EAE 08 Metamorphic Rocks (12)

Question 1

What is the impact of bulk composition?

Answer 1

• 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

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Question 2

At Granulite Facies P-T, which minerals are found in metamorphosed limestone?

Answer 2

The limestone bulk composition grows

• Calcite
• Garnet
• Diopside
• Wollastonite

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Question 3

At Granulite Facies P-T, which minerals are found in metamorphosed mudstone?

Answer 3

At high T mudstones will grow:

• Garnet
• Sillimanite
• Biotite
• Plagioclase
• Quartz
• Orthopyroxene
• Cordierite
• Spinel
• Sapphirine
• Osumilite

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Question 4

What are 5 different ways of causing metamorphism on Earth?

Answer 4

1. Regional metamorphism
2. Contact metamorphism
3. Cataclastic metamorphism = at fault zones
4. Hydrothermal metamorphism (metasomatism) = hot fluids flow through and chemically change rocks = Mid-Ocean Ridges
5. Impact metamorphism = when meteorites hit

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Question 5

What is Regional Metamorphism?

Answer 5

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

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Question 6

What is Contact Metamorphism?

Answer 6

• 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…

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Question 7

Compressional Tectonic Settings

Describe Subduction zones.

Answer 7

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

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Question 8

Compressional Tectonic Settings

Describe Subduction Impacts.

Answer 8

Releases water → hydration melting of overlying mantle

Eclogite is denser than mantle → “slab pull” force of plate tectonics

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Question 9

Compressional Tectonic Settings

Describe Continental Collision

Eg, the collision of India into Asia

Answer 9

• 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

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Question 10

Compressional Tectonic Settings

What are the Impacts of Continental Collision?

Answer 10

• 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

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Question 11

What is the impact on metamorphism of extension?

Answer 11

• 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

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Question 12

Extension Tectonic Settings

Describe Rifting

Answer 12

• 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

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Question 13

Extension Tectonic Settings

What is the impact of Rifting?

Answer 13

• 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

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Question 14

What is a Metamorphic Rock?

Answer 14

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?

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Question 15

What is metamorphism?

Answer 15

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

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Question 16

Definition

Metamorphism

Answer 16

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

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Question 17

What is the link between pressure and temperature?

Answer 17

The natural increase in temperature with depth is the geothermal gradient, averages ~25°/km

However from 60°/km - 10°/km depending on tectonic setting

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Question 18

What are the primary sources of heat in the earth?

Answer 18

• Mainly: radioactive decay of U, K and Th
• A bit of: crystallisation of the Earth’s Inner Core

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Question 19

What is tectonic setting?

Answer 19

• 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

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Question 20

How do Changes in Fluid Chemistry impact rocks?

Answer 20

• 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

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Question 21

What causes changes in texture?

Answer 21

• 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

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Question 22

What do we call this alignment of minerals?

Answer 22

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

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Question 23

Names for mineral shapes . . . .

Roughly round grains

Answer 23

Equant

This applies to ALL rock types

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Question 24

Names for mineral shapes . . . .

Long thin grains

Answer 24

Elongate

This applies to ALL rock types

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Question 25

Names for mineral shapes . . . .

Flat grains

Answer 25

Platy

This applies to ALL rock types

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Question 26

What sort of rock is Shale?

Answer 26

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

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Question 27

What happens to shale with increased Temperature and Pressure?

Answer 27

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

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Question 28

What happens to Shale/Slate with more Temperature & Pressure?

Answer 28

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

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Question 29

What happens to Shale/Slate/Schist with more Temperature & Pressure?

Answer 29

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

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Question 30

How do changes in minerals progress?

Answer 30

• 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)

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Question 31

What happens to mudstone (pelite) as its travels along the Geotherm?

Stage 1

Answer 31

• 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…

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Question 32

What happens to mudstone (pelite) as its travels along the Geotherm?

Stage 2 (after mudstone)

Answer 32

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)

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Question 33

What happens to mudstone (pelite) as its travels along the Geotherm?

Stage 2 (after micas)

Answer 33

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)

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Question 34

What happens to mudstone (pelite) as its travels along the Geotherm?

Stage 3 (after micas)

Answer 34

• 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

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Question 35

What is ‘bulk composition’?

Answer 35

The overall chemical composition of the rock

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