
Crustal melting under H2O saturated conditions occurs in the upper amphibolite facies.
The granulite facies, therefore occurs only in H2O deficient rocks, either dehydrated lower crust or areas with high XCO2 in the intergranular fluid.
Low P/T series :- It is characterized by high heat flow orogenic belts, rift areas or contact metamorphism. The sequence may be low pressure version of medium P/T series with cordierite and/or andalusite in aluminous rocks.
The sequence is (zeolie facies) – albite-epidote hornfels facies – hornblende-hornfels facies– pyroxene-hornfels facies – sanidinite facies. Sanidinite facies are rare and generally localised, requiring the introduction of greate heat to very shallow level.
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