The distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the Acadian Orogeny to have been approximately those of the Al 2 SiO 5 invariant point, thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the Al 2 SiO 5 invariant point of Holdaway (1971). Four fluid-independent equilibria yield P-T estimates consistent with Holdaway (1971). Two fluid-dependent equilibria yield internally consistent but unrealistically high temperatures. Our data are consistent with the conclusions that: (1) only pyrope-grossular mixing in the quaternary garnet system is significantly non-ideal; and (2) the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point P-T conditions.--Modified journal abstract.

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