The Caledonides of the British Isles--reviewed. 1979. Geological Society of London. Late Precambrian igneous activity in Southern Britain

Ireland and the Channel Islands. This activity included calc-alkaline intrusive and volcanic complexes which, although showing important regional differences, have many similar features. Recent U-Pb and Rb-Sr isotopic ages indicate that this activity was broadly contemporaneous at ca 600_+50 Ma. The initial Sr isotope ratios of these igneous rocks indicate that they largely represent new additions to the continental crust in an island arc-continental margin setting during a major period of crustal growth in south Britain. Igneous, metamorphic and sedimentary rocks of Precambrian age occur as small outcrops in Eng-land, Wales and SE Ireland and rocks of similar character form the Channel Islands (Fig. 1). Scattered borehole occurrences in N Wales, cen-tral and E England indicate that similar rocks underlie much of S Britain. Recent studies of these rocks have been reviewed in Dunning (1975). Geological and isotope age data indicate that these rocks were largely formed during a late Precambrian orogenic episode, at c.600 _+ 50 Ma. Here I review the geological character of the igneous rocks belonging to this episode and argue that these rocks were formed in a system of small ocean basins and island arcs, the growth and accretion of which represented a major episode of crustal growth in S Britain. These rocks there-fore formed the floor of the lower Palaeozoic 'geosyncline ' and hence experienced further deformation and igneous activity during the later Caledonian orogeny. The largest outcrop of Precambrian rock within S Britain is in N Wales (Shackleton 1975). In the Mona complex of Anglesey and Lleyn (Fig. 1) a thick sequence of flysch-type sedimen-tary rocks (the 'Bedded Succession') was deposited during the latest Precambrian. The sediments are associated with ophiolitic rocks suggestive of an oceanic environment: chert and manganiferous shale, ocean-floor tholeiitic basalt and a gabbro-serpentinite suite which resemble successively deeper components of the oceanic crust. These rocks were folded, metamorphosed and intruded by igneous rocks of intermediate to acid composition at c.600 Ma (the Coedana gra