Note that this subdivides the chemographic diagram into 5 subtriangles, labeled AE

1

• Note that this subdivides the chemographic
  diagram into 5 sub-triangles, labeled (A)-(E)

• x-xy-x2z
• xy-xyz-x2z
• xy-xyz-y
• xyz-z-x2z
• y-z-xyz

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3
Below the isograd
Bulk rock composition
A B ? C D
At the isograd
Above the isograd
This is called a tie-line flip, and results in
new groupings in the next metamorphic zone
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• ACF Diagram

5

• The ACF Diagram

• The three pseudo-components are all calculated on
  an atomic basis
• 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 range of P and T (the kyanite zone in
  the Scottish Highlands)

Green area is bulk composition of metabasaltic
rocks
What are the common assemblages for kyanite zone
(amphibolite facies)??
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• AFM Diagram

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• AFM Diagram

A  Al2O3 K  K2O F  FeO M  MgO
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• AFM Diagram

Project from a phase that is present in the
mineral assemblages to be studied Usually
muscovite (Ms) or K-feldspar (Kfs)
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• AFM Diagram

11
Chapter 28 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

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Chlorite Zone

• lower greenschist facies
• 300 400C

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Biotite Isograd

• tie-line flip (discontinous) reaction type

• Chl Kfs ? Bt phengitic Ms

• 400 425C

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P-T grid
? Biotite Isograd
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Chapter 28 Metapelites
Biotite Zone

• 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 500C

P,T increasing
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Garnet Isograd
Part 1

• tie-line flip (discontinous) reaction type

• Cld Bt ? Grt Chl

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Chapter 28 Metapelites
Garnet Isograd
Part 2

• Continuous reaction type (over a range of P-T)
  involving solid solution

• Chl Bt ? Grt Mg-rich Chl Mg-rich Bt

• This is the garnet isograd for almost all common
  metapelites

• 525 555C

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P-T grid
?? Garnet Isograd
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Chapter 28 Metapelites
Staurolite Isograd
Part 1

• terminal point reaction type
  (chloritoid-out disappearance of chloritoid)

• Cld ? Grt Chl St

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Chapter 28 Metapelites
Staurolite Isograd
Part 2

• tie-line flip (discontinous) rxn

• Chl Grt ? St Bt

• This is the staurolite isograd for almost all
  common metapelites

• 575 600C

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P-T grid
?? Staurolite Isograd
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Part 1
Kyanite Isograd

• tie-line flip (discontinous) rxn

• St Chl ? Ky Bt

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Chapter 28 Metapelites
Part 2
Kyanite Isograd

• terminal point reaction type
  (staurolite-out disappearance of staurolite)

Ky

• St ? Grt Bt Ky

• This is the kyanite isograd for almost all
  common metapelites

• 625-650C

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P-T grid
? Kyanite Isograd
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Chapter 28 Metapelites
Sillimanite Isograd

• polymorphic transition

Sil

• Ky ? Sil

• 650 - 700C

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P-T grid
? Sillmanite Isograd
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Chapter 28 Metapelites
K-feldspar Isograd (2nd sillimanite)

• dehydration reaction

• Ms Qtz ? Sil Kfs H2O

• gt700 - 750C

Kfs

• liberated H2O may cause partial melting

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P-T grid
11 K-feldspar Isograd
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Granulite Facies

• presence of cordierite and/or Opx (depending on
  P)

• Bt Sil ? Grt Crd H2O Bt
  Qtz ? Opx Kfs H2O

• tie-line flip/dehydration reaction

• gt750 - 800 C

• liberated H2O may cause partial melting

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Migmatites

• migmatite mixed rock part igneous, part
  metamorphic

• Breakdown of muscovite and biotite at high
  grades may cause partial melting

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Chapter 28 Metapelites
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Review and P-T Application
AFM basics
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2. Getting P-T constraints
chl
gar
bio
Example Over what P-T range is the assemblage
GarChlBio stable?
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Step 1 find AFM range for assemblage
Where in P-T space does this assemblage occur?
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Step 2 use AFM labels to find P-T field
This is the only part of P-T space where
garchlbio can coexist
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Granulite
Amphibolite
Greenschist
Kfs Zone
Sil Zone
Ky Zone
St Zone
Grt Zone
Bt Zone
Chl Zone
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