|Metamorphic facies in Plate Tectonics|
An overview of metamorphism in relation to tectonic regimes:
The metamorphic facies series encountered in different tectonic regimes or settings can be summarized as follows, and are shown schematically on Figs. :
- Ridges and rift valleys: characterized by high geothermal gradients contact and ocean floor metamorphism.
- Areas of magmatic activity; volcanic - plutonic complexes: greenschists amphibolites granulites.
- Areas of crustal thickening and mountain building: greenschists amphibolites granulites and type B eclogites (particularly if there are magmatic intrusions).
- Subduction zones: Characterized by low geothermal gradients: zeolite pumpellyite-actinolite facies /lawsonite albite facies blueschist facies type C eclogites.
The movement of tectonic plates transports sediment and rocks into different geologic setting—these changes can result in metamorphism, particularly in zones where tectonic plates are converging, as in a subduction zone or where continental plates converge, pushing up high mountain ranges while material below the mountains are pushed down under increasing temperature and pressure condition. These are large scale regions that experience a wide range of conditions through time, and is called regional metamorphism. Areas that are subjected to definable conditions, in part based on the minerals formed, are called metamorphic facies (Figure). Examples include:
|Temperature and pressure effects on metamorphism|
- Moderate pressure + low temperature = Greenschist facies
- High pressure + low temperature = Blueschist facies
- Moderate pressure + high temperature = Amphibolite facies
- Highest temperature + high pressure = Granulite facies
- High temperature + highest pressure = Ecologite facies
Even though the rock material may have the same chemical composition, their mineral compositions, texture, and appearance are different, and the rocks they comprise are classed into rocks of different metamorphic grades (metamorphic facies). Over the past century, scientists have studies the distribution and occurrences of minerals in the field as well as having manufactured them in laboratories settings, simulating the pressures and temperatures within the earth where different grades of metamorphism takes place. As a result, the temperature and pressure ranges of formation (and destruction) of many minerals, and the metamorphic rocks they form, are well known (Figure ).
|Generalized illustration of a convergent plate|
boundary showing regional metamorphism.
Examples of rocks from different metamorphic facies are illustrated below. It is important to note that the rocks may have been exposed to the different grades of metamorphism, but the minerals present in those rocks are based on the elemental composition of the protolith from which it formed. For example, a metamorphic rock formed in the "amphibolite facies" (or "amphibolite" grade) may or may not contain the mineral amphibole (a common mafic mineral). It may not contain amphibole if there wasn't enough mafic mineral component in the original protolith from which it formed, but it has experienced heat and pressure in the range where amphibolite forms.
Ecologite is a high-grade metamorphic rock containing granular minerals, typically red garnet mixed with grains of pyroxene, quartz, and feldspars and other metamorphic minerals formed under great pressure.
Granulite is a high-grade metamorphic rock containing granular minerals, typically red garnet mixed with grains of pyroxene, quartz, and feldspars formed under great heat without melting.