7.4 Metamorphic facies and pressure-temperature conditions
2 min read•july 22, 2024
Metamorphic facies are key to understanding the pressure and temperature conditions rocks experienced during transformation. They reveal the environments where rocks formed, from shallow burial to deep , helping geologists reconstruct a region's metamorphic history.
Each facies has unique mineral assemblages that form under specific conditions. By analyzing these minerals and using P-T diagrams, geologists can determine metamorphic grade and intensity. This information provides crucial insights into tectonic settings and burial depths of metamorphic rocks.
Metamorphic Facies and Pressure-Temperature Conditions
Significance of metamorphic facies
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- Provide insight into pressure and temperature conditions during metamorphism allows geologists to understand the environment in which the rocks formed
- Help understand tectonic settings and burial depths of metamorphic rocks such as shallow burial in low-grade or deep burial in subduction zones
- Allow reconstruction of metamorphic history of a region by analyzing the spatial distribution and changes in facies over time
Characteristics of metamorphic facies
- forms under low pressure and temperature conditions characterized by zeolites, prehnite, and pumpellyite minerals
- forms under low to medium pressure and temperature conditions characterized by , actinolite, epidote, and albite minerals
- forms under medium to high pressure and temperature conditions characterized by , , and minerals
- forms under high pressure and temperature conditions characterized by , plagioclase, and garnet minerals
- forms under very high pressure and high temperature conditions characterized by ( pyroxene) and garnet minerals
Interpretation of P-T diagrams
- Display stability fields of metamorphic minerals and facies with temperature on x-axis and pressure on y-axis
- Metamorphic facies plotted as regions on the diagram show the pressure and temperature conditions under which they form
- Can be used to determine the metamorphic grade and intensity of metamorphism a rock has undergone
Concept of metamorphic grade
- Refers to intensity or degree of metamorphism with low-grade at lower temperatures and pressures and high-grade at higher temperatures and pressures
- Low-grade metamorphism corresponds to zeolite and greenschist facies indicating shallow burial or low-grade regional metamorphism
- Medium-grade metamorphism corresponds to amphibolite facies indicating moderate to deep burial in orogenic belts
- High-grade metamorphism corresponds to granulite and eclogite facies indicating deep burial in the lower crust or subduction zones
Tectonic settings from facies
- Zeolite and greenschist facies indicate low pressure and temperature conditions found in shallow burial settings or low-grade regional metamorphism (burial metamorphism)
- Amphibolite facies indicate medium to high pressure and temperature conditions associated with regional metamorphism in orogenic belts suggesting moderate to deep burial depths (Barrovian metamorphism)
- Granulite facies indicate high pressure and temperature conditions found in the lower crust or deep portions of orogenic belts suggesting deep burial depths and high-grade regional metamorphism (Abukuma metamorphism)
- Eclogite facies indicate very high pressure and high temperature conditions associated with subduction zones or ultra-high pressure metamorphism suggesting very deep burial depths, typically in subduction settings (Franciscan metamorphism)
Key Terms to Review (25)
Amphibolite facies: Amphibolite facies refers to a specific set of metamorphic conditions characterized by moderate to high temperatures and pressures, typically ranging from about 500 to 700 degrees Celsius and pressures from 4 to 8 kilobars. This facies is primarily associated with the presence of amphibole minerals, particularly hornblende, and is indicative of a metamorphic environment where the original rock, or protolith, was subjected to significant alteration without melting.
Chlorite: Chlorite is a green, sheet silicate mineral that is commonly found in metamorphic rocks and plays a significant role in metamorphic facies. This mineral forms under specific pressure-temperature conditions, often associated with low-grade metamorphism, and is indicative of particular metamorphic environments.
