Metamorphic Processes

• Regional Metamorphism

  • Result of high pressure and high temperatures under Earth’s surface
  • Consequence of the Geothermal Gradient and the weight of the overlying rock hence grade increases with depth
  • Common under mountainous areas
• Regional metamorphic rocks show foliation - an alignment of crystals along axes as they crystallised under directed tectonic stresses
• • Low-grade regional metamorphism: SLATE
  • Forms 5-15km depth, high pressure, low temperature conditions
  • Clay minerals recrystallise into chlorite and biotite mica and are coarser grained than original parent rock
  • Flat and platy minerals align themselves at right angles to the principal stress direction to form slaty cleavage. May have relict sedimentary structures
• • Medium-grade regional metamorphism: SCHIST
  • Higher temperatures 300-500 degrees at depths of 15-25km
  • Higher temperature results in larger crystals such as garnet and quartz
  • Garnet crystals occur as porphyroblasts up to 5mm in diameter which distort the foliation
  • Coarse foliation known as schistose texture
• • High-grade regional metamorphism: GNEISS
  • Higher temperatures 500-700 degress and depths of 25-40km
  • Higher temperature results in larger crystals over 2mm and growth of new minerals
  • Minerals segregated into layers to produce a coarse foliation known as gneissose banding. Composition similar to granite
• Contact Metamorphism

  • Result of high temperatures close to Earth’s surface as a consequence of heat from an igneous intrusion
  • Depends on several factors such as size of the igneous body
  • Small intrusions will have baked margins, larger intrusions will contain more obvious changes that increase in grade
  • Zone of metamorphism - metamorphic aureole
• • Low grade contact metamorphism: SPOTTED ROCK
  • Temperature 200-300 degrees
  • New minerals occur as oval spots 2-5mm in diameter
  • Spots have overgrown and included grains of original rock
  • May have relict structures such as laminations and fossils
• • Medium grade contact metamorphic rock: ANDALUSITE SLATE
  • Temperature 300-500 degrees
  • Chiastolite needles develop (porphyroblasts) up to 2cm long
  • Needles show random orientation as they have crystallised in absence of directed pressure
• • High grade contact metamorphic rock: HORNFELS
  • Temperature 500-600 degrees
  • Hornfelsic texture - granoblastic
  • Completely recrystallised
  • No evidence of relict structures
• Dynamic Metamorphism

  • Pressure is the dominant factor causing solid state recrystallisation - consequence of pressure from a FAULT
  • Result of mainly mechanical deformation associated with fault movement
• • Mylonite
  • Foliated, very fine grained
  • Variable mineralogy
• • Fault breccia
  • Consists of angular fragments of rock in a matrix of powdered rock
  • Crystallisation of calcite or quartz can act like a cement