A review of geological origins and relationships in the Ballantrae Complex, SW Scotland

Abstract

SynopsisA substantial history of research has established the Ballantrae Complex as an ophiolite assemblage, generated within the Iapetus Ocean and accreted at the margin of Laurentia as that ocean began to close. Radiometric dates and graptolite biostratigraphy provide an age range from Neoproterozoic (rarely and questionably) to early Ordovician (commonly and securely). Components of the complex formed in a range of oceanic environments: within-plate oceanic islands, island arcs and backarc spreading zones. Overall, the various lines of evidence point to an early Arenig marginal basin, incorporating some older elements, which was tectonically disrupted and compressed during an obduction event driven by arc-continent collision. Despite the general consensus as to the age and origin of the complex there remain areas of uncertainty. It is highly probable that some components of the complex that are now structurally juxtaposed originated at a considerable distance from each other both horizontally and vertically. Further, the sporadic presence of high-grade metamorphic rocks -notably blueschists -suggests a complicated and probably polyphase structural history, whilst the relative importance of serpentinite diapirism versus compressional and strike-slip tectonics remains unresolved.

A plate tectonic scenario for the Iapetus and Rheic oceans

“…The destruction of this passive margin during the late Early to Middle Ordovician Grampian orogeny was due to the accretion of a Cambrian to Early Ordovician intraoceanic arc that is thought to be preserved among the ophiolitic and arc complexes along the south margin of the Grampian terrane of northern Ireland and Scotland, and beneath the younger cover of the Midland Valley graben (Cooper et al, 2011;Chew and Strachan, 2014). Assembly of this Cambrian to Early Ordovician intraoceanic arc complex may have included the incorporation of a minor outboard assemblage based on the occurrence of the CambroOrdovician Ballantrae ophiolite to the south of the Midland Valley terrane in Scotland (Chew and Strachan, 2014;Stone, 2014). Following accretion of the intraoceanic arc, a Middle Ordovician to early Silurian accretionary complex was constructed to the southeast of the Midland Valley terrane, indicating that subduction initiated beneath the northern British Isles in the Middle Ordovician and continued into the Silurian (Leggett, 1987;Waldron et al, 2008).…”

Chromitite petrogenesis in the mantle section of the Ballantrae Ophiolite Complex (Scotland)

“…Cr-spinel Cr# exist ( Fig. 1; Stone and Smellie, 1988;Stone, 2014). In this study, we present detailed new petrological and mineral chemical observations on the chromitites from the BOC.…”

“…This led to extensive debate about the tectonic provenance of the BOC; proposed settings involve melt generation in an island arc with a back-arc basin, a MOR, a volcanic arc and the involvement of a mantle plume in the formation of the ophiolite (Wilkinson and Cann, 1974;Jones, 1977;Lewis and Bloxam, 1977;Thirlwall and Bluck, 1984;Oliver and McAlpine, 1998;Smellie and Stone, 2001). In addition, it has been suggested that the mantle portion of the BOC has been processed in different tectonic settings (Stone, 2014). In particular, the two broad serpentinite belts that comprise the BOC mantle have different petrological characteristics and sparse data published on the podiform chromitites hosted in each suggests significant differences in chromitite…”

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…If the Ballantrae Complex formed through compression of an arc to back-arc zone formed in such a way, it would obviate the otherwise necessary long-distance obduction of the ophiolite across the fore-arc (Stone 2014). Another important assumption in Figure 12a is the inclusion within the fore-arc crust of the magnetic continental basement unit recognised by Kimbell & Stone (1995) as the source of the 'Galloway High' (Figure 11), and identified as 'Novantia' by Armstrong & Owen (2001).…”