Metamorphic Rocks

• Metamorphism


  Set of processes by which pre-existing rocks undergo solid-state change in response to changes in physical and/or chemical environment (i.e. changes in pressure and temperature and/or reaction with hydrothermal fluids)
• Metamorphic rock


  A product of metamorphism (solid-state change); crystalline at high grade; characterised by minerals not present in the original rock (protolith) i.e. by minerals that form (grow) during metamorphism; often characterised by a texture defined by the preferred orientation of the new (metamorphic) minerals (foliation) and/or changes in the size of mineral grains

  
• Causes of metamorphism


  • Temperature (heating of the protolith)
  • Lithostatic Pressure (due to burial)
  • Differential Stress (due to applied force)
  • Hydrothermal (chemically active) fluids passing through the rock

  
• Agents of metamorphism


  • Heat - the energy that "powers" metamorphic reactions
  • Lithostatic Pressure - due to the weight and density of overlying rocks; homogeneous
  • Differential stress - inhomogeneous and in compression is responsible for the preferred orientation of minerals to produce foliation
  • Chemically active fluids - accelerate chemical reactions

  
• Metamorphic processes


  1. Recrystallization: changes the shape and size of grains but mineral remains the same

  2. Phase Change: response to changes in P and T, minerals have same composition but different crystal structure

  3. Neocrystallization: growth of new minerals that differ in composition from those of the protolith

  
• Types of metamorphism


  • Regional metamorphism - characterises large areas and forms in response to combinations of increased temperature, pressure and differential stress
  • Contact or thermal metamorphism - due to heating without significant change in pressure and without differential stress
  • Dynamic metamorphism (shear metamorphism) - a response to blocks of rock moving past each other along shear surfaces/zones
  • Burial metamorphism - occurs at depths of >8km in sedimentary basins related to lithostatic pressure and the geothermal gradient
  • Shock metamorphism - phase changes in quartz due to energy released by meteor impacts

  
• Foliated metamorphic rocks


  • Slate
  • Phyllite
  • Schist
  • Gneiss

  
• Non-foliated metamorphic rocks


  • Hornfels
  • Quartzite
  • Marble
  • Amphibolite

  
• Metamorphism takes place between ~250 and 850C though locally up to 1200C
• Metamorphic mineral assemblages reflect P-T conditions
• Presence of hydrothermal fluids can accelerate metamorphic reactions
• Metamorphic grade


  An informal reference to describe the degree or intensity of metamorphism: low-grade (250-400C), medium grade (400-600C), high-grade (>600C)
• Prograde metamorphism


  Metamorphism that occurs as temperature and pressure steadily increase

  
• Retrograde metamorphism


  Metamorphism that takes place during steady decrease in temperature and pressure

  
• Metamorphic facies


  A set of metamorphic mineral assemblages indicative of a range of temperature and pressure

  
• Main metamorphic facies


  • Zeolite
  • Hornfels
  • Greenschist
  • Amphibolite
  • Granulite
  • Blueschist
  • Eclogite
• Contact metamorphism occurs wherever large volumes of magma intrudes cold rocks

  
• Regional metamorphism is a response to a set of dynamic changes in the physical environment of rocks; occurs where processes responsible for these changes are most active - along convergent plate boundaries
• At mid-ocean ridges cold seawater flows into the fractures oceanic crust and is heated up by magma below the ridge, causing hydrothermal metamorphism of the oceanic crust (basalt)