Location Road cuts are on the approach ramp to New Hampshire 101 from New Hampshire 101A (Fig. 1), 2.2 mi (3.5 km) east of intersection of New Hampshire 101A and 13 (center of Milford) and 0.5 mi (0.8 km) west of intersection of New Hampshire 101A and 122. Because the approach ramp is a busy thoroughfare, vehicles should be parked at the north end of the outcrop, either on the shoulder of New Hampshire 101A or in the parkinglot of a nearby store about 300 ft (100 m) east on 101A. Thethree outcrops can be reached on foot; there is space of about 10to 20 ft (3 to 6 m) between the edge of the road and the verticalfaces.

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.

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.

Geologic mapping and U-Pb geochronology by ion microprobe on zircon, titanite, and monazite in the New Milford quadrangle, western Connecticut indicate Meso-proterozoic events at ca. 1.3, 1.05, and 0.99 Ga in the Laurentian basement rocks. Pink granite gneiss (1311 ± 7 Ma) intruded a paragneiss sequence during the early stages of the Elzevirian orogeny. During the Ottawan orogeny, syn-tectonic anatexis produced a belt of stromatic migmatite at 1057 ± 10 Ma. Ottawan igneous activity included syn-tectonic intrusion of abundant sills of biotite granite gneiss, dated at 1050 ± 14 and 1048 ± 11 Ma, and intrusion of the Danbury augen granite at 1045 ± 8 Ma. Overgrowths on igneous zircon and metamorphic zircon in hornblende gneiss indicate that terminal Grenville metamorphism occurred at ca. 993 ± 8 Ma. Late Ordovician syn-tectonic events included intrusion of a leucogranite dike into the Brookfield Gneiss at 453 ± 6 Ma and intrusion of the Candlewood Granite at 443 ± 7 Ma. A monazite age from the Candlewood Granite of 445 ± 9 Ma agrees with the zircon age. A second phase of migmatization in the basement rocks is associated with the injection of numerous granitic leucosomes at 444 ± 6 Ma along the margin of the Candlewood Granite. Titanite ages from 431 to 406 Ma indicate several high-grade heating events from the Silurian to the Early Devonian. The lack of Grenville-age titan-ite in the basement suggests that post-Taconian heating was sufficient to completely reset old titanite in the massif.