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.

Late Proterozoic igneous rocks in the Boston-Avalon zone of eastern Massachusetts are dominated by voluminous calc-alkalic intrusives and extrusives that yield radiometric ages between 600 and 650 Ma. This “main phase” Avalonian volcano-plutonic series, which includes the Dedham Granite and Lynn Volcanics, was emplaced over a time span that was both preceded and followed by separate episodes of mafic volcanism. The Middlesex Fells Volcanic Complex temporally preceded the emplacement of the Dedham and Lynn, whereas the Brighton Volcanics, part of the stratified section of the Boston Basin, clearly postdate them. Geochemical study of the mafic members of the Middlesex Fells Volcanic Complex, which were regionally and thermally metamorphosed to the greenschist facies, reveals that these alkaiic and transitional basalts have a geochemical signature characteristic of modern continental rift zones. In contrast, flows, pillows, and pyroclastics of the basalts, basaltic andesites, and andesites of the Brighton Volcanics are high-alumina basalts and andesites of calc-alkaline character resembling those from modern subduction-related magmatic regimes. Magmatism thus provides evidence of the changing tectonic regime during the late Proterozoic in the Avalon terrane of southeastern New England. An initial period of continental rifting preceded the extensive calc-alkaline magmatism associated with the Avalonian orogeny. Subsequently, the terrane experienced a brief period of continental subduction-zone-related magmatism before becoming a stable shelf in the early Paleozoic.