Metamorphism
Metamorphic Rocks & the Rock Cycle
Types of Metamorphism
Patterns of
Metamorphic
Grade
Mineral Assemblages in
Metamorphic Rocks
METAMORPHIC ROCKS
Metamorphic
Textures
Index Minerals
Phase Diagrams
Factors affecting type
of metamorphic rock
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
List down as many metamorphic rocks as you can remember:
• spotted rock
• hornfels
• marble
• metaquartzite
• slate
• phyllite
• schist
• gneiss
Metamorphic Rocks
Which ones are foliated and which ones are unfoliated?
• spotted rock
• hornfels
• marble
unfoliated
• metaquartzite
• slate
• phyllite
• schist
• gneiss
foliated
Metamorphic Rocks
Which can be formed due to contact metamorphism & which can
be formed due to regional metamorphism?
• spotted rock
• hornfels
• marble
unfoliated
Contact
metamorphism
• metaquartzite
• slate
• phyllite
• schist
foliated
Regional
metamorphism
• gneiss
• mylonite/fault breccia
Dynamic
metamorphism
Metamorphic Processes
• Mineral alignment
• Recrystallisation
• New minerals formed from metamorphic reactions
Metamorphic Reactions
Low temperature minerals
Change in stability
High temperature minerals
PROGRADE REACTION
reactants
temp by 550 °C
products
(increase in metamorphic grade)
muscovite + quartz = feldspar + sillimanite + water vapour
products
temp by 550 °C
reactants
(decrease in metamorphic grade)
RETROGRADE REACTION
Low temperature minerals
Change in stability
High temperature minerals
Metamorphic Reactions
Low temperature minerals
Change in stability
High temperature minerals
PROGRADE REACTION
reactants
temp by 550 °C
products
Escapes from
rock
(increase in metamorphic grade)
muscovite + quartz = feldspar + sillimanite + water vapour
Metamorphic
mineral
Phase Diagrams
Kyanite stability field.
Phase boundary
Sillimanite stability field.
Andalusite stability field.
Al2SiO5
All have same formula but each
mineral has a different crystal
structure = POLYMORPH
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.
• Classify metamorphic rocks into two groups.
Put these metamorphic rocks into 2 groups – unfoliated &
foliated.
Unfoliated
• schist
1. Fault breccia
• mylonite
2. Spotted rock
• fault breccia
3. Hornfels
4. Marble
• spotted rock
5. Metaquartzite
• hornfels
• slate
Foliated
• phyllite
1. Schist
• marble
2. Mylonite
• gneiss
3. Slate
• metaquartzite
4. Phyllite
• migmatite
5. Gneiss
6. 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
Marble
Spotted rock
Metaquartzite
Hornfels
Contact
Schist
Slate
Phyllite
Gneiss
Migmatite
Regional
Fault Breccia
Mylonite
Dynamic
• 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
Increasing metamorphic grade
migmatite
• 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 -
Localised deformation along
fault zones.
Contact metamorphism -
Heat from igneous
intrusions.
Regional metamorphism -
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
Clay
Andalusite
Clay
Andalusite
Retrograde reaction
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
large, angular fragments (fault breccia)
fine-grained matrix (fault gouge)
Mylonite
Fault Breccia
elongation of minerals
foliation
Contact
Metamorphism
A
B
E
G
D
C
F
Contact
Metamorphism
A
B
F
E
D
C
H
Interlocking quartz,
feldspar & mica
grains (granoblastic)
F
Cordierite minerals
(porphyroblastic)
Recrystallised mica
minerals (unfoliated)
E
G
Interlocking coarse
calcite crystals
(granoblastic)
H
Partially
recrystallised
clay minerals
(unfoliated)
Interlocking quartz
grains (granoblastic)
D
B
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
Regional Metamorphism
Schistosity
(foliation)
Schist
Slaty cleavage
(foliation)
Clay &
mica
Phyllite
Slate
Increasing
metamorphic grade
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
Regional Metamorphism
Granite
Gneissose Banding
(foliation)
Migmatite
Gneiss
Increasing
metamorphic grade
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.
• Identify & describe the metamorphic textures
associated with the different types of metamorphism.
2mm
Thin section of a metamorphic rock
1. Describe the texture of this rock.
2. Name this rock.
3. How did this rock form?
• Identify & describe the metamorphic textures
associated with the different types of metamorphism.
Thin section of a metamorphic rock
Cordierite
Andalusite
Clay
1. Describe the texture of this rock.
2. Name this rock.
3. How did this rock form?
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
Spotted Rock
Hornfels
CONTACT METAMORPHISM
Metamorphic Mineral Isograds
Index minerals
North
Kyanite zone
Key:
Staurolite zone
Kyanite
Staurolite
Garnet
Biotite
Garnet zone
Chlorite
Biotite zone
Chlorite zone
isograd
Contact Metamorphism
UNALTERED COUNTRY
ROCKS
Metamorphic aureole
Increasing
metamorphic grade
andalusite
cordierite
GRANITE
sillimanite
• concentric metamorphic zones
• associated with a batholith
• index minerals (andalusite, cordierite & sillimanite)
• hornfels at contact (granoblastic texture)
Increasing
metamorphic grade
Regional Metamorphism
• 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
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.