Contact Metamorphism: Contact metamorphism is a type of metamorphism that occurs when rocks are heated and altered due to their proximity to hot magma or lava. This process typically leads to localized changes in the mineral composition and texture of the surrounding rocks, resulting in the formation of metamorphic rocks that reflect the conditions near the intrusive igneous structures that caused the metamorphism. The relationship between contact metamorphism and these structures highlights how heat transfer from magma can dramatically influence nearby rock formations.
Eclogite facies: Eclogite facies is a specific set of metamorphic conditions characterized by high pressure and moderate to high temperature, typically found at depths of around 30 to 70 kilometers in the Earth's crust. This facies is primarily composed of garnet and omphacite, reflecting the unique mineral assemblage that forms under such extreme conditions. The presence of eclogite facies indicates significant tectonic processes such as subduction, where oceanic plates are forced down into the mantle, leading to the transformation of basaltic rocks into eclogite.
Foliation: Foliation is a textural feature in metamorphic rocks characterized by the parallel alignment of mineral grains or layers, which occurs due to directed pressure during metamorphism. This alignment often results in a layered appearance and can significantly influence the physical properties of the rock, such as its strength and how it breaks. The development of foliation is closely linked to the metamorphic processes that occur under varying pressure and temperature conditions.
Garnet: Garnet is a group of silicate minerals that are commonly found in metamorphic rocks, known for their distinctive crystal shapes and hardness. These minerals can form under high pressure and temperature conditions, making them important indicators of metamorphic processes and conditions. Garnets often appear as red, but they can come in a variety of colors, depending on their chemical composition.
Geotherm: A geotherm is a graphical representation that shows how temperature changes with depth in the Earth's crust. It is important for understanding the thermal gradient, which indicates how heat flows through geological formations and is essential for determining the metamorphic conditions under which rocks can change. The shape of a geotherm can reveal significant information about tectonic activity, geothermal resources, and the stability of different minerals in metamorphic processes.
Granulite facies: Granulite facies is a metamorphic facies characterized by high temperatures and moderate to high pressures, typically resulting in the formation of granulite, a high-grade metamorphic rock. This facies reflects a specific range of pressure-temperature conditions, often associated with deeper crustal levels where rocks undergo significant metamorphism due to tectonic processes and heat from the Earth's interior.
Greenschist facies: Greenschist facies is a set of metamorphic conditions characterized by moderate temperature and pressure, where minerals such as chlorite, albite, and actinolite are typically formed. This facies indicates a specific range of metamorphic conditions usually found in subduction zones or areas of regional metamorphism, reflecting the geological processes that transform pre-existing rocks into metamorphic rocks.
High-pressure: High-pressure refers to conditions in which the pressure exerted on rocks is significantly greater than atmospheric pressure, usually occurring deep within the Earth's crust. This term is crucial for understanding metamorphic processes, where rocks are subjected to intense pressure and temperature, resulting in the formation of metamorphic rocks with unique characteristics and mineral compositions.
Hornblende: Hornblende is a complex inosilicate mineral that belongs to the amphibole group, characterized by its dark color and elongated prismatic crystals. It plays an important role in metamorphic geology as it forms under specific pressure-temperature conditions, reflecting the metamorphic facies of the rocks in which it is found.
Lineation: Lineation refers to the linear features or structures within a rock that indicate the direction of stress during deformation. This concept is crucial in understanding the fabric of metamorphic rocks, as lineation helps geologists decipher the conditions under which these rocks formed, including pressure-temperature conditions and tectonic forces.
Low-temperature: Low-temperature refers to conditions that are generally associated with metamorphic processes occurring at relatively mild thermal regimes, typically below 300°C (572°F). These conditions lead to specific mineral assemblages and texture changes in rocks without the extreme heat often associated with high-grade metamorphism. Low-temperature metamorphism is essential for understanding the formation of certain metamorphic facies and the physical changes that occur in response to varying temperature and pressure conditions.
Marjorie E. Rice: Marjorie E. Rice was a prominent American geologist known for her work in metamorphic geology and the study of metamorphic facies. Her research contributed significantly to the understanding of the relationship between pressure-temperature conditions and the formation of metamorphic rocks, helping to define key concepts in the field.
