Silurian provenance variation in the Southern Uplands terrane, Scotland, assessed using neodymium isotopes and linked with regional tectonic evolution

The progressive changes in the provenance of Silurian greywacke turbidites in the Southern Uplands terrane reflect geotectonic events at the Laurentia n continental margin during closure of the Iapetus Ocean. In the northern G ala Group, juvenile andesitic detritus in some beds gives epsilon Nd values no lower than -4.2, more commonly, quartzo-feldspathic greywackes have eps ilon Nd values in the -5.5 to -6.7 range, produced by the mixing of juvenil e plutonic and Proterozoic basement detritus during are unroofing. In the s outhern (younger) Gala Group, Proterozoic epsilon Nd values range down from -7.7 to -11.2 with only sporadic evidence for a juvenile component. An abr upt change is seen between the Gala Group and its tectonostratigraphical su ccessor, the Hawick Group. In the latter, epsilon Nd values have a compact range between -4.7 and -6.6, indicating the renewed dominance of a more juv enile, plutonic provenance. Regional variations in the Sr/Rb ratio suggest that this was more evolved than the source of the Gala Group plutonic mater ial. The Wenlock greywackes of the Riccarton Group have epsilon Nd values i n the range -5.1 to -7.8, overlapping with the Hawick Group and with coeval greywackes from both the Midland Valley and Lakesman terranes. Overall, th e data support proposals that the Iapetus Ocean had effectively closed by m id-Silurian times. Conversely, data from greywacke boulders in the basal Ol d Red Sandstone conglomerate of the Midland Valley terrane militate against its Wenlock juxtaposition with the Southern Uplands.