Download Chemographic Diagrams for Metamorphic Rocks

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