HIGH PRECISION U-PB ZIRCON DATES FOR EDIACARAN GRANITOID ROCKS IN SE NEW ENGLAND: REVISED MAGMATIC CHRONOLOGY AND CORRELATION WITH OTHER AVALONIAN TERRANES

Southeastern New England is generally equated with peri-Gondwanan Avalonian terranes in Newfoundland and Maritime Canada where ca. 620 Ma magmatism is a characteristic component of Neoproterozoic sequences. This correlation is deeply rooted in published U-Pb zircon upper-intercept dates of 630 ± 15 Ma from the Dedham and Milford granites surrounding Boston, Massachusetts and 621 ± 8 Ma from the Esmond Granite in Rhode Island. These dates (obtained by Robert Zartman, Richard Naylor and Don Hermes in the 1980’s) revolutionized the understanding of southeastern New England geology by demonstrating the widespread presence of Precambrian rather than Paleozoic granitoid rocks as previously assumed. None of these ages, however, meets the standards of modern geochronology.

Revised dates are reported here for all of the above plutons and for the Fall River Granite in SE Massachusetts. Our new U-Pb analyses on single zircons pre-treated by the chemical abrasion technique (CA-TIMS of Mattinson, 2005) produce weighted mean ²⁰⁶Pb/²³⁸U dates (2-sigma errors) of 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, and 604.4 ± 1.2 Ma for the Fall River Granite. Data from the Esmond Granite show considerable scatter between 599.86 ± 0.26 Ma and 598.37 ± 0.45 Ma, indicating a complex crystallization history that would require more work to resolve. We regard 599 ± 2 Ma as a reasonable estimate of the crystallization age for this body. These dates are not only significantly more precise than the previously published dates, but also lie entirely outside the time span that the previous dates imply.

Our results document a series of closely spaced intrusive events that contributed to the batholithic expanse of granitoid rocks in southeastern New England. Integrating the new results with less precise, previously published dates from the Westwood, Cohasset, and Dedham Granite North of Boston suggests that the collective Avalonian plutonic suite developed between about 610 Ma and 590 Ma, a much shorter interval than inferred from previous U-Pb dates. 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.