Metamorphism

Metamorphic Rocks & the Rock Cycle

Types of Metamorphism

Mineral Assemblages in Metamorphic Rocks

Metamorphic Textures

Factors affecting type of metamorphic rock

Index Minerals

Patterns of Metamorphic

Grade

METAMORPHIC ROCKS

Phase Diagrams

Metamorphic Rocks

By the end of this lesson you will be able to:

• Classify metamorphic rocks into two groups.

• Explain the concept of metamorphic grade.

• Describe the causes of the different types of metamorphism.

• Explain how metamorphic reactions occur during metamorphism.

Metamorphic Rocks

• spotted rock

• hornfels

• marble

• metaquartzite

• slate

• phyllite

• schist

• gneiss

List down as many metamorphic rocks as you can remember:

Metamorphic Rocks

• spotted rock

• hornfels

• marble

• metaquartzite

• slate

• phyllite

• schist

• gneiss

unfoliated

foliated

Which ones are foliated and which ones are unfoliated?

Metamorphic Rocks

• spotted rock

• hornfels

• marble

• metaquartzite

• slate

• phyllite

• schist

• gneiss

unfoliated

foliated

Which can be formed due to contact metamorphism & which can be formed due to regional metamorphism?

Contact metamorphism

Regional metamorphism

• mylonite/fault breccia

Dynamic metamorphism

Metamorphic Processes

• Mineral alignment

• Recrystallisation

• New minerals formed from metamorphic reactions

Metamorphic Reactions

muscovite + quartz = feldspar + sillimanite + water vapour

reactantsproducts

temp by 550 °C

(increase in metamorphic grade)

PROGRADE REACTION

temp by 550 °C

(decrease in metamorphic grade)

RETROGRADE REACTION

reactantsproducts

Low temperature minerals

High temperature minerals

High temperature minerals

Low temperature minerals

Change in stability

Change in stability

Metamorphic Reactions

muscovite + quartz = feldspar + sillimanite + water vapour

reactantsproducts

temp by 550 °C

(increase in metamorphic grade)

PROGRADE REACTION

Low temperature minerals

High temperature minerals

Change in stability

Escapes from rock

Metamorphic mineral

Phase Diagrams

Andalusite stability field.

Kyanite stability field.

Sillimanite stability field.

Phase boundary

All have same formula but each mineral has a different crystal structure = POLYMORPHAl2SiO5

Metamorphism & Metamorphic Reactions

1. Define metamorphism.

2. What does the term metamorphic grade mean?

3. Why do metamorphic reactions occur?

4. What do these reactions typically form?

5. Explain prograde and retrograde reactions using the muscovite + quartz metamorphic reaction.

6. In reality most metamorphic reactions do not return to their original starting point. Why?

7. Metamorphic reactions are slow. What could cause them to speed up and how is this done?

8. What is a phase diagram and what does it show? (note: explain these words - phase/phase transformation/ stability field - using the kyanite, andalusite & sillimanite example)

9. Why might a mineral be termed metastable?

10. Describe the differences between the three types of metamorphism.

Metamorphic Rocks

The intended learning outcomes from last lesson were to be able to:

• Classify metamorphic rocks into two groups.

• Explain the concept of metamorphic grade.

• Describe the causes of the different types of metamorphism.

• Explain how metamorphic reactions occur during metamorphism.

Put these metamorphic rocks into 2 groups – unfoliated & foliated.

• Classify metamorphic rocks into two groups.

• schist• mylonite• fault breccia• spotted rock• hornfels• slate• phyllite• marble• gneiss• metaquartzite• migmatite

Unfoliated

1. Fault breccia2. Spotted rock3. Hornfels4. Marble5. Metaquartzite

Foliated

1. Schist2. Mylonite3. Slate4. Phyllite5. Gneiss6. Migmatite

• Classify metamorphic rocks into two groups.

Put these metamorphic rocks into a Venn diagram to show whether they are formed by contact metamorphism or regional metamorphism, or by both.

• schist• mylonite• fault breccia• spotted rock• hornfels• slate• phyllite• marble• gneiss• metaquartzite• migmatite

Contact Regional

SchistSlatePhylliteGneissMigmatite

Spotted rockHornfels

MarbleMetaquartzite

Dynamic

Fault BrecciaMylonite

• Explain the concept of metamorphic grade.

Put these regional metamorphic rocks into a linear order to show increasing metamorphic grade.

• schist• slate• phyllite• migmatite• gneiss

slate phyllite schist gneiss migmatite

Increasing metamorphic grade

• Describe the causes of the different types of metamorphism.

Write the cause of the metamorphic change next to the types of metamorphism below.

Dynamic metamorphism -

Contact metamorphism -

Regional metamorphism -

Localised deformation along fault zones.

Heat from igneous intrusions.

Heat & pressure due to orogenesis.

• Explain how metamorphic reactions occur during metamorphism.

Use clay & its polymorph andalusite to show how metamorphic reactions occur.

Prograde reaction

Retrograde reaction

Clay Andalusite

AndalusiteClay

Metamorphic Rocks

By the end of this lesson you will be able to:

• Identify & describe the metamorphic textures associated with the different types of metamorphism.

• Explain how these metamorphic textures form.

