Regional metamorphism
Introduction
Regional metamorphism refers to the process by which rocks undergo transformation due to high pressures and temperatures over large areas, typically associated with orogenic events. This type of metamorphism is distinct from contact metamorphism, which occurs over smaller areas due to the heat from nearby magma bodies. Regional metamorphism results in the recrystallization of minerals, changes in rock texture, and the formation of new metamorphic minerals.
Geological Setting
Regional metamorphism commonly occurs in convergent plate boundaries, where tectonic plates collide and create immense pressure and heat. These conditions are prevalent in orogenic belts, regions where mountains are formed. The process can affect vast regions of the Earth's crust, often extending over hundreds or thousands of square kilometers.
Metamorphic Grade
The intensity of regional metamorphism is classified by metamorphic grade, which ranges from low to high. Low-grade metamorphism occurs at relatively low temperatures and pressures, typically resulting in the formation of minerals such as chlorite and muscovite. High-grade metamorphism occurs at higher temperatures and pressures, leading to the formation of minerals like garnet and sillimanite.
Low-Grade Metamorphism
Low-grade metamorphism is characterized by temperatures between 200°C and 320°C and pressures of 2-4 kilobars. Rocks subjected to these conditions often exhibit slaty cleavage and contain minerals such as chlorite, muscovite, and biotite. Common rock types include slate and phyllite.
Medium-Grade Metamorphism
Medium-grade metamorphism occurs at temperatures between 320°C and 450°C and pressures of 4-7 kilobars. This grade is marked by the development of schistosity and the presence of minerals like garnet, staurolite, and kyanite. Typical rocks include schist and quartzite.
High-Grade Metamorphism
High-grade metamorphism takes place at temperatures above 450°C and pressures exceeding 7 kilobars. Rocks at this grade often exhibit gneissic banding and contain minerals such as sillimanite, feldspar, and cordierite. Common high-grade metamorphic rocks include gneiss and migmatite.
Metamorphic Facies
Metamorphic facies are groups of minerals that form under similar pressures and temperatures. They provide valuable information about the metamorphic conditions and the tectonic setting. Key facies associated with regional metamorphism include:
Greenschist Facies
The greenschist facies is characterized by low to moderate temperatures (300-450°C) and pressures (2-10 kilobars). Typical minerals include chlorite, actinolite, and epidote. Rocks in this facies often exhibit a greenish color due to the presence of chlorite.
Amphibolite Facies
Amphibolite facies occur at higher temperatures (450-700°C) and pressures (5-10 kilobars). Minerals such as hornblende, plagioclase, and garnet are common. Rocks in this facies are typically dark and dense, often forming amphibolite.
Granulite Facies
The granulite facies represents high-temperature (700-900°C) and moderate to high-pressure (5-12 kilobars) conditions. Characteristic minerals include orthopyroxene, clinopyroxene, and plagioclase. Granulite facies rocks are typically coarse-grained and exhibit a granular texture.
Textural Changes
Regional metamorphism induces significant textural changes in rocks. These changes include foliation, lineation, and recrystallization.
Foliation
Foliation is the alignment of platy minerals, such as mica, within a rock. This texture is a result of differential stress, which causes minerals to reorient perpendicular to the direction of maximum pressure. Foliation is commonly observed in slate, phyllite, schist, and gneiss.
Lineation
Lineation refers to the alignment of elongated minerals or mineral aggregates within a rock. This texture is often seen in rocks that have undergone significant deformation, such as those found in shear zones.
Recrystallization
Recrystallization involves the growth of new mineral grains that are stable under the metamorphic conditions. This process can result in the development of larger, more equant grains and the obliteration of original rock textures.
Metamorphic Reactions
Metamorphic reactions are chemical processes that occur during regional metamorphism, leading to the formation of new minerals. These reactions are driven by changes in temperature, pressure, and the presence of fluids.
Dehydration Reactions
Dehydration reactions involve the loss of water from hydrous minerals, such as chlorite and muscovite. An example of a dehydration reaction is the transformation of chlorite to garnet and biotite.
Decarbonation Reactions
Decarbonation reactions involve the release of carbon dioxide from carbonate minerals, such as calcite and dolomite. These reactions are common in marble and calc-silicate rocks.
Solid-Solid Reactions
Solid-solid reactions involve the transformation of one mineral to another without the involvement of fluids. An example is the conversion of kyanite to sillimanite at high temperatures.
Tectonic Implications
Regional metamorphism provides valuable insights into the tectonic history of an area. The distribution of metamorphic facies and the orientation of foliations and lineations can reveal information about past tectonic events, such as continental collision and subduction.
Economic Importance
Metamorphic rocks formed through regional metamorphism can host valuable mineral deposits. For example, schist and gneiss can contain economically important minerals such as gold, graphite, and talc. Additionally, the study of regional metamorphism can aid in the exploration of hydrothermal ore deposits.