Rare earth element geochemistry of Lower Palaeozoic turbidites in the British trans-Iapetus zone: provenance patterns and basin evolution

Abstract

Synopsis Rare earth element (REE) signatures provide a potentially powerful tool for evaluating the provenance and tectonic setting of turbidites throughout the geological record. The acquisition of REE data for the Southern Uplands of Scotland may thus assist in resolving uncertainties regarding the depositional regime and evolution of this important Lower Palaeozoic turbidite sequence. In this paper REE spectra for a comprehensive stratigraphic suite of Southern Uplands greywackes are presented. These highlight a juvenile volcanogenic component (probably derived from a southerly source and deposited in a back-arc environment) in the Leadhills Group and northern part of the Gala Group, conspicuous by the lack of a Eu/Eu* anomaly. A more evolved ‘granitic’ REE signature, showing distinct affinities to the Lake District Windermere Supergroup, is evident in the southern sector of the Gala Group and the Hawick Group. These results are compatible with structural models which indicate a transition from a back-arc basin to a foreland basin during the mid–late Llandovery closure of the Iapetus Ocean. Comparisons between the Southern Uplands data and the REE signatures of modern turbidites from a variety of tectonic settings are of some value for determining the nature of the Lower Palaeozoic depositional environment. However, the approach must be used with caution in view of the clear temporal adjustments of sedimentary REE signatures resulting from changing crustal evolution processes on timescales ranging from 10 6 to 10 9 years.