Detrital zircons from syntectonic sandstones in Scotland and Newfoundland: A record of the closing of Iapetus

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Waldron, John۱; Floyd, James۲; Van Staal, Cees۳; McNicoll, V.۳; Simonetti, Antonio۱; Heaman, Larry۱
Holding Date 04 October 2008

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. Zircons extracted from Late Ordovician and Early Silurian quartzose wackes from several fault-bounded tracts in the northern, and central belts of the Southern Uplands were analyzed by LA-MC-ICPMS. Age populations range from Paleoarchean to Early Silurian. The age distributions of Precambrian zircon are closely similar to those from the Taconian orogen of Newfoundland and the Grampian orogen of Scotland, and indicate a variety of ultimate sources in Laurentia. 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 a subduction-accretion model for the Southern Uplands.
Analogous sandstones in Newfoundland are found adjacent to the Red Indian line, where they record deposition on oceanic and continental fragments that approached Laurentia from Late Ordovician onward. Zircons from the Badger Group in Newfoundland were analyzed by SHRIMP. In contrast with the Southern Uplands, these show abundant Early Paleozoic ages, suggesting sources in recently emplaced ophiolites and contemporary arc volcanics from an 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 the proximity of Ganderian basement at the present day.