THE TECTONIC CONSEQUENCES OF HIGH-PRESSURE GRANULITE AND ULTRAHIGH-PRESSURE ROCKS IN THE ACADIAN/NEOACADIAN CENTRAL MAINE TERRANE

Several different metamorphic rocks recording extreme temperatures and pressures crop out in the Brimfield Schist of the Acadian/Neoacadian Central Maine Terrane (CMT) of Connecticut. Ultrahigh-temperature granulites^[1] point to high levels of heat flow in the mid- to lower crust during orogenesis. The recent discovery of a ~1.8 GPa/1,040 °C high-pressure granulite in the Brimfield Schist^[2] revealed that significantly deeper rocks are exhumed in the thrust slices of the CMT than were previously recognized in the New England Appalachians.

Newly-discovered metasedimentary gneisses from the Brimfield Schist contain garnets with crystallographically-oriented exsolution lamellae of silicates. Lamellae assemblages comprise quartz, amphibole, sodium-phlogopite, apatite, ilmenite, rutile, and mica. Lamellae have crystallographic orientation relationships (COR) with garnet, and garnet and lamellae together preserve the chemical signal of majoritic garnet, indicating equilibration at ultrahigh-pressure (UHP) conditions of ≥5 GPa followed by exhumation and exsolution^[3]. The extreme pressures recorded by the gneisses warrant comparisons to the deeply buried UHP rocks of the European Caledonides which formed concurrently to the Acadian/Neoacadian Appalachians. The continent-continent collision of Baltica with Laurentia formed the Caledonides, whereas Laurentia-microcontinent collisions were responsible for the Acadian and Neoacadian orogenies. The presence of UHP gneisses in the CMT requires renewed scrutiny of the processes responsible for exhuming rocks from the base of the orogenic root during Acadian/Neoacadian orogenesis and should further stimulate comparative tectonic studies of the Appalachians and Caledonides.

References

[1] Ague et al. Geology 2013. [2] Keller and Ague J.J. Am. Mineral. 2018. [3] Keller and Ague Science Advances 2020 (accepted).