Eclogite-facies metapelites of the Worcester Mountains: A new Taconic high-pressure locality in the Appalachian orogen

Polymetamorphosed mafic and pelitic schists of the Worcester Mountain Complex in Stowe, Vermont, northeastern United States, contain a record of subduction zone metamorphism during the Ordovician Taconic orogeny. These rocks occur south of, and along strike of, the Taconic high- and ultrahigh-pressure rocks of Belvidere Mountain and Tillotson Peak. Because the Worcester Mountain Complex preserves minerals that formed during the Taconic orogeny, this key location is ideal to test the hypothesis that additional localities in the northern Appalachian Mountains reached high-pressure metamorphic conditions during the Taconic orogeny. We present trace-element thermobarometric data on inclusions in garnet (Zr-in-rutile; Ti-in-quartz), phengite and biotite 40Ar/39Ar data, and phase equilibria calculations for pelitic schist samples from the Worcester Mountain Complex. Results indicate garnet growth occurred during two distinct episodes that can be correlated with regionally recognized structural fabrics. Garnet cores nucleated at amphibolite-facies conditions (∼0.8 GPa and ∼520 °C), whereas garnet mantles/rims grew at eclogite-facies conditions ranging from 1.35 GPa to 1.75 GPa and 630−695 °C. Phengitic white mica (up to 3.22 Si atoms per formula unit) and biotite preserve Taconic 40Ar/39Ar ages (469−459 Ma) that are concordant with 40Ar/39Ar ages on glaucophane from Tillotson Peak. Our peak pressure estimates are significantly higher than previous constraints and further demonstrate that rocks within the Taconic suture zone in the northern Appalachian Mountains preserve evidence for eclogite-facies metamorphism. Reevaluation of high-pressure metamorphic rocks within the Taconic suture zone in the Appalachian orogen is warranted to document their distribution, and to determine how they formed and how they were exhumed.