The Acadian Orogeny in the Northern Appalachians

摘要

The Acadian orogeny, involving deposition of clastic wedges, deformation, metamorphism, magmatism, and exhumation, is limited in time to the Devonian, and in space to the northern mainland Appalachians. Conventional interpretations attribute it to collision between Laurentia and Avalon or Meguma terranes. However, advances in paleogeography indicate that the Avalon and Meguma terranes were accreted to Laurentia in the Late Ordovician and Early Silurian, which was synchronous with closure of the Iapetus Ocean. On the other hand, the Rheic Ocean remained open to the south. In this context, we propose that the Acadian orogeny developed on an Andean-type margin, and attribute it to flattening of the subduction zone as a consequence of collision of an oceanic plateau surrounding a plume. This model explains: (1) SE to NW diachronism in the onset of deformation throughout the Devonian; (2) the development of a -400-380 Ma magmatfic arc gap in Maritime Canada that was abruptly terminated in the Meguma terrane at ~380-370 Ma by (3) intrusion of voluminous felsic magmatism and plume-related lamprophyres as the plume thermally eroded the oceanic lithosphere, causing melting of the lower crust; (4) accompanying regional high-T, low-P metamorphism related to thermal anomalies above a plume; (5) emplacement of gold deposits and associated siderophile elements, possibly derived from fluid circulation above an ascending plume; and (6) rapid Late Devonian exhumation of -10 km attributed to dynamic uplift over the plume. As the plume head migrated northward, the anomalously intense bimodal magmatism shifted into the Cobequid Highlands (Avalon terrane) at -360 Ma, and then to the Magdalen Islands, where -330 Ma plume-related magmatism occurred above a high-density, lower crustal lens interpreted as plume-derived underplated mafic rocks. Late Carboniferous formation of the Maritimes basin is attributed to cooling of this decapitated plume head.