Rare earth geochemistry of Arenig cherts from the Ballantrae Ophiolite and Leadhills Imbricate Zone, southern Scotland: implications for origin and significance to the Caledonian Orogeny

Abstract

Rare earth element (REE) data from low to mid-Arenig cherts are used to test competing models for the early Ordovician evolution of the Laurentian margin in the northern British Isles. Cherts from the Ballantrae Ophiolite Complex have chondrite-normalized REE patterns typical of continental margin settings with LREE enrichment, a slight negative Eu anom and shale and chondrite-normalized La/Yb values of 0.97–1.41 and 7.78–11.4 respectively. This pattern, together with a large positive chondrite-normalized Ce anom (1.44–1.70), is virtually identical to that found in radiolarian chert of the Gascoyne Abyssal Plain, in the Timor Sea. Cherts from the Raven Gill Formation within the Leadhills Imbricate Zone, Northern Belt, Southern Uplands have typical continental margin REE patterns, chondrite-normalized Ce anom (0.9–1.21) and Eu anom (0.61–0.79) values indicating that they formed closer to the continental margin than those from Ballantrae. Shale and chondrite-normalized and La/Yb values of 0.95–1.27 and 4.92–13.88 respectively confirm this interpretation. It is concluded that the Ballantrae ophiolite formed in a rifted–arc basin above a northwards dipping, intra-oceanic subduction zone. The modest depth of burial of the Raven Gill Formation precludes it being part of a marginal basin which was subsequently trapped as the Ballantrae Ophiolite was obducted in the late Arenig. The Arenig rocks of the Leadhills Imbricate Zone represent an allochthonous terrane accreted to the western extension of the Midland Valley in Ireland in pre-Caradoc times. Here it formed the basement to the Southern Uplands basin. Palaeontological evidence places this basin adjacent to Pomeroy, Co. Tyrone in the early Caradoc. Sinistral strike-slip faulting, from the late Ashgill transported the Southern Uplands Terrane to its present location, a distance of less than 250 km.