Rare earth elements in chert clasts as provenance indicators in the Ordovician and Silurian of the Southern Uplands of Scotland

Abstract

Rare earth element (REE) patterns in chert clasts provide both an indication of the environment of deposition of the source succession and a means of recognising their provenance. The REE compositions of chert clasts in three Ordovician and Silurian conglomerates in the Southern Uplands of Scotland are compared to the REE fingerprints of their possible sources. The REE patterns of the clasts show that they were derived from successions deposited close to a continental margin. The absence of clasts with large positive cerium anomalies suggests that the Ballantrae Complex was not exposed and supplying material to the basin from at least the mid-Caradoc onwards although a single chert clast from the lower Llandovery conglomerate on Pinstane Hill has an almost identical REE pattern to a black chert from the Ballantrae Complex. A chert clast from a monomict chert conglomerate in the mid-Caradoc Kirkcolm Formation has an REE signature identical to that of bedded cherts in that formation, indicating an intrabasinal provenance. The REE patterns of a chert clast from the Caradoc Blackcraig Formation and of some of the clasts from the Pinstane conglomerate lie at the extreme end of those of the bedded Arenig and Caradoc cherts in the Southern Uplands but the REE patterns of other clasts from Pinstane and one from Caradoc-Ashgill Shinnel Formation lie beyond this, suggesting an extrabasinal source.