🌋Physical Geology Unit 7 – Metamorphic Rocks and Processes

What Are Metamorphic Rocks?

• Form from pre-existing rocks that undergo changes in their physical and chemical properties due to exposure to high temperatures, pressures, or chemically active fluids
• Metamorphism occurs without melting the original rock, distinguishing it from igneous processes
• Require temperatures between 150°C and 795°C and pressures up to 1500 MPa to form
• Composed of recrystallized minerals that are stable under the new metamorphic conditions
• Classified based on their texture and mineral composition, which reflect the conditions of metamorphism
• Common examples include marble (metamorphosed limestone), quartzite (metamorphosed sandstone), and schist (metamorphosed shale)
• Play a crucial role in understanding the Earth's tectonic history and the conditions deep within the Earth's crust

Types of Metamorphism

• Regional metamorphism occurs over large areas and is associated with mountain-building events and plate tectonic processes
  • Affects extensive regions of the Earth's crust
  • Results from high pressures and temperatures caused by deep burial or tectonic compression
• Contact metamorphism occurs when magma intrudes into surrounding rock, causing localized heating and metamorphism
  • Limited to the area immediately surrounding the intrusion
  • Characterized by high temperatures but relatively low pressures
• Hydrothermal metamorphism involves the interaction of hot, chemically active fluids with rocks
  • Fluids are often rich in dissolved ions and can cause significant chemical changes in the rocks
  • Common in areas of volcanic activity or near hydrothermal vents
• Cataclastic metamorphism results from mechanical deformation and fracturing of rocks without significant chemical changes
  • Occurs along fault zones or areas of intense shearing
  • Leads to the formation of fault breccias and mylonites
• Burial metamorphism occurs due to the increasing pressure and temperature experienced by sedimentary rocks as they are buried deeper in the Earth's crust
  • Gradual process that can lead to the formation of low-grade metamorphic rocks (slates, phyllites)

Key Metamorphic Processes

• Recrystallization involves the growth of new minerals from the original rock-forming minerals without melting
  • Driven by changes in temperature, pressure, and chemical environment
  • Results in the formation of metamorphic textures and fabrics
• Metamorphic differentiation is the segregation of chemical components within a rock during metamorphism
  • Can lead to the formation of distinct mineral layers or bands (foliation)
  • Occurs due to differences in the mobility and solubility of elements under metamorphic conditions
• Pressure solution involves the dissolution of minerals at points of high stress and their reprecipitation in areas of lower stress
  • Contributes to the development of foliation and the removal of original sedimentary or igneous textures
• Metasomatism is the chemical alteration of rocks through the introduction or removal of elements by fluids
  • Can result in significant changes in the mineral composition of the rock
  • Often associated with hydrothermal metamorphism and the formation of ore deposits
• Deformation mechanisms, such as dislocation creep and diffusion creep, play a crucial role in the development of metamorphic textures and structures
  • Dislocation creep involves the movement of defects (dislocations) through the crystal lattice
  • Diffusion creep occurs through the diffusion of atoms or ions within the rock

Metamorphic Textures and Structures

• Foliation is the planar arrangement of minerals or aggregates within a metamorphic rock
  • Develops perpendicular to the direction of maximum compressive stress
  • Types of foliation include slaty cleavage, schistosity, and gneissic banding
• Lineation refers to the linear alignment of minerals or aggregates within a metamorphic rock
  • Can form due to the preferred orientation of elongated minerals or the alignment of fold axes
  • Provides information about the direction of stress or fluid flow during metamorphism
• Porphyroblasts are large, well-formed crystals that grow within a finer-grained metamorphic matrix
  • Often contain inclusions of the surrounding matrix, preserving a record of the metamorphic history
  • Can be used to determine the relative timing of metamorphic events (pre-, syn-, or post-tectonic growth)
• Reaction rims are narrow zones of distinct mineral composition that form around pre-existing grains or at the interface between different minerals
  • Result from chemical reactions between the minerals and the surrounding metamorphic environment
  • Provide insights into the changes in pressure, temperature, and fluid composition during metamorphism
• Metamorphic differentiation can lead to the formation of compositional banding
  • Alternating layers of different mineral compositions (leucosome and melanosome in migmatites)
  • Reflects the segregation of chemical components during high-grade metamorphism

