Regional Metamorphism

Regional Metamorphism

1. ·

   • Associated with destructive plate margins during subductionand continental collision
   • Occurs on a regional scale and involves thousands of cubic kilometres of rock

2. Regional Metamorphism –Temperature/Pressure Range

3. Regional Metamorphic Rocks show Foliation

   • Regional metamorphic rocks show foliation, a banding/layering/alignment of crystal long axes as they crystallised under directed (tectonic) stress
     • • Arrows indicate directed stress
       • Pink and blue mica crystals aligned parallel to each other produces the foliation

4. Regional Metamorphism of Argillaceous Rocks

   • • Argillaceous rocks are rich in clay minerals and include siltstone, mudstone, clay and shale
     • Different metamorphic rocks are formed from argillaceous rocks under increasing temperature and pressure conditions

5. Low Grade Regional Metamorphism -Slate

   • • Chlorite and biotitemica are platy minerals have their long axes aligned and at right angles to the principal stress direction to form slatycleavage
   • • Forms 5-15 km depth at high pressure, low temperature (<300°c) conditions
     • Clay minerals recrystallizeinto chlorite and biotitemica and are coarser grained than the original clay minerals

6. ·

   • Slaty cleavage direction
   • Principal stress directions
   • Slate is coarser grained and crystalline than the shale from which it has been derived but the crystals are too small to be seen with the naked eye
   • Platy crystals of chlorite and biotitehave their long axes aligned parallel and at right angles to the principal stress direction

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