ABSTRACT Southeastern New England is largely composed of Ediacaran granitoid and related volcanic rocks formed during the main phase of arc-related magmatism recorded in West Avalonian lithotectonic assemblages extending through Atlantic Canada to eastern Newfoundland. In situ Lu-Hf analyses presented here for zircons from the Dedham, Milford, and Esmond Granites and from the Lynn-Mattapan volcanic complex show a restricted range of εHf values (+2 to +5) and associated Hf- T DM model ages of 1.3–0.9 Ga, assuming felsic crustal sources. The most evolved granites within this suite lie in a belt north and west of the Boston Basin, whereas upfaulted granites on the south, as well as the slightly younger volcanic units, show more juvenile Hf isotopic compositions. Similar inferences have been drawn from previously published Sm-Nd isotopic signatures for several of the same plutons. Collectively, the isotopic compositions and high-precision U-Pb geochronological constraints now available for southeastern New England differ in important respects from patterns in the Mira terrane of Cape Breton Island or the Newfoundland Avalon zone, but they closely resemble those documented in the Cobequid and Antigonish Highlands of mainland Nova Scotia and New Brunswick’s Caledonia terrane. Particularly significant features are similarities between the younger than 912 Ma Westboro Formation in New England and the younger than 945 Ma Gamble Brook Formation in the Cobequid Highlands, both of which yield detrital zircon age spectra consistent with sources on the Timanide margin of Baltica. This relationship provides the starting point for a recent model in which episodic West Avalonian arc magmatism began along the Tonian margin of Baltica and terminated during diachronous late Ediacaran arc-arc collision with the Ganderian margin of Gondwana.

Terminal Neoproterozoic (Ediacaran) granitoid rocks underlie most of the Southeastern New England Avalon Zone. Major- and trace-element analyses on representative samples from Massachusetts and Rhode Island corroborate earlier interpretations that these rocks were formed in a subduction-related setting. New crystallization ages from the same suite are 609.5 ± 1.1 Ma, 609.1 ± 1.1 Ma, and 608.9 ± 1.2 Ma for units of the Dedham Granite; 606.3 ± 1.2 Ma for the Milford Granite; 604.4 ± 1.2 Ma for the Fall River Granite; and 599 ± 2 Ma for the Esmond Granite (2σ errors, including internal and external uncertainties). The Avalonian magmatic interval defined by these and other reliable dates is ca. 610–590 Ma, which is considerably shorter and younger than previously thought. These dates provisionally link the southeastern New England Avalon zone with the Antigonish and Cobequid Highlands in northern mainland Nova Scotia as distinctive blocks in the Northern Appalachian Avalonian collage.