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

Abstract

SynopsisRegional geochemical data have been systematically collected across the British Caledonides and interpreted, using single and multi-element 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.

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…Stone et al (1999 identified two element assemblages as being of particular value in tracking geologicallycontrolled changes in sediment composition: the basic-ultrabasic elements (Cr-Ni-Mg-VTi), and the feldspar association (Rb-Sr-K-Ba), both of which had potential as lithostratigraphical mapping surrogates. Breward et al (2011) were able to extend the element distribution patterns seen in the Southern Uplands westward into the DownLongford inlier, Ireland, utilising similar geochemical data acquired there as part of the TELLUS project.…”

Regional geochemical patterns in SE Scotland: source lithology, inheritance and glacial overprinting

“…The implication is that stream sediment composition reflects the Lower Palaeozoic bedrock geology and that glacial smearing effects are minimal (Stone et al 2006 and references therein). Stone et al (1999) Leeder (1973Leeder ( , 1976, citing the evidence of Smith (1967), concluded that southwesterly-directed currents predominated, thus supporting his concept of an interior basin, although this model was disputed by Paterson et al (1976) who favoured an eastward flowing river system. Despite the undoubted introduction of some 'foreign' sediment, the persistence of the CaO geochemical lineament strongly suggests that much of the material forming the Stratheden Group 'Upper Old Red Sandstone' lithologies of the Tweed Basin was derived from the local Silurian sandstones.…”

“…The regional tectono-stratigraphical significance of the geochemical patterns has been discussed by Stone et al (1999 who have identified two element assemblages as being of particular value in tracking geologically-controlled changes in sediment composition: the basic-ultrabasic elements (Cr-Ni-Mg-V-Ti), and the feldspar association (Rb-Sr-K-Ba).…”

“…However, we stress that the relatively low sampling density means that the positions of the identified lineaments should be regarded as indicative rather than precise. It should also be noted that different elements may be responding to different influences, so much so that some lineaments appear to intersect.The regional tectono-stratigraphical significance of the geochemical patterns has been discussed by Stone et al (1999 who have identified two element assemblages as being of particular value in tracking geologically-controlled changes in sediment composition: the basic-ultrabasic elements (Cr-Ni-Mg-V-Ti), and the feldspar association (Rb-Sr-K-Ba).However, the direct spatial associations between the patterns of element distribution in stream sediment and changes in the underlying bedrock outcrop do not necessarily mean that the two media have the same compositions. The significances of relative enhancement or depletion from rock to sediment have been explored by Stone et al (2003Stone et al ( , 2006.…”

Regional geochemical comparisons from the Lower Palaeozoic, Southern Uplands–Down-Longford terrane in Northern Ireland and Scotland

“…A series of previous studies have identified several element assemblages as being of value in tracking geologically-controlled changes in sediment composition across the Southern Uplands (Stone et al, 1993(Stone et al, , 1999. Of particular value under appropriate circumstances are the basic-ultrabasic element suite Cr-Ni-Mg-V, the feldspar association Rb-Sr-K-Ba, the resistate mineral association Ti-Y-Zr-La-Cr and the base metal association Cu-Pb-Zn-As.…”