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

Abstract

Dolerite dykes in Connemara and Murrisk that yield c. 315 Ma K-Ar ages vesiculated in varying degrees at the present exposure level. This incipient degassing suggests that the dyke-contemporary land surface lay not more than several hundred metres above the present-day surface. The West Connacht peneplain was cut across Dalradian and Ordovician-Silurian rocks during mid-late Devonian, following which it was buried under Devonian-Carboniferous sedimentary strata since almost entirely removed. Triassic-Jurassic rifting of the central Atlantic basin produced a strong thermal imprint on the rocks of the west of Ireland, an event assumed to have been accompanied by crustal uplift and faulting to produce horst-graben terrain. Ensuing erosion throughout Jurassic-Cretaceous time stripped off much or all of the late Palaeozoic cover. If and where the stripping penetrated into the sub-Carboniferous peneplain, then to that extent the present West 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 greater extent than their late Palaeozoic precursors. This is consistent with geologically estimated shallower levels of emplacement into the contemporary crust, assuming similar magmatic steam overpressures at given depths. Any thickness of Chalk over West Connacht cannot have been significant in terms of overburden pressure. Late Tertiary uplift of the West Connacht peneplain produced the present Connemara-Murrisk plateau which has been deeply dissected by Pleistocene glaciation.