topic 6 ( metamorphic zone )

• low Metamorphism

   

  Temperature - 150oC – 400oC

  Pressure - 0 – 2 kbar

  Depth- 0 – 6 km
• Medium Metamorphism

   

  Temperature - 400oC – 600oC

  Pressure -2 – 6 kbar

  Depth - 2 – 6 km
• High Metamorphism

   

  Temperature - > 600oC

  Pressure - > 6 kbar

  Depth - > 20 km
• Isograds

  Lines on geologic maps marking the first appearance of a particular index mineral
• Metamorphic / Barrovian Zones

  Region bounded by two isograd lines, recording important Temperature changes during metamorphism
• Chlorite Zone 

  • Region bounded by the Chlorite and Biotite Isograds.
  • Records metamorphic grades sufficient for the first appearance of chlorite mineral group
  • Chlorite, sericite, quartz, muscovite, albite, pyrophyllite
  • Chlorite-bearing slate, chlorite-sericite phyllite, chlorite-sericite schist
• Biotite Zone

  • Region bounded by Biotite and Almandine Isograds
  • Biotite, sericite, quartz, albite, chlorite, muscovite; forms from chemical reaction involving chlorite, muscovite, quartz, magnetite, rutile
  • Sericite-biotite phyllite, Biotite schist
• Almandine Zone 

  • Bounded by Almandine and Staurolite isograd
  • Garnet schist, Garnet-mica schist
  • Garnet (commonly as porphyroblast), biotite, muscovite, magnetite, quartz, albite, oligoclase; from chemical reaction of chlorite and magnetite
• Staurolite Zone

  • Bounded by Staurolite and Kyanite Isograds
  • Staurolite-mica schist, Staurolite-garnet-mica schist
  • Staurolite, quartz, almandine, biotite, muscovite, potassium feldspar (from breakdown of muscovite); from chemical reaction of almandine, chlorite, muscovite
  • Staurolite only exists in its zone
• Kyanite Zone 

  • Bounded by Kyanite and Sillimanite Zones
  • Kyanite schist, Kyanite-mica schist
  • Kyanite, biotite, muscovite, almandine, cordierite, quartz
• Sillimanite Zone 

  • Marks the highest temperature zone 
  • Sillimanite schist, sillimanite gneiss, cordierite gneiss
  • Sillimanite, biotite, muscovite, cordierite, quartz, oligoclase, orthoclase
• Problem with Metamorphic Zones:

  • Only recognized if protoliths are Pelitic composition
  • Less useful for non-pelitic rocks, rocks in subduction zones, or in contact with magmatic intrusions; different mineral assemblages are formed
  • Eskola (1915, 1920, 1939) developed a more reliable and comprehensive approach called Metamorphic Facies