ARC MAGMATISM IN THE APPALACHIANS-CALEDONIAN OROGEN

Geologic, isotopic, paleontologic and paleomagnetic evidence leaves little doubt that there were several independent, generally short-lived arc systems in Iapetus during the Paleozoic. Suprasubduction zone magmatism in Iapetus varied from Lower to Upper Cambrian (515-490 Ma) infant arc ophiolites and associated primitive oceanic island-arc fragments that were formed nearly coevally both near the Laurentian and Gondwanan margins when Iapetus achieved its maximum width to Lower Ordovician to Lower Silurian magmatic and volcanic arcs that were largely constructed on microcontinents present out-board of the margins of Laurentia, Baltica and Gondwana. Interaction between peri-Gondwana Ganderia and the primitive Penobscot arc during the Tremadoc (485-480 Ma) produced a relatively soft collision, mainly represented by melanges. Although it was always assumed that the Penobscot arc was emplaced from west to east in present coordinates, there is evidence that the arc was emplaced from east to west and related to closure of a small oceanic seaway between Ganderia and another peri-Gondwanan fragment (Bras d'Oria?). A new arc (Popelogan) formed on the leading edge of Ganderia during the early Arenig and started to drift rapidly north after it rifted-off, presumbaly due to slab rollback. The Popelogan arc started to collide with the Notre Dame arc built on the Dashwoods microcontinent, just outboard of the Laurentian margin, during the Late Ordovician (c. 450 Ma). Closure of the marginal basin between the accreted Popelogan arc and Ganderia took place between the Ashgill and Llandovery (445-430 Ma) and produced a second phase of calc-alkaline magmatism in the Notre Dame arc. The Laurentia (Dashwoods)-Ganderia collision may also have been the cause of the final suturing between the Dashwoods microcontinent and Laurentia during the Early Silurian. Diachronous closure of the remaining Iapetus seaway between the leading edge of Laurentia (upper plate) and Avalonia took place mainly during the Early Silurian (440-425 Ma) and produced the Kingston-coastal volcanic arc on Ganderian basement, which is preserved along the eastern seaboard of North America. Its collision and the subsequent Early Devonian accretion of Meguma (c. 415 Ma), which likely formed part of the Armorican terrane assemblage (ATA), were probably the cause for most of the Early Devonian deformation and metamorphism throughout the Northern Appalachians.