1. AFM Diagram
Due to extensive Mg-Fe solid solution in biotite and garnet, much of the area is dominated by 2-phase fields with tie- lines (really 4-phase when we include the Qtz and Mu projection phases)
Although we can easily plot ideal mineral formulas on the ACF and AKF diagrams, in order for a real mafic phase to be plotted on an AFM diagram we must know Mg/(Fe+Mg), which can only be determined by chemical analysis of the minerals, generally performed using the electron microprobe
If analyses are unavailable, we can approximate the correct positions on the basis of typical relative Mg/(Fe+Mg), based on our knowledge of numerous analyses of these minerals available in the literature. From these we know that Mg-enrichment occurs typically in the order: cordierite > chlorite > biotite > staurolite > garnet

2. At different P-T conditions, the diagrams change
Other minerals become stable
Different arrangements of the same minerals (different tie-lines connect different coexisting phases)
Use to graphically show important isograd reactions
low P-T
high P-T

3. A + B  C + D Below the isograd Bulk rock composition At the isograd
low P-T
A + B  C + D
At the isograd
Above the isograd
Note that phi = 4 at the isograd with crossing tie-lines
Then have new groupings :
A + C + D or
B + C + D
No new minerals become stable- simply different associations
The groupings follow from the reaction:
If A > B then B consumed first, and A remains with new C & D -> A + C + D
C + D cannot coexist below the isograd, and A + B cannot coexist above it
If a chemographic diagram is a projection, the approach still works, but you will have to balance the reaction with other components
For example, if the previous diagram is projected from quartz, SiO2 will have to be added to one side of the A + B = C + D reaction to balance it properly
high P-T
This is called a tie-line flip, and results in new mineral assemblages in the next metamorphic zone

4. different diagrams are separated by metamorphic reactions
Review and P-T Application
AFM basics
each diagram represents stable assemblages at fixed P & T
different diagrams are separated by metamorphic reactions
different assemblages = different bulk X

5. 2. Getting P-T constraints
chl
gar
bio
Example:
Over what P-T range is the
assemblage Gar+Chl+Bio stable?

6. H I J Step 1: find AFM range for assemblage
Where in P-T space does this assemblage occur?

7. kya sill and Step 2: use AFM labels to find P-T field H to J
This is the only part of P-T space where gar+chl+bio can coexist
H to J
Al2SiO5 in nearby rx could further restrict P&T
kya
sill
and

8. Metapelites Metapelites = metamorphosed mudstones and shales
Distinguishing chemical characteristics: high Al2O3 and K2O, and low CaO
Reflect the high clay and mica content of the original sediment and lead to the dominance of muscovite and quartz throughout most of the range of metamorphism
High proportion of micas ® common development of foliated rocks, such as slates, phyllites, and mica schists
the petrogenesis of pelites is represented well in AKF and AFM diagrams

9. Barrovian Zones in Metapelites
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Kyanite zone
Sillimanite zone
K-feldspar zone

10. Variable P-T Conditions in a Convergent Plate Setting
Low P, high T (contact)
high P, low T (“blueschist”)
high P and T (regional)
Barrovian
Barrovian Series rocks typical of regional metamorphism at mid- to lower crust in mountain belts

11. Barrovian Series metapelites have kyanite
The molar volume of cordierite is also quite high, indicating that it too is a low-pressure mineral
The geothermal gradient in this northern district was higher than in Barrow’s area, and rocks at any equivalent temperature must have been at a lower pressure
This lower P/T variation has been called Buchan-type metamorphism. It too is relatively common
Miyashiro (1961), from his work in the Abukuma Plateau of Japan, called such a low P/T variant Abukuma-type
Both terms are common in the literature, and mean essentially the same thing

12. Barrovian
Buchan
Dutchess Co. Trip

14. Barrovian Zones in Metapelites
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Kyanite zone
Sillimanite zone
K-feldspar zone

15. P-T grid
Barrovian Series

16. Metamorphic “zones” based on metapelites can give relatively high resolution P-T estimates
Greenschist
Amphibolite
Granulite
Kfs Zone
Sil Zone
Ky Zone
Barrovian Series
St Zone
Grt Zone
Bt Zone
Chl Zone
P-T grid

17. Chlorite Zone
lower greenschist facies
300 – 400°C

18. Biotite Isograd tie-line flip (discontinuous) reaction type
Chl + Kfs  Bt + phengitic Ms
400 – 425°C

19. “P-T” grid
 = Biotite Isograd

20. Chapter 28: Metapelites Biotite Zone middle to upper greenschist
Continuous reactions (over a range of P-T) involving solid solution
gradual expansion of Ms-Bt-Chl triangle to include more pelite compositions
middle to upper greenschist
400 – 500°C
P,T increasing

21. Garnet Isograd Part 1 tie-line flip (discontinuous) reaction type
Cld + Bt  Grt + Chl
~500°C

22. Chapter 28: Metapelites Garnet Isograd Part 2 P,T increasing
Continuous reaction type (over a range of P-T) involving solid solution
Chl + Bt  Grt + Mg-rich Chl + Mg-rich Bt
P,T increasing
This is the garnet isograd for almost all common metapelites
525 – 555°C

23. “P-T” grid
 = Garnet Isograd

24. Staurolite Isograd Chapter 28: Metapelites Part 1
terminal point reaction type (“chloritoid-out”; disappearance of chloritoid)
Cld  Grt + Chl + St
~550°C

25. Chapter 28: Metapelites Staurolite Isograd Part 2
tie-line flip (discontinuous) rxn
Chl + Grt  St + Bt
This is the staurolite isograd for almost all common metapelites
550 – 600°C

26. “P-T” grid
 = Staurolite Isograd

27. Kyanite Isograd Part 1 tie-line flip (discontinous) rxn
St + Chl  Ky + Bt
~625°C

28. Chapter 28: Metapelites Kyanite Isograd Part 2
terminal point reaction type (“staurolite-out”; disappearance of staurolite) Ky
St  Grt + Bt + Ky
This is the kyanite isograd for almost all common metapelites °C

29. “P-T” grid
 = Kyanite Isograd

30. Chapter 28: Metapelites Sillimanite Isograd polymorphic transition Ky
Ky  Sil °C

31. “P-T” grid
 = Sillmanite Isograd

32. K-feldspar Isograd (“2nd sillimanite isograd”)
Chapter 28: Metapelites
breakdown of muscovite; dehydration reaction
Ms + Qtz  Sil + Kfs + H2O
~750°C
liberated H2O may cause partial melting
Kfs

33. “P-T” grid
11 = K-feldspar Isograd

34. Granulite Facies Breakdown of biotite; dehydration reactions
presence of cordierite and/or Opx (depending on P)
Bt + Sil  Grt + Crd + H2O Bt + Qtz  Opx + Kfs + H2O
> ° C
liberated H2O may cause partial melting

35. Migmatites migmatite = “mixed rock”; part igneous, part metamorphic
Breakdown of muscovite and biotite at high grades may cause partial melting

36. Metamorphic “zones” based on metapelites can give relatively high resolution P-T estimates
Greenschist
Amphibolite
Granulite
Kfs Zone
Sil Zone
Ky Zone
St Zone
Grt Zone
Bt Zone
Chl Zone