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.