1. Pelitic Schists: Fun with AFM Diagrams
AFM basics
Getting P-T constraints
Comparison with mafic rocks
Interpreting complex assemblages
Sample localities… c
Jane Selverstone, University of New Mexico

2. different diagrams are separated by univariant reactions
1. AFM basics
each diagram represents stable assemblages at fixed P & T
different diagrams are separated by univariant reactions
different assemblages = different bulk X

3. Topological changes from 500-650°C at constant P=6 kbar
Watch for terminal and nonterminal reactions….

4. AFM requirement: Qtz + Musc
kspar
sill
musc
qtz
…but musc breaks down at high T
Musc + Qtz = AS + Kspar + H2O
gar
kspar
sill
Kspar
AFM requirement: Qtz + Musc

5. If you don’t see musc, look for Kspar!! Muscovite
Musc projection
Kspar projection
If you don’t see musc, look for Kspar!!
Muscovite
K-feldspar

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

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

8. kya sill 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 rx could further restrict P&T
kya
sill
and

9. 3. Correlation with mafic rocks
blueschist
greenschist
granulite
amphibolite
eclogite
epid amphib
3. Correlation with mafic rocks
What is the equivalent metamorphic facies?
epidote amphibolite
What assemblage would be present in interlayered mafic rocks?
epid/czo + hbl+ plag

10. staur gar chl bio 4. “Too many AFM minerals”:
Interpreting complex assemblages
staur
gar
chl
bio
What are the possible interpretations??

11. ? …observations lead to interpretations
a) Reaction frozen in, V=1
b) Extra component, e.g. Mn, Ca in Grt or Zn in St
c) Retrograde mineral, e.g. chlorite
d) Other possibilities…
…observations lead to interpretations ?
…look for textural evidence of rxn
…chl replacing Grt, St, or Bt; don’t plot chl
…no textural evidence of rxn

12. Insert info here on the tectonic settings/metamorphic histories of your own samples