TOWARDS BETTER METAMORPHIC PEAK P-T ESTIMATES: COMBINING FORWARD AND BACKWARD THERMODYNAMIC MODELING

Pseudosections (forward thermodynamic modeling) are a powerful technique to estimate peak P-T conditions, P-T paths, and reaction history. Results are not affected by garnet fractionation at <5% garnet. The technique is most useful in rocks with low variance assemblages because these typically have small P-T ranges. We present a series of MnNCKFMASH pseudosections used to constrain the metamorphic peak P-T conditions in metamorphic rocks of the Cascades Core (WA) and Picuris Range (NM). In the Cascades Core, traditional thermobarometry (backward thermodynamic modeling) gives uncertainties that overlap with P-T estimates for initial garnet growth, obscuring garnet core to rim P-T paths. Pseudosection mineral assemblages can provide a more precise peak P-T estimate and a garnet core to rim P-T path. For example in 01NC015b, the peak temperature range is narrowed from 110 to 40ºC (680-720ºC) and the peak pressure range is narrowed from 2 to 1 kbar (8.5-9.5 kbar). In the Picuris range pseudosections were constructed in unusual rock compositions where traditional thermobarometry is not useful. The results of the peak P-T estimates in these rock compositions are comparable to those of pelitic rock compositions using traditional thermobarometry. In sample 00NM01, the pseudosection was used to constrain the temperature of formation to 500-550 ºC. Pseudosections can be used to estimate P-T conditions provided that the analyzed bulk composition is the effective rock composition at the time of the metamorphic peak conditions. Assuming closed system behavior, the presence of strongly zone minerals (e.g. garnet and plagioclase) would change the effective rock composition from that of the bulk rock composition. Garnet fractionation in a pelitic rock from the Cascades Core with garnet fractionation up to 5% garnet produces negligible changes in the P-T location of the peak mineral assemblage field. P-T pseudosections are characterized by large area - high variance assemblage fields and by small area - low variance assemblage fields. In the samples studied, assemblage fields with variance of 4 or less can provide reasonably precise P-T estimations, while those with variance of 5 or more are too broad to be useful for P-T determinations, but can be used to estimate maximum P or T where paragenesis are not useful for thermobarometry.