UHP, UHT, AND HPG METAMORPHISM IN THE ACADIAN/NEOACADIAN OROGEN, APPALACHIANS: INDICATORS AND IMPLICATIONS

Metamorphic rocks of the Appalachian Orogen have been studied for over 100 years, but modern work continues to shape petrological and tectonic understanding of this world-class mountain belt. In the last decade, study of the Acadian/Neoacadian part of the orogenic belt has yielded discoveries of metamorphic rocks formed at extreme conditions, such as ultrahigh-pressure (UHP) metasedimentary gneisses (Keller and Ague 2020), ultrahigh-temperature (UHT) granulites (Ague et al. 2013; Axler and Ague 2015), and high-pressure granulites (HPGs; Keller and Ague 2018). These findings have prompted reconsideration of long-standing models for Appalachian orogenesis and brought exciting interdisciplinary problems, such as relamination of subducted metasedimentary material, to discussions of Appalachian tectonism. We compare the extreme Acadian metamorphic rocks to those from other mountain belts worldwide, such as from the sibling Caledonian Orogen, and use the constraints on P-T conditions from these Acadian examples to discuss processes at and below the orogenic root during Acadian/Neoacadian deformation. We also discuss signpost indicators of extreme metamorphism from the Acadian rocks that may yield further illuminating discoveries in the Appalachians as a whole.