A model for the Lower Palaeozoic evolution of the southern margin of the early Caledonides of Scotland and Ireland

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Abstract
Synopsis: Consideration of the stratigraphical and structural development of the relationship between the early and late Caledonides lends some support to the view that the early Caledonides of Scotland and Ireland lay on a leading plate edge above a northward-dipping descending plate. An early Ordovician trench just north of the Longford/Down Massif and Southern Uplands was the probable site of consumption of an oceanic plate which carried Ordovician/Silurian spilite-argillite-chert-turbidite sequences. The Highland Zone of high temperature metamorphism and gabbroic and granitic intrusion probably lay above a zone of calc-alkaline magma ascent, generated by partial fusion of the crust of a descending plate at depths over 100 km. During Ordovician and early Silurian times the Midland Valley was a southward, oceanward, thickening sedimentary wedge built on deformed Dalradian terrain in the north and on upthrust wedges of early Ordovician oceanic crust and mantle (Ballantrae Complex) in the south. At this time flysch wedges were built out southwards into the ocean and, beginning in late Upper Llandovery times, were progressively deformed to produce a growing land area (Cockburnland). By early Devonian times the sediments of the oceanic region were strongly deformed, forming the lands of the late Caledonides, and the Midland Valley was a graben receiving high rank metamorphic detritus from the Highlands and low rank metamorphic detritus from the Southern Uplands. Ophiolite complexes throughout the Appalachian/Caledonian Orogen are dominantly of early Ordovician age. By analogy with Tethyan ophiolite complexes and the processes involved in the genesis of marginal basins behind, and within, island arc complexes, the Appalachian/Caledonian ophiolites are considered to have originated by sea-floor spreading behind and within Ordovician island-arc complexes.