U-Pb DETRITAL ZIRCON GEOCHRONOLOGICAL CONSTRAINTS ON THE EARLY SILURIAN COLLISION OF GANDERIA AND LAURENTIA ALONG THE DOG BAY LINE: THE TERMINAL IAPETAN SUTURE IN THE NEWFOUNDLAND APPALACHIANS

摘要

The Dog Bay Line is a major Silurian terrane boundary in the Exploits Subzone of the Newfoundland Appalachians. Late Ordovician-Early Silurian rocks northwest of the Dog Bay Line, the Badger and Botwood groups, contain detritus sourced exclusively from Laurentia. These rocks were deposited upon peri-Gondwanan volcanic arc terranes that were accreted to Laurentia in the Middle Ordovician. The Davidsville and Indian Islands groups southeast of the Dog Bay Line have stratigraphic links to peri-Gondwanan terranes and were deposited during the Late Ordovician-Early Silurian upon the peri-Gondwanan margin of Iapetus and were then accreted to Laurentia in the Early Silurian. A change from Paleozoic-dominated (Badger Group) to Meso- and Neoproterozoic-dominated (Botwood Group) detritus in sequences northwest of the Dog Bay Line is attributed to the Middle Ordovician collision and exhumation of peri-Laurentian arc terranes of the Notre Dame Subzone with Laurentia. Unroofing and erosion of these accreted arc terranes re-exposed Laurentian basement and deposited detritus from the latter into the Botwood Group. Salinic orogenesis resulting from the collision of Ganderia with Laurentia resulted in obduction and erosion of the accreted Victoria and Exploits arcs and deposition of the detritus into a f orearc basin on Laurentia. The absence of zircons in the 510 to 550 Ma and 1520 to 1600 Ma age range northwest of the Dog Bay Line and no 1600 to 1700 Ma zircons southeast of the Dog Bay Line suggests the presence of a Silurian arm of the lapetus Ocean (Tetagouche-Exploits basin) that separated Laurentia from peri-Gondwanan microcontinents of Ganderia and Avalonia. The change in Late Ordovician deep marine turbidites to Early Silurian stable-shelf rocks and non-marine, subaerial sediments east of the Dog Bay Line parallels that on the Laurentian margin and indicates the destruction and subsequent closure of lapetus. The upper sequences of the Botwood Group (Roger-son Lake Formation) contain 700 to 800 Ma zircons that are atypical of Laurentia, but are common in peri-Gondwanan terranes, and suggest that the youngest rocks of the Botwood Group may post-date closure of the Dog Bay Line and transgress the suture as an overlap sequence. The presence of Silurian orogenesis on both the Laurentian and peri-Gondwanan margins of lapetus is consistent with the closure of the Tetagouche-Exploits basin and lapetus Ocean by the Late Silurian. Laurentia and the Ganderian microcontinent were involved in a continent-continent collision suggesting that rocks along the Dog Bay Line represent the last known occurrence of lapetus in the Newfoundland Appalachians.