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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.

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