ABSTRACT The Avalon terrane of southeastern New England is a composite terrane in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia, and New Brunswick, Canada, based on rock types and ages, U-Pb detrital zircon signatures of metasedimentary rocks, and Sm-Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U-Pb laser ablation–inductively coupled plasma–mass spectrometry analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ca. 647–643 and ca. 745–733 Ma age populations and minor zircon up to ca. 3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane, or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, Massachusetts, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, Massachusetts, and as far west as Warwick, Rhode Island, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or it may have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone.

Location The locality consists of three sites along the western marginof Narragansett Bay, in southern Rhode Island (Figs. 1 and 2). Stop 1 is in the Wickford 7½-minute Quadrangle, while the otherstops are in the Narragansett Pier 7½-minute Quadrangle. Stop 1. Jamestown, Conanicut. Take Rhode Island 138to the eastern end of the Jamestown Bridge, which connects thewestern side of Conanicut Island to the eastern shore of Narragansett Bay. Park near the Jamestown Shores Motel and take thepath down to the shoreline exposures beneath the bridge. At thetime of this writing the bridge is being replaced with a new one, approximately 330 ft (100 m) to the north; however, the directionsgiven here should still apply in the future. There is freeaccess and outcrops are best seen at low tide. Stop 2. Stook Hill, Saunderstown. From Stop 1, proceedwest on Rhode Island 138, over the Jamestown Bridge, to theintersection of Rhode Island 138 and 1A. Continue west on Rhode Island 138 for 1.35 mi (2.2 km) to the Stook Hill roadcuts (Fig. 3), where access is also free. Stop 3. Cormorant Point, Narragansett. Return east to the intersection of Rhode Island 138 and 1A, and take 1A (Boston Neck Road) south 5.8 mi (9.3 km). Turn left (east) and takethe secondary road 0.8 mi (1.3 km) (or seven “roadhumps„) east, and park along the road. The shoreline exposures (Fig. 4) maybereached by walking approximately 0.25 mi (0.4 km) farther to the south,