OVERINTERPRETATION OF METAMORPHIC PARAGENESES FROM MINERAL ASSEMBLAGE DIAGRAM ANALYSIS, EXAMPLES FROM THE CONNECTICUT VALLEY SYNCLINORIUM, VERMONT

Metapelitic samples from the Connecticut Valley Synclinorium (CVS) display typical St-Ky zone mineral assemblages with potentially deceptive paragenetic sequences. Samples TM-921 and TM-675 contain large 6-7 mm garnet porphyroblasts with ilmenite inclusions in the core and rutile inclusions in the rim. Conventional petrographic and mineral assemblage diagram (MAD) analysis would lead to the conclusion that garnet nucleated in equilibrium (EQ) at the garnet isograd and grew over a clockwise prograde P-T path, during which the rutile isograd was passed and rutile became the stable Ti-oxide phase. An entirely different interpretation is determined when considering disequilibrium conditions and thermobarometric methods that are independent of equilibrium.

To check the validity of this interpretation, quartz inclusion in garnet (QuiG) Raman barometry and zirconium in rutile (ZiR) thermometry were used to independently determine the P-T conditions of garnet and rutile nucleation. The results suggest a different order of events, where garnet and rutile nucleate close to peak P-T conditions after considerable overstepping. Further complications arise when considering the paleoassemblage from which garnet nucleated. Despite reaching the EQ stability fields for both garnet and rutile, garnet must have nucleated from a metastable assemblage containing Chl + Qtz + Ms + Bt + Pl + Ilm ± Ep, as determined by identifying matrix and inclusion phases, suggesting the stability fields from EQ MADs are misleading in this case. Together these indicate that the correct paragenetic interpretation is that garnet and rutile nucleated near peak, but there is no unique P-T path that may be determined from MADs.

This would lead to a conclusion that the P-T path may have crossed the isograd at any number of P-T conditions, and is only loosely constrained by the likely paleoassemblage fields. While this may be the case, comparing MADs with independent thermobarometers provides superior constraints on the P-T of garnet nucleation. More importantly, the magnitude of overstepping necessary for garnet can be determined, which cannot be observed from MADs alone. When combined with independent thermobarometers, MADs may be used to make new observations about prograde metamorphism and determine whether equilibrium nucleation occurred.