Caledonian and related events in Scotland

Abstract

The Scottish Caledonides, sited near the triple-junction between Laurentia, Amazonia and Baltica, is divided into at least five discrete blocks, each with a history incompatible with that of the block now lying adjacent to it. With the exception of the Hebridian margin, with its extensional Torridonian basins and Cambrian passive margin sequence, all blocks have undergone terrane-scale movements. The Moine, Central Highland Division and the bulk (if not all) of the Grampian Group have shared a common regional metamorphism, involving thickening and uplift, atc. 800 Ma andc. 450–480 Ma. This is incompatible with their being in the extensional regime that appears to characterise much of Neoproterozoic Laurentia. They, along with the polymetamorphic Dalradian block, now replace a region of passive margin and an unknown width of attendant Iapetus oceanic crust. These metamorphic blocks are grossly out-of-place.The Midland Valley is a severely contracted block of ancient crust, once fringed by extensive oceanic basins to the N and S. An Ordovician–Devonian arc was founded on this older craton, and supplied sediment to basins on either side of it. This arc, during its Lower Palaeozoic life, matured, finally to yield relatively quartz-rich sediment, but was re-activated during the Devonian. An arc, similar to that of the Midland Valley, also supplied sediment to the Southern Uplands. Metamorphic debris in the Southern Uplands had a provenance in either this arc-basement or in a basement somewhere along the orogen. No metamorphic detritus in the Highland Border Complex has yet been demonstrated to have a Dalradian source.Much of Scotland was assembled in a strike-slip regime. Evidence for strike-slip tectonics can be seen from the Late Proterozoic through to the Devonian. In periods of transtension, basins opened to accumulate sediment; in periods of transpression, those sediments were compressed and uplifted to yield sediments to successor basins. In the Neoproterozoic, during the phase of transpression, the basins were buried and metamorphosed, but during the Palaeozoic the basins were at a much higher level and escaped metamorphism.A substantial volume of the Neoproterozoic–Palaeozoic sediment that accumulated in Scotland was derived from two orogens, both of which were sited some distance away. During the Neoproterozoic, the Grenville orogen was the main source, and in later (Devonian) time sediment accumulated in Scotland from the major, Late Palaeozoic continent–continent collision of Greenland–Scandinavia. These two external sources were augmented by a substantial contribution of sediment supplied from the Midland Valley arc or its lateral equivalent and by mild uplifts within the Scottish basements.

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

“…Alternatively, if the Knockormal pyroxenite is an isolated block in a sediment-hosted mélange, then it suggests an episode of mantle uplift and erosion prior to formation of the Ballantrae Complex, during an interval when the Laurentian margin of Iapetus was apparently passive (Bluck in Oliver et al 2002). It is worth remembering that Bluck (1978Bluck ( , 1983Bluck ( , 2001 has consistently argued that more than one accretion/obduction event has contributed to the development of the Ballantrae Complex, as considered in more detail below.…”

“…Thirlwall & Bluck 1984;Stone & Smellie 1990;Smellie & Stone 1992, 2001) it became clear that the volcanic parts of the Ballantrae complex were indeed polygenetic (Figure 2). Of the various models developed to accommodate all of the disparate components within a broadly arc to back-arc situation, two are summarised in Figure 3.…”

“…Firstly, Bluck (1985, 2001 envisaged a series of obduction/accretion events alternating with phases of renewed marginal basin extension and volcanism so that different components of the complex were formed at different times as the geotectonic regime changed and evolved.…”

“…Thereafter, as analytical techniques improved, the wide range of arc, BABB-like and within-plate rocks present was firmly established and incorporated into an inclusive model of volcanic arcs and a back arc spreading zone (Stone & Smellie 1990;Smellie & Stone 1992, 2001Smellie et al 1995). Some geochemical signatures are more definitive than others, and so the discovery of boninites amongst the Games Loup arc lavas (Smellie et al 1995) was regarded as particularly compelling evidence for the eruption of those lavas at a relatively primitive island arc.…”

“…Bluck (2001, and has envisaged a series of obduction/accretion events alternating with phases of renewed marginal basin extension and volcanism. Other authors have sought to accommodate all elements of the Ballantrae complex within a single, evolving arcback-arc basin; see for example Stone & Smellie (1990) and Smellie & Stone (2001), whose model of an early-formed arc split by back-arc spreading to accommodate a range of supra-subduction and within-plate components, is reproduced in simplified form in Figure 3 (a and b).…”