Lecture 5: Metamorphic facies concept - McGill...
Transcript of Lecture 5: Metamorphic facies concept - McGill...
Lecture 5: Metamorphic facies concept
Metamorphic rock names include parts from all three effects that metamorphism can have on a rock: mineralogy, composition and fabric
composition terms: meta-pelite; meta-granite; meta-basaltadd mineralogy: garnet-staurolite meta-pelite; hornblende-plagioclase meta-basaltadd fabric name: slate; gneiss; granofels
final name includes all three (if known): ‣ garnet-staurolite meta-pelite schist‣ hornblende-plagioclase meta-basaltic gneiss‣ garnet-pyroxene migmatite‣ plagioclase augen mylonite
Now need a final component: metamorphic grade Ideally the name of a metamorphic rock gives a (rough) indication of its overall grade, and these grade-related names can be established in the field
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Barrovian and Buchan style metamorphism
Barrow zones:• sillimanite zone• kyanite zone• garnet zone• biotite zone• chlorite zone
Buchan zones:• andalusite zone
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Estimators of grade: mineral zonesBecause minerals have a limited stability in pressure and temperature, their presence or absence can be used as an indicator of the general P-T conditions in a rock
However, this doesn’t only depend on P and T - bulk composition also pays a role: you cannot make a garnet in a pure quartzite or marble
from Bucher and Frey 2002
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Metamorphic facies (after Eskola 1915)
Characteristic paragenesis of minerals that give a first-order indication of P-T
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Metamorphic facies: minerals depend on compoThe characteristic set of minerals in a meta-pelitic greenschist is going to be different from that in an equivalent-grade meta-basalt.
from Bucher and Frey 2002 5
Metamorphic facies: boundaries are gradational
from Bucher and Frey 2002
Boundaries between the facies are gradational, because the positions of mineral reactions in P-T space, and hence changes in mineralogy, depend on the bulk rock composition.
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Metamorphic facies: minerals for different protoliths
from Bucher and Frey 2002 7
Bathozones and bathograds (Carmichael 1978)For a given bulk rock composition, the appearance of an indicator mineral can be a good indicator of P and/or T. Carmichael proposed six P-zones for the meta-pelites, allowing for a quick estimate of pressure from thin section mineral parageneses:
Note that these zones are essentially all within the amphibolite facies
Bathograds:
5 → 6: qtz + ab + ms + sil → Kfs + ky + liq
4 → 5: qtz + ms + st + sil → bt + gt + ky + fl
3 → 4: and → ky or sil2 → 3: bt + gt + and + fl → qtz + ms + st + sil1 → 2: Kfs + and + fl → qtz + ms + sil
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Bathozones and bathograds (Carmichael 1978)From these bathograd reactions we can now define bathozones with a characteristic mineral paragenesis. Essentially this is a subdivision of the amphibolite facies valid for meta-pelitic protoliths
Bathograds:
5 → 6: qtz + ab + ms + sil → Kfs + ky + liq
4 → 5: qtz + ms + st + sil → bt + gt + ky + fl
3 → 4: and → ky or sil2 → 3: bt + gt + and + fl → qtz + ms + st + sil1 → 2: Kfs + and + fl → qtz + ms + sil
Note that these zones are essentially all within the amphibolite facies
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Bathozones and bathograds (Carmichael 1978)
from Bucher and Frey 2002
Is this contact metamorphism ?10
Metamorphic pseudosectionsCan extent this by calculating the positions of these reactions for the exact bulk-rock composition of the sample we are looking at. This is called a pseudosection. It is a section through the positions of mineral reactions in P-T-X space at fixed X.
11Guiraud et al. JMG 2001
Metamorphic pseudosectionsCan extent this by calculating the positions of these reactions for the exact bulk-rock composition of the sample we are looking at. This is called a pseudosection. It is a section through the positions of mineral reactions in P-T-X space at fixed X.
