EAE 08 Metamorphic Rocks (12) Flashcards
What is the impact of bulk composition?
- This is a major control on which minerals can grow in response to metamorphism
- In order to make a specific metamorphic mineral, you need the right ingredients…
- Continental crust = very heterogeneous = wide range of compositionsk
EAE 10aa
At Granulite Facies P-T, which minerals are found in metamorphosed limestone?
The limestone bulk composition grows
- Calcite
- Garnet
- Diopside
- Wollastonite
EAE 10ab
At Granulite Facies P-T, which minerals are found in metamorphosed mudstone?
At high T mudstones will grow:
- Garnet
- Sillimanite
- Biotite
- Plagioclase
- Quartz
- Orthopyroxene
- Cordierite
- Spinel
- Sapphirine
- Osumilite
EAE 10ac
What are 5 different ways of causing metamorphism on Earth?
- Regional metamorphism
- Contact metamorphism
- Cataclastic metamorphism = at fault zones
- Hydrothermal metamorphism (metasomatism) = hot fluids flow through and chemically change rocks = Mid-Ocean Ridges
- Impact metamorphism = when meteorites hit
EAE 10ad
What is Regional Metamorphism?
Occurs over large areas of the crust
Associated with orogenic (mountain) belts
Accompanied by deformation (often compressional)
Increase in Temperature and Pressure
Dominant style of metamorphism on Earth
EAE 10ae
What is Contact Metamorphism?
- Associated with igneous intrusions
- High heat flow from the intrusion into surrounding cooler country rocks = Increased T, unchanged P
- Often no deformation
- Typically in narrow (metres to kilometres) zones around intrusive rocks
- Forms hornfels - has “sugary” textures…
EAE 10af
Compressional Tectonic Settings
Describe Subduction zones.
Blueschists to Eclogites
- Thermally, subduction zones are simple: dense cold material is thrust under more buoyant, cool material very rapidly (geologically speaking!!!)
- The down-going slab contains basalts and some sediments
- Dense cold material is thrust under warmer material faster than it can heat up
EAE 10ag
Compressional Tectonic Settings
Describe Subduction Impacts.
Releases water → hydration melting of overlying mantle
Eclogite is denser than mantle → “slab pull” force of plate tectonics
EAE 10ah
Compressional Tectonic Settings
Describe Continental Collision
Eg, the collision of India into Asia
- Collision zone results in thickening of the crust … Cold crust is pushed to depths of up to 80km
- Huge range of protoliths: sediments, volcanics from previously existing arcs, fragments of subducting slabs, etc
- Scale is large - hundreds to thousands of km
- Time plays a central role:
- Initially have high P low T metamorphism due to crustal thickening
- Over time, the geotherm will attempt to return to equilibrium - id new rocks will heat up
EAE 10ai
Compressional Tectonic Settings
What are the Impacts of Continental Collision?
- Rocks closest to the centre of the collision get buried the deepest
- Those at the edges get buried only a small amount
- Greater systematic variation in metamorphic T and P over a large area…
- Highest T and P is in the centre of the collision zone, lowest at the margins
EAE 10aj
What is the impact on metamorphism of extension?
- Metamorphism associated with extension is characterised by increases in T and decreases in P due to bringing hot material closer to the surface of the Earth
- Extension carried to extremes results in new ocean basins
EAE 10ak
Extension Tectonic Settings
Describe Rifting
- Rifting can be caused by thermal plumes within the mantle rising up underneath a continent
- Rifting causes the crust to thin, making deeper crustal levels rise slightly
- When the mantle rises it melts and the melt then migrates into the lower crust where it causes heating and metamorphism
EAE 10al
Extension Tectonic Settings
What is the impact of Rifting?
- High Temperature-Low Pressure metamorphism, typified by amphibolites and granulites
- Associated with basin formation, or with mountain range collapse typical of the final phase of regional metamorphism
- Get similar effect from increasing the heat flow through the crust, or increasing the heat production of the crust
EAE 10am
What is a Metamorphic Rock?
Greek: Metamorphoun - to transform, change shape
- When rocks are subjected to changes in Pressure and Temperature, the minerals within change_
- This makes a completely new rock?
EAE 10an
What is metamorphism?
When anything is subjected to changes in Pressure and Temperature, the chemical structure changes
Organic matter - carbon compounds, water
heat ⇩ pressure
Oil - new carbon compounds
heat ⇩ pressure
Graphite - crystalline carbon
heat ⇩ pressure
Diamond - carbon polymorph
EAE 10ao
Definition
Metamorphism
The process by which the mineralogy and texture of a rock is modified
By changes in temperature, pressure and fluid chemistry within the Earth
- New minerals grow in response to changes in Pressure, Temperature (P, T) and fluid chemistry
- Some minerals will be recrystallised and their composition can change
EAE 10ap
What is the link between pressure and temperature?
