Metamorphic facies in Plate Tectonics
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.
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:
- 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 ).
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.
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