Abstract

New U-Pb zircon ages of selected rocks from the Bronson Hill anticlinorium in southern New England are as follows: Microcline gneiss of the Dry Hill Gneiss in the Pelham dome, believed to be metamorphosed alkali rhyolite, is confirmed as Late Proterozoic, yielding an age of 613 ± 3 Ma. Another sample shows strong evidence of zircon inheritance with a minimum mean age of 1402 Ma. Plagioclase-rich gneisses with associated amphibolites (Swanzey, Pauchaug, Monson, and Fourmile gneisses), now believed to represent metamorphosed intrusive igneous rocks, yield ages from 454 to 442 +3/-2 Ma, thus ranging from early Late through latest Ordovician, consistent with other absolute ages of the Oliverian plutonic series in the region. Quartz-phyric rhyolite from the upper member of the Ammonoosuc Volcanics and rhyolite tuff from the Partridge Formation are dated at 453 ± 2 Ma and 449 +3/-2 Ma, respectively, indicating that eruption of felsic lavas in the overlying volcanic section spanned an interval of approximately 4 m.y. Additional zircon ages are reported from a crosscutting but deformed tonalite dike in the Swanzey Gneiss (381 ± 3 Ma) and a coarse-grained gabbro from the Prescott Intrusive Complex (407 +3/-2 Ma) intrusive into the Partridge Formation. Metamorphic overprinting of the region during both the Acadian and Alleghanian orogenies is indicated by U-Pb titanite ages of 357 ± 5 Ma and 292 ± 5 Ma from a tonalite gneiss on Prescott Peninsula and the Dry Hill Gneiss, respectively.

The above ages 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.

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