Metamorphism and Metamorphic Rocks

Metamorphism & Metamorphic Rocks

  • Metamorphic Rocks:
    • Definition: Rocks that have undergone solid-state alteration of preexisting rocks (protoliths).
    • Components:
    • Meta = change
    • Morphe = form
    • Types of protoliths: Igneous, sedimentary, or metamorphic.

Protoliths

  • Protoliths:
    • Definition: The original rock that is altered during metamorphism.
    • Changes:
    • Texture and mineralogical changes.
    • Influenced by temperature, pressure, tectonic stress, and reactive water.
    • Example:
    • Limestone transforms into marble.

Changes Due to Metamorphism

  • Changes in Mineralogy:

    • Example:
    • Red shale (protolith) → Gneiss (metamorphic rock)
    • Composition Changes:
      • Red shale consists of quartz, clay, and iron oxide.
      • Gneiss is composed of quartz, feldspar, biotite, and garnet.

  • Changes in Texture:

    • Formation of intergrown and interlocking mineral grains due to pressure and temperature conditions.

Foliation

  • Foliation:
    • Definition: Planar fabric in metamorphic rocks defined by the alignment of platy minerals or alternating light/dark bands.
    • Impact: Provides strength and could lead to weakness along which rocks may break.

Metamorphic Processes

  • Recrystallization:

    • Process where minerals change size and shape without altering composition.
    • Example: Limestone → Marble.

  • Neocrystallization:

    • Formation of new minerals from existing minerals in the protolith.
    • Example: Garnet mica schist from a shale protolith.

  • Pressure Solution:

    • Mineral grains partially dissolve under pressure; ions migrate and reprecipitate in low-pressure areas.

  • Plastic Deformation:

    • Softening and deformation of mineral grains under high temperature and pressure without breaking.

  • Overprinting:

    • Multiple agents can alter a rock, including heat, pressure, and fluids. Not all agents are required for metamorphism.

Polymorphs

  • Polymorphs:
    • Minerals with the same chemical composition but different crystal structures due to varying pressure and temperature conditions.
    • Examples:
    • Andalusite (low T and P)
    • Kyanite (low T, high P)
    • Sillimanite (high T and P)

Hydrothermal Fluids

  • Hydrothermal Fluids:
    • Hot water with dissolved ions and volatiles that enhance metamorphism and lead to metasomatism.

Foliation Characteristics

  • Foliation Development:
    • Causes: Differential stress and presence of platy minerals that lead to a parallel fabric in metamorphic rocks.

Composition and Banding

  • Compositional Banding:
    • Developed through high-temperature shearing or solid-state differentiation, leading to light and dark layering in metamorphic rocks (e.g., gneiss).

Migmatites

  • Migmatites:
    • Rocks that are partially melted gneiss showcasing both metamorphic and igneous characteristics.

Metamorphic Grade

  • Metamorphic Grade:
    • A measure of the intensity of temperature and pressure conditions leading to alteration.
    • Categories:
    • Low-grade: slight alterations
    • High-grade: intense alterations.

Metamorphic Paths

  • Prograde vs. Retrograde Metamorphism:
    • Prograde: Involves changes due to increasing pressure and temperature as rocks are buried.
    • Retrograde: Occurs as deep-seated rocks return to the surface, needing hydrothermal fluids to sustain alteration.

Geothermal Gradient

  • Geothermal Gradient:
    • Varies in geological settings, influencing the characteristics of heat flow, stress, and aqueous interactions.

Types of Metamorphism

  • Types and Settings:
    • Thermal: Heating by a plutonic intrusion.
    • Burial: Deep burial pressures in basins.
    • Dynamic: Shearing effects in fault zones.
    • Regional: Pressure and temperature alterations due to orogenesis.
    • Hydrothermal: Hot water alteration processes.
    • Subduction: High-pressure, low-temperature alterations.
    • Shock: Extremely high pressure from impact events.