PROVENANCE OF THE NARRAGANSETT BASIN IN NEW ENGLAND USING DETRITAL ZIRCON ANALYSIS

The Narragansett basin stretches from the eastern edge of Rhode Island into parts of southeastern Massachusetts and contains the only Carboniferous strata in New England, making it a rare sedimentary record of the transition from the Acadian to the Alleghenian orogeny. Sedimentary rocks within the basin are continentally derived and were deposited on plutonic and metamorphic basement rocks of the Avalon terrane. The lower units of the basin include the Wamsutta Formation, Pondville Conglomerate, Sachuest Arkose, and Purgatory Conglomerate, although their relationships to one another are not entirely clear. These units are overlain by the Rhode Island Formation and Dighton Conglomerate. Plant fossils indicate that most of these units are Pennsylvanian in age (Murray et al., 2004), although the base of the Wamsutta Formation contains a Devonian rhyolite lava (Thompson and Hermes, 2003). The basin was affected by Alleghenian deformation and metamorphism shortly after deposition ceased.

Detrital zircons ages from the four lower units all show main age peaks around 600-620 Ma and a minor one around 400 Ma, although Wamsutta and Purgatory differ slightly in that they also contain a scattering of >1 Ga grains. The Rhode Island Formation is defined by detrital zircon ages between 425 and 490 Ma while the apparently youngest unit of the basin, the Dighton Conglomerate, has age peaks at 640 and 760; both units also have a few >1 Ga grains. None of the units of the basin seem to contain zircon derived from the underlying rhyolite lava. Trace element compositions of zircons also distinguish the Rhode Island Formation from the other units in that zircons from the former appear to be derived from more evolved and more reduced melts than the other units. Hf isotope analyses yield epsilon Hf(i) values primarily between -5 and +5 for the main Paleozoic and Neoproterozoic ages populations.

Analysis of possible sediment sources is ongoing, but our preliminary interpretation is that most detritus is drawn from southern sources, including proximal plutons and older metasedimentary rocks along the southern coast, although distal African sources cannot be discounted. Conversely, the Rhode Island Formation appears to consist of material derived from plutonic belts to the north, near the MA-NH border.