Arenig to Wenlock regional metamorphism in the Paratectonic Caledonides of the British Isles: a review

Caledonides in the British Isles were metamorphosed by a variety of processessuch as sedimentary burial in volcanic arcs, inter-arc basins and fore-arc and back-arc basins (e.g. Wales, SE and W Ireland, the Lake District and the Midland Valley of Scotland), tectonic burial in an accretionary prism (e.g. Ballantrae, the Southern Uplands and the Longford-Down Massif), hydrothermal alteration associated with the generation of oceanic crust (e.g. Ballantrae and the Highland Border Complex, the Southern Uplands and the Tyrone spilites), dynamothermal metamorphism accompanying obduction of oceanic mantle and crust (e.g. Ballantrae and the Highland Border complexes) and tectonic burial by nappes (e.g. the NW Foreland beneath the Moine Thrust). The region hassuffered low-grade regional metamorphism ranging from zeolite facies through prehnite-pumpellyite and pumpellyite and pumpellyite-actinolite facies into greenschist facies, with the localized development of amphibolite and granulite facies. Prior to the commencement of International Geological Correlation Programme (IGCP) Proj-ect 27, The Caledonide Orogen, in 1975 little was known about the regional metamorphism of the Paratectonic Caledonides of the British Isles. The Paratectonic Caledonides are defined as those tectonized lower Palaeozoic rocks lying S of the Highland Boundary Fault (Read 1961, Dewey 1969). Most workers regarded the region as unmetamorphosed compared with the Orthotec-tonic Caledonides (defined as those tectonized lower Palaeozoic rocks lying to the N of th