MAGMATISM DURING THE SALINIC, ACADIAN AND NEOACADIAN OROGENIES

The lifespan of the Acadian Orogeny in the Northern Appalachians varies considerably (between 80 and 25 m.y. long) depending on how it is defined. A re-evaluation based on kinematic evolution and geological characteristics rather than just time, indicates that the Acadian Orogeny should be reserved for the tectonic events related to the Avalonia-composite Laurentia collision, which started c. 421 Ma and was finished by at least 395 Ma (Late Silurian-late Early Devonian). The Late Ordovician to Late Silurian compressional events (450 – 423 Ma), which are related to accretion of various elements of Ganderia to Laurentia, are referred to as the Salinic Orogeny. The Middle to Late Devonian events (395-360 Ma), corresponding with the docking of Meguma, are referred to as the Neoacadian. Geological evidence indicates that Ganderia, Avalonia and Meguma were three independent Gondwanan terranes, each of which exhibit distinct Early Paleozoic geological and tectonic evolutions. Silurian magmatism in the Northern Appalachians is related to formation of two arc systems (s.l.): 1) Notre Dame arc during Salinic closure of the Tetagouche-Exploits backarc basin, and 2) Coastal volcanic arc during Acadian convergence between composite Laurentia (including Ganderia) and Avalonia. Early Devonian Acadian orogenesis is related to shallow underthrusting of Avalonia. Its associated magmatism is Laramide-like and mainly related to dehydration of the underthrusted Avalonian slab and subsequent melting of the overlying mantle and crust following tectonic erosion of most of the lithospheric mantle during flat slab subduction. Middle-Late Devonian Neoacadian orogenesis is related to wedging of Laurentia's leading edge (Avalonia), obduction of Meguma and shallow underthrusting of the Rheic plate, possibly due to interaction with a plume. Its magmatism may be related to plume subduction, Laramide-like processes or a combination of both.