WITHIN-PLATE TECTONIC SETTING OF PALEOZOIC ALKALIC SUITES IN SE NEW ENGLAND, USA: CONSTRAINTS FROM CA-TIMS U-PB ZIRCON GEOCHRONOLOGY AND PALEOMAGNETISM

Southeastern New England is best known as the southernmost segment of the peri-Gondwanan “West” Avalonian terrane, composed here of 610-585 Myr arc-related magmatic and sedimentary assemblages overlain by platform deposits extending into the Cambrian. Mid-Paleozoic bimodal alkalic plutons and minor volcanic rocks succeed the Avalonian sequence in a NE-trending belt from Rhode Island into the Gulf of Maine. This magmatism is well known for anorogenic, “within-plate” geochemical signatures, yet why it recurred during more than 70 Myr in the same narrow zone remains unclear. New high-precision U-Pb geochronology (CA-TIMS method) integrated with a compilation of published U-Pb dates and paleomagnetically determined latitudes are used to construct the history of within-plate magmatism during West Avalonia’s northward drift into collision with Laurentia.

West Avalonia moved from 65°S at 490 Myr (paleopole from Nahant, MA) to 41°S at 460 Myr (paleopole from Dunn Point, NS) at a rate of 8-10 cm/yr and slowed to 5-6 cm/yr as it progressed to 31°S at 441 Myr (paleopole from Cape St. Mary’s, NFL). The earliest alkalic suite in southeastern New England, 444 ± 3 Myr alkalic gabbro and related syenite reported near Danvers, MA, was emplaced at this near-Laurentian paleolatitude. By 430 Myr, as proxied by data from the East Mendips Inlier, England for lack of a West Avalonian pole at this time, SE New England was in position to dock with previously accreted arc fragments of the Ganderian Nashoba terrane. The documented dextral component to this this convergence is proposed as a mechanism for intra-plate rifting that localized Danvers plutonism as well as younger alkalic intrusive and volcanic rocks from localities north and south of Boston, MA. The latter have yielded ²⁰⁶Pb/²³⁸U dates that range generally between ca. 426 Myr and 378 Myr (with 2σ analytical uncertainties < 1 Myr) but cluster into three discrete episodes: Late Silurian Cape Ann Granite, associated with syenite and mafic intrusives; Early Devonian Quincy Granite and related volcanic rocks; Late Devonian Wenham Monzonite and Peabody Granite. Respectively, these magmatic phases immediately precede, coincide with and post-date the 421-400 Myr Acadian Orogeny widely accepted as the time of West Avalonian terrane accretion throughout the northern Appalachians.