Late Palaeozoic and Palaeocene magmatic intrusion levels in West Connacht and inferences for palaeotopography

Dolerite dykes in Connemara and Murrisk that yield c. 315 Ma K-Ar ages vesi culated in varying degrees at the present exposure level. This incipient de gassing suggests that the dyke-contemporary land surface lay not more than several hundred metres above the present-day surface. The West Connacht pen eplain was cut across Dalradian and Ordovician-Silurian recks during mid-la te Devonian, following which it was buried under Devonian-Carboniferous sed imentary strata since almost entirely removed. Triassic-Jurassic rifting of the central Atlantic basin produced a strong t hermal imprint on the rocks of the west of Ireland, an event assumed to hav e been accompanied by crustal uplift and faulting to produce horst-graben t errain. Ensuing erosion throughout Jurassic -Cretaceous time stripped off m uch or all of the late Palaeozoic cover. If and where the stripping penetra ted into the sub-Carboniferous peneplain. then to that extent the present W est Connacht peneplain has an end-Mesozoic sculpture imprinted on it. Palaeocene dolerite dykes in Connemara, Murrisk and North Mayo, associated with initiation of the North Atlantic basin, degassed to a generally greate r extent than their late Palaeozoic precursors. This is consistent with geo logically estimated shallower levers of emplacement into the contemporary c rust. assuming similar magmatic steam overpressures at given depths. Any th ickness of Chalk over West Connacht cannot have been significant in terms o f overburden pressure. Late Tertiary uplift of the West Connacht peneplain produced the present Connemara-Murrisk plateau which has been deeply dissec ted by Pleistocene glaciation.