Carboniferous dikes of West Connacht, Ireland

Abstract

Vigorous Carboniferous extensional tectonism and associated igneous activity is a feature of Ireland and the neighbouring areas of Newfoundland and Britain. In West Connacht, Ireland, dolerite dikes of late Carboniferous age are newly recognised and distinguished from Tertiary dolerites. In the N, the 320 Ma-old Logmor dike trends N–S across the Ordovician South Mayo Trough, and has an evolved, mildly alkaline basalt geochemistry. In the S, in Connemara, a diffuse and arcuate swarm of ENE–NE trending, 305 Ma-old dolerite dikes, the Teach Doite swarm, cuts the Galway Granite batholith and its envelope of Dalradian gneisses. These more tholeiitic dolerites reveal the mineralogical and geochemical effects of varying degrees of pervasive hydrothermal alteration, imposed on an original magmatic composition similar to that of mid-ocean ridge basalts. Variscan overprinting on k–Ar ages from West Connacht dolerites clusters at 300, 245 and 205 Ma. Overprinting is also a feature of much of the Galway Granite. Carboniferous diking in West Connacht may have responded to nascent opening of the proximate North Atlantic, its pattern related to domal uplift adjacent to the rifted zone.

Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

“…The pre-Tertiary dolerite dykes of W Connacht (Mitchell & Mohr 1987) comprise the Logmor and Teach Doite (TD) Carboniferous and Paleocene dykes in W Connacht: thick plain trace = Carboniferous; thin ticked trace = Paleocene. Sample localities are box enclosed numbers (prefixed by IC in the text).…”

“…suites which run north and south of the Connemara Dalradian belt respectively. Locations and mineral modes of most samples analysed in this study are given in Mitchell & Mohr (1987): additional data are given in Table 1.…”

“…Averaging 6 m wide, it dips steeply westward, switching to east only where traversing the Caledonian Maum Valley Fault Zone. The dyke experienced local ESE faulting during intrusion (Mitchell & Mohr 1987). Logmor dyke exposures can be traced to within 750-100 m below the sub-Visean peneplain (Dewey & McKerrow 1963), suggesting that it was intruded relatively near-surface, depending upon the thickness of overlying Carboniferous strata which was probably < 2 k m (Sevastopulo 1981).…”

“…Table 1 Localities and mineral modes of samples not previously described by Mitchell & Mohr (1987 Hb 13, Q 3 , Epl Ep2…”

Spinel lherzolite and other xenoliths from a dolerite dyke in southwest Donegal

“…The dyke is considerably richer in K 2 O, Rb, Sr, Nb and Zr than any of the Tertiary Antrim Lava Series basalts (Lyle, 1980(Lyle, , 1985 or Tertiary dolerite intrusions from western Ireland (Mohr, 1982). However, the K 2 O and Rb are closely similar to those of Carboniferous Logmor dykes from West Connacht, as are the high total Fe and low Na 2 O/K 2 O (Mitchell & Mohr, 1987). The La/Nb ratio, close to 0.5, is considerably lower than values for the Tertiary Antrim basalts (approximately 3; see Lyle, 1985 a, b), or for Tertiary dykes from Ardgour (approximately 1.4; see Morrison, Hendry & Leat, 1987).…”