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Concept of Index Minerals Index Minerals
Chlorite, biotite, garnet, kyanite, sillimanite
Only exist over a narrow P-T range
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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)
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Biotite Isograd• Chlorite + K-feldspar Biotite + Muscovite (phengitic)
• 400 – 425°C
Chemographic Diagrams
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Chemographic Diagrams
Plot the following “minerals”
on an “XYZ” diagram
xz, xyz, and yz2
• Graphical representation of the chemistry of mineral assemblages in metamorphic rocks
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Note that this subdivides the diagram into 5 sub-triangles
2-phase tie line3-phase field
What is the stable mineral assemblage in (A)?
x-xy-x2z
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(A) through (E) might represent the protolith bulk chemistry
2-phase tie line3-phase field
xy-x2z-xyz
What is the stable mineral assemblage if protolith chemistry = (B)?
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A diagram in which some minerals exhibit solid solution
2-phase tie lines
3-phase field
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2-phase tie lines
3-phase field
if protolith chemistry = (f), What is the stable mineral assemblage?
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if protolith chemistry = (f), What is the stable mineral assemblage?
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Graphical representation of the chemistry of mineral assemblages in metamorphic rocks
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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 - 3.3 P2O5 F = FeO + MgO + MnO
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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)??
1) Plag + Alm + Hbl
2) Plag + Hbl
3) Plag + Hbl + Diop
4) Hbl + Alm
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Different protoliths have different assemblages at specific P-T conditions
metabasaltsmetapelites
metacarbonates
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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
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A + B C + D
This is called a tie-line flip, and results in new mineral assemblages in the next metamorphic zone
At the isograd
Above the isograd
Below the isograd
Bulk rock compositionlow P-T
high P-T
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Biotite Isograd• Chlorite + K-feldspar Biotite + Muscovite (phengitic)
• 400 – 425°C
Chemographic Diagrams
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AFM Diagram
Muscovite and quartz must be present in the assemblage
What is the assemblage if protolith is “x”?
Sil + St + Bt + Qtz + Ms
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each diagram represents stable assemblages at fixed P & T
different diagrams are separated by metamorphic
reactions
different assemblages = different bulk X
AFM basics
Basic P-T Application
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Example: Over what P-T range is the assemblage Gar+Chl+Bio stable?
chl
gar
bio
Getting P-T constraints
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Where in P-T space does this assemblage occur?
Step 1: find AFM range for assemblage H
I J
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This is the only part of P-T space where gar+chl+bio can coexist
Al2SiO5 in nearby rocks could further restrict P&T
Ky
Sil
And
H to J
Step 2: use AFM labels to find P-T field
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blueschist
greenschist
granulite
amphibolite
eclogite
H to J