Abstract The contemporary rifted margin off Nova Scotia was formed over the Meguma and Avalon Appalachian terranes in response to plate motions between Africa and North America (Figs. 1 and 2; Williams and Hatcher, 1983; Haworth, 1981). Rifting began in the Late Triassic and produced rift basins beneath the present continental shelf. Of these basins, the Orpheus sub-basin which lies on the Avalon-Meguma boundary is a prominent example. Regionally the rift basins are not uniformly distributed throughout the Nova Scotian margin and southern Grand Banks region. Most are found in the Avalon terrane, rather than the Meguma terrane. Evidence of Triassic mafic volcanism is exemplified by the North Mountain basalts and the Shelburne dike in Nova Scotia, and most of the known occurrences of volcanic rocks appear to lie well inland of the present ocean-continent transition (Poole, 1970; Papezik and Barr, 1981). Triassic rifting was accompanied by the deposition of non-marine clastics within the rift basins. During the Early Jurassic, thick salt beds developed beneath much of the present shelf and slope (Jansa and Wade, 1975). In Early-Middle Jurassic time a deepening, less restricted marine environment led to the construction of carbonate platforms (Jansa, 1981). It was during this time that sea-floor spreading began east of the rifted North American continent. This rift-drift transition does not appear to be marked by a period of uplift, erosion, or by the development of a clear breakup unconformity After the onset of sea floor spreading, the margin subsided and over 10 km of Jurassic

The Vermont Appalachians expose metamorphosed magmatic rocks ranging in age from Late Proterozoic to Cretaceous. Geochemistry, in concert with stratigraphic, structural, and metamorphic studies, reveals the origins of the magmatic rocks. Late Proterozoic–Early Cambrian dikes and greenstones in western Vermont formed during rifting of the Laurentian continent to form the Iapetus Ocean. Cambrian serpentinized peridotite represents forearc suprasubduction zone ophiolitic fragments. Cambrian to Ordovician amphibolites and felsic gneisses were formed as part of the Shelburne Falls volcanic arc. The Early Ordovician Mount Norris intrusive suite formed in an extensional setting in the vicinity of the Shelburne Falls arc. Silurian extensional magmatism at ca. 420 Ma is represented by the Comerford intrusive suite of dikes and small metamorphosed gabbro/diorite bodies in northeastern Vermont. Extension in the Silurian occurred behind a volcanic arc, perhaps because of slab detachment. Late Devonian granitoid bodies cut metamorphosed Silurian sedimentary units in northeastern Vermont. They probably formed as a result of delamination of lithosphere and consequent partial melting of mantle lithosphere and lower crust following continental collision. The last episode of magmatism in Vermont is represented by lamprophyric dikes and small alkaline bodies, which intruded at 130–110 Ma. Formation of the Late Proterozoic to Late Devonian magmatic rocks is explained in plate models involving continental rifting to produce the Iapetus Ocean in the Late Proterozoic, followed by subduction processes to close that ocean in stages from the Early Ordovician to Devonian time.