The Late Caledonian granites of Donegal are all intruded into metasediments of the Dalradian Supergroup of Neoproterozoic age, which were metamorphosed and deformed during the Grampian Phase of the Caledonian orogeny at c. 470-460 m.y. They were intruded in a singular pulse well after the main tectonic event, apparently peaking at 407-402 m.y.; importantly after the strong collision of Laurentia with Baltica on closure of the Iapetus Ocean. The plutons are mainly made up of granodiorite and granite, and are all ‘I’ type, but different to Cordilleran ‘I’ types of the eastern Pacific margin. Major element chemistry indicates they are high-K calc-alkaline rocks with a large range in SiO2 content. However three of the plutons (Fanad, Thorr, Ardara), have very high Ba and Sr contents, even higher than Mainland Scotland counterparts; they are high Ba-Sr plutons. Three plutons (Barnesmore, Rosses, Trawenagh Bay) are evolved and are low-Ba-Sr types, while one (Main Donegal) has atypical, intermediate characteristics. The origin of the magmas is still much debated; here we suggest slab breakoff on Iapetus Ocean closure accounts for the special compositions of these magmas and the other major features of Late Caledonian granitic magmatism, including the singular intrusion peak and the associated appinite-lamprophyre suite.

The Late Caledonian granites of Donegal are all intruded into metasediments of the Dalradian Supergroup of Neoproterozoic age, which were metamorphosed and deformed during the Grampian Phase of the Caledonian orogeny at c. 470–460 m.y. They were intruded in a singular pulse well after the main tectonic event, apparently peaking at 407–402 m.y.; importantly after the strong collision of Laurentia with Baltica on closure of the Iapetus Ocean. The plutons are mainly made up of granodiorite and granite, and are all 'I' type, but different to Cordilleran ‘I’ types of the eastern Pacific margin. Major element chemistry indicates they are high-K calc-alkaline rocks with a large range in SiO2 content. However three of the plutons (Fanad, Thorr, Ardara), have very high Ba and Sr contents, even higher than Mainland Scotland counterparts; they are high Ba–Sr plutons. Three plutons (Barnesmore, Rosses, Trawenagh Bay) are evolved and are low-Ba–Sr types, while one (Main Donegal) has atypical, intermediate characteristics. The origin of the magmas is still much debated; here we suggest slab breakoffon Iapetus Ocean closure accounts for the special compositions of these magmas and the other major features of Late Caledonian granitic magmatism, including the singular intrusion peak and the associated appinite–lamprophyre suite.