Pressure and Temperature Estimation in Greenschist-Facies Metapelites: An Example From the Variscan Belt in Sardinia

Abstract

Phase equilibrium modelling is a powerful method for obtaining pressure–temperature (P–T) estimates in metapelites; however, the advantages, disadvantages, and limitations of applying this technique to greenschist-facies pelites have received little attention. This study uses two greenschist-facies metapelites from the Nappe Zone in Sardinia (Italy) to examine differences between currently available thermobarometric techniques. First, this work compares P–T estimates from chlorite–K-white mica–quartz–water and K-white mica–quartz–water multiequilibrium approaches, the latter integrated with previously published T_max results from Raman spectroscopy on carbonaceous material. Consistent P–T conditions are retrieved from these methods for both samples, with P–T estimates suggesting a difference in peak metamorphic conditions between the Internal and External Nappe Zones of Sardinia (peak P–T conditions of 0.6–0.8 GPa, 420°C–450°C and 0.4–0.5 GPa, 370°C–400°C for the Internal and External Nappes, respectively). Second, phase equilibrium modelling was used to assess the ability of six different sets of solution models, alongside their associated thermodynamic datasets, to constrain P–T conditions in the same samples, which are characterized by high-variance mineral assemblages. Significant differences were found between the six thermodynamic databases, and important discrepancies were recognized between the observed and predicted mineral assemblages and compositions (e.g., Si in K-white mica). In only one sample, three databases were able to predict the observed natural assemblage. The compositional isopleths for K-white mica, from the same databases, do not intersect, offering no assistance in constraining P–T conditions. However, one database predicted chlorite's natural composition, yielding a P–T estimate consistent with multiequilibrium methods: ~0.9 GPa, ~350°C–400°C for the Internal Nappe Zone and ~0.5 GPa, ~300°C–350°C for the External Nappe Zone. Although P–T conditions constrained using calculated isopleths from greenschist-facies metapelites and databases optimized for modelling low-grade to medium-grade pelitic systems may be reliable, they should be applied cautiously and compared with estimates from inverse and independent approaches.