DETRITAL ZIRCONS FROM SYNTECTONIC SANDSTONES IN SCOTLAND AND NEWFOUNDLAND: SIMILARITIES AND CONTRASTS IN APPALACHIAN/CALEDONIDE CONVERGENCE HISTORY

Late Ordovician and Early Silurian sandstones in the Southern Uplands Terrane of Scotland were deposited during closure of the Iapetus Ocean between Laurentia and peri-Gondwanan fragments including Ganderia and Avalonia. Their tectonic setting is controversial, with different authors supporting models involving subduction-accretion, extensional continental-margin development, or back-arc basin deposition. Analogous sandstones in Newfoundland are found adjacent to the Red Indian line, where they record synsubduction deposition on the Middle Ordovician Victoria arc and older Ganderian arc basement, which had accreted to Laurentia during the Caradoc. Convergence between the remaining, still isolated peri-Gondwanan fragments in Iapetus and Laurentia has been interpreted as diachronous, with collision probably earlier in Canada than in Scotland.

Zircons extracted from quartzose wackes from several fault-bounded tracts in the Southern Uplands were analyzed by LA-MC-ICPMS. Age populations are closely similar to those from the Grampian and Taconian orogens, with a range from Paleoarchean to late Ordovician. Paleozoic zircon is rare, suggesting that contemporary volcanic activity, and ophiolitic rocks on the margin, were minor inputs. The mafic-sourced Portpatrick Formation, previously interpreted to represent early derivation from a Peri-Gondwanan arc fragment, yields zircon populations almost identical to the quartzose wackes, suggesting Laurentian provenance. Overall, the results are most consistent with forearc-basin - accretionary wedge models for the Southern Uplands.

Zircons from the Badger Group in Newfoundland were analyzed by SHRIMP. In contrast with the Southern Uplands, these show abundant Early Paleozoic ages, consistent with sources in emplaced suprasubduction zone ophiolites and contemporary arc volcanics built upon the active Laurentian margin. Precambrian zircons, though less abundant, show relative proportions that are close to those of the Scottish samples, indicating derivation from the Taconian/Grampian orogen. There is little evidence of Gondwanan provenance, despite deposition above the Victoria arc and its Ganderian basement.