The natural increase in temperature with depth is the geothermal gradient, averages ~25°/km
However from 60°/km - 10°/km depending on tectonic setting
EAE 10aq
What are the primary sources of heat in the earth?
- Mainly: radioactive decay of U, K and Th
- A bit of: crystallisation of the Earth’s Inner Core
EAE 10ar
What is tectonic setting?
- Different rocks form at different P and T conditions in the crust…
- Can identify them using minerals
- With some Metamorphic Rocks, you can work out the T and P conditions it formed under
→ tectonic setting
EAE 10as
How do Changes in Fluid Chemistry impact rocks?
- Hydrothermal fluids are found throughout the Earth’s crust
- Common chemicals: H₂O, CO₂ (CH₄, H₂S)
- The presence or absence of these fluids strongly affects the chemical stability of many minerals
- Example:
Many minerals contain H₂O
If there is no H₂O present, “hydrous minerals” are not able to form
K-Feldspar + H₂O → Muscovite
EAE 10at
What causes changes in texture?
- Minerals are affected by stress
- Long thin minerals or flat minerals align along axis of least stress
i.e, at right angles to direction of squashing
Analogy: deck of cards slide to the side - Round minerals can be elongated (squashed) by stress
Analogy: ball of play-doh squished flat
EAE 10au
What do we call this alignment of minerals?
Any alignment of minerals is called a “Foliation”
- At low T, foliation causes rocks to break along flat planes = slaty cleavage
- At higher T, foliation makes a visible alignment of minerals = e.g, gneissosity, schistosity, depending on rock type
EAE 10av
Names for mineral shapes . . . .
Roughly round grains
Equant
This applies to ALL rock types
EAE 10aw
Names for mineral shapes . . . .
Long thin grains
Elongate
This applies to ALL rock types
EAE 10ax
Names for mineral shapes . . . .
Flat grains
Platy
This applies to ALL rock types
EAE 10ay
What sort of rock is Shale?
Shale is a sedimentary rock, NOT a metamorphic rock
Shale is made of clay - the grains are too small to see = ancient lake or ocean
A rock before it is metamophosed is called a Protolith
EAE 10az
What happens to shale with increased Temperature and Pressure?
It is now Slate
Increased T and P changes the look of the rock
A fine-grained cleaved metamorphic rock
The cleavage is the result of the alignment of fine-grained platy minerals, such as muscovite or chlorite micas
EAE 10ba
What happens to Shale/Slate with more Temperature & Pressure?
It is now Schist
A strongly foliated crystalline metamorphic rock
The foliation is the result of the alignment of platy and elongate minerals, such as muscovite, chlorite or biotite mica, and in some rocks, the amphibole mineral hornblende
EAE 10bb
What happens to Shale/Slate/Schist with more Temperature & Pressure?
It is now Gneiss
A coarse grained metamorphic rock consisting of alternating bands of light and dark minerals
Note:
As temperature increases the size of individual mineral grains increases. Higher T makes rocks that are more coarsely crystalline
EAE 10bc
How do changes in minerals progress?
- At increasing temperatures and pressures minerals re-organise their lattices, or chemically react with one another, to form an energetically more stable configuration
- This tends to be simpler crystal structures, and denser new minerals, or can think of as minerals with fewer bonds (lower polymerisation)
EAE 10bd
What happens to mudstone (pelite) as its travels along the Geotherm?
Stage 1
- Clay, quartz & detritus falls onto the seafloor with water in between the grains - clay has LOTS of water
- More sediments settle - water is squeezed out - calcite cements grains together = mudstone = sedimentary rock…
EAE 10be
What happens to mudstone (pelite) as its travels along the Geotherm?
Stage 2 (after mudstone)
Micas
Further increases in P and T (e.g., sedimentary burial, or continental collision) result in the formation of micas - muscovite, biotite or chlorite (less water, still quite a bit)
EAE 10bf
What happens to mudstone (pelite) as its travels along the Geotherm?
Stage 2 (after micas)
Micas to amphiboles and similar
Higher T and P result in reactions breaking down the micas, releasing some H₂O, to produce structurally simpler silicates, e.g., garnets, cordierite, staurolite & amphiboles (still a little bit of water)
Note:
“Amphibole” refers to a whole family of different minerals with similar crystal structure (double chain silicates) - e.g., Hornblende (greeny-black) & Glaucophane (blue)
EAE 10bg
What happens to mudstone (pelite) as its travels along the Geotherm?
Stage 3 (after micas)
- At high T and P, the last hydrous minerals break down to pyroxenes and other anhydrous minerals, releasing the last of the H₂O
- Pyroxenes are even simpler and denser mineral structures again: they contain no water bound in the mineral structure
E.g. augite, orthopyroxene
EAE 10bh
What is ‘bulk composition’?
The overall chemical composition of the rock
EAE 10bi
