Deformation and metamorphism of Dalradian rocks and the evolution of the Connemara Cordillera

Dalradian metasediments in the Cur district of north-east Connemara have had a complex history of deformation and metamorphism. The earliest schistosity present formed during biotite-grade metamorphism and may be mimetic after a compaction fabric. Subsequently, the rocks attained garnet grade and major D 2 folds developed. This deformation involved an initial buckling followed by a coaxial homogeneous flattening and the structures produced were probably initially upright. After D 2 , staurolite, and locally kyanite, grew under moderately high pressure conditions. Continued increase in metamorphic grade was, however, apparently accompanied by regional uplift and erosion, for staurolite breakdown occurred at lower pressures than those required to form kyanite. Furthermore, the highest grade metamorphism was accompanied by a steepening of the thermal gradient, since breakdown of the assemblage staurolite + muscovite + quartz produced andalusite at high structural levels, but sillimanite deeper down. D 3 deformation began after the initial uplift at the peak of metamorphism and produced two major northward-facing nappes thrust over a basal fold-nappe. The nappes root to the south of the Corcogemore Mountains but continued uplift in south Connemara faulted out the root zones as later nappes developed. After cooling, broad open D 4 folds were formed and the Connemara Schists were thrust up and to the south over lower grade rocks. Uplift of the Connemara region may have been complementary to the subsidence of the Mayo Trough to the north, in which case the oldest Ordovician rocks in South Mayo may have been deposited at the same time that the peak of metamorphism was attained in Connemara.