Lower Palaeozoic metamorphism in the Moine-Dalradian belt of the British Isles

Abstract

Summary Greenschist- to amphibolite-facies mineral assemblages occur which are of Proterozoic as well as lower Palaeozoic age in the Moine rocks but are only Lower Palaeozoic in the Dalradian rocks. The widespread age of high-grade metamorphism in the Dalradian is 520–490 Ma (Grampian), whilst in the Moines N of the Great Glen high-grade Lower Palaeozoic metamorphism is dated at 460–440 Ma. In Moine rocks, Grampian deformation and metamorphism may be absent N of the Great Glen but present S of the Great Glen, and this may indicate substantial movement on the Great Glen Fault between about 500 and 450 Ma. In addition to Moine metamorphism and deformation, the 460–440 Ma period appears to encompass diverse and extensive tectonic activity near the southern margin of the belt: thrusting in Connemara, Highland Border Downbend formation in Scotland and ophiolite emplacement in the Shetland Isles. A set of probably basement-controlled tectono-metamorphic domains in the Scottish Dalradian is suggested.