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1 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
2 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
The Caledonides of northern Britain are commonly divided into an assemblage of terranes based on the interpretation of geology in relation to major fault structures. This paper incorporates metallogenic concepts into the terrane model and uses systematic regional geochemical data to constrain models of terrane configuration and evolution: analyses of one-, two- and three-component geochemical images are used to identify changes in the levels of single elements and their ratios across the orogen. The distribution of sedimentary exhalative (SEDEX) ore deposits and volcanogenic massive sulphide, and mesothermal and epithermal gold mineralization is shown to reflect the presence of long-lived metallogenic provinces which have distinctive geochemistry reflecting their origin as zones of crustal extension and lithospheric thinning.
The geochemistry of the northern Caledonides reflects the predominantly lower crustal nature of the Laurentian continental margin, now exposed in the Lewisian foreland and Caledonized Lewisian inliers in the Moine succession. In contrast, the Avalonian Plate has a more evolved geochemistry, although the ratios of elements associated with basic rocks suggests that they were less fractionated during emplacement into the thinner crust of Avalonia and the Iapetus Suture Zone. The predominantly psammitic Moine Supergroup and the Grampian Group Dalradian have an evolved chemistry consistent with derivation from a higher crustal level than that of the exposed Lewisian complex; basin-scale variations, such as the proportion and composition of arkosic material, are also evident. The regional pattern was intermittently disrupted by the influx of mantle material during deposition of the mineralized Argyll and Southern Highland groups (Dalradian), and later as intrusions were emplaced from early Ordovician (Grampian Orogeny) to early Devonian times.
South of the Southern Upland Fault, the geochemical patterns suggest a complex interplay of sedimentary provenance and depositional environments during the final stages of collision between Laurentia and Avalonia. Provenance changes in early Silurian times probably reflect the initiation of major tectonism in the Scandian Orogen to the northeast and, more locally, a change from a mineralized, extensional back-arc- to a foreland-basin environment as the Laurentian plate overrode Avalonia. Very similar geochemical trends extending from the highest Silurian strata in the Southern Uplands into the Windermere Supergroup of the English Lake District are consistent with sequential deposition in the same foreland basin.
The principal crustal boundaries indicated by the geochemical data are the Moine Thrust Zone, the Grampian-Appin groups’ boundary, the Highland Boundary Fault and the Southern Upland Fault. The geochemical evidence provides little support for the Great Glen Fault as a significant terrane boundary, while the Iapetus Suture is shown to be a complex transition zone extending across the Southern Uplands and into the south of the Lake District. Within the former, the Moffat Valley Fault is identified as a major structure of regional significance.
Throughout the orogen there is a direct relationship between geochemistry, tectonic setting and the presence of significant metalliferous mineralization.
