TACONIAN SUBDUCTION: PRESSURE, TEMPERATURE PATHS PRESERVED IN METAMORPHOSED MAFIC ROCKS FROM NORTHERN VERMONT AND ADJACENT QUEBEC

To explore the tectonic history preserved in metamorphosed mafic rocks from northern Vermont and adjacent Quebec, we present their pressure, temperature paths as determined from modal space calculations and geothermobarometry. In northern Vermont and Quebec low temperature, medium – high pressure metamorphism is preserved in extensional volcanics of the Tibbit Hill Fm. Higher temperature metamorphism of similar age (470 Ma) is medium pressure in the Underhill Fm. and increases to medium high to high pressure facies east across the Honey Hollow Fault to the Hazens Notch Fm. Medium high pressure metamorphism also extends east of the Burgess Branch Fault, and to the south, the Whitcomb Summit Thrust. In the Worcester Mountains west of the Coburn Hill Thrust, however, progressive metamorphism is medium pressure. Subsequent retrograde, greenschist facies metamorphism occurs from the Tibbit Hill Fm. east to the Stowe Fm.

The highest pressure in this terrane is recorded in MORB from the Tillotson Peak Complex (TPC) where inclusions in garnet porphyroblasts record progressive metamorphism along the reaction chlorite + albite + quartz = glaucophane + paragonite + H₂O. Using mineral analyses and the computer program TWEEQ (Berman, 1994), 11Kb, 500^oC was calculated for garnet cores and 13Kb, 560^oC for garnet rims in contact with omphacite. North of Hazens Notch lower temperature, medium-high pressure facies series metamorphism is preserved. Amphibole cores similar in composition to sodic-calcic amphibole in the TPC occur in the Belvidere Mountain Complex to the south (dated at 500 Ma) and in the Hazens Notch, Pinney Hollow and Ottauquechee Fms. farther south. The juxtaposition of these P-T paths can be explained by the imbrication of thrust slices along a Taconian subduction zone.