A regional geochemical assessment of some terrane relationships in the British Caledonides

Abstract

Synopsis Regional geochemical data have been systematically collected across the British Caledonides and interpreted, using single and multielement image analysis, to provide geochemical constraints on terrane relationships. In the Hebridean Terrane, the Lewisian rocks show the characteristics of a large ion lithophile element (LILE) depleted lower crustal complex and seem unlikely to have been the source for the bulk of the overlying Torridonian clastic rocks. The psammite-dominated Moine Supergroup and the Dalradian rocks of the Grampian Group have closely similar evolved chemistries consistent with derivation from a higher crustal level than that of the currently exposed Lewisian complex. Across the Scottish Highland terranes the principal geochemical discontinuities coincide with the Moine Thrust Zone, the boundary between the Grampian Group and the Appin and Argyll Group Dalradian rocks, and the Highland Boundary Fault. The geochemical evidence provides little support for the Great Glen Fault as a significant terrane boundary. South of the Southern Upland Fault, which also coincides with a major geochemical discontinuity, a range of elements indicates division of the Southern Uplands terrane into northern and southern zones. These are transitional between the Orlock Bridge and Moffat Valley faults and reflect a change in sedimentary provenance during the early Silurian. Basic and ultrabasic elements, especially Cr, show exceptionally high levels over early Llandovery strata possibly indicating derivation from Iapetus Ocean crust obducted further NE as the Scandian Orogen was initiated. The geochemical characteristics of the southern part of the Southern Uplands Terrane continue into the Windermere Supergroup of the English Lake District, emphasising the continuity of mid- to late-Silurian deposition across the Laurentia–Avalonia (Iapetus) suture zone.