1. Concept of Index Minerals
Chlorite, biotite, garnet, kyanite, sillimanite
Only exist over a narrow P-T range

2. Geologic Mapping of Metamorphic Terranes
Index minerals are mapped into “zones” with equivalent P-T conditions
Boundaries between zones are called “isograds” (lines of equal P-T)

3. Chemographic Diagrams
Biotite Isograd
Chlorite + K-feldspar  Biotite + Muscovite (phengitic)
400 – 425°C
Chemographic Diagrams

4. Chemographic Diagrams
Graphical representation of the chemistry of mineral assemblages in metamorphic rocks
Plot the following “minerals”
on an “XYZ” diagram
xz, xyz, and yz2
Suppose you had a small area of a metamorphic terrane in which the rocks correspond to a hypothetical 3-component system with variable proportions of the components x-y-z
The rocks in the area are found to contain 6 minerals with the fixed compositions x, y, z, xz, xyz, and yz2

5. x-xy-x2z Note that this subdivides the diagram into 5 sub-triangles
2-phase tie line
3-phase field
What is the stable mineral assemblage in (A)?
x-xy-x2z
A diagram like this is a compatibility diagram, a type of phase diagram commonly employed by metamorphic petrologists
Any point within the diagram represents a specific bulk rock composition
The diagram determines the corresponding mineral assemblage that develops at equilibrium
For example, a point within the sub-triangle (E), the corresponding mineral assemblage corresponds to the corners = y - z - xyz
Any rock with a bulk composition plotting within triangle (E) will develop that same mineral assemblage

6. (A) through (E) might represent the protolith bulk chemistry
2-phase tie line
3-phase field
What is the stable mineral assemblage if protolith chemistry = (B)?
xy-x2z-xyz
A diagram like this is a compatibility diagram, a type of phase diagram commonly employed by metamorphic petrologists
Any point within the diagram represents a specific bulk rock composition
The diagram determines the corresponding mineral assemblage that develops at equilibrium
For example, a point within the sub-triangle (E), the corresponding mineral assemblage corresponds to the corners = y - z - xyz
Any rock with a bulk composition plotting within triangle (E) will develop that same mineral assemblage

7. A diagram in which some minerals exhibit solid solution
2-phase tie lines
3-phase field
Minerals x(y,z) and x2(y,z) show limited solid solution of components y and z on one type of lattice site.
Mineral x(y,z) allows more y in the lattice than does mineral x2(y,z)
Minerals (xyz)ss and zss (the subscript denotes solid solution) show limited solid solution of all three components
Click
Suppose a bulk rock composition is in the shaded field of the mineral (xyz)ss
phi = 1, but the system is not degenerate
Due to the variable nature of the composition of the phase, C must still equal 3 and the phase rule tells us that F = C - phi + 2 = 4
Thus P, T, and any 2 of the 3 components in the phase are independently variable
The shaded area of mineral (xyz)ss is thus an area (compositionally divariant), and our single-phase rock can have any composition within the shaded solid solution limits
phi < C in this case because of the solid solution and compositional variance

8. if protolith chemistry = (f), What is the stable mineral assemblage?
2-phase tie lines
3-phase field
As in the previous example, there are two coexisting phases (xyz)ss and zss
The composition of the two minerals that correspond to bulk rock composition (f) are indicated by the two shaded dots at the ends of the tie-line through (f)
phi = 2 and C is still 3, so F = = 3
Since P and T are independently variable, that means the composition of each phase is univariant, and must vary along the lines where the bundles of tie- lines end
Although the composition of (xyz)ss can vary anywhere in the shaded area, the composition of (xyz)ss that coexists with Zss is constrained to the edge of the area facing z
A degree of freedom is thus lost as a phase is gained
Likewise the composition of zss that coexists with (xyz)ss is constrained to a portion of the edge of the shaded zss area

9. if protolith chemistry = (f), What is the stable mineral assemblage?
In such situations phi = 3, and C = 3, as predicted by the mineralogical phase rule
Since F = 2 and corresponds to P and T the phase rule tells us that all of the compositional variables for each phase are fixed

10. Graphical representation of the chemistry of mineral assemblages in metamorphic rocks
Fig illustrates the positions of several common metamorphic minerals on the ACF diagram. Note: this diagram is presented only to show you where a number of important phases plot.
It is not specific to a P-T range and therefore is not a true compatibility diagram, and has no petrological significance

11. Chemographic Diagrams for Metamorphic Rocks
Most rocks/minerals contain the major elements: SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO, MnO and H2O such that number of components in the system is large.
Three components is the maximum number that we can easily deal with in 2-D (ie. a triangular diagram)
Some simplifying methods: (lumping of components)
A = Al2O3 + Fe2O3 - Na2O - K2O
C = CaO P2O5
F = FeO + MgO + MnO
All 9 is clearly too complex. Must simplify if we want to display the system in a convenient graphical way
Four components requires 3-D tetrahedra, and we lose even a semi- quantitative sense of depth in the diagram
More than four components is much too complex to be useful

13. A typical ACF compatibility diagram, referring to a specific P-T condition (the kyanite zone in the Scottish Highlands)
Green area is bulk composition
of metabasaltic (mafic) rocks
What are the common assemblages for kyanite zone (amphibolite facies)??
Plag + Alm + Hbl
Plag + Hbl
Plag + Hbl + Diop
Hbl + Alm
Plot all phases and connect coexisting ones with tie-lines
The composition of most mafic rocks fall in the hornblende-plagioclase field or the hornblende- plagioclase-garnet triangle, and thus most metabasaltic rocks occur as amphibolites or garnet amphibolites in this zone
More aluminous rocks develop kyanite and/or muscovite and not hornblende
More calcic rocks lose Ca-free garnet, and contain diopside, grossularite, or even calcite (if CO2)
We again see how the diagram allows us to interpret the relationship between the chemical composition of a rock and the equilibrium mineral assemblage

14. Different protoliths have different assemblages at specific P-T conditions
metacarbonates
metabasalts
metapelites
Plot all phases and connect coexisting ones with tie-lines
The composition of most mafic rocks fall in the hornblende-plagioclase field or the hornblende- plagioclase-garnet triangle, and thus most metabasaltic rocks occur as amphibolites or garnet amphibolites in this zone
More aluminous rocks develop kyanite and/or muscovite and not hornblende
More calcic rocks lose Ca-free garnet, and contain diopside, grossularite, or even calcite (if CO2)
We again see how the diagram allows us to interpret the relationship between the chemical composition of a rock and the equilibrium mineral assemblage

15. 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

16. 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

17. Chemographic Diagrams
Biotite Isograd
Chlorite + K-feldspar  Biotite + Muscovite (phengitic)
400 – 425°C
Chemographic Diagrams

18. AFM Diagram Muscovite and quartz must be present in the assemblage
What is the assemblage if protolith is “x”?
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
Sil + St + Bt + Qtz + Ms

19. different diagrams are separated by metamorphic reactions
Basic 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

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

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

22. Ky Sil 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 rocks could further restrict P&T Ky
Sil
And

23. blueschist
greenschist
granulite
amphibolite
eclogite
H to J