EAE 08 Metamorphic Rocks (12) Flashcards by Andrew crisp

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

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

The limestone bulk composition grows

Calcite
Garnet
Diopside
Wollastonite

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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

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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

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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

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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…

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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

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Compressional Tectonic Settings

Describe Subduction Impacts.

Releases water → hydration melting of overlying mantle

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

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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

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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

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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

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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

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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

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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?

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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

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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

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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

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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

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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

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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

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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

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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

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Names for mineral shapes . . . .

Roughly round grains

Equant

This applies to ALL rock types

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Names for mineral shapes . . . .

Long thin grains

Elongate

This applies to ALL rock types

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Names for mineral shapes . . . . ## Footnote **Flat grains**

Platy ## Footnote 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 ## Footnote 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 ## Footnote 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 ## Footnote 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) ## Footnote _{EAE 10bd}

What happens to mudstone (pelite) as its travels along the Geotherm? ## Footnote **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… ## Footnote _{EAE 10be}

What happens to mudstone (pelite) as its travels along the Geotherm? ## Footnote **Stage 2 (after mudstone)**

Micas ## Footnote 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? ## Footnote **Stage 2 (after micas)**

Micas to amphiboles and similar ## Footnote 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? ## Footnote **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 ## Footnote _{EAE 10bi}