Introduction to Metamorphic Rocks Flashcards
How do metamorphic rocks form
- minerals in the protolith are no longer in equilibrium with their environment so they remobilise and recrystallise
- this occurs in the solid state
- New minerals form, or same minerals in different direction (chemistry don’t change)
- the changes that occur are adjustments of the protolith to new environmental conditions
Three main agencies that facilitate change in protolith
- temperature
- pressure
- chemically active pore fluids
Three types of metamorphism
- Contact
- Dynamic (cataclastic)
- Regional
What is the metamorphic grade
- Refers to the relative intensity of metamorphism
- describes relative T & P and grade increases under high T & P
- grade indicated by mineral content i.e. low grade is characterised by hydrous minerals e.g. chlorite, medium grade is characterised by less hydrous e.g. mica minerals and high grade is non-hydrous e.g. garnet, pyroxene
Contact Metamorphism
- caused by localised heating and associated alteration of country rock by an igneous intrusion
- temperature most important agent; fluids also important
- relatively shallow depth
- produces granular rocks
- heating occurs around intrusion in metamorphic aureole
- resulting rock dependant on form of protolith
Example of contact metamorphic rock - hornfels
- Mudstone -> Hornfels
- dark, finely crystalline
- no foliation (granular) because no directed pressure
What happens in contact metamorphism of protoliths with mainly one mineral
- remobilisation of the mineral which recrystallises to fill previous void spaces
- any previous structure is destroyed (e.g. bedding)
- granular rocks form
- leads to tight interlocking coarse crystals, no pore spaces and increased density
Example of contact metamorphic rock - Marble
- limestone -> marble
- impurities cause colour
Example of contact metamorphic rock - Quartzite
- sandstone with high quartz content (SiO2) -> quartzite
- impurities cause colour
Dynamic Metamorphism
- occurs due to rupturing and movement of rocks along a fault
- rocks near fault are often intensely deformed and reconstituted
- localised metamorphism is caused by high pressures due to directed stress
- shear stress aligns minerals as they recrystallise
Dynamic metamorphism in shallow depths
Brittle rocks become mechanically pulverised; gouge, breccia, cataclasite
Dynamic metamorphism at greater depths
Ductile rocks become metamorphic forming: Mylonite; blueschist; pseudotachylite
Examples of dynamic metamorphic rocks - Mylonite
- foliated and very fine-grained with porphyroblasts
Regional Metamorphism
- occurs under high temperature and high pressure conditions
- deep burial and/or large internal forces
- huge volume affected over large areas
- considerable deformation, as well as recrystallisation
- directed pressure (differential stress) forms mostly foliated rocks
- can be associated with igneous rocks such as granites (remelting)
Pressure and foliation
- platy minerals tend to grow with sheet structures perpendicular to direction of maximum pressure
- Acicular (needle-like) minerals can also align
- shearing can also influence texture
- impart planar structure to rock
Low metamorphic grade leads to _______ for example _______
‘salty cleavage’
‘Slate’
Medium metamorphic grade leads to ______ for example ______
‘schistosity’
‘schist’
High metamorphic grade leads to ______ for example _______
‘gneissic layering’
‘gneiss’
Slate
- low metamorphic grade
- continuous foliation; very fine-grained
- protolith mudstone, basalt
- mainly chlorite, mica minerals; earthy lustre
- preferred orientation of sheet silicates cause the rock to easily break along the planes parallel to the sheet silicates - salty cleavage
Phyllite
- low metamorphic grade; higher than slate
- continuous foliation; fine grained; curvy foliation - crenulated
- protolith is mudstone, basalt
- mainly chlorite, mica minerals; silky lustre
- preferred orientation of sheet silicates cause the rock to easily break along the planes parallel to the sheet silicates
Schist
- medium metamorphic grade
- continuous foliation (visible); fine to med-grained
- protolith is mudstone, igneous rocks
- mica minerals, chlorite, quartz, plagioclase
- sheet silicates cause foliation; quartz and plagioclase have no preferred orientation
Gneiss
- high metamorphic grade
- discontinuous foliation, segregated mineral bands, med to coarse-grained
- protolith felsic igneous rock
- fledspar, quartz, pyroxene, amphibole
- At high T & P silicate structures breakdown, other minerals dominate and dark and light coloured minerals segregate forming “gneissic layering”
Migmatite (‘mixed rock’)
- very high metamorphic grade; extreme conditions cause some parts of the rock to melt (aided by water)
- foliation large scale; med to coarse-grained; wavy
- protolith felsic igneous rock (often granite)
- feldspar, garnet, amphibole etc.
- essentially a coarse gneiss with non-foliated igneous layers; transitional between metamorphic and igneous rock
