Age and setting of the Bronson Hill magmatic arc: a re-evaluation based on U-Pb zircon ages in southern New England

New U-Pb zircon ages of selected rocks from the Bronson Hill Anticlinorium in southern New England are presented. Those and other zircon ages, as well as petrography and geochemistry, of the Fourmile, Monson, Swanzey, Pauchaug, and other plagioclase-rich gneisses of the Bronson Hill Anticlinorium suggest that they represent the plutonic root of a calc-alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in Late Ordovician time (ca. 454-442 Ma). The physically overlying cover sequence of chemically different tholeiitic arc-volcanic and sedimentary rocks of the Ammonoosuc Volcanics and Partridge Formation, previously thought to rest unconformably on the plagioclase-rich plutonic gneisses, is now shown to have had a magmatic history overlapping with them. The zircon ages in the plutonic gneisses and the cover sequence indicate that both are the same age or slightly younger than the Late Ordovician (Caradocian) emplacement of the Giddings Brook slice of the Taconian allochthons, and too young to have formed above an early and medial Ordovician subduction zone. -from Authors