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
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
Topological changes from 500-650°C at constant P=6 kbar Watch for terminal and nonterminal reactions….
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
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
2. Getting P-T constraints chl gar bio Example: Over what P-T range is the assemblage Gar+Chl+Bio stable?
H I J Step 1: find AFM range for assemblage Where in P-T space does this assemblage occur?
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
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
staur gar chl bio 4. “Too many AFM minerals”: Interpreting complex assemblages staur gar chl bio What are the possible interpretations??
? …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
Insert info here on the tectonic settings/metamorphic histories of your own samples