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

Synopsis 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 (c. 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 c. 1000 Ma and is thought to represent detritus shed from a Grenvillian (c. 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 & 2 can be found at http://www.geolsoc.org.uk/SUP18370苏格兰中部山谷（Midland Valley）南部兰多维列世（Llandovery）至温洛克世（Wenlock）砂岩中的碎屑锆石（detrital zircon）种群概要显示，此类沉积岩的再旋回造山带物源（recycled orogenic provenance）整体呈现双峰式特征，包含年轻的下古生界（Lower Palaeozoic）组分与以中元古界（Mesoproterozoic）为主的古老组分。下古生界组分以阿伦尼格期（Arenig）/兰维尔恩期（Llanvirn）（约475 Ma）锆石为主，这类锆石被推断源自中部山谷地体（Midland Valley terrane）内部的火山-侵入岩源区（volcanic-plutonic source）。砂岩中占主导的中元古界组分年龄约为1000 Ma，被认为来自中部山谷地体格林威尔造山期（Grenvillian，约1000–1800 Ma）基底的剥蚀碎屑。太古宙（Archaean）锆石的稀缺性，排除了格兰扁变质地体（Grampian metamorphic terrane）达勒德超群（Dalradian Supergroup）作为沿中部山谷南部边缘分布的奥陶-志留纪盆地（Ordovician–Silurian basins）沉积物供给源的可能。碎屑锆石种群的年龄分布并不符合火山弧复合体（volcanic-arc complex）的简单剥露（unroofing）模型。相反，其指向在持续的（从兰多维列世至少至早泥盆世）左旋走滑变形（sinistral strike-slip deformation）过程中，断裂围限的火山岩与侵入岩解体岩块发生周期性抬升，而这一过程控制了中部山谷地体南侧的盆地演化、沉积相分布与变形作用。附录1与附录2可于http://www.geolsoc.org.uk/SUP18370获取。