Abstract

Comparison of the basement rocks of Britain reveals major contrasts to the N and S of the Iapetus suture (Solway-Shannon line). North of the suture, polymetamorphic high-grade basement schists and gneisses and igneous rocks date from about 2900 Ma to c. 400 Ma, the latter age corresponding to closure of the Iapetus ocean. By contrast, S of the suture, in England and Wales, the basement is dominantly low grade metamorphic rock, together with intrusive and extrusive igneous, and sedimentary rocks. There is little geological or geochemical evidence for the presence of basement older than c. 900 Ma below this area and the predominant expression of crustal growth in England and Wales is indicated by radiometric ages between c. 700 Ma and 450 Ma, reflecting late Precambrian-Palaeozoic magmatism and metamorphism.

Palaeomagnetic data from Scotland demonstrate that the crust in northern Britain developed as an integral part of the Laurentian Shield during mid-Proterozoic times (1600–1000 Ma). The palaeomagnetic record from the pre-Ordovician rocks of England and Wales shows that this area was unrelated to the Laurentian Shield prior to closure of the Iapetus Ocean. The apparent polar wander (a.p.w.) path is that of a distinct microplate involved in rapid growth of Iapetus during early Cambrian times, and emplaced in its present configuration adjacent to northern Britain as a result of subduction of the ocean basin between Cambrian and Silurian times. We conclude that whereas the basement to the north of the Iapetus suture formed an essentially continental unit from 2900 to 400 Ma, the basement of England and Wales is a result of crustal growth from about 900 Ma to 400 Ma and formed by accretion of island arcs, associated accretionary prisms and fore-arc basin sediments within the Iapetus Ocean.

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