APPALACHIAN-CALEDONIAN ARC TERRANES: WHAT WAS WHERE AND WHEN?

While the latitudinal positions of the main palaeocontinents involved in the Appalachian-Caledonian orogeny (Laurentia, Baltica, Avalonia and Gondwana) are now relatively well established, considerable debate remains as to the number of arc terranes present and their paleogeographic affinities. Recent paleogeographic models have become much more sophisticated, and depict a detailed, and complex, history of terrane migration and collision within the Iapetus Ocean. Several studies from Early Ordovician units from the Notre Dame subzone in Newfoundland yielded evidence for arc volcanism at low peri-Laurentian latitudes (e.g. Lawrence Head and Moreton's Harbour), which complements the established Laurentian faunal provinciality of related rocks in the same subzone. These Early Ordovician arc rocks can be correlated with some confidence with arc sequences in the western Irish Caledonides, where recent palaeomagnetic evidence also indicates low peri-Laurentian latitudes (Mweelrea Formation), and where Grampian Orogenesis can be shown to have been a very short-lived Llanvirn event. However, not all younger low-latitude magmatic arcs may necessarily have been built on the Laurentian margin. For example, Caradocian volcanic rocks of the Winterville and Bluffer Pond Formations in the Exploits subzone of northern Maine also yielded paleolatitudes adjacent to Laurentia, although strong geological correlations with similar rocks in northern New Brunswick (Popelogan arc) and other geological data in Maine and New Brunswick suggest that these Middle Ordovician volcanic rocks formed originally near the Gondwanan side. The latter is consistent with palaeomagnetic data from the other rocks in the Exploits subzone of the Northern Appalachians, which indicate that Arenig-Llanvirn magmatism (e.g. Tetagouche, Summerford, Roberts Arm and Chanceport Groups) took place at higher latitudinal, peri-Avalonian and/or intra-Iapetan settings. The available paleomagnetic data leave little doubt that the Appalachian-Caledonian Orogen preserves remnants of several arc systems that were active within Iapetus during the Ordovician. Accretion of these arcs to Laurentia during the Ordovician has led to confusion and miscorrelation between arcs along the length of the orogen.