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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.