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1B ESB > Metamorphic Petrology I (L36-43) > Flashcards

Metamorphic Petrology I (L36-43) Flashcards

1
Q

What information is gained from metamorphic petrology?

A

P-T-X conditions
Deformation fabrics
Time constraints
Understand tectonic processes

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2
Q

What are the five principal settings of metamorphism?

A 
Regional (continent-continent collision)
Seafloor
Subduction zone
Contact
Regional (subduction zone)
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3
Q

Why does metamorphism occur?

A

T change
P change
Composition (X) change

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4
Q

What are the three lithospheric thermal properties?

A 
Conductivity (K)
Heat production (A)
Heat flow (q)
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5
Q

Subduction zone thermal regime

A

Hotter than normal T in back arc

Colder than normal T in subducted plate

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6
Q

Collision zone thermal regime

A

Hotter than normal T in zone of accretion

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7
Q

Contact metamorphism thermal regime

A

Hotter than normal T around intrusion

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8
Q

Define migmatites

A

Metamorphic rocks that are partially melted

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9
Q

Define Ultra-high T (UHT) metamorphism

Name an indicator of UHT

A

T > 900C

Sapphirine + quartz

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10
Q

What are the aluminosilicate polymorphs?

Which PT conditions are each at?

A

Kyanite - low P
Sillimanite - high P
Andalusite - low P, medium T

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11
Q

What defines ultra-high pressure (UHP) metamorphism?

A

Coesite

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12
Q

What are the five principal rock protoliths?

A 
Mudstones
Sandstones
Carbonates
Mafic igneous
Felsic igneous
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13
Q

Mudstones:
Rock name?
Typical minerals?
Bulk composition?

A

Metapelite
Muscovite, chlorite, bt, gt, qz, staurolite, Al-silicates, cordierite
High Al2O3, K2O, SiO2

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14
Q 
Muscovite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
KAl2[AlSi3O10](OH,F)2
Sheet silicate
Monoclinic
Shiny silver fabric forming flakes
Colourless, low relief, basal cleavage
3rd order δ, speckled extinction
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15
Q 
Chlorite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
(Mg,Fe)5Al[AlSi3O10](OH)8
Sheet silicate
Monoclinic
Fabric-forming green sheen
Greem, low/med relief, basal cleavage
1st order/anomalous δ, common retrograde of gt or bt
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16
Q 
Biotite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
K(Mg,Fe)3[AlSi3O10](OH,F)2
Sheet silicate
Monoclinic
Black plates forming fabric
Brown pleochroism, med relief, basal cleavage, pleochroic haleos
3rd order δ, speckled extinction
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17
Q 
Garnet:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
(Fe,Mg,Ca,Mn)3Al2[SiO4]3
Isolated tetrahedra
Cubic
Dark red porphyroblasts
High relief, inclusion rich
Isotropic
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18
Q 
Staurolite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
(Fe,Mg)2Al9O6[SiO4]4(O,OH)2
Isolated tetrahedra
Monoclinic
Brown porphyroblasts, cruciform
Yellow, high relief, inclusion rich with qz
High 1st order δ
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19
Q 
Kyanite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A

Al2SiO5
Isolated tetrahedra
Triclinic
Blue blades
High relief, two good cleavages, tabular habit
High 1st order δ, length slow, simple twins

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20
Q 
Andalusite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
Al2SiO5
Isolated tetrahedra
Orthorhombic
Square prisms/needles
Med relief, 2 90 degree cleavages, graphite cross
Low 1st order δ, length fast
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21
Q 
Sillimanite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
Habit
A 
Al2SiO5
Isolated tetrahedra
Orthorhombic
White needles
Med relief, good cleavage
2nd order δ, length slow
Prismatic or fibrolitic
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22
Q 
Cordierite:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
(Fe,Mg)2Al4Si5O18
Ring silicate
Orthorhombic
Spotted appearance
Low relief, poikiloblastic, often altered at margins, yellow pleochroic haloes
1st order δ, can have sector trilling
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23
Q

What is the consequence of T changes in metamorphism lagging behind tectonic changes?

A

Clockwise P-T-t paths

24
Q

Why is equilibrium best maintained in prograde metamorphism?

A

Reaction rates increase with T
Devolatilisation is easier than adding during cooling
Fluids are catalysts

25
Q

How may prograde history be preserved?

A

Mineral zoning
Mineral inclusions
Pseudomorphs

26
Q

What is retrograde evolution typically limited by?

A

Water availability

27
Q

What are the Barrow’s zones?

Which minerals are in them?

A 
All have muscovite and qz
Chlorite zone: chl
Biotite zone: bi + chl
Garnet zone: g + bi + chl
Staurolite zone: st + g + bi
Kyanite zone: ky + st + g + bi
Sillimanite zone: sill + st + g + bi
28
Q

Define isograd

How are they determined in field?

A

Lines on a map separating zones of differing grade

Demarcating where a new mineral first appears

29
Q

What is the basis for constructing phase diagrams?

