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

Abstract

Abstract An occurrence of blueschistfacies 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 garnetbarroisite amphibolite of the Belvidere Mountain Complex in Vermont. Pressuretemperature pseudosection and mineral composition isopleth calculations demonstrate that the Belvidere Mountain Complex blueschistfacies mineral assemblage of glaucophane–magnesio-hornblende–omphacite–chlorite–rutile–quartz–clinozoisite–garnet was stable at ~1.65–2.0 GPa and ~450–480 °C. Garnetabsent amphibolite with barroisite and chlorite inclusions in clinozoisite records highpressure 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 blueschistfacies mineral assemblages could be revealed in the Appalachians with detailed analysis of retrogressed rocks.