Metasomatism: Metasomatism is the process of chemical alteration of a rock by the introduction or removal of chemical components, typically through hydrothermal fluids. This process can lead to significant changes in the mineral composition and texture of the rock, influencing its properties and behavior under varying pressure-temperature conditions. Metasomatism often occurs in metamorphic environments, linking it to the broader concepts of metamorphic processes and the classification of metamorphic facies based on specific pressure-temperature conditions.
Mid-ocean ridges: Mid-ocean ridges are underwater mountain ranges formed by the process of seafloor spreading, where tectonic plates pull apart and magma rises to create new oceanic crust. These features are significant because they represent the boundary between diverging tectonic plates, playing a crucial role in plate tectonics, geological processes, and the formation of metamorphic rocks under specific pressure and temperature conditions.
Omphacite: Omphacite is a clinopyroxene mineral that is commonly found in high-pressure metamorphic rocks, especially eclogites. This mineral is significant because it forms under specific pressure-temperature conditions that are characteristic of certain metamorphic facies, particularly in subduction zones. Its presence indicates the unique conditions that lead to the transformation of rocks under intense geological processes.
Parent Rock: Parent rock refers to the original rock from which a metamorphic rock is formed through the process of metamorphism. This term is crucial for understanding how different types of rocks evolve, as the characteristics of the parent rock heavily influence the properties and textures of the resulting metamorphic rock. Additionally, in igneous processes, parent rocks can be related to the source material from which magma is generated, connecting to the classification and textural aspects of igneous rocks.
Plagioclase: Plagioclase is a group of feldspar minerals that are important components in many igneous and metamorphic rocks. These minerals vary in composition from sodium-rich albite to calcium-rich anorthite and are crucial in understanding the metamorphic facies and pressure-temperature conditions of rocks. Plagioclase minerals can influence the texture and color of rocks, helping to identify the conditions under which they formed.
Pyroxene: Pyroxene is a group of important rock-forming silicate minerals commonly found in igneous and metamorphic rocks. They typically exhibit a single chain structure of silicon-oxygen tetrahedra and are characterized by their dark color, two directions of cleavage at nearly 90 degrees, and high density. This mineral group plays a vital role in the classification of igneous rocks and understanding metamorphic conditions, as it provides insights into the mineral composition and the processes that formed these rocks.
Regional metamorphism: Regional metamorphism is the process where rocks undergo significant changes in mineralogy and texture due to high pressure and temperature over large areas, typically associated with tectonic forces. This type of metamorphism is important as it helps in understanding the formation of various rock types and their association with geological processes like mountain building and plate tectonics.
Subduction Zones: Subduction zones are regions of the Earth's crust where one tectonic plate moves under another and sinks into the mantle. These zones are critical areas of geological activity, leading to the formation of deep ocean trenches, volcanic arcs, and earthquakes, as well as influencing metamorphic processes and rock classification.
T. Scott McLennan: T. Scott McLennan is a prominent geologist known for his contributions to understanding metamorphic facies and their relationships to pressure-temperature conditions in metamorphic rocks. His research focuses on the transformation of minerals under varying geophysical conditions, helping to establish the connections between mineral assemblages and the specific environments in which they form.
Thermal gradient: A thermal gradient refers to the rate at which temperature changes with depth in the Earth, typically measured in degrees Celsius per kilometer. This concept is crucial in understanding how temperature variations affect metamorphic processes and the stability of mineral assemblages within different metamorphic facies, revealing the environmental conditions under which rocks form and change.
Zeolite facies: Zeolite facies refers to a specific category of metamorphic rocks that form under low-temperature and low-pressure conditions, typically associated with the alteration of volcanic ash or tuff. This facies is characterized by the presence of zeolites, which are hydrous aluminosilicate minerals, indicating a significant degree of hydration and the influence of fluids during metamorphism.