U-PB ZIRCON AGE FOR THE DUNN POINT VOLCANICS, ARISAIG, NOVA SCOTIA: IMPLICATIONS FOR THE DRIFT OF AVALONIA AND THE EVOLUTION OF THE IAPETUS AND RHEIC OCEANS

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 and structural and isotopic data indicate that Avalonia had accreted to Laurentia by the Early Silurian. Late Ordovician-Early Devonian rocks of the Arisaig Group, Antigonish Highlands, Nova Scotia, consist of bimodal volcanic rocks overlain by a thick sequence of fossiliferous siliciclastics. The oldest rocks of this group have been interpreted to reflect local extension and basin development reflecting the oblique collision between Avalonia and Laurentia. Until now, the only published geochronological information for the volcanic rocks was a Rb-Sr whole rock age which suggested eruption occurred between ca. 420-435 Ma, but was considered unreliable due to younger alteration effects. Here, we present U-Pb data obtained on zircons from rhyolite of the Dunn Point Formation collected near Arisaig which indicate a crystallization age of 460.0 ± 3.4 Ma, and, significantly, reassigns the formation to the Middle Ordovician.

The new, precise age results together with petrological data imply that the Dunn Point volcanism developed on a microcontinent outboard from both Laurentia and Gondwana, possibly in a setting analogous to modern New Zealand. A Llanvirn age for these felsic rocks reconciles apparently conflicting paleomagnetic data and inferred paleolatitude for Avalonia in the Ordovician-Silurian. Hodych and Buchan (1998) determined a paleolatitude of 32°S±8° for Early Silurian mafic sills in the Avalon terrane of Newfoundland whereas Johnson and Van der Voo (1990) determined a paleolatitude of 41°S±5° for the Dunn Point volcanics, which were thought to be the same age. These data can be reconciled by 10 degrees northward movement of Avalonia between 460 and 440 Ma and can be used to chart the northward drift of Avalonia. As Laurentia lay at ~20°S and the northern Gondwanan margin at ~60°S between 460 and 440 Ma (e.g. MacNiocaill and Smethurst, 1994; Torsvik et al., 1996), this would imply about 5.5 cm/yr for the latitudinal component of the convergence between Avalonia and Laurentia and divergence between Gondwana and Avalonia during this interval.