The minor intrusions and structure of the Carlingford complex, Eire

The Carlingford volcanic complex has an intense swarm of minor intrusions associated with it. Field work demonstrates that the swarm is composite, having involved the repeated injection of magma as dykes, cone sheets and complementary cone sheets in at least three rythmic cycles. Three major rock series are represented in these bodies: the tholeiitic basalt - craignurite series, the alkali basalt series and the high alumina basalt series, other rock types such as felsites, quartz feldspar porphyries and lamprophyres are present in minor proportions. Analysis of the structure of individual swarms suggests that basaltic magma was injected into the crust as an elongate ridge like body culminating in a cupola beneath the complex, from which cone sheet and radial dyke swarms emerged; linear dyke swarms arose from the ridge crest. Detailed geochemical investigations show, that the main magma series were derived by fractionation from basaltic parents; that two fractionation paths are represented in the alkali basalt series (the trachy basalt - tristanite series and hawaiite - mugearite - benmorite series); that craignurites represent a true fractionation product of tholeiitic magma and are petrochemically distinct from icelandites. Basalt fractionation at Carlingford was dominated by the separation of olivine and plagioclase at low pressure while the petrochemistry of the rocks suggests that the three parental stems - tholeiitic basaft, alkali basalt and high alumina basalt, were derived from a common parent by fractionation at higher pressure. Time relations indicate that common parent to be olivine tholeiite magma which it is suggested was formed at depths in excess of 60 km. Miscellaneous rock types, including felsites,lamprophyres and quartz feldspar porphyries are shown to have been derived by melting of crustal rocks during the injection of basic magma within the complex.