Zircon age constraints on the provenance of Llandovery to Wenlock sandstones from the Midland Valley terrane of the Scottish Caledonides

<strong>Synopsis</strong> Detrital zircon populations within the Llandovery to Wenlock sandstones of the southern Midland Valley of Scotland indicate that the recycled orogenic provenance for these sedimentary rocks was essentially bimodal, comprising a younger Lower Palaeozoic component and an older predominantly Mesoproterozoic component. The Lower Palaeozoic contribution is dominated by Arenig/Llanvirn (<em>c</em>. 475 Ma) zircons interpreted as having been derived from a volcanic-plutonic source located within the Midland Valley terrane. The dominant Mesoproterozoic component within the sandstones is <em>c</em>. 1000 Ma and is thought to represent detritus shed from a Grenvillian (<em>c</em>. 1000–1800Ma) basement to the Midland Valley terrane. The scarcity of Archaean zircons precludes the Grampian metamorphic terrane Dalradian Supergroup as a supplier of sediment to the Ordovician–Silurian basins located along the southern margin of the Midland Valley. The age profiles of detrital zircon populations do not fit with a simple model of unroofing of a volcanic-arc complex. Rather they point to the periodic uplift of fault-bound, dismembered blocks of volcanic and plutonic rocks during a prolonged (Llandovery through to at least early Devonian) period of sinistral strike-slip deformation, and it was this which controlled basin development, sedimentary facies distribution and deformation along the southern side of the Midland Valley terrane. Appendices 1 &amp; 2 can be found at http://www.geolsoc.org.uk/SUP18370