Newly recognized blueschist-facies metamorphism (glaucophane-omphacite-garnet), Belvidere Mountain Complex, northern Appalachians

An occurrence of blueschist-facies metamorphism in the Appalachian orogen is newly recognized in northwestern New England, United States. Inclusions of glaucophane and omphacite occur in a relict garnet core from a retrogressed garnet-barroisite amphibolite of the Belvidere Mountain Complex in Vermont. Pressure-temperature pseudosection and mineral composition isopleth calculations demonstrate that the Belvidere Mountain Complex blueschist-facies mineral assemblage of glaucophane–magnesio-hornblende–omphacite–chlorite–rutile–quartz–clinozoisite–garnet was stable at ~1.65–2.0 GPa and ~450–480 °C. Garnet-absent amphibolite with barroisite and chlorite inclusions in clinozoisite records high-pressure epidote-amphibolite–facies metamorphism at ~1.0–1.4 GPa and ~515–550 °C. These new findings quantify deep subduction of the Belvidere Mountain Complex during the Cambrian to Ordovician Taconic orogenic cycle and suggest that more blueschist-facies mineral assemblages could be revealed in the Appalachians with detailed analysis of retrogressed rocks.