Metamorphism 9-11 Flashcards
Metamorphism
Recrystallisation without melting under heat and/or pressure
Thermal or Contact metamorphism
- High-temperature low pressure
- Rocks near an igneous intrusion will be heated and recrystallized; metamorphic ‘aureole’
Metastability
Rocks brought out of their regions of stability where they are formed take a long time to break down as will not have the activation energy to recrystallize
Regional metamorphism
Metamorphism formed due to conditions in a region under the earth
Burial at depths
Low-temperature high-pressure metamorphism
Formed in subduction when the rock does not have time to heat up but is exposed to great pressures
Paired metamorphic belts
Igneous intrusions above subduction zones and belts formed by high pressure subduction often come together
Facies
Formed under identical conditions with different minerals
Idioblastic (or Idiomorphic)
Crystals completely bounded by their own crystal faces; the metamorphic equivalent of euhedral.
Metapsammitic
Describes rocks formed by the metamorphism of sandstones
Dynamic metamorphism
Dominant processes involve deformation and recrystallisation rather than temperature change
Migmatitic
Partly metamorphic and partly igneous in appearance, commonly formed by partial melting in metamorphism
Metasomatic
Describes a metamorphic process involving mass transfer and change of bulk chemical composition
Protolith
Original rock changed by metamorphism
Lithostatic load
Pressure on rocks caused by the weight of rocks above it
3 agents of metamorphism
Temperature
Pressure
Fluids
Cataclasitic
Rock ground and crushed during faulting
Prograde vs Retrograde
Prograde metamorphism is when minerals adapt to more extreme conditions
Retrograde metamorphism is when minerals adapt to less extreme conditions
Isochemical metamorphism
New minerals are formed from others without large-scale migration of chemicals
Recrystalisation of strained minerals
Same mineral recrystallises in a different way to allow it to unstrain e.g. quartz
Formation of new textures / Foliation
Minerals are realligned in response to deformarion and form in a pattern
Low grade Regional Metamorphism
Rock types: Slate
Temperatures: 200-350
Pressures: 2-3 kbar (6-10km)
Index minerals: Muscovite, Chlorite (250-325), Biotite (325-375)
Medium grade Regional Metamorphism
Rock types: Schist
Temperatures: 350-550
Pressures: 3-4 kbar (10-15km)
Index minerals: Garnet (425-550), Staurolite (525-600)
High grade Regional Metamorphism
Rock types: Gneiss
Temperatures: >550
Pressures: >4kbar (>15km)
Index minerals: Sillimanite (>550), Kyanite (>4kbar)
Pellitic Protolith
Protolith fine grained sedimentary rock e.g. mudstone
Garnet colour for composition in metamorphism
Red - Almandine IRON
Dark - Pyrope MAGNESIUM
Green - Grossular CALCIUM-ALUMINUM
Change in texture through regional metamorphic grade
Slatey: strong cleavage and foliation
Schistosity: prominant foliations and banding with minerals visible to the naked eye
Gneiss: diffuse bands with segraegated different compositions and larger coarse minerals
Index minerals
Cholrite-Biotite-Garnet-Staurolite-Sillimanite/Kyanite
Staurolite only forms if there is iron!
Aluminum Silicates triple point
514 C, 4kbars
Low temperature low pressure facies
Zeolite
High temperature low pressure facies (e.g. Contact metamorphism)
Hornfels
Low temperature High pressure facies (e.g. subduction)
Blueschist
Very High temperature (some pressure) facies
Granulite
Very high pressure (some heat) facies
Eclogite
2 middle facies in order of increasing temperature
Greenschist
Amphibolite
Quartzite
Metamorphosed sandstone
Marble
Metamorphosed limestone
Metamorphic aureole factors:
Size of intrusion - will take longer to cool
Magma temperature - hotter magmas (more mafic usually) will affect the area more
Depth of intrusion - shallow intrusions will cause more change as the rocks around will be cool
Metamorphic zones in conact metamorphism of pelites
1) Inner Hornfels Zone (C) a granoblastic texture completly recrystalised with interlocking quartz, corderite and biotite surrounding the intrusion (A) and the chilled margin (B).
2) Intermediate Porphyroblastic zone (D) Large andalusite porphyroblasts, and cordierite in slate
3) Outer Spotted slate zone (E) Small spots of metamorphic minerals form. This is surrounted by unaffected mudstone (F)
