Dating events in the Metamorphic Caledonides: impressions of the symposium held in Edinburgh, September 1971

Abstract

Synopsis The symposium comprised seventeen contributions on the following subjects: palaeontological evidence for the age of the Dalradian, the Ordovician of western Ireland and other formations relevant to the dating of events (2 papers); the sequence of tectonothermal events in parts of the Moine Thrust Belt and the Moine Nappe (4 papers); deformation rates (1 paper); isotope ages and their relation to events in the Moinian and Dalradian areas (4 papers); early events in the Scandinavian Caledonides and their isotope dating (3 papers); and wider geotectonic aspects, including a plate tectonics synthesis (1 paper), a comparison of Newfoundland and Ireland (1 paper), and new palaeomagnetic evidence for the location of the main continental masses in the Palaeozoic (1 paper). The symposium failed to settle the question of the significance of the Precambrian isotope ages obtained from various parts of the Moine outcrop. Several new Precambrian ages were put forward to augment those already published, but at the same time further geological evidence was adduced to show that the earliest tectonic structures in the Moinian were post-Arenig. Evidence was presented to show that Dalradian sedimentation in Scotland and Ireland north of the Highland Boundary Fault lasted from perhaps 700 m.y. ago (top of the Lower Dalradian = 660 m.y.) through the Cambrian into the Lower Ordovician (Arenig or Llanvirn). The main folding and metamorphism of the Dalradian north of the Highland Boundary Fault occurred around 500–505 m.y. ago. In western Ireland south of the Highland Boundary Fault, Dalradian sedimentation ceased before the Arenig and the main polyphase folding and metamorphism occurred about 520 m.y. ago. The younger D 3 dedformation phase in the Scottish Dalradian may be about 460 m.y. old and possibly equivalent to the pre-Silurian deformation of the Ordovician of western Ireland. It now seems probable that early Caledonian folding in the Scandinavian Caledonides is confined to the North Cape and Bergen regions and may be about 530 m.y. old. Elsewhere in Scandinavia the main Caledonian folding is of Upper Silurian date; K-Ar ages around 435 m.y. are reported in the Trondheim area. The Varanger Tillite is 660 m.y. old. Palaeomagnetic evidence was put forward which demonstrated the existence of a vast Hercynian ocean which closed around the beginning of the Carboniferous, but there is no evidence for a Caledonian ocean between the presently exposed forelands in Britain that could have been wider than 2000 km.