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

1

Barrovian and Buchan style metamorphism

Barrow zones:• sillimanite zone• kyanite zone• garnet zone• biotite zone• chlorite zone

Buchan zones:• andalusite zone

2

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

3

Metamorphic facies (after Eskola 1915)

Characteristic paragenesis of minerals that give a first-order indication of P-T

4

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


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.

6

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

8

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

9

Bathozones and bathograds (Carmichael 1978)


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.

12

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

13

Pseudosections: dependence on bulk composition

chl ep ccpl q ru


amp chlpl q ilm

ampchl

ep pl

q ilm

chl ep ccpl q ilm

ampchl epgt q ilm


ampchl

ep ilm q


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


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

14

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


amp chlpl q ilm

ampchl

ep pl

q ilm

chl ep ccpl q ilm

ampchl epgt q ilm


ampchl

ep ilm q


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


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

15

lecture 5: metamorphic facies concept - mcgill...

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