CONTIGUOUS RATHER THAN DISCRETE PALEOZOIC HISTORIES FOR THE AVALON AND MEGUMA TERRANES BASED ON DETRITAL ZIRCON DATA

The Paleozoic evolution of the Avalon and Meguma terranes is crucial to the understanding of the Appalachian orogen. In the Avalon terrane of Nova Scotia, Ordovician-Early Devonian rocks consist of bimodal volcanics at the base (Dunn Point Formation) disconformably overlain by ca. 1900 m of fossiliferous siliciclastics (Arisaig Group) which contain Llandoverian to Lochkovian fossils. U-Pb zircon data from a rhyolite yields 460.0 ± 3.4 Ma for the Dunn Point Formation, and together with paleomagnetic data suggest development on a microcontinent at 30^o S, outboard from both Laurentia (20^oS and Gondwana (60^oS), possibly in a rifted arc setting. Geochemical, Sm-Nd isotopic tracer, and detrital zircon age data for Arisaig Group clastic rocks contrast with underlying Avalonian units, indicating a provenance source other than Avalonian basement. These rocks are characterized by negative eNd (-4.8 to -9.3), TDM > 1.5 Ga, abundant 620-520 Ma zircons, with lesser concentrations at ca. 0.9-1.2 Ga and 1.5-2.2 Ga. Archean zircons are minor. The Arisaig Group is inferred to be primarily derived from Baltica-Laurentia, with increasing input from more ancient basement in the early Devonian. Detrital zircon populations from coeval strata (White Rock and Torbrook formations) of the Meguma terrane also contain abundant ca. 620-520 Ma zircons and an important Mesoproterozoic population (1.0 to 1.4 Ga), that strongly suggests contiguity with Avalonia by the Late Ordovician-Early Silurian. These coeval clastic rocks are interpreted to have been deposited adjacent to the trailing edge of Avalonia-Meguma during Appalachian accretionary events. As Avalonia had accreted to Laurentia-Baltica by the Late Ordovician, these data suggest that the Meguma terrane also resided along the same (northern) margin of the Rheic ocean at that time. This interpretation is supported by the absence of a Cambro-Ordovician accretionary event, the lack of intervening suture zone ophiolitic units, and the similarity of Avalonian and Meguma basement Nd isotopic signatures in Paleozoic igneous suites. This conclusion implies that the Siluro-Devonian Acadian orogeny was not related to collision of the Meguma terrane with the Laurentian margin. Instead, we suggest that the Acadian orogeny occurred in an Andean-type setting.