MID-PALEOZOIC ARC ACCRETION ON THE EASTERN SIDE OF THE APPALACHIAN OROGEN, EASTERN MASSACHUSETTS AND ADJACENT AREAS

Neoproterozoic to early Paleozoic peri-Gondwanan arcs make up the eastern portion of the Appalachian orogen in eastern Massachusetts and adjacent areas. These arcs were largely ribbon microcontinents juxtaposed with terranes to the west during the mid-Paleozoic, although a model for this juxtaposition is still unclear. The easternmost and latest accreted Avalon terrane is largely underlain in southeastern New England by plutonic and volcanic rocks associated with ~ 600 Ma Avalonian continental arc-type magmatism. To the west, inboard of this terrane, is the Nashoba terrane, a late Neoproterozoic to Cambro-Ordovician oceanic (?) arc-backarc complex, now metamorphosed to the upper amphibolite facies. The Avalon and Nashoba terranes may or may not have amalgamated before accretion to North America. The boundary between them is marked by the brittle Bloody Bluff fault that likely includes late (Mesozoic?) normal fault motion. This fault overprints the wide ductile NW-dipping Burlington mylonite zone that is thought to be the suture zone. Petrofabrics within this mylonite zone indicate oblique sinistral top-to-the-south movement of Nashoba over Avalon. The timing of this movement in eastern MA is currently best constrained between mid-Silurian and mid-Devonian. Calc-alkaline diorite and tonalite plutons of this age in the Nashoba terrane and in the eastern Merrimack belt, and the presence of basaltic andesites in the Newbury Volcanics indicate that subduction, at least on some scale, was still active as the Nashoba and Avalon terranes approached Laurentia. The Avalon terrane was experiencing only A-type magmatism during the Siluro-Devonian. The Avalon terrane is only weakly metamorphosed and appears to have largely escaped Acadian deformation. High-grade metamorphism began in the Nashoba terrane at ~ 425 Ma. This was overprinted by a second high-grade, migmatite-forming event at ~ 395 Ma. An additional Neoacadian (?) metamorphic event is recorded between ~ 380-350 Ma. The end of this period temporally overlaps the youngest granitic magmatism in the terrane and coincides with hornblende 40Ar/39Ar cooling ages. In eastern Massachusetts, crystalline and previously deformed or metamorphosed rocks seem to have largely escaped the effects of the Alleghanian orogeny at the end of the Paleozoic.