Tectonic significance of a Llanvirn age for the Dunn Point volcanic rocks, Avalon terrane, Nova Scotia, Canada: implications for the evolution of the Iapetus and Rheic Oceans

Abstract

The Paleozoic evolution of Avalonia is crucial to the understanding of the development of the Appalachian orogen and the Iapetus and Rheic Oceans. Paleomagnetic, faunal, structural and isotopic data indicate that Avalonia had accreted to Laurentia by the Early Silurian. In the Avalon terrane of Nova Scotia, Ordovician–Early Devonian rocks consist of bimodal volcanic rocks at the base (Dunn Point Formation) disconformably overlain by a thick sequence of fossiliferous siliciclastics (Arisaig Group) which contain Llandoverian to Lochkovian fossils. Until now, the only published age data for the volcanic rocks yielded an imprecise Rb–Sr whole rock isochron age of 421±15 Ma and the volcanism has been interpreted to reflect local extension and basin development related to the oblique collision between Avalonia and Laurentia after closure of the Iapetus Ocean. We present U–Pb zircon data from a rhyolite which indicate an age of 460.0±3.4 Ma for the Dunn Point Formation, an age that requires a re-interpretation of its tectonic setting. The Dunn Point volcanism probably developed on the Avalonian microcontinent outboard from both Laurentia and Gondwana, possibly in a rifted arc setting, analogous to the modern Taupo Zone in northern New Zealand. A Llanvirn age for these felsic rocks also reconciles apparently conflicting paleomagnetic data and inferred paleolatitude for Avalonia in the Ordovician–Silurian and implies 10° northward movement of Avalonia between 460 and 440 Ma and about 5.5 cm/year for the latitudinal component of the convergence between Avalonia and Laurentia (i.e. the destruction of the Iapetus Ocean) and divergence between Gondwana and Avalonia (development of the Rheic Ocean) during this interval.