Introduction to Metamorphic Facies

This ppt includes brief information about metamorphic facies
like, what is metamorphic facies?
How it is classified? etc.

1. Introduction to Metamorphic F acies By : Haroon Muhammed MS C . Geology 2020 MIT - MAHE

2. metamorphic facies Eskola (1920) first prosed the concept of metamorphic facies for subdividing the entire P-T range of metamorphism in number of selected domains, primarily on the basis of occurrences of stable mineral assemblages developing in rocks of different bulk composition. Where he introduced 8 facies . Later in (1958) Fyfe , Turner, Coombs et al. (1959 ) updated it by adding four more facies .

3. Definition Metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperature conditions.

4. Concept of Metamorphic facies Rocks of identical bulk composition having identical mineral assemblages must have metamorphosed in identical P-T condition of metamorphism and surely belong to same metamorphic facies . Rocks of a specific bulk chemical composition metamorphosed under different facies condition will definitely produce different mineral assemblages.

5. In any given metamorphic setting there can be a variety of protolith exposed to metamorphism. While these rocks will be exposed to the same range of pressure and temperatures conditions within that setting, the metamorphic rock that results will depend on the protolith . A convenient way to indicate the range of possible metamorphic rocks in a particular setting is to group those possibilities into metamorphic facies The unique sets of minerals that form in a reference material at one P and T can be compared with assembages of minerals that formed in other rock compositions under the same P and T conditions.

6. Metamorphic facies A metamorphic facies is not a single rock-type but a wide range of minerals that form under similar P-T conditions. A general facies diagram was developed and names of each facies are based on those mineral assemblages that develop when a mafic bulk composition undergoes various P-T conditions.

7. Metamorphic Facies Zeolite Facies - zeolites Prehnite-Pumpelleyite Facies - Prehnite + pumpellyite Greenschist Facies – chlorite + albite + epidote + actinolite Amphibolite Facies -plagioclase + hornblende + garnet Granulite Facies - orthopyroxene + clinopyoxene + plag + hornblende + garnet Blueschist Facies - glaucophane + lawsonite or epidote (+ albite + chlorite) Eclogite Facies - pyroxene + garnet

8. Albite-epidote hornfelsic Facies Hornblende hornfelsic Facies Pyroxene hornfelsic Facies Sanidinite Facies

9. D iagram shows the different metamorphic facies . The axes on the diagram are temperature, depth and pressure; the depth within the Earth will determine how much pressure a rock is under, so the vertical depth axis is also a pressure axis. Each facies represents a range of temperature and pressure conditions where particular types of metamorphic rocks will form.

10. The different metamorphic facies are defined by the mineralogical composition of a rock. When the temperature or pressure in a rock body change, the rock can cross into a different facies .

11. Metamorphic facies Basalt are considered as reference material due to their abundance. A basalt metamorphosed during subduction to high pressures at low temperatures recrystallizes into a rock containing glaucophane , lawsonite , and albite; glaucophane is is blue to black in hand sample and lavender to blue under the microscope. Because of their distinctive bluish coloration, such samples are called blueschists . The same rock type metamorphosed at more moderate pressures and temperatures in the range of 400–500 °C (752–932 °F) would contain abundant chlorite and actinolite , minerals that are green both in hand sample and under the microscope, and would be referred to as a greenschist .

12. Geothermal gradient The dashed lines represent the geothermal gradients in different environments. G eothermal gradient describes how rapidly the temperature increases with depth in Earth. In most areas the rate of increase in temperature with depth is 30 °C/km . In volcanic areas (yellow dashed line), the geothermal gradient is more like 40 to 50 °C/km, so the temperature rises much faster as you go down. Along subduction zones, the cold ocean lithosphere keeps temperatures low, so the gradient is typically less than 10 °C/km.

13. tectonic process and metamorphic facies Different types of tectonic processes produce different associations of metamorphic facies in the field. For example, regions associated with subduction of oceanic material beneath either oceanic crust or continental crust are characterized by blueschist , greenschist , and eclogite facies rocks, whereas areas thought to reflect continent-continent collision are more typically distinguished by greenschist and amphibolite facies rocks (see also subduction zone). Still other regions, usually containing an abundance of intrusive igneous material, show associations of low-pressure greenschist , amphibolite, and granulite facies rocks.

14. Metamorphic facies series Akiho Miyashiro, develop the concept of metamorphic facies series. Miyashiro described the three facies associations given above as high-pressure, medium-pressure, and low-pressure facies series.

15. Reference John D. Winter Brittanica (William S. Fyfe) K Bucher, University of Freiburg, Freiburg, Germany Tulane.edu

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