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Calculated for:
K2O-FeO-MgO-Al2O3-SiO2-H2O
Considering: Chl, Bt, Gt, St, Al-Sil, Cltd, Crd
Not considering: Opx, Cpx, Pl, melt
In excess: H2O, Qtz, Ms
http://serc.carleton.edu/research_education/equilibria/pseudosections.html
Pseudosections: dependence on bulk composition
chl ep ccpl q ru
chl ep pl ccdol q ru
amp chlpl q ilm
ampchl
ep pl
q ilm
chl ep ccpl q ilm
ampchl epgt q ilm
amp chl eppl gt q ilm
ampchl
ep ilm q
amp chl ccep gt q ilm
amp plgt ilm q
amp pl gtcpx ilm q
amp chl plgt q ilm
melt pl amp gt cpx
ilm q
900Temperature (oC)
Pres
sure
(kb
ar)
10001
3
5
7
9
500 600 700 800
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30
300 400
Sample 512256greenschist Nuuluk
1Ti-Na-Ca-Fe-Mg-Al-Si-H-O
saturated in H2O-CO2 fluid, XCO2 = 0.2
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Pseudosections: dependence on bulk composition
chl ep ccpl q ru
chl ep pl ccdol q ru
amp chlpl q ilm
ampchl
ep pl
q ilm
chl ep ccpl q ilm
ampchl epgt q ilm
amp chl eppl gt q ilm
ampchl
ep ilm q
amp chl ccep gt q ilm
amp plgt ilm q
amp pl gtcpx ilm q
amp chl plgt q ilm
melt pl amp gt cpx
ilm q
900Temperature (oC)
Pres
sure
(kb
ar)
10001
3
5
7
9
500 600 700 800
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30
300 400
Sample 512256greenschist Nuuluk
1
9 30
amp pl gtcpx ilm q
amp pl gt cpx ilm
amp plgt ilm
chl ampdol plq ru amp pl
gt q ilm
chlamppl gtq ilm
chlamppl gt ilm
chl amppl q ilm
chl amppl dol gt
q ilm
chl amppl dolq ilm
chl plgt q
ilm dol
amp pl gtdol q ilm
amp chl plgt dol q ru
chl pl gtdol q ilm ru
melt amppl gt cpx
q ilm
meltamp plgt cpx
ilm
melt pl gtcpx ilm
melt plcpx ilm
melt gtcpx ilm
Pres
sure
(kb
ar)
Ti-Na-Ca-Fe-Mg-Al-Si-H-Osaturated in H2O-CO2 fluid
1
3
5
7
9
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30chlampdol
mgs pl ru q
chlampdolmgsab
q ru
chl ampdol abq ru
chl dolpl q ru
chl dolab q ru
chl dol pl qilm
chl dolpl q ru
ilm
Sample 512184greenstone Nuuluk
amp pl gtcpx ilm q
melt plgt cpxilm q melt
pl gtcpxilm
melt gt cpx q ilm
meltpl
cpx ilm
meltgt cpxilm
chl dolpl q ru
chl ep dolpl q ru
chl dolpl ilm q
dol chl amp plilm q
chl amppl ilm q
chl ep dolpl ilm q
dol chlamp pl qgt ilm
chl dolpl gtilm q
chl amppl gt ilm q
amp plgt ilm q
dol plgt ilm
q
dol amppl gt ilm q
dol amp pl gt cpxilm q
pl gt cpxilm q
Pres
sure
(kb
ar)
1
3
5
7
9
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30Sample 510866greenschist Nuuluk
b
melt breaking throuhg fold hinge
partial melt
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Overlapping pseudosection fields
900Temperature (oC)
Pres
sure
(kb
ar)
10001
3
5
7
9
500 600 700 800
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30
300 400
2
7
6
34
5
1
gt-amp-pl-qampibolite
gt-amp-cpx-q granulite
peritectic gt + melt reaction
chl ± ep ± actgreenschist
tourmalinethermometry
chl-amp ± ep
amphibolite
chl ep ccpl q ru
chl ep pl ccdol q ru
amp chlpl q ilm
ampchl
ep pl
q ilm
chl ep ccpl q ilm
ampchl epgt q ilm
amp chl eppl gt q ilm
ampchl
ep ilm q
amp chl ccep gt q ilm
amp plgt ilm q
amp pl gtcpx ilm q
amp chl plgt q ilm
melt pl amp gt cpx
ilm q
900Temperature (oC)
Pres
sure
(kb
ar)
10001
3
5
7
9
500 600 700 800
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30
300 400
Sample 512256greenschist Nuuluk
amp pl gtcpx ilm q
amp pl gt cpx ilm
amp plgt ilm
chl ampdol plq ru amp pl
gt q ilm
chlamppl gtq ilm
chlamppl gt ilm
chl amppl q ilm
chl amppl dol gt
q ilm
chl amppl dolq ilm
chl plgt q
ilm dol
amp pl gtdol q ilm
amp chl plgt dol q ru
chl pl gtdol q ilm ru
melt amppl gt cpx
q ilm
meltamp plgt cpx
ilm
melt pl gtcpx ilm
melt plcpx ilm
melt gtcpx ilm
Pres
sure
(kb
ar)
Ti-Na-Ca-Fe-Mg-Al-Si-H-Osaturated in H2O-CO2 fluid
1
3
5
7
9
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30chlampdol
mgs pl ru q
chlampdolmgsab
q ru
chl ampdol abq ru
chl dolpl q ru
chl dolab q ru
chl dol pl qilm
chl dolpl q ru
ilm
Sample 512184greenstone Nuuluk
amp pl gtcpx ilm q
melt plgt cpxilm q melt
pl gtcpxilm
melt gt cpx q ilm
meltpl
cpx ilm
meltgt cpxilm
chl dolpl q ru
chl ep dolpl q ru
chl dolpl ilm q
dol chl amp plilm q
chl amppl ilm q
chl ep dolpl ilm q
dol chlamp pl qgt ilm
chl dolpl gtilm q
chl amppl gt ilm q
amp plgt ilm q
dol plgt ilm
q
dol amppl gt ilm q
dol amp pl gt cpxilm q
pl gt cpxilm q
Pres
sure
(kb
ar)
1
3
5
7
9
2
4
6
8
Approxim
ate depth (km)
5
15
25
10
20
30Sample 510866greenschist Nuuluk
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