The geology of the Clifden district, Connemara Co. Galway, Ireland and present understanding of Connemara geology

Abstract:The Clifden district, illustrated with an accompanying new detailed 1:12,500 geological map and fold trace map, includes most of the stratigraphy of the Connemara Dalradian and is used to summarise present understanding of the older geological history of Connemara in the light of new structural work and recent age determinations. The Grampian metamorphism peaked with sillimanite formation during and after the late D2 syntectonic intrusions of metagabbros at 475–470Ma and the formation of the major D2 Derryclare fold which was then repeatedly folded by numerous major D3 folds synchronous with the injection in the south of the 467Ma Quartz diorite gneiss suite, maintaining the high-grade metamorphism. Late D3 saw the general initiation of cooling and uplift, the latter pronounced in eastern Connemara above and around the subterranean gathering of the Oughterard Granite magma, plus movement on the late D3 Renvyle-Bofin slide, generating an area of lower pressure metamorphism with cordierite and andalusite not found in the Clifden area.This was rapidly followed, or the ending was even partially overlapped, by D4 folding creating the Connemara Antiform and Synform and Joyces Antiform (and ‘the steep belt’) through which the later now exposed phases of the 462Ma Oughterard Granite intruded. Cooling continued up to and after the ~454 ± 4Ma Ordovician D5 Mannin Thrust brought the Dalradian rocks and their contained metagabbros and gneisses southwards onto the 474Ma metarhyolites of the Delaney Dome but the displacement on the thrust may have been small. The dome formed by a combination of the last D5 movements which folded the thrust producing the E-W D5 Mannin Antiform, which itself was later deformed by N-S D6 folds which passed into faults as the rocks became cooler and more brittle. The unusual N-S folding became possible because the Delaney Dome was not underlain by E-W striking Dalradian as everywhere else in Connemara. D6 complementary NE-SW sinistral and NW-SE dextral faults combined with reworked E-W D3 slides were late Ordovician to at least 438Ma, Telychian. D6 movements opened a basin in the eroded basement, which combined with the extraordinarily high sea-level after the late Ordovician (Hirnantian) glaciation ended, received Silurian sediments, sometimes during continued faulting. Sedimentation was abruptly terminated at the end of the Wenlock, by a short-lived but major~426Ma Scandian D7 folding of the Silurian rocks and the basement, tightening the Connemara and Joyces D4 folds (and possibly also the Mannin Antiform) which became composite structures. D7 finished before the long continued (from 425Ma) D8 sinistral transpression to sinistral transtension which accompanied the intrusions of the circular Omey (422–3Ma), Inish (423–4Ma) and Roundstone Granites (420 ± 4Ma) and the bulk of the following intrusions (~415–374Ma) of the Galway Granite batholith.