THE FINAL CLOSURE OF IAPETUS: NEW U-PB ZIRCON GEOCHRONOLOGICAL CONSTRAINTS FROM THE COLLISION OF GANDERIA AND LAURENTIA ALONG THE DOG BAY LINE, NEWFOUNDLAND
A change from Paleozoic dominated (Badger Group) to Meso- and Neoproterozic dominated (Botwood Group) detritus in sequences NW of the DBL is attributed to the Middle Ordovician collision and exhumation of peri-Laurentian arc terranes (Notre Dame Subzone) with Laurentia. Unroofing and erosion of the accreted arc terrane re-exposed Laurentian basement which eroded providing detritus for the Botwood Group. Accrection of peri-Gondwanan arc terranes (Exploits Subzone) in the Silurian resulted in erosion and deposition of the detritus into a fore-arc basin on the Laurentian margin.
The absence of zircons in the ca. 510-550 Ma and 1550-1600 Ma age range in rocks NW of the DBL and no 1600-1700 Ma zircons SE of the Dog Bay Line suggests the presence of a Silurian arm of the Iapetus Ocean that separated Laurentia from peri-Gondwanan microcontinents of Ganderia and Avalonia. The change from Late Ordovician deep marine turbidites to Early Silurian stable-shelf rocks and non-marine sediments SE of the DBL parallels that on the Laurentian margin and indicates the destruction and subsequent closure of Iapetus. The upper sequences of the Botwood Group contain 750-800 Ma zircons that are atypical of Laurentia but are common in Gondwana and suggest that the youngest rocks of the Botwood Group post-date closure of the DBL and transgress the suture as an overlap sequence.
The presence of Silurian orogenesis on both the Laurentian and peri-Gondwanan margins of Iapetus is consistent with closure of the Iapetus Ocean by the Late Silurian. Laurentia and Ganderia were subsequently involved in a continent-continent collision suggesting that rocks along the DBL represent the last known occurrence of Iapetus in the Appalachian Orogen.