The nature and timing of orogenic activity in the caledonian of the British Isles / Природа и сроки орогенной активности в Каледонии на Британских островах

The British sector of the Caledonide-Appalachian fold belt, which bridges the Iapetus plate closure zone, has a range of plutonic, volcanic and volcaniclastic lithologies with characteristic space-time variations. The map of Caledonian igneous rocks in the British Isles (Plate 1) is compiled from many sources, some of which predate the use of methods of classification which are based largely on chemical data. In such cases, whilst extensive re-mapping continues, the compilers have had to resort to the collection and analysis of representative samples from the areas in question, the collection sites being based on existing maps. Thus, whilst the references to the mapped areas indicate specific publications, the chemical data on which the coding is based are commonly derived from compilations such as Stillman & Williams (1979) and Stillman & Francis (1979) for volcanic rocks and Sutherland & Pankhurst (1982), Le Bas (1982), Hall (1972) and Brown et al. (1981) for plutonic rocks. 
The coding system, which is explained fully in the Plate 1 legend, is that accepted by the Plutonics-Volcanics International Working Group of IGCP Project 27 and is intended to provide a shorthand description by which the user of the map may ascertain as much information as possible about the igneous rocks of each locality, many of which cover too small an area on the 1:2 million scale of the map to be reconizable on any conventional system of colour and symbol designation. Given the small scale of Plate 1 the chemical symbols portrayed are a compromise in some cases, and cannot do full justice to the wide petrochemical diversity of certain small extrusive complexes or intrusive centres. Limitations were also imposed by a rigid colour scheme denoting age and whilst, for some igneous bodies, high precision radiometric data are unambiguous, other suites have ages falling across the systematic colour boundaries, and yet others are either poorly dated or have attracted no radiometric work. All Rb-Sr dates have been adjusted, where necessary, to accord with the Steiger- Jager (1977) decay constant as recommended by the IUGS Subcommission for Geochronology, viz. (,,l Rb = 1.42 • 10 11 a-l). Plate 1 does not indicate the more conjectural interpretations used in most modern plate-tectonic magmatic models, but reference to such interpretations may be gathered from the appropriate publications (notably papers such as Stillman & Williams (1979), Ryan et al. (1980), Stillman & Francis (1979), Max & Long (1979) and Fitton et al. (1982)). A feature common to all such interpretations is the relationship between Caledonian magmatism and the history of the Iapetus ocean. However, there is no conclusive indication on the map of the suture which is believed to represent the junction between the leading edges of the two continental plates that underwent accretion, both sedimentary and igneous, whilst being brought together by the closure of the ocean throughout the Lower Palaeozoic (Phillips et al. 1976). A wide variety of evidence indicates that suturing took place in late Ordovician times approximately through southern Scotland and Central Ireland (Leggett et al. 1979). The closure is believed to have been effected by oceanic lithosphere subduction both northwards and southwards and to have been accompanied and followed by substantial and repeated lateral transgressive movement, all of which has resulted in the almost total loss of oceanic crust except for the Ballantrae ophiolite complex of southern Scotland. No unequivocal ophiolite fragments have yet been recognized in Ireland though recent studies suggest that some may exist in Tyrone and in western Ireland (Ryan et al. 1983). The proposed trace of the Iapetus suture is largely concealed beneath the Lower Carboniferous of the Irish midlands, yet there is an increasing body of evidence, both structural and geophysical, to confirm its existence. Its position, running northeastward across Ireland from the Shannon estuary to the east coast at Clogher Head some 55 km north of Dublin, can hardly now be regarded as conjectural. Similarly, geophysical, structural and sedimentological data indicate that the suture passes south of the Southern Uplands of Scotland from the Solway Firth to the coast near Berwick <...>