GANDERIA, AVALONIA, AND THE SALINIC AND ACADIAN OROGENIES

New SHRIMP-analyses of zircon grains from Cambrian arenites in New Brunswick and Maine constrain the location of the Gander-Avalon terrane boundary, which is critical to understanding the Early Silurian Salinic and Early Devonian Acadian orogenies. Whereas peri-Gondwanan Ganderia and Avalonia have similar arc-dominated Neoproterozoic basements, their Lower Paleozoic rocks and tectonic evolutions differ significantly. In Ganderia, Middle Cambrian through Tremadoc arenites and shales overlie Lower Cambrian arc rocks, which were locally accompanied by coeval tectonism. In Avalonia, however, shale-dominated Lower Cambrian to Ordovician strata indicate a stable platform. Arenite samples collected from the Baskahegan Lake, Calais, Ellsworth, and Matthews Lake Formations contain abundant Early Cambrian and Late Neoproterozoic detrital zircon grains derived, probably, from local basement as well as important populations of Mesoproterozoic (1.2 -1.6 Ga) and Early Paleoproterozoic (2.0 - 2.3 Ga) zircon grains, which suggest an Amazonian provenance. We propose that the Gander-Avalon zone boundary coincides with the Caledonia Fault in New Brunswick, lies southeast of Grand Manan Island (Stanley Brook granite is c. 535 Ma), which is due to sinistral offset along the Oak Bay fault, and emerges southeastward from the Gulf of Maine as the Bloody Bluff Fault.

At ~430 Ma, the bulk of Ganderia accreted to Laurentia along its northern margin due to closure of the Middle Ordovician Tetagouche backarc basin, which caused the Salinic orogeny. Around this time (440-425 Ma), the coastal volcanic/Mascarene arc/backarc complex developed on its southern margin above a northwest-subducting Avalonian plate. Inversion of the Mascarene backarc basin at ~ 420 Ma signals the start of the Avalonia-Ganderia collision, which caused the Acadian orogeny.