Common Metamorphic Minerals

• Garnet is a common metamorphic mineral that forms under a wide range of pressure and temperature conditions
  • Almandine (Fe-rich) and pyrope (Mg-rich) are common in regional metamorphic rocks
  • Grossular (Ca-rich) and andradite (Ca-Fe-rich) are often found in contact metamorphic environments
• Mica minerals, such as biotite and muscovite, are abundant in many metamorphic rocks
  • Biotite is a dark, Fe- and Mg-rich mica that forms under medium to high-grade conditions
  • Muscovite is a light-colored, K-rich mica that is stable over a wide range of metamorphic conditions
• Amphiboles, particularly hornblende, are common in medium to high-grade metamorphic rocks
  • Form under a range of pressure and temperature conditions and can be used as indicators of metamorphic grade
• Kyanite, sillimanite, and andalusite are polymorphs of Al2SiO5 that form under different pressure and temperature conditions
  • Kyanite forms at high pressures and low to medium temperatures
  • Sillimanite forms at high temperatures and pressures
  • Andalusite forms at low pressures and medium to high temperatures
• Chlorite is a low-grade metamorphic mineral that forms under greenschist facies conditions
  • Commonly found in metamorphosed mafic rocks and hydrated ultramafic rocks (serpentinites)
• Epidote is a Ca-Al silicate mineral that forms under low to medium-grade metamorphic conditions
  • Often associated with the metamorphism of mafic rocks and the alteration of plagioclase feldspar

Metamorphic Facies and Grades

• Metamorphic facies are groups of metamorphic rocks that form under similar pressure and temperature conditions
  • Defined by the presence of specific mineral assemblages that are stable under those conditions
  • Common facies include zeolite, greenschist, amphibolite, granulite, and eclogite facies
• Metamorphic grade refers to the intensity of metamorphism, which is related to the pressure and temperature conditions
  • Low-grade metamorphism occurs at relatively low temperatures and pressures (zeolite to greenschist facies)
  • Medium-grade metamorphism occurs at moderate temperatures and pressures (amphibolite facies)
  • High-grade metamorphism occurs at high temperatures and pressures (granulite and eclogite facies)
• Pressure-temperature (P-T) paths describe the changes in pressure and temperature that a rock experiences during metamorphism
  • Can be determined by studying the mineral assemblages and textures of metamorphic rocks
  • Provide insights into the tectonic settings and burial/exhumation history of metamorphic terranes
• Index minerals are minerals that are diagnostic of specific metamorphic facies or grades
  • Examples include chlorite (greenschist facies), garnet (amphibolite facies), and sillimanite (granulite facies)
  • Used to determine the pressure and temperature conditions of metamorphism

Plate Tectonics and Metamorphism

• Subduction zones are major sites of regional metamorphism
  • Cold, dense oceanic crust is subducted beneath lighter continental or oceanic crust
  • Subducted rocks experience increasing pressure and temperature, leading to metamorphism
• Collision zones, such as continent-continent or arc-continent collisions, result in the formation of extensive metamorphic belts
  • Rocks are subjected to high pressures and temperatures due to crustal thickening and deep burial
  • Examples include the Himalayas (India-Asia collision) and the European Alps (Africa-Europe collision)
• Rifting and extension of the continental crust can lead to metamorphism
  • Thinning of the crust results in the upwelling of hot mantle material, causing heating and metamorphism of the overlying rocks
  • Often associated with the formation of metamorphic core complexes
• Hotspots and mantle plumes can cause localized metamorphism
  • Upwelling of hot mantle material leads to heating and metamorphism of the overlying crust
  • Examples include the Bushveld Complex (South Africa) and the Stillwater Complex (Montana, USA)
• Metamorphic rocks provide valuable information about the tectonic history of an area
  • Pressure-temperature paths and metamorphic ages can be used to reconstruct the burial and exhumation history of metamorphic terranes
  • Metamorphic rocks can also preserve evidence of past plate tectonic configurations and collisional events

Real-World Applications

• Metamorphic rocks are widely used as building materials and decorative stones
  • Marble is used for sculptures, monuments, and architectural applications (Taj Mahal, Parthenon)
  • Slate is used for roofing, flooring, and decorative purposes
• Metamorphic rocks host a variety of economically important mineral deposits
  • Metamorphosed limestones and dolostones can host lead-zinc deposits (Mississippi Valley-type deposits)
  • Metamorphosed ultramafic rocks (serpentinites) are a source of asbestos, talc, and magnesite
• Metamorphic rocks can serve as reservoirs for groundwater and hydrocarbons
  • Fractured and foliated metamorphic rocks can have significant porosity and permeability
  • Metamorphic basement rocks can act as source rocks for overlying sedimentary basins
• Metamorphic rocks provide insights into the Earth's deep crustal and mantle processes
  • High-pressure metamorphic rocks (eclogites) can be used to study subduction zone processes
  • Ultrahigh-pressure metamorphic rocks (UHP) provide evidence for the deep subduction and exhumation of continental crust
• Metamorphic rocks are crucial for understanding the evolution and stability of the continental crust
  • Metamorphic processes contribute to the differentiation and stabilization of the crust over geological time scales
  • Metamorphic rocks record the pressure-temperature-time paths of crustal materials, providing insights into the tectonic history of continents