Grade and time of metamorphism in the Caledonide Orogen of Britain and Ireland

Abstract

Abstract Plate 3 depicts three major belts distinguished by their style and age of metamorphism: the orthotectonic and paratec-tonic zones of the Caledonides and the Hercynides. Age of metamorphism, shown on the inset map (Plate 3), in part reflects the areal distribution of these zones and refers to the timing of the main phase or peak of metamorphism. The orthotectonic zone is, therefore, shown to be metamorphosed during the earliest Ordovician although recrystaHization was almost certainly taking place from the late or even mid-Cambrian to the Silurian. In the paratec-tonic belt the main metamorphism is shown as occurring in the period Lower Ordovician-to-Silurian although recrystahization may well have continued into the Lower Devonian. The Lower Devonian rocks within the central and southern Caledonides have suffered folding and weak metamorphism normally before Middle Devonian times and have been differentiated on the map. The southern limit of significant Caledonian deformation is normally taken as the northern edge of the Midland Block (F. W. Dunning, pers. comm.). However, it is believed that weak Caledonian metamorphism affected rocks south of this line. Similarly, recrystaHization associated with the Hercynian extended well to the north of the so-called Hercynian front, the northern limit of strong deformation (F. W. Dunning, pers. comm.). There is, therefore, some doubt as to whether the main recrystaHization in pre-Carboniferous rocks of South Wales and the English Midlands relates to the late-Caledonian or to the Hercynian.