Metamorphic Rock Grades
Metamorphic rock grades describe the intensity of metamorphism in terms of temperature and pressure conditions. They are generally classified into low-grade, intermediate-grade, and high-grade metamorphism.
Metamorphic rocks form from pre-existing rocks (protoliths) that undergo changes in mineralogy, texture, and chemical composition due to variations in temperature, pressure, and chemical environment. The degree or intensity of metamorphism is termed the metamorphic grade, while the specific set of mineral assemblages that form under particular conditions of pressure and temperature are referred to as metamorphic facies.
Metamorphic rock grades are classified based on the intensity of metamorphism, which is determined by the temperature and pressure conditions a rock experiences. These grades help in understanding the metamorphic history and the tectonic processes involved. Here’s a detailed explanation of each grade, including their occurrence, characteristics, index minerals, and rock examples:
Low-Grade Metamorphism
Occurrence:
Low-grade metamorphic rocks typically form at shallow depths within the Earth's crust where temperatures and pressures are relatively low.
Common in areas subjected to regional metamorphism due to tectonic forces, such as mountain-building regions and areas adjacent to mid-ocean ridges.
Characteristics:
- Low temperatures (generally below 400°C) and relatively low pressures.
- Fine-grained textures due to the low temperatures that do not allow for significant crystal growth.
- Low-grade metamorphic rocks often retain many features of the original parent rock.
- Characterized by slaty cleavage, where the rock can be split into thin layers.
- Quartz and feldspar are often present but are not exclusive to low-grade metamorphism.
Low-Grade Metamorphism Index Minerals:
- Chlorite: Green, platy mineral that forms under low temperatures.
- Muscovite: Light-colored mica.
- Biotite: Dark-colored mica, although it can also form at higher grades.
Associated Metamorphic Facies:
- Zeolite Facies
- Prehnite-Pumpellyite Facies
Low-Grade Metamorphism Rock Examples:
Slate: A fine-grained, foliated rock formed from shale. Characterized by its ability to break into thin, parallel layers.
Phyllite: Similar to slate but with a slightly higher grade, Slightly coarser than slate with a silky sheen, also derived from shale or mudstone. exhibiting a glossy sheen due to the presence of fine-grained mica.
Intermediate-Grade Metamorphism
Occurrence:
Forms at greater depths than low-grade metamorphic rocks, usually associated with regional metamorphism in mountainous regions where tectonic plates converge.
Also found in the core zones of mountain ranges and areas undergoing moderate to deep burial.
Characteristics:
- Moderate temperatures (between 400°C and 600°C) and pressures.
- Medium to coarse-grained textures.
- Distinct foliation (schistosity), where the rock splits into thin, irregular sheets.
- Increased recrystallization and development of new mineral assemblages.
- More pronounced foliation and banding compared to low-grade rocks.
Intermediate-Grade Metamorphism Index Minerals:
- Garnet: Typically red to brown, forms at intermediate to high temperatures and pressures.
- Staurolite: Often brown, forming prismatic crystals.
- Kyanite and andalusite: Aluminous silicate minerals that form under intermediate-grade conditions.
- Biotite: Dark mica, prevalent at this grade.
Associated Metamorphic Facies:
- Greenschist Facies
- Amphibolite Facies
Intermediate-Grade Metamorphism Rock Examples:
Schist: Medium to coarse-grained, with pronounced foliation and visible minerals such as mica, garnet, or staurolite.
Amphibolite: Typically derived from basalt or gabbro, characterized by amphibole (hornblende) and plagioclase.
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| A guide to the types of metamorphic rocks that form from different parent rocks at different grades of regional metamorphism. |
High-Grade Metamorphism
Occurrence:
Forms at significant depths within the Earth's crust, often in the roots of mountain belts where pressures and temperatures are highest.
Common in areas of continental collision and subduction zones.
High-Grade Metamorphism Characteristics:
- High temperatures (above 600°C) and high pressures.
- Coarse-grained textures with well-developed foliation and banding.
- Extensive recrystallization leads to the formation of high-grade minerals.
- Recrystallization leads to the growth of large, visible mineral grains.
- High-grade metamorphism can result in partial melting, producing migmatites.
High-Grade Metamorphism Index Minerals:
- Sillimanite: High-temperature aluminosilicate mineral.
- Kyanite: Can also form at high pressures, although it starts to appear at intermediate grades.
- Quartz and feldspar: Major components, especially in granitic gneisses.
- Pyroxenes: Common in high-grade mafic rocks.
- Garnet (stable across a wide range of conditions but indicative of high-grade when large)
Associated Metamorphic Facies:
- Granulite Facies
- Eclogite Facies
High-Grade Metamorphism Rock Examples:
Gneiss: A high-grade, Coarse-grained, foliated rock with distinct banding caused by segregation of mineral layers (distinct banding of light and dark minerals). Composed of quartz, feldspar, and mica.
Granulite: Formed under extremely high temperatures and pressures, typically consisting of feldspar, quartz, and high-temperature minerals like pyroxene.
Migmatite: A rock that exhibits features of both igneous and metamorphic processes, often showing partial melting with light-colored granitic material interspersed with darker metamorphic rock.
Summary Table
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| Metamorphic Rock Grades summary |
Other Factors Affecting Metamorphic Grade:
Parent rock composition: The original rock type influences the metamorphic minerals formed.
Differential stress: The direction and intensity of pressure can affect the orientation of minerals and the development of foliation.
Fluids: The presence of fluids can accelerate metamorphic reactions and influence mineral growth.
Understanding these grades helps geologists interpret the conditions under which the rock formed and the geological history of the region.