The metamorphism of the Dalradian rocks of western Ireland and its relation to tectonic setting

Abstract

The metamorphic evolution of Dalradian rocks exposed in the NW Mayo, Ox Mountains and Connemara inliers of western Ireland is reviewed, and new data and revised calculations are presented. There is evidence at a single locality for an early episode of moderately high-pressure metamorphism with the production of crossite—epidote schists. Subsequent Barrovian-style metamorphism overprinted this almost everywhere in NW Mayo and resulted in further heating to produce chlorite—biotite and garnet zones with the garnet isograd near 400—450 °C, and a staurolite—kyanite zone for which conditions were P = 8 ± 2 kbar (1 bar = 10 ⁵ Pa) and T = 620 ± 30 °C. In Connemara, rare staurolite—kyanite schists are now inferred to have formed at lower pressures than those in the Barrovian zones because ilmenite is stable rather than rutile. The metamorphic zones now mapped result from regional scale heating by synorogenic intrusions exposed to the south, and took place at pressures of 4-6 kbar. There is, however, strong evidence to suggest progressive uplift during this heating phase, especially in the south. It is concluded that the thermal evolution of complex areas such as these can usefully be broken down into a series of modules, each corresponding to metamorphism in a distinct tectonic setting. The sequence identified in the Irish Dalradian is: (i) recrystallization during burial in a low heat flow setting; (ii) thermal relaxation and possibly uplift; and, in Connemara only, (iii) further heating and uplift, probably in the roots of a volcanic arc.