The Highland Border Complex, Scotland: a paradox resolved

Abstract

A previously inexplicable difference between the Highland Border Complex, Scotland, and its correlative in Ireland, the Clew Bay Complex, is that rocks of Caradoc–Ashgill age occur only in the former. We reject evidence from supposed chitinozoa for this dichotomy: no sedimentary rocks of proven age younger than Arenig occur in the Highland Border Complex. Consequentially, the stratigraphy must be totally recast, and the ‘exotic terrane model’ replaced by one in which a largely autochthonous Highland Border Complex in stratigraphical continuity with the Dalradian (Grampian terrane), was overridden by the Highland Border ophiolite (Midland Valley terrane). Tectonic models for SE Laurentia are thereby considerably simplified.

The Ocean – Continent Transition Zones Along the Appalachian – Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean

“…3) is a metasedimentary succession that was deposited on the eastern margin of Laurentia during the late Neoproterozoic and Early Cambrian. Existing constraints imply that the base is younger than 800 Ma and that the age ranges to at least 510 Ma (Smith et al 1999;Tanner and Sutherland 2007). It comprises a thick sequence of lithologically diverse metasedimentary and mafic volcanic rocks, along with three distinct glacigenic units that are correlated with widespread Neoproterozoic glaciations (McCay et al 2006).…”

“…The serpentinite olistoliths occur at a similar stratigraphic level (Upper Easdale -Tayvallich Subgroup) for over 500 km along strike in the Dalradian of Ireland and Scotland and are associated with a change from shallow-to deep-water sedimentary strata and the first major sequence of rift-related basaltic volcanic rocks. This evidence was interpreted by Chew (2001) to suggest that the serpentinite olistoliths represented protrusions of serpentinized mantle onto the seafloor that were generated in Easdale Subgroup times during a phase of major crustal extension leading to the formation of an OCT. A series of poorly exposed fault-bound slivers of ophiolitic rocks (termed the Highland Border Ophiolite [HBO] in Scotland; Tanner and Sutherland 2007) crop out within the Highland Boundary -Fair Head -Clew Bay fault zone in Scotland and western Ireland. These rocks have traditionally been regarded as Late CambrianEarly Ordovician Penrose-type ophiolite complexes that were dismembered following obduction during the early stages of the Grampian Orogeny.…”

How the Neoproterozoic S-isotope record illuminates the genesis of vein gold systems: an example from the Dalradian Supergroup in Scotland

“…The sequence was deposited along the developing east Laurentian passive margin during a period of ocean widening (Anderton 1985). The Dalradian Supergroup has a depositional history spanning the Neoproterozoic (Cryogenian) to mid-Cambrian (Tanner & Sutherland 2007;Stephenson et al 2013) and comprises marineclastic sedimentary rocks with occasional carbonate beds and minor volcanic rocks (Stephenson et al 2013). The oldest rocks are psammites and semi-pelitic schists deposited in an extensional basin, collectively called the Grampian Group (Figs 1 & 2).…”

“…The sequence is generally considered to be mid-Neoproterozoic (Cryogenian) to at least mid-Cambrian in age (Tanner & Sutherland 2007;Stephenson et al 2013). However, there are only limited horizons where precise chronostratigraphy is available (Dempster et al 2002;Rooney et al 2011).…”

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

“…Bluck 1984Bluck , 2001Bluck , 2010). An alternative view has now linked most of the sedimentary rocks of the complex with the uppermost parts of the Dalradian succession in the adjacent Scottish Highlands (Tanner 1995;Tanner & Sutherland 2007;Cawood et al 2012), leaving mainly igneous rocks in an enigmatic Highland Border Complex. Even the superficially similar cherts found in several of the HBC outcrops prove, from the evidence of their rare-earth geochemistry, to have formed in markedly different geotectonic environments (Tanner et al 2013).…”