Dynamic Metamorphism

Fault Breccia

Mylonite

elongation of minerals

foliation

large, angular fragments (fault breccia)

fine-grained matrix (fault gouge)

Contact Metamorphism

AB

CDE

FG

Contact Metamorphism

AB

CDE

GH

H D BF

Interlocking coarse calcite

crystals (granoblastic)

Interlocking quartz grains (granoblastic)

Interlocking quartz, feldspar &

mica grains (granoblastic)

Partially recrystallised clay minerals (unfoliated)

Cordierite minerals

(porphyroblastic)

Recrystallised mica minerals (unfoliated) E

F

Metamorphic Texture

• Recrystalline• Interlocking• Medium-grained • Granoblastic• Non-foliated

Metamorphic Minerals

• Quartz• Feldspar• Mica• Sillimanite

Metamorphic Texture

• Recrystalline• Interlocking• Medium-grained • Granoblastic• Non-foliated

Metamorphic Minerals

• Calcite

Schist

Increasing metamorphic grade

Slate

Phyllite

Regional Metamorphism

Schistosity (foliation)

Slaty cleavage(foliation)

Clay & mica

Metamorphic Texture

• Partially recrystalline• Fine-grained • Foliated• Slaty cleavage• (porphyroblastic)

Metamorphic Minerals

• Quartz• Clay• Mica• Pyrite (porphyroblasts)

Metamorphic Texture

• Partially recrystalline• Fine-grained • Foliated• Slaty cleavage

Metamorphic Minerals

• Quartz• Clay• Chlorite Mica

Metamorphic Texture

• Recrystalline• Interlocking• Medium-grained • Foliated• Schistosity• Porphyroblastic

Metamorphic Minerals

• Quartz• Feldspar• Mica• Garnet

Gneiss

Migmatite

Granite

Increasing metamorphic grade

Regional Metamorphism

Gneissose Banding

(foliation)

Metamorphic Texture

• Recrystalline• Interlocking• Coarse-grained • Foliated• Gneissose banding

Metamorphic Minerals

• Quartz• Feldspar• Hornblende• Augite

Metamorphic Rock Identification

Checklist for describing metamorphic rocks A – H:

1.Texture

• Recrystalline

• Interlocking

• Grain size (coarse >2mm, medium 2mm-0.05mm, fine <0.05mm)

• Granoblastic

• Porphyroblastic

• Foliated (slaty cleavage, schistosity, gneissose banding)

2.Mineralogy

• Quartz, feldspar, hornblende, augite

• Chlorite mica, muscovite mica, biotite mica, andalusite, garnet

A. B. C.

D. E. F.

Metamorphic Rocks

The intended learning outcomes from last lesson were to be able to:

• Identify & describe the metamorphic textures associated with the different types of metamorphism.

• Explain how these metamorphic textures form.

Thin section of a metamorphic rock

• Identify & describe the metamorphic textures associated with the different types of metamorphism.

1. Describe the texture of this rock.

2mm

2. Name this rock.

3. How did this rock form?

Andalusite

Cordierite

• Identify & describe the metamorphic textures associated with the different types of metamorphism.

1. Describe the texture of this rock.

2. Name this rock.

3. How did this rock form?

Thin section of a metamorphic rock

Clay

Metamorphic Rocks

By the end of this lesson you will be able to:

• Interpret pressure-temperature graphs.

• Recognise the type of metamorphism from the patterns of changing metamorphic grade.

• Use index minerals to indicate the direction of increasing metamorphic grade & the location of metamorphic mineral isograds.

P-T Graph

Slate

Phyllite

Schist

Gneiss

Igneous

Spotted Rock Hornfels

CONTACT METAMORPHISM

Fault-b

recciaM

ylonite

DYN

AM

IC M

ETA

MO

RPH

ISM

REGIONAL METAMORPHISM

Metamorphic Mineral Isograds

Chlorite

Biotite

Garnet

Staurolite

Kyanite

Key:

isograd

Increasing

metam

orphic grade

Index mineralsNorth

Chlorite zone

Biotite zoneGarnet zone

Staurolite zone

Kyanite zone

Contact Metamorphism

• concentric metamorphic zones

• associated with a batholith

• index minerals (andalusite, cordierite & sillimanite)

• hornfels at contact (granoblastic texture)

GRANITE

Metamorphic aureole

UNALTERED COUNTRY ROCKS

Increasing metamorphic grade

Increasing metamorphic grade

andalusite

cordierite

sillimanite

Regional Metamorphism

Increasing metamorphic grade

• linear metamorphic zones

• not associated with a batholith

• index minerals (muscovite mica, chlorite mica, biotite mica, garnet, staurolite, kyanite & sillimanite)

• foliated textures (slaty cleavage, schistosity & gneissose banding)

Increasing metamorphic grade

Phase Diagrams

Using the phase diagram:

1.Which mineral is stable at 700°C and 400 Mpa?

2.At what temperature do andalusite & sillimanite coexist in equilibrium at a pressure of 200 Mpa?

3.Which mineral is high temperature, low pressure metamorphic mineral?

4.Which mineral is high pressure, low pressure?

Metamorphic Rocks

By the end of this unit you should now be able to:

• Classify metamorphic rocks into two groups.

• Explain the concept of metamorphic grade.

• Describe the causes of the different types of metamorphism.

• Explain how metamorphic processes occur during metamorphism.

• Identify & describe the metamorphic textures associated with the different types of metamorphism.

• Explain how these metamorphic textures form.

• Name metamorphic rocks from their metamorphic texture.

• Interpret P-T graphs and phase diagrams.

• Recognise the type of metamorphism from the patterns of changing metamorphic grade.

• Use index minerals to indicate the direction of increasing metamorphic grade & the location of metamorphic mineral isograds.