SPECULATIONS ON THE PROVENANCE AND TECTONIC EVOLUTION OF THE PERI-GONDWANAN TERRANES IN THE APPALACHIAN-CALEDONIDE OROGEN

The provenance and tectonic evolution of the Gondwana-derived terranes Ganderia, Avalonia, Megumia and Carolinia, in the Appalachian and Caledonide orogens is a contentious issue with many unknowns remaining. Ganderia, by a process of elimination, probably had Amazonian provenance during the Late Neoproterozoic to Early Paleozoic, drifting away from Amazonia near the end of the Middle Cambrian due to back-arc rifting and thus opening the Rheic Ocean. It accreted piecemeal to Laurentia starting in the Early Ordovician and finishing in the Silurian. Avalonia has a markedly different provenance and tectonic evolution. Together with Carolinia, it probably formed an assemblage of arc-backarc terranes situated at high southerly latitudes in the Mirovoi Ocean during the Cyrogenian but moved into the progressively widening gap between Baltica and Amazonia and began colliding obliquely with Ganderia and other parts of West Gondwana during the Ediacaran to Early Cambrian. In the process, parts of Ganderia may have been transferred to Avalonia (e.g., southern England?), whereas Carolinia seems to have been captured by Ganderia. The provenance of Meguma may have been North Africa or Armorica and its stratigraphy, magmatism, and tectonic evolution suggest it was isolated from Avalonia during most of the Early Paleozoic. Whether or not the Meguma terrane as preserved in Nova Scotia has links with similar rocks in Wales remains uncertain. Lack of direct knowledge about pre-Ediacaran basement in all of these terranes remains an obstacle in unravelling their origin and inter-relationships.