Metamorphism Flashcards
(24 cards)
Outline
- It is a solid-state process leading to changes in mineralogy and/or texture (for example, grain size or preferred orientation of minerals)
- Metamorphism takes place when a rock is subjected to a new physical or chemical environment in which its existing mineral assemblage is no longer the most stable. Can be a sedimentary, igneous or pre existing metamorphic rock
- Normally the bulk composition of the rock is largely unchanged except for the loss, or sometimes gain of volatile constituents, such as water or carbon dioxide
Why do rocks recrystallise?
• To achieve lowest ‘potential energy’
• Gibbs free energy
• Rock moves from unstable to metastable state then an activation energy moves it to a stable phase
o Density (molar volume) is important. So graphite recrystallises as diamond at high pressure (P) because this results in a volume reduction
Controls of metamorphism:
• Chemical composition of original rock
o Depends on intensity of metamorphism what the original rock will become o Shale → Slate → Schist → Gneiss o Rhyolite → Slate or Schist → Gneiss o Granite → Schist → Gneiss o Basalt → Amphibolite or Schist o Limestone → Marble o Sandstone → Quartzite
Controls of metamorphism:
• Heat (temperature)
o Usually the most important factor
o Temp is related to depth
o Drives reactions that consume unstable minerals
o Produces new minerals that are stable under the new conditions
o Promotes recrystallization → increased grain size
- Larger surface/volume ratio of a mineral → lower stability
- Increasing temperature eventually overcomes kinetic barriers to recrystallization, and fine aggregates coalesce to larger grains.
Controls of metamorphism:
• Pressure
o Lithostatic pressure is uniform stress (hydrostatic)
o Deviatoric pressure = unequal stress in different directions
o Stress is an applied force acting on a rock (over a particular cross-sectional area)
o Strain is the response of the rock to an applied stress (= deformation)
o Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblages
Controls of metamorphism:
• Time
o Mineral growth often occurs in response to burial, slow changes in temperature and pressure
o Kinetics of new mineral growth also may be very slow.
o New minerals may grow just mm’s over a timescale of a million years
Controls of metamorphism:
• Fluids
o Evidence for the existence of metamorphic fluids:
o Fluid inclusions
o Fluids are required for hydrous or carbonate phases
o Reactions involving volatiles (H2O, CO2) occur at temperatures and pressures that are different to anhydrous systems
Contact metamorphism?
• Proximity to igneous intrusions
o Heat next to cold rocks
• Result of thermal (and sometimes fluid) effects of hot magma intruding cooler shallow rocks
• - Occurs over a wide range of pressures, including very low, commonly mid- to upper-crust (0 to 10 km depth)
• Contact aureole
o Aureole – a zone around an intrusion made of igneous rocks
o Heats country rock
o The size and shape of an aureole is controlled by:
o The nature of the pluton
o The nature of the country rocks
Regional metamorphism?
• Occurs over a large area
• Burial – 15-30 degree increase per km
o Burial metamorphism caused by increased temperature and pressure due to burial in deep sedimentary basins
o Does not involve deformation
Orogenic metamorpism?
increasing pressure and temperature due to tectonic deformation
o Caused by increased temperature and pressure due to crustal thickening during continental collision
o Metamorphism often associated with deformation
o Crustal thickening at convergent plate boundaries
o Foliated rocks are a characteristic product
o Most orogenic belts have several episodes of deformation and metamorphism, creating a more complex polymetamorphic pattern
Sea floor metamorphism?
- Wide range of temperatures at relatively low pressure
- Metamorphic rocks exhibit considerable alteration, notably loss of Ca, Mg and Si and gain of Na, K and base metals
- These changes can be accounted for by exchange between hot basalt and seawater
Schistosity?
- A preferred orientation of mineral grains or grain aggregates produced by metamorphic processes
- Aligned minerals are coarse grained enough to see with the unaided eye
- The orientation is generally planar
Genesis?
- A metamorphic rock displaying gneissose structure
- Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers
- Gneisses may also be lineated, but must also show segregations of felsic-mineral-rich and dark-mineral-rich concentrations
Non-foliated rocks?
- Granofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation)
- Hornfels: is a type of granofels that is typically very fine-grained and compact, and occurs in contact aureoles. Hornfelses are tough, and tend to splinter when broken.
Porphyroblastic
o means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast
Metamorphism of pelatic sediments?
- Mudstones and shales: very fine grained clastic sediments derived from continental crust
- The mineralogy of pelitic sediments is dominated by fine Al-K-rich phyllosilicates, such as clays, white micas and chlorite, all of which may comprise more than 50% of the original sediment
- Other common constituents include quartz (10-30%)
- High proportion of micas leads to common development of foliated rocks, such as slates, phyllites, and mica schists
Barriovian Zone mineral assemblages:
- Chlorite zone. slates or phyllites, and typically contain chlorite, muscovite, quartz and albite
- Biotite zone. phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite
- Garnet zone. Schists with almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase
- Staurolite zone. Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist
- Kyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite
- Sillimanite zone. Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite.
Metamorphic Facies:
- More broad and harder to identify
- Barrovian zones more sensitive to P-T changes
Greenschist (green)
• Metamorphism of mafic rocks is first evident in the greenschist facies, which correlates with the chlorite and biotite zones of associated pelitic rocks
• Typical minerals include chlorite, albite, actinolite, epidote, quartz
Amphibolite (blue)
• Greenschist to amphibolite facies transition involves two major mineralogical changes, formation of:
• Ca-rich plagioclase
• Al-rich Amphibole
Granulite Facies
- Mafic rocks generally melt at somewhat higher temperatures than pelites
- If water is removed by the earlier dehydration then the remaining mafic rock may become depleted in water
- Hornblende breaks down and orthopyroxene + clinopyroxene appear
Hyrdration; lose water; higher grades lose amphibole
Metamorphic facies?
High P series – blueschist -eclogites.
Medium P/T – greenschist, amphibolite, granulite
Low P/high T – contact hornfels
- The mafic rocks develop conspicuous and definitive mineral assemblages under high P/T conditions.
- High P/T gradients characterize subduction zones.
- Mafic blueschists are easily recognizable by their color, and are useful indicators of ancient subduction zones
- Eclogites have a high density: subducted basaltic oceanic crust becomes more dense than the surrounding mantle.
- The blueschist facies in mafic rocks is characterized by the presence of a sodic blue amphibole stable only at high pressures (glaucophane)
- Presence of glaucophane is diagnostic of high pressures.
Important stuff about polymorphs?
Al2SiO5 Polymorphs
• Kyanite: High pressure, regional
• Siliminaite: High temps, both metamophisms
• Andaluiste: Low pressures, 15 or above, commonly contact
Glaucophane = mafic igneous rock, high pressures: subduction zones
Changes to protoliths:
Sandstone: • Low-grade = Quartzie o Interlocking grains o Increase in grain size • High Grade = same
Limestone • Low-grade = Marble o Increase in grain size o Interlocking grains • High-grade = same
Shale, siltstone, greywacke • Slate: o Increase in grain size o Slaty cleavage (foliation) • High grade: o Increase in grain size o Schistosity/banding o New minerals (garnet)
Pre-kinematic?
Inherited from an earlier deformation
Compressed around the deformation (opposite)
Pressure shadow develops
Post-kinematic?
Identical
Syn-kinematic?
Rotation of porphyroblasts
Deformation did not outlast porphyroblast growth
