Abstract

New paleomagnetic results were obtained from several units in the Central Mobile belt of the north-central Newfoundland Appalachians. Combined with recently published data, the results place important limits on interpretations of early Paleozoic paleogeography and evolution of the Appalachian-Caledonian orogen. The Ordovician Appalachian margin of North America, located at low paleolatitudes and facing roughly south, was separated from the southern margin of Iapetus (Avalon-Armorica) by ∼3500 km. Primary directions in pillowed volcanic rocks of the Moreton's Harbour Group that are part of an Early Ordovician island arc show that these rocks were formed near the North American margin, at a paleolatitude of 12°S. Early to Middle Ordovician volcanic rocks of the Roberts Arm, Chanceport, and Summerford groups, on the other hand, were extruded at paleolatitudes of 30°-40°S. Unconformably overlying volcanics of the Silurian Botwood Group were extruded subaerially at a paleolatitude of 24°S. Combined with tectonostratigraphic data, these results suggest the following tectonic history. In Early Ordovician time, northerly subduction of the main Iapetus basin was accompanied by the formation of a volcanic arc and a back-arc basin near the North American margin. Middle Ordovician emplacement of ophiolites on the North American margin reflects the closure of this back-arc basin (Iapetus I), and continued northerly subduction in Silurian time closed the main basin (Iapetus II).

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