THE MORETOWN FORMATION: EVIDENCE FOR THE DASHWOODS TERRAIN IN THE NEW ENGLAND APPALACHIANS

The Appalachian passive margin of Laurentia was tectonically stable from Early Cambrian to Early Ordovician, when collision with the Notre Dame-Shelburne Falls arc initiated westward thrusting of continental margin rocks at ca. 470-460 Ma. However, Early Ordovician magmatism and deformation in the Laurentian Dashwoods terrain in Newfoundland was coeval with passive margin sedimentation. Waldron and van Staal (2001) explained this paradox by suggesting that Dashwoods was a peri-Laurentian microcontinent separated from Laurentia by a narrow sea during Neoproterozoic rifting. Subduction under Dashwoods began in Cambrian time, but tectonism was confined to Dashwoods, and did not destabilize the passive margin to the west.

Based on inheritance in zircon, Karabinos et al. (1998) suggested that Shelburne Falls arc magmas mixed with Laurentian crust, and they showed an east-dipping subduction zone under a Laurentian-derived ribbon continent. However, direct evidence for an analogous peri-Laurentian terrain in the New England Appalachians is obscure due to younger Paleozoic deformation. The Moretown Formation, which extends from northern VT to southern MA, may have formed on the margin of Dashwoods, and be roughly equivalent to the Fleur de Lys Supergroup in Newfoundland. Lithologically diverse, the Moretown Fm. was interpreted as a Middle Ordovician fore-arc deposit in early plate tectonic models. Yet it is predominantly quartz-rich clastic metasediment, including quartzite and quartz-pebble conglomerate. It was intruded by mafic and felsic magmas of diverse geochemistry and age, including the 479 ± 8 Ma Hallockville Pond Gneiss in Massachusetts (Karabinos and Williamson, 1994), and 484 ± 4 and 496 ± 8 Ma tonalites in southern Vermont (Ratcliffe et al., 1997). Furthermore, detrital zircons from the Moretown Fm. indicate a Grenvillian source. Thus, the Moretown Fm. was intruded by arc-derived magmas in the Cambrian, derived from Grenvillian crust, and is distinct from coeval sediments on the Laurentian margin to the west. The oldest intrusive rocks in the Moretown Fm. probably predate the Shelburne Falls arc and likely formed in an older subduction zone under a micro-continent. An older subduction history may also explain enigmatic Cambrian Ar-hornblende cooling ages in northern Vermont (Laird et al., 1993).