Detrital Avalonian zircons in the Laurentian Southern Uplands Terrane, Scotland

The Siluro-Ordovician Southern Uplands terrane occupies a key position in the Caledonian Orogen, yet its genesis is controversial. Marginal basin, back-arc and forearc tectonic regimes, operative at the Laurentian margin of the Iapetus Ocean, have all been invoked. Fresh andesitic detritus within turbidite sandstones has, until now, been assumed to provide evidence for an Ordovician, supra-subduction volcanic arc, a central feature of most models. However, high-precision TIMS U-Pb and laser ablation data for detrital zircons from the sandstone prove Neoproterozoic volcanism at 557±6 Ma (2σ) and 613±12 Ma (2σ). The complex crystallisation history recorded by the zircons shows assimilation of 1043±7 Ma (2σ), Grenvillian basement into the andesite magma. No zircons have been found which overlap the Caradoc depositional age of the host sedimentary rocks, so undermining all extant terrane models. The age profile of the detrital zircons is typical of Gondwana/Avalonia. This has important implications for the palaeogeography of the Iapetus Ocean during the Ordovician, since it requires the introduction of Avalonian detritus into a sedimentary basin marginal to Laurentia.