Metamorphic Rock Flashcards
metamorphic change (3)
Neo-crystallization
Foliation
Re-crystallization
Plastic deformation
mineral grains soften and deform
increased temperature and pressure
dont break, behave plastically
Agents of metamorphism (4)
1) Heat (T)
2) Pressure (P)
3) Compression and shear
4) Hot water
Sources of heat (3)
1) Geothermal gradient
2) Magmatic intrusions
3) Tectonic compression
Where does metamorphism occur?
2-12 Kbars = 6.5-40km
Kinds of differential stress (2)
Normal stress
Shear stress
Normal stress
operates perpendicular to surface
Tension-compression
Shear stress
moves one part of the material sideways
causes material to be smeared out
Shape changes due to differential stress (2)
Equant: equal in all dimensions
Inequant: not equal in all dimensions
Do metamorphic rocks prefer equant of inequant dimensions
Inequant dimensions
How does preferred mineral orientation develop?
- Shear rotation and flattening
- Shear flattens grains in a manner similar to compression
- Shear rotates grains into alignment
Hydrothermal alteration is called _______
Metasomatism
Major divisions of Metamorphic rocks (2)
1) foliated
2) non-foliated
Foliated metamorphic rocks
- through going planar fabric
- crystallized with differential stress
- planty minerals
Non-foliated metamorphic rocks
- no planar fabric
- crystalized without differential stress
- equant minerals only
Foliation
- Parallel planar surfaces
- Streaked or striped appearance
- Foliated rocks often break along foliation planes
Slate
Fine grained
low grade metamorphic shale
Schist
Fine to course rock with larger Mica’s
medium to high grade metamorphism
shistosity (type of foliation)
Large non-mica minerals
porphyroblasts
Gneiss
distinct banded foliation
light bands: Feldspar/quartz
dark bands: biotite/amphibole
compositional banding development (3)
1) Original layering in the protolith
2) Extensive high T shearing
3) Metamorphic differentiation
Migmatite
Partially melted gneiss
feature of igneous and metamorphic rock
Hornfels
alteration by heating (contact metamorphism)
baked contact by igneous rock
Amphibolite
protolith of gabbro or basalt
High is Mg and Fe
foliation under differential stress
Quartzite
pure quartz in composition
formed by alteration in quartz sandstone
conchoidal fractures
Marble
crystalline calcite or dolomite
limestone or dolostone protolith
Broad compositional classes (4)
1) Pelitic
2) Mafic
3) Calcareous
4) Quartzo
Pelitic class
derived from shale protolith
Al rich so form Micas
Mafic
basalt or gabbro protolith
Calcareous
carbonate protolith
Quartzo-feldspathic
granite protolith
Metamorphic grade is a measure of ______
Intensity
Low metamorphic grade
High metamorphic grade
Low T and P
High T and/or P
Prograde metamoprphism
Metamorphism with increasing T and P
burial process
Retrograde metamorphism
Metamorphism with decreasing T and P
Requires addition of H2O
Rocks move towards surface
Metamorphic Facies
Grew in association with other rock in the same P and T
Derived from the same:
1) protolith
2) P and T conditions
Types of Metamorphic environments (7)
1) Thermal
2) Burial
3) Dynamic
4) Regional
5) Hydrothermal
6) Subduction
7) Shock
Thermal metamorphism
- Thermal baked contact
- Grades of alteration form aureoles around the pluton
- contact between magma and other rock
Burial metamorphism
As sediments are buried in sedimentary basin: T increases due to geothermal gradient; P increase due to overburden
~8-15Km deep
Dynamic metamorphism
breakage of rock due to shear forces at fault zone
Shallow (<15km): Fault Breccia
Deep (<15km): Fault Mylonite
Regional metamorphism
collisions form huge “mobile belts”
Compression adds to mountain ranges
creates foliated rocks
Hydrothermal metamorphism
hot water deformation
mid-ocean ridge magma
Subduction metamorphism
bluesheist facies
trenches and accretionary prisms
High P - Low T
- Glaucophale: Blue amphibole
Shock Metamorphism
asteroid
compressional shockwave
heat vapourizes large amount of ground
- Coesite, schisovite
Exhumation
how rocks return to the surface
1) uplift
2) extensional collapse
3) erosional unrooting
Where are most metamorphic rocks found?
continental interiors
