Metamorphic Petrology I (L36-43) Flashcards
(55 cards)
1
Q
What information is gained from metamorphic petrology?
A
P-T-X conditions
Deformation fabrics
Time constraints
Understand tectonic processes
2
Q
What are the five principal settings of metamorphism?
A
Regional (continent-continent collision) Seafloor Subduction zone Contact Regional (subduction zone)
3
Q
Why does metamorphism occur?
A
T change
P change
Composition (X) change
4
Q
What are the three lithospheric thermal properties?
A
Conductivity (K) Heat production (A) Heat flow (q)
5
Q
Subduction zone thermal regime
A
Hotter than normal T in back arc
Colder than normal T in subducted plate
6
Q
Collision zone thermal regime
A
Hotter than normal T in zone of accretion
7
Q
Contact metamorphism thermal regime
A
Hotter than normal T around intrusion
8
Q
Define migmatites
A
Metamorphic rocks that are partially melted
9
Q
Define Ultra-high T (UHT) metamorphism
Name an indicator of UHT
A
T > 900C
Sapphirine + quartz
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
11
Q
What defines ultra-high pressure (UHP) metamorphism?
A
Coesite
12
Q
What are the five principal rock protoliths?
A
Mudstones Sandstones Carbonates Mafic igneous Felsic igneous
13
Q
Mudstones:
Rock name?
Typical minerals?
Bulk composition?
A
Metapelite
Muscovite, chlorite, bt, gt, qz, staurolite, Al-silicates, cordierite
High Al2O3, K2O, SiO2
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
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
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
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
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
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
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
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
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
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
How may prograde history be preserved?
Mineral zoning
Mineral inclusions
Pseudomorphs
26
What is retrograde evolution typically limited by?
Water availability
27
What are the Barrow's zones?
| Which minerals are in them?
```
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
Define isograd
| How are they determined in field?
Lines on a map separating zones of differing grade
| Demarcating where a new mineral first appears
29
What is the basis for constructing phase diagrams?
Graphical representation of eqm relationships between phases in model system
30
Outline Gibb's phase rule
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
W.r.t. Gibb's phase rule, what do the terms say about an assemblage?
Invariant
Univariant
Divariant
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
In a phase diagram, which phase is the most stable in P-T space?
The one with the smallest Gibbs Free Energy (G)
33
What does a total phase diagram show?
| How many axes are there?
All P-T-X information for an n-component system
| n + 1 axes
34
What are the graphical analysis approaches for lower dimension representations of total phase diagrams?
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
What is the model system approach for lower dimension representations of total phase diagrams?
Simplified sets of system components to describe commonly occuring rock types
I.e. Metapelites = KFMASH
36
Define compatibility diagram
When specific to a P-T condition, phases in equilibrium joined by tie-lines
37
What are the two reaction types in phase diagrams?
D = A + B + C: terminal, included-phase
| D + C = A + B: crossed tieline
38
How is projection used to lower the dimensions in a total phase diagram?
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
How can combining components be used to lower the dimensions in a total phase diagram?
Give an example
```
If system components behave similarly, can combine
ACF diagram:
A = Al2O3 - Na2O
C = CaO
F = FeO + MgO
```
40
What is the problem with trying to combine FeO and MgO as one component?
Mg and Fe frequently behave independently
41
How is the total phase diagram for pelites reduced to 3 components?
Approximate to NCKFMASH
Low CaO and Na2O so ignore
H2O and Qz in excess
Project from muscovite, then ignore and use AFM
42
What is the univariant reaction on the staurolite isograd?
g + chl = st + bi
| Balanced with xs Ms on LHS and Qz and H2O on RHS
43
What is the univariant reaction on the kyanite isograd?
st + chl = bi + ky
44
What is the univariant reaction on the sillimanite isograd?
ky = sill
45
```
Chloritoid:
Formula
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
```
(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
```
Tourmaline:
Silicate type
Crystal system
Hand specimen identifiers
PPL identifiers
XPL identifiers
```
```
Ring silicate
Trigonal
Black striated prisms
Pleochroic, commonly zoned, med relief
3rd order δ, straight extinction
```
47
Outline the garnet zone
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
Outline the staurolite isograd
(st) and (bi) triangles have met
g + chl + st + bi in equilibrium
Univariant reaction from crossing tie lines:
g + chl = st + bi
49
Outline the staurolite zone
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
How do the Barrow zones and Stonehaven Coast differ in the pelite petrogenetic conditions?
Barrow zones = higher P
| Stonehaven Coast = lower P
51
Describe the simple thermal model for the evolution of rocks
Doubling continental crust gives a saw-tooth profile
Evolves with time (thermal relaxation)
Initial negative thermal gradient rapidly eliminated
No heat production assumed
52
How can the simple thermal model for the evolution of rocks be improved?
Include advection terms
| Include radiogenic heating terms
53
What information can be obtained from Pressure-Temperature-time (PTt) paths for a crustal doubling situation?
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
What is a metamorphic geotherm or metamorphic field gradient?
Locus of T(max) points on the P-T paths for measured rocks
55
Give an example of how phase models can break the Gibbs Phase rule
Garnet frequently stabilised by extra components, not in the KFMASH model system (e.g. CaO or MnO)
