The basement and Dalradian rocks of the North Mayo inlier

Basement rocks of the North Mayo inlier are unique in Ireland in displaying a series of Palaeoproterozoic to Mesoproterozoic orthogneisses (Annagh Gneiss Complex) affected by the polyphase Grenville Orogeny. Together with their cover of Dalradian rocks, they represent the Laurentian margin of the remnant Rodinia supercontinent, whose protracted break-up started in the Tonian (1000-720 Ma), and ultimately led to the opening of the Iapetus Ocean in the Ediacaran (635–541 Ma). The magmatic manifestation of the opening of the Iapetus Ocean is seen in North Mayo as a c. 600 m.y. old basaltic dyke swarm represented as dolerite intrusions that cut both the Annagh Gneiss Complex basement and its Dalradian cover. It is inferred that these dykes fed volcanic deposits as the Dalradian sediments accumulated on the extending continental margin. Hyperextension of the Laurentian margin during Iapetus opening is manifest by the presence in the Dalradian rocks on Achill of megablocks of serpentinite sourced from the sub-continental lithospheric mantle, as well as exposures of the basement itself. Indirect evidence that the Annagh Gneiss Complex is both the source of abundant Dalradian detritus, as well as its basement, is now strongly supported by the recent discovery of a probable original unconformity between them. Closure of the Iapetus Ocean resulted in the collision of the Annagh Gneiss Complex basement and its Dalradian cover with intra-oceanic arc terranes to the south. Consequent shortening and tectonic burial led to the development of Alpine-scale fold nappes and both Barrovian and contemporaneous blueschist-facies metamorphism—another unique feature of this sector of the Grampian orogen. The basement rocks also experienced Barrovian metamorphism as well as extensive deformation during the Grampian orogeny (c. 470–460 Ma) with the Iapetus-related dyke swarm acting as a structural marker allowing Grampian and Grenvillian events to be separated. Late Grampian tectonism resulted in modification (downbending) of the Grampian nappe pile close to the Laurentian margin, which was also the locus of late Caledonian (Devonian) strike slip motion.