Lecture Nineteen - Metamorphic processes and occurrences III Flashcards
Why and how are different metamorphic rocks created?
The range of conditions where metamorphism occurs is different for different geological settings.
Different geological settings produce varying:
1) Geothermal gradiente - rate of change of temperature with depth.
2) Amounts of differential stress.
3) Amounts of interaction with hydrothermal fluids.
Where does metamorphism occur?
1) Contact (thermal) metamorphism.
2) Burial metamorphism.
3) Metamorphism along faults (cataclastic).
4) Regional metamorphism (mountain building).
5) Hydrothermal metamorphism.
6) Impact (shock) metamorphism.
7) Subduction zones metamorphism.
8) Metamorphism in the mantle.
Explain contact (thermal) metamorphism.
When hot magmas rise up into the shallow crust, the surrounding country rock is cooler.
1) The magma cools.
2) The country rock heats up.
3) Hydrothermal fluids circulate through the intrusion and coutry rock (can be an important source of economic deposits).
The country rock can undergo contact (thermal) metamorphism.
The highest grade rocks are nearer the intrusion, progressively lower grade the further away from the intrusion.
The metamorphic rocks that form around an igneous intrusion is called a metamorphic aureole or contact aureole.
There is no differential stress so non-foliated metamorphic rocks are produced - usually hornfels, sometimes quartzite and/or marble.
Generally etamorphism is localised (small scale) unless there is a very large intrusion.
Contact metamorphism follows line one in the rock facies diagram.
Explain burial metamorphism.
When sediments accumulate and build up in a sedimentary basin, the pressure increases from the weight of the overlying layers.
The temperature also increases due to the geothermal gradient.
Upper few km -> diagenesis (low T and P).
Depths greater than ~8km -> T high enouhg for low grade burial metamorphism to occur.
Low grade, non-foliated metamorphic rocks can form.
Common for organic molecules of oil to break up at this depth -> when oil driller hit this depth (burial metamorphism) they stop drilling.
Burial metamorphism follows lines 3 and 5 in the metamorphic facies diagram.
Explain metamorphism along faults.
At shallow depths, when pieces of rock slide or shear past others (in faults), they can fracture, breaking up into smaller pieces or powder.
At depth, it is warm and rocks can deform plastically when shear movement along the fault takes place.
During this process the minerals in the rocks recrystallise -> dynamic metamorphism (no change in T or P requires, but a change in T or P can occur).
The rock that is formed is called a mylonite with a foliation that is parallel to the fault.
This type of metamorphism occurs anywhere where faulting occurs -> in all tectonic settings.
Explain regional metamorphism.
Mountain builing tectonic settings:
1) Continental margins (subduction of oceanic lithosphere beneath continental lithosphere) = The Andes.
2) Continent-continent collision (two pieces of continental lithophere collide) = The Himalayas.
- > Rocks originally near the surface can end up at huge delths. Changes that occur to the protolith.
1) It heats up (due to the geothermal gradient).
2) Higher pressures are experienced.
3) Differential stresses are generated.
Foliated metamorphic rock (e.g. slate at shallow depths, schist/gneiss’ deeper).
This type of metamorphism affects a large area -> regional metamorphism.
Exhumation via erosion can eventually remove rock and expose long belts of metamorphic rocks formed in this way.
Regional metamorphism follows line 3 in the metamorphic facies diagram.
Explain hydrothermal metamorphism.
Occurs where cold water sinks through cracks at the Earth’s surface down to depths wher it heats up -> hydrothermal fluid.
These hot fluids then rise up and start to metamorphose rocks - water passes through cooler country rock which causes it to precipitate out minerals into the country rock.
Common in mid oceans ridges where hot magma heats up cold ocean water.
The hot fluids cause hydrothermal metamorphism of ocean floor basalt -> chlorite is produced giving a greenish colour.
The fluids then escape through vents into the sea = black smokers.
Explain impact (shock) metamorphism.
Meteorite impacts are one way we can create (local) high pressure and temperature over very short time scales.
Heat produced can melt or vapourise rock at impact and compression from shock wave causes ultra high pressure minerals can form as well as ‘shocking’ existing ones.
Meteorite impacts can metamorphose the rocks on which they lands -> shock metamorphism.
One way of identifying the occurences and location of meteorite impacts is to look for the metamorphism effects on the Earth rocks that are produced.
Explain subduction zone metamorphism.
Subduction zone subject rocks to high pressure, low temperature metamorphism.
This happens near the convergence of the two plates int he accretionary prism/wedge.
Low temperature -> cool ocenaic crust getting subducted.
Blueschist develop (blue colour from glaucophane) and at extreme depths/pressure, eclogite may form.
Subduction zone metamorphism follows line 5 in the metamorphism facies diagram.
Explain metamorphism in the mantle.
We don’t often see mantle rocks exposed at the Earth’s surface.
But mantle rocks are udner great P and high T -> multipe periods of metamorphism must have occured.
Peridotite from the upper mantle can cool and sink, until at ~440km depth, olivine crystals undergo a phase transition to new minerals that are stable at higher pressures.
Other phase transitions happen at ~500 km and 660km depth and others deeper still.
The reverse can happen is this peridotite gets heated up, becomes buoyancy and can rise back up through the mantle forming minerals stable at lower pressures.
Where are metamorphic rocks found at the surface?
Large areas of metamorphic rocks crop out in continental shields.
Shield = older portion of a continent where you find extensive precambrian rocks (>540 ma) exposed at the surface (overlying rock eroded away).
The oldest rocks on Earth occur at shields.
Can only see them when:
1) They were uplifted and the younger Palaeozoic, Mesozoic and Cenozoic rocks are eroded away.
OR.
2) Rivers cut down deeply to expose basement rocks - E.g. Grand Canyon.
What is exhumation?
Involves natural processes ‘digging up’ rocks out of the Earth after a period of burial/rocks not being accessable.
Different processes to procuce this:
1) Orogenesis = Mountain building from convergence of lithosphere can produce an upwards ‘squeezing’ of rocks.
2) During orogenesis, this lower portion of the thickened continental crust warms up, gets weaker and can start to collapse -> extensional collapes. This causes thinning of the crust and deeper parts end up closer to the surface.
3) Erosion of the surface through weathering, landslides river flow, glacial flow -> exposing deeper rocks.
4) As the weight of overlying rock is reduced, underlying rock rises isostatically (like the deck of a ship as its cargo is removed).
