Rare earth element and La-Th-Sc analysis of cherts from the Highland Border Complex, Scotland: Geochemical determination of the sedimentary environment in greenschist facies rocks

摘要

New geological maps of the Highland Border Complex (HBC) at four classical localities across Scotland - Stonehaven; River North Esk; Loch Ard Forest, Aberfoyle; and North Glen Sannox, Arran - are presented as part of this study. Cherts sampled from the HBC at these localities have rare earth element (REE) profiles and La-Th-Sc values that are comparable with published data from past and present-day oceanic settings. These new data are used to determine the palaeoenvironment, and plate tectonic setting, of the Highland Border cherts, and to shed further light on the controversial relationship between the HBC and the Neoproterozoic-(?)Lower Ordovician Dalradian Supergroup (ss) on the SE margin of Laurentia. Bedded cherts from the HBC at Loch Ard and Glen Sannox preserve REE profiles and La-Th-Sc values characteristic of continental margin/arc deposits sourced from a cratonic area with granites and gneisses, whereas cherts from Stonehaven originated from close to a mid-ocean ridge. Conversely, jaspers from the North Esk yield non-systematic results. Whilst not resolving the HBC controversy, the REE and La-Th-Sc results demonstrate that bedded cherts preserve a geochemical record of their depositional environment, despite having been strongly deformed, cut by penetrative fabrics and microveins, and affected by greenschist facies regional metamorphism.