Dykes, minor igneous intrusions, Palaeogene, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
M R Cooper and T P Johnston
Throughout the early Palaeogene (Palaeocene) the continental crust of Northern Ireland was in a state of tectonic extension . Magma, of mainly basaltic composition, was intruded into the dilated crust and formed linear dyke swarms in northwest Britain  and Northern Ireland (P947872). A small number of dykes in Co. Fermanagh are multiple intrusions up to 30 m wide compared to the normal range of 1–5 m. Some of these dykes clearly acted as long-term conduits for eruptive volcanism that was evidently more extensive than the present outcrop would indicate. Age dating of the Doraville Dyke in Co. Fermanagh at 69Ma raises the possibility that its intrusion occurred in the Late Cretaceous .
Dyke intrusion took place throughout the period of early Palaeogene magmatism and pre- and post-dates the central complexes of Carlingford, Slieve Gullion and the Mourne Mountains. The basalt lavas of the Antrim Lava Group were erupted from linear fissures fed by major dykes which have mostly been eroded or remain concealed. Nevertheless, the Upper Basalt Formation is cut by a combination of dykes and elongate dyke-like plugs such as Slemish [D 222 054] and Craigcluggan [D 294 080] .
A major dyke swarm extends southeastwards across the north of Ireland (P947872) and represents a continuation of the Donegal-Kingscourt dyke swarm that reach from Rockall, through Ireland to Anglesey and North Wales and into the Midlands of England. Regional doming and rifting between Greenland and northwest Europe in the Late Cretaceous and early Palaeogene resulted in crustal dilation and the opening of fractures. Initially, the fractures lay parallel to a northeast-southwest trending rift zone and although followed by dykes in Greenland and the Faeroe Islands were unable to completely release tensional stresses caused by the doming.
In Ireland, the dyke swarm intruded a complementary set of northwest-southeast trending fractures that subsequently developed on the margin of the North European Plate. It is most likely that the source of these dykes was located northwest of Ireland. The dykes transect all major structural elements and only rarely are offset by contemporaneous or later faults. Dyke thickness is extremely variable and while thin examples may not be identifiable beyond one-exposure, mega-dykes, 10–100 m wide, can be traced for tens of kilometres at outcrop and on regional magnetic anomaly maps . Numerous examples in Counties Fermanagh and Tyrone intrude Devonian and Carboniferous rocks, such as the 27 km long Cuilcagh-Glenfarne Dyke ,  and the 30–35 km long Irvinestown Dyke which transects the western end of the Fintona Block . Most of the mega-dykes are moderately fresh, feldsparphyric olivine basalt and dolerite. The multiple nature and gabbroic core of the Doraville Dyke is exposed in the quarry [H 200 558] southwest of Irvinestown and illustrates a long history of intrusion.
The minor dykes in Counties Fermanagh and Tyrone are usually of olivine basalt or dolerite composition. Most have been altered in the zeolite facies, and contain saponite and carbonate, the most common alteration products, occurring in small circular vesicles. However, the vesicles occasionally contain silica (chalcedony) making these dykes unusually hard, such as the example intruding the Carboniferous Ballyshannon Limestone Formation in the quarry [H 243 630] 6.4 km east of Kesh.
Within the dyke swarm are rare examples of olivine-free basalt and olivine-rich picro-dolerite. A small number of dykes intruding the Dalradian rocks northeast of Lough Derg have relatively evolved trachybasalt or rhyolite compositions. They include a 2.5 m wide quartz trachyte or rhyolite dyke in the River Derg [H 096 765] and a 2.7 m wide trachybasalt dyke with fresh andesine phenocrysts at Tievenameenta [H 152 756]. On the east bank of the Owenboy Burn at Owenboy [H 123 768] an intrusion of porphyritic rhyolite forms a 1.75 m thick sheet inclined 40° to the southeast and has thin margins of pitchstone. The upper 1.5 m is crudely columnar jointed while the lower part is flow-banded and intensely jointed.
In Co. Down, the Palaeogene dyke swarms follow a NNW-orientation and are localised around Slieve Gullion and the Mourne Mountains, between Ardglass and Hillsborough and in the Belfast area. There appears to be a discontinuity between the swarms in Co. Down and southeast Co. Antrim that may reflect late Palaeogene dextral strike-slip on the Southern Upland Fault . The dykes in north Co. Down consist predominantly of olivine basalt and olivine dolerite with examples of silica-rich, olivine-free tholeiitic basalt.
Dykes pre- and post-date the intrusion of the Mourne Mountains granites . The younger dyke swarm includes olivine-bearing and olivine-free (tholeiitic) basalt, andesite, dacite, quartz-feldspar and feldspar-rich porphyry and felsite. Dykes are well exposed on the coast south of Newcastle. A number of composite intrusions occur in this area, the most notable example being the Glasdrumman Cone Sheet (P948039). The intrusion outcrops (P947873) on the shore at Glasdrumman Port [J 380 225] and extends for over 20 km along the east and southeast sides of the Eastern Mournes Centre. It is concentric to the margin of the Eastern Mournes Centre, dips to the north and west, and consists of quartz-feldspar porphyry with thin basaltic margins.
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