New U-Pb age determinations and field observations combined with published field relations refine the late Taconian and pre-Acadian history of the New England Appalachians of southwestern Connecticut. Foliation within the Camerons Line shear zone was developed after 514 ± 11 Ma but prior to 445 ± 1.5 Ma. This structure probably experienced renewed movement in the Early Silurian (437 ± 2 Ma), as suggested by new xenotime growth in protomylonite. Metasedimentary and metavolcanic rocks east of Camerons Line were deposited, were isoclinally folded, and reached temperatures in excess of about 550 °C before emplacement of small dioritic plutons of the 453 ± 3 Ma Brookfield plutonic series. Igneous titanites in Brookfield plutons cooled below U and Pb diffusion (e.g., 525 ± 25 °C) in the latest Ordovician to earliest Silurian and were not reset in the Acadian. The Newtown gneiss is a composite magmatic complex composed of quartz diorite (446 ± 2 Ma), granodiorite (436 ± 2 Ma), and granite (438 ± 2 Ma). Low 207Pb/204Pb feldspar isotopic compositions and 1150 Ma zircon inheritance in the Newtown gneiss and Brookfield plutonic series are consistent with interaction with Grenvillian crust or crustally derived material.
We propose that the Newtown and Harrison gneisses, and possibly the Brookfield plutonic series, make up the plutonic roots of a Late Ordovician to Early Silurian (454–438 Ma) magmatic arc complex generated during subduction along the eastern North American continental margin. Magmatic activity was coeval with that in the Bronson Hill anticlinorium to the east (ca. 454–442 Ma).