REFINED AGES OF PALEOZOIC ROCKS IN RHODE ISLAND AS CONSTRAINTS ON THE FORMATION OF THE APPALACHIAN HOPE VALLEY TERRANE IN SOUTHEASTERN NEW ENGLAND

The geology of southeastern New England has proved difficult to understand despite a century of intensive study. In recent years, new isotopic data has revealed fundamental flaws in the long accepted regional stratigraphy. Integrated petrologic, geophysical, and geochronological studies necessitate radical tectonic re-thinking in the New Hampshire and Massachusetts portion of the Southeastern New England Avalon Zone. In Southeastern New England the Late Proterozoic basement can be divided into distinct terranes, the Esmond-Dedham and the Hope Valley. The Esmond-Dedham terrane consists of Late Proterozoic to middle Palaeozoic rocks, including the Late Proterozoic Ponaganset, granitic augen gneiss and the Devonian Scituate granite gneiss of western and central Rhode Island. The Hope Valley terrane contains Late Proterozoic Hope Valley alaskite and other gneisses, as well as intrusions of the Permian Narragansett Pier granite, but no rocks of middle Palaeozoic age. This separation is deeply rooted in published U-Pb zircon upper-intercept dates obtained by Robert Zartman, Richard Naylor and Don Hermes in the 1980’s. None of these ages, however, meets the standards of modern geochronology.

Here we present revised dates for four key areas of both terranes in southern Rhode Island, namely Ten Rod granite (TRG), Hope Valley alaskite (HVA), Stepping Stone gneiss (SSG) and Scituate granite (SG). Our new preliminary CA-TIMS U-Pb analyses on single zircons allowed us to separate 4 separated magmatic events over the area: ~506 Ma (TRG), ~365 Ma (SSG), ~345 Ma (SG), and ~336 Ma (HVA). These dates are not only significantly more precise than the previously published dates, but also lie entirely outside the time span that the previous dates imply.

Our results document indicate beside a close Avalonian affinity of the Ten Rod granite, a series of closely spaced, short lived intrusive events that contributed to the development of the Hope Valley terrane. Our data also suggest a similarity, on the basis of ages of basement and geochemistry, between the Hope Valley terrane and the Putnam-Nashoba terrane to the north. The present geographical relationship of the Putnam-Nashoba terrane and Bronson Hill anticlinorium suggests that they may be reflective of similar tectonic environments may be parts of the same terrane.