Abstract

The Late Precambrian rocks of Scotland and those of England and Wales lie on opposite sides of the Iapetus suture, and they represent the contrasting products of Late Precambrian evolution in the marginal zones of separate continents. They are separated by a wide belt of Paleozoic sediments. The Scottish region provides a section through thick ensialic clastic rocks of the Caledonide fold belt, from the foreland into the deep level of the orogen; whereas the region of England and Wales represents the weakly metamorphosed volcanic arc and trench environment of a continental margin.

In Scotland, the basement of the Caledonide orogen consists of the Lewisian complex of gneisses which at the time formed part of the Labrador-Greenland craton. The gneisses yield Archean ages of 2900-2700 m.y., and Aphebian ages of reworking of 1800-1700 m.y. In the foreland region of the Caledonides, the Lewisian complex is unconformably overlain by an assemblage of largely unmetamorphosed continental rift deposits, the “Torridonian,” in which an angular unconformity separates red beds deposited at c. 1000 and 800 m.y.

The main fold belt comprises metasediments which for more than a century have been referred to as the “Moine” or the “Moine Series” and the metasediments of the Dalradian Supergroup. In the “Moine,” we recognize two assemblages of immature psammitic and semipelitic sediments. The older assemblage represents the Grenville cycle, its rock having been converted into schists and migmatitic gneisses at c. 1100-1000 m.y. Subsequently, the Lewisian gneisses and the rocks of this Grenville complex formed a basement to the sediments of the younger assemblage which was deposited between c. 1000 and 750 m.y., and which appear to be temporally equivalent to the Torridonian red beds of the foreland. This younger assemblage, in turn, has been metamorphosed and deformed during an event between c. 780 and 750 m.y., the “Morarian” event, which however, seems to have been of localized extent, so that the thick Moine psammites grade into the more variable rocks of the overlying Dalradian Supergroup without any major stratigraphical break. The Dalradian rocks were not affected by Precambrian tectonometamorphic activity, and their deposition straddles the Precambrian-Cambrian time boundary. The lower rocks were deposited as shelf sediments, but towards the top of the succession conditions had changed to those of a marginal trough, with the accumulation of thick turbidites with volcanic horizons. Subsequently, all the Dalradian and Moine rocks of the fold belt were strongly affected by the Caledonian Orogeny.

In contrast to the very long duration of Late Precambrian ensialic sedimentation in Scotland, the Late Precambrian rocks of England and Wales are mainly continental marginal deposits of the Cadomian orogenic cycle of latest Precambrian to earliest Cambrian age. Rocks of similar age are present in the western European mainland. Archean and Aphebian rocks, which are limited to small inliers in southeastern Ireland, the Channel Islands and Brittany, formed the continental basement to a series of trough sediments (largely olistostromes) associated with extensive continental calc-alkaline volcanic activity which continued from Late Precambrian times into the Lower Cambrian. A phase of folding and high P low T metamorphism before 600 m.y. probably represents a marginal basin closure. All the rocks were involved in the end-Silurian Caledonian Orogeny.

This marked contrast in the orogenic history of regions now juxtaposed may be explained if the Iapetus ocean was of long standing in Late Precambrian time. It would furthermore appear that the orogenic trends of the Late Precambrian set the pattern for Phanerozoic evolution in the North Atlantic region.