TECTONIC SETTING(S) OF DEVONIAN MAGMATIC ROCKS IN SE NEW ENGLAND AND THE GULF OF MAINE

Analyses of accretionary history in the northern Appalachians are complicated by the presence of magmatic suites with overlapping ages but contrasting tectonic settings. Devonian magmatism, for example, appears to reflect both Acadian and Neoacadian events. Acadian events include ca. 415-395 Ma progressive shallowing of the subducted Avalonian slab after the juxtaposition of Avalonia and Ganderia and ca. 395-375 Ma break-off of that slab; Neoacadian docking of Meguma and outboard Rheic Ocean subduction under Meguma began in the Middle Devonian and continued into the Carboniferous (ca. 395-350 Ma). Expanding databases of U-Pb geochronology and geochemistry from Devonian plutonic and volcanic rocks of Massachusetts and Rhode Island and their offshore counterparts in the Gulf of Maine are beginning to demonstrate their place in this scenario.

Early Devonian plutons between ca. 417 Ma and ca. 400 Ma in SE New England include newly dated alkalic granite and syenogranite (Cashes Ledge Igneous Suite) in the Gulf of Maine and the Quincy Granite in Massachusetts (erroneously constrained as Ordovician by early U-Pb methods). Within-plate characteristics of these rocks as well as the ca. 392 Ma Salem Gabbro suggest that an extensional regime had already developed in this part of Avalonia during the Acadian and that it continued throughout Neoacadian convergence of Meguma and Avalonia.

Late Devonian magmatic rocks in SE New England (including the Scituate Plutonic Suite, rhyolite from the Wamsutta Formation, the Peabody Granite, felsic tuffs from the Gulf of Maine and minor gabbro) formed between 384 Ma and 370 Ma, within the interval of Meguma docking. However, all of these suites are dominated by peralkaline, A-type and within-plate compositions that contrast with volcanic arc and syn-collisional signatures of Meguma granites. These geochemical distinctions suggest that Avalonia in SE New England occupied a back-arc position during Meguma docking and Rheic Ocean subduction. Extensional shear zones confined to the Scituate pluton and neighboring units are consistent with dextral motions at this time. Contemporaneous arc-related plutons in more inboard terranes (e.g. ca. 382 Ma Webhannet Granite in the Merrimack belt and ca. 385 Ma Straw Hollow Diorite in the Nashoba terrane) may record Avalonian slab break-off.