REFINED AGES OF PALEOZOIC PLUTONS AS CONSTRAINTS ON AVALONIAN ACCRETION IN SOUTHEASTERN NEW ENGLAND

Avalonian basement in southeastern Massachusetts and Rhode Island is dotted with Paleozoic alkalic plutons reflecting Ordovician, Silurian, Devonian and Carboniferous magmatic episodes according to published U-Pb zircon dates, some of which are crude by modern standards. New CA-TIMS single-zircon U-Pb (weighted mean ²⁰⁶Pb/²³⁸U) dates of 425.97 ± 0.64 Ma for the Cape Ann Granite and 378.08 ± 0.62 Ma for the Peabody Granite represent significant refinements in both precision and accuracy over previous multi-grain results (450 ± 25 Ma and 380 ± 15 Ma, respectively). Such refinements are essential for properly assessing the Paleozoic accretionary history of the Southeastern New England Avalon Zone with respect to the adjacent Nashoba terrane to the west and the coastal Maine along strike to the north.

The Silurian age of the Cape Ann Granite and work in progress on the Quincy Granite, along with re-calibration of the geologic time scale cast doubt on Ordovician magmatism in southeastern New England. Rather, these developments strengthen the pattern of Siluro-Devonian igneous activity known from previous geochronology (427 ± 2 Ma Lexington pluton; 417 ± 6 Ma Franklin pluton, 392 ± 4 Ma Salem gabbro-diorite), as well as fossiliferous Newbury volcanic rocks. This magmatic episode coincides with deformation, metamorphism and plutonism in the Nashoba terrane that has been interpreted in terms of the amalgamation of Ganderian and Avalonian elements prior to docking with Laurentia. Coastal Maine experienced similar activity as composite Avalon converged with and ultimately overthrust inboard sequences of the Fredericton Trough and Central Maine Basin (Acadian orogeny of some workers). Geochemical dissimilarities between Newbury and coastal Maine volcanic rocks have suggested that Siluro-Devonian convergence involved subduction with both frontal arc and extensional back-arc components, and the alkalic characteristics of the plutons in the Southeastern New England Avalon Zone may reveal a deeper level of the latter.

The Peabody Granite falls with other Late Devonian igneous rocks in southeastern New England (378 ± 3 Ma diorite at Waltham, 370 ± 7 Ma Scituate plutonic suite, 373 ± 2 Ma Wamsutta rhyolite). Bodies of comparable age seal the Avalonian terrane boundary in coastal Maine.