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Pelitic Schists: Fun with AFM Diagrams

c. Pelitic Schists: Fun with AFM Diagrams. AFM basics Getting P-T constraints Comparison with mafic rocks Interpreting complex assemblages Sample localities…. Jane Selverstone, University of New Mexico. each diagram represents stable assemblages at fixed P & T.

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Pelitic Schists: Fun with AFM Diagrams

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

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

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

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

  5. 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. H to J kya sill Al2SiO5 in nearby rx could further restrict P&T and 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

  9. eclogite blueschist epid amphib greenschist amphibolite granulite 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. 4. “Too many AFM minerals”: Interpreting complex assemblages staur gar chl bio What are the possible interpretations??

  11. d) Other possibilities… ? …look for textural evidence of rxn …chl replacing Grt, St, or Bt; don’t plot chl …no textural evidence of rxn …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

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

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