USE OF PSEUDOSECTIONS FOR PREDICTING METAMORPHIC MINERAL ASSEMBLAGES IN PELITIC ROCKS: EXAMPLES FOR AN AVERAGE PELITE

MnNaCaKFMASH pseudosections, constructed for pelite compositions are useful for predicting metamorphic mineral assemblages and reaction history. Pseudosections were constructed using THERMOCALC (Powell & Holland, 1988) and thermodynamic data of Holland & Powell (1998). Activity models are from Holland & Powell (1998) and THERMOCALC models extended for Mn. Three pseudosections were constructed: I) average biotite zone pelite composition from the Waterville Formation Maine, II) #I with Al₂O₃ concentration increased from 16.88 to 19.65%, and III) #I with K₂O concentration decreased from 3.7 to 2%.

Pseudosection mineral assemblages mostly agree with nature; however, naturally occurring staurolite+cordierite and staurolite+andalusite are not predicted by pseudosections. This may result from modeled Al concentrations and/or Mg/(Fe+Mg). Higher Al and lower Mg/(Fe+Mg) would favor staurolite+andalusite; however, higher Al and higher Mg/(Fe+Mg) would favor cordierite stability. Higher Mg and Al could move a bulk composition to the Mg side of the staurolite-cordierite tie-line in AFM space (Pattison et al., 1999); therefore, allowing the possibility of a staurolite+cordierite+aluminum silicate field. However, our pseudosection prediction of no staurolite+cordierite stability is compatible with a 'polymetamorphic' interpretation of this assemblage (Pattison et al., 1999).

Pseudosection I predicts zoisite at T <425°C @ 3.3 kbar and <525°C @ 7 kbar. Garnet forms at 450-460°C above 5 kbar and would grow with pronounced decrease in Ca after consumption of zoisite along an isobaric heating path. Positive slope of the zoisite-out reaction and predicted garnet zonation is in agreement with observations made in calcic pelites from Vermont (Menard & Spear, 1993) and southeastern Alaska (Stowell & Menard, 1996).

Aluminum silicate minerals are stable at T >540°C @ 2.5 kbar and T >600°C @ 7 kbar. Therefore, andalusite+sillimanite+kyanite would not be stable. This prediction is in agreement with KFMASH phase equilibria (Pattison, 1997) and observations from natural assemblages (Larson & Sharp, 2000). Increased Al (e.g., Pseudo II) shifts aluminum silicate-in reactions to lower T and may stabilize andalusite+sillimanite+kyanite at very high Al concentrations.