Regional Metamorphism
Regional metamorphism is a type of metamorphism that occurs over a wide area, typically hundreds to thousands of square kilometers. It is caused by the combined effects of heat, pressure, and stress associated with large-scale tectonic processes, such as mountain building and continental collisions.
The force of the collision causes rocks to be folded, broken, and stacked on each other, so not only is there the squeezing force from the collision, but from the weight of stacked rocks. The deeper rocks are within the stack, the higher the pressures and temperatures, and the higher the grade of metamorphism that occurs.
During regional metamorphism, rocks are buried deep within the Earth's crust, where they are subjected to temperatures of hundreds of degrees Celsius and pressures that can reach several kilobars. These extreme conditions cause the rocks to recrystallize and form new minerals.
Rocks that form from regional metamorphism are likely to be foliated because of the strong directional pressure of converging plates.
Regional metamorphism occurs when rocks are buried deep in the crust. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. Because burial to 10 km to 20 km is required, the areas affected tend to be large.
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| Regional Metamorphism |
Most regional metamorphism takes place within continental crust. While rocks can be metamorphosed at depth in most areas, the potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to great depths.
An example would be the Himalayan Range. At this continent-continent convergent boundary, sedimentary rocks have been both thrust up to great heights (nearly 9,000 m above sea level) and also buried to great depths.
At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary. Water within the crust is forced to rise in the area close to the source of volcanic heat, and this draws more water in from farther out, which eventually creates a convective system where cold seawater is drawn into the crust and then out again onto the sea floor near the ridge.
The passage of this water through the oceanic crust at 200° to 300°C promotes metamorphic reactions that change the original pyroxene in the rock to chlorite and serpentine. Because this metamorphism takes place at temperatures well below the temperature at which the rock originally formed (~1200°C), it is known as retrograde metamorphism.
The rock that forms in this way is known as greenstone if it isn’t foliated, or greenschist if it is. Chlorite and serpentine are both “hydrated minerals” meaning that they have water (as OH) in their chemical formulas. When metamorphosed ocean crust is later subducted, the chlorite and serpentine are converted into new non-hydrous minerals (e.g., garnet and pyroxene) and the water that is released migrates into the overlying mantle, where it contributes to flux melting.
At a subduction zone, oceanic crust is forced down into the hot mantle. But because the oceanic crust is now relatively cool, especially along its sea-floor upper surface, it does not heat up quickly, and the subducting rock remains several hundreds of degrees cooler than the surrounding mantle. A special type of metamorphism takes place under these very high-pressure but relatively low-temperature conditions, producing an amphibole mineral known as glaucophane, which is blue in colour, and is a major component of a rock known as blueschist.
If you’ve never seen or even heard of blueschist, it’s not surprising. What is surprising is that anyone has seen it! Most blueschist forms in subduction zones, continues to be subducted, turns into eclogite at about 35 km depth, and then eventually sinks deep into the mantle — never to be seen again.
Regional metamorphism also takes place within volcanic-arc mountain ranges, and because of the extra heat associated with the volcanism, the geothermal gradient is typically a little steeper in these settings (somewhere between 40° and 50°C/km). As a result higher grades of metamorphism can take place closer to surface than is the case in other areas.
Rather than focusing on metamorphic rock textures (slate, schist, gneiss, etc.), geologists tend to look at specific minerals within the rocks that are indicative of different grades of metamorphism.
Some common minerals in metamorphic rocks are shown in Figure, arranged in order of the temperature ranges within which they tend to be stable. The upper and lower limits of the ranges are intentionally vague because these limits depend on a number of different factors, such as the pressure, the amount of water present, and the overall composition of the rock.
Metamorphic Grade
Metamorphic grade refers to the intensity of metamorphism a rock has undergone, which is determined by factors such as temperature, pressure, and the duration of metamorphic conditions. It's typically classified into low-grade, medium-grade, and high-grade metamorphism.
Metamorphic grade:
The Scale: Metamorphic grade ranges from low to high. Low-grade metamorphism indicates minimal change, while high-grade signifies a dramatic transformation of the original rock.
Temperature and Pressure: The grade is directly linked to the temperature and pressure the rock experiences. Higher temperatures and pressures lead to higher metamorphic grades.
Index Minerals: Geologists use specific minerals, called index minerals, to identify metamorphic grade. These minerals only form under certain temperature and pressure conditions. For example, chlorite is an indicator of low-grade metamorphism, while garnet suggests a higher grade.
Classifications of metamorphic grade
Low-grade metamorphism: This occurs at relatively low temperatures and pressures and typically involves the alteration of minerals without complete recrystallization. Examples of low-grade metamorphic rocks include slate and schist.
Medium-grade metamorphism: This involves higher temperatures and pressures compared to low-grade metamorphism. Rocks experiencing medium-grade metamorphism often exhibit more pronounced mineral changes and may show some evidence of recrystallization. Examples include gneiss.
High-grade metamorphism: This occurs under conditions of very high temperatures and pressures, often deep within the Earth's crust or even in the mantle. Rocks subjected to high-grade metamorphism can undergo extensive recrystallization, leading to the formation of minerals like garnet, staurolite, and kyanite. Examples include migmatite and granulite.
The metamorphic grade of a rock can provide insights into the tectonic history and geological conditions it has experienced over time.
Metamorphic Facies
Metamorphic grade is often described alongside metamorphic facies, which refers to the specific mineral assemblages that form under certain pressure-temperature conditions. There are various facies, like greenschist facies (medium-grade) and amphibolite facies (high-grade).
Metamorphic facies include
Zeolite facies: This facies forms at low temperatures and low to moderate pressures. Minerals commonly found in zeolite facies include zeolites, chlorite, and prehnite.
Blueschist facies: Formed at low temperatures but relatively high pressures, typically in subduction zones. Minerals present may include glaucophane, lawsonite, and epidote.
Greenschist facies: Characterized by low to moderate temperatures and pressures. Common minerals include chlorite, biotite, and amphibole.
Amphibolite facies: Formed under moderate to high temperatures and pressures. Minerals often found in this facies include amphibole, plagioclase feldspar, and garnet.
Granulite facies: This facies forms at high temperatures and pressures, often in deeply buried continental crust. Common minerals include pyroxene, plagioclase feldspar, and garnet.
Eclogite facies: Characterized by very high pressures and moderate to high temperatures, typically formed in subduction zones at great depths. Key minerals include omphacite (a sodium-rich pyroxene) and garnet.
These facies represent different segments of the pressure-temperature space in which rocks undergo metamorphism.
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