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

Plot the following “minerals”

on an “XYZ” diagram

xz, xyz, and yz2

• Graphical representation of the chemistry of mineral assemblages in metamorphic rocks

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

(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)?

A diagram in which some minerals exhibit solid solution

2-phase tie lines

3-phase field

2-phase tie lines

3-phase field

if protolith chemistry = (f), What is the stable mineral assemblage?

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

metabasaltsmetapelites

metacarbonates

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

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

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

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

Example: Over what P-T range is the assemblage Gar+Chl+Bio stable?

chl

gar

bio

Getting P-T constraints

Where in P-T space does this assemblage occur?

Step 1: find AFM range for assemblage H

I J

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

blueschist

greenschist

granulite

amphibolite

eclogite

H to J