LOOK AT YOUR ROCK: USING DETAILED PETROGRAPHY TO GROUNDTRUTH THERMODYNAMIC MODELING IN THE WEPAWAUG SCHIST, THE MIDDLETOWN FORMATION, AND BEYOND

Since its inception, the field of metamorphic petrology has used equilibrium thermodynamics as a framework for understanding rock evolution (e.g. Goldschmidt, 1914). However, the dual complications of disequilibrium and open- system processes have been recognized just as long (e.g. Eskola 1920, Boeke 1915). Both Robert Wintsch and the rocks he studies can be can be described as often out-of-equilibrium and occasionally affected by fluid. Fortunately, Bob’s most important lesson provides an effective means to identify and understand these complexities: Look at your rock.

With this in mind, two examples from Connecticut are discussed. The first locale is the Wepawaug Schist in the Orange-Milford belt where both disequilibrium cleavage textures (Growden et al. 2013) and extensive fluid infiltration (Ague 2002) are observed. The metacarbonate layers are composed of an ankerite-albite zone, a biotite zone, a Ca-amphibole zone, and a Diospide zone, from lowest to highest-grade (e.g. Hewitt 1973). Closed-system pseudosection models completely fail to reproduce the observed mineral assemblages, while pseudosections accounting for aqueous fluid infiltration successfully reproduce mineral assemblages and compositions. Remarkably, closed-system models underestimate the evolution of metamorphic carbon-dioxide by more than 500%. By accounting for fluid infiltration-driven decarbonation, Stewart & Ague (2018) estimate a regional carbon-dioxide flux that may have been sufficient to perturb climate in the mid- to late- Devonian.

To the northeast, amphibolites of the Middletown formation in the Bronson Hill terrane exhibit dramatic disequilibrium textures. Within the foliation, bands of tremolite + albite appear to crosscut the assemblage pargasite + garnet. Both amphiboles and garnets exhibit distinct chemical zonation. Preliminary modeling suggests that the two different assemblages did not form at the same P-T conditions and that each represents a local equilibrium at a scale of less than ~1 cm.