A

Graphical representation of eqm relationships between phases in model system

30
Q

Outline Gibb’s phase rule

A

P + F = C + 2
P = number of phases
C = minimum number of independent components to describe the system
F = degrees of freedom in the system

31
Q

W.r.t. Gibb’s phase rule, what do the terms say about an assemblage?
Invariant
Univariant
Divariant

A

Invariant: F=0, stable at a point in P-T space
Univariant: F=1, stable along a reaction in P-T space
Divariant: F=2, stable in a field in P-T space

32
Q

In a phase diagram, which phase is the most stable in P-T space?

A

The one with the smallest Gibbs Free Energy (G)

33
Q

What does a total phase diagram show?

How many axes are there?

A

All P-T-X information for an n-component system

n + 1 axes

34
Q

What are the graphical analysis approaches for lower dimension representations of total phase diagrams?

A

T-X section = fixed P
P-T pseudosection = shows assemblages for a fixed composition
P-T projection = shows invariant points and univariant reactions
Compatibility diagrams

35
Q

What is the model system approach for lower dimension representations of total phase diagrams?

A

Simplified sets of system components to describe commonly occuring rock types
I.e. Metapelites = KFMASH

36
Q

Define compatibility diagram

A

When specific to a P-T condition, phases in equilibrium joined by tie-lines

37
Q

What are the two reaction types in phase diagrams?

A

D = A + B + C: terminal, included-phase

D + C = A + B: crossed tieline

38
Q

How is projection used to lower the dimensions in a total phase diagram?

A

If a phase is present in all assemblages over the P-T range, can project from that phase to reduce the dimensions
If the projection phase is also an end-member component, ignore that component and state ‘in excess’

39
Q

How can combining components be used to lower the dimensions in a total phase diagram?
Give an example

A 
If system components behave similarly, can combine
ACF diagram:
A = Al2O3 - Na2O
C = CaO
F = FeO + MgO
40
Q

What is the problem with trying to combine FeO and MgO as one component?

A

Mg and Fe frequently behave independently

41
Q

How is the total phase diagram for pelites reduced to 3 components?

A

Approximate to NCKFMASH
Low CaO and Na2O so ignore
H2O and Qz in excess
Project from muscovite, then ignore and use AFM

42
Q

What is the univariant reaction on the staurolite isograd?

A

g + chl = st + bi

Balanced with xs Ms on LHS and Qz and H2O on RHS

43
Q

What is the univariant reaction on the kyanite isograd?

A

st + chl = bi + ky

44
Q

What is the univariant reaction on the sillimanite isograd?

45
Q 
Chloritoid:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A

(Fe,Mg,Mn)2Al4Si2O10(OH)4
Isolated tetrahedra
Tri/monoclinic
Black platy
Pale blue to yellow pleochroism, med relief
Anomalous 1st order δ, lamellar twinning and sector zoning, length fast

46
Q 
Tourmaline:
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
A 
Ring silicate
Trigonal
Black striated prisms
Pleochroic, commonly zoned, med relief
3rd order δ, straight extinction
47
Q

Outline the garnet zone

A

Below the staurolite isograd
Respond to increasing T by continuous reactions:
(st) chl = g + bi
(bi) chl + st = g
As T rises, triangles migrate to the right
(st) triangle catches up with (bi)
g+chl two-phase field width reduced

48
Q

Outline the staurolite isograd

A

(st) and (bi) triangles have met
g + chl + st + bi in equilibrium
Univariant reaction from crossing tie lines:
g + chl = st + bi

49
Q

Outline the staurolite zone

A

Above the staurolite isograd
Two divariant continuous reactions:
(chl) g = st + bi
(g) chl = st + bi
As T rises, both triangles migrate to the right
(g) triangle moves faster than (chl) triangle
st + bi two-phase field opened

50
Q

How do the Barrow zones and Stonehaven Coast differ in the pelite petrogenetic conditions?

A

Barrow zones = higher P

Stonehaven Coast = lower P

51
Q

Describe the simple thermal model for the evolution of rocks

A

Doubling continental crust gives a saw-tooth profile
Evolves with time (thermal relaxation)
Initial negative thermal gradient rapidly eliminated
No heat production assumed

52
Q

How can the simple thermal model for the evolution of rocks be improved?

A

Include advection terms

Include radiogenic heating terms

53
Q

What information can be obtained from Pressure-Temperature-time (PTt) paths for a crustal doubling situation?

A

T(max) occurs at different times for different depths, so peak assemblages do
Sil-grade conditions can be reached during regional metamorphism, without an external heat source

54
Q

What is a metamorphic geotherm or metamorphic field gradient?

A

Locus of T(max) points on the P-T paths for measured rocks

55
Q

Give an example of how phase models can break the Gibbs Phase rule

A

Garnet frequently stabilised by extra components, not in the KFMASH model system (e.g. CaO or MnO)

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