Geochemistry of the tertiary volcanism of Northern Ireland

The Antrim Plateau (Northern Ireland) forms the southwestern part of the British Tertiary Igneous Province and is dominated by basaltic lavas. Previous work divided the province into three stratigraphic formations, viz. the Lower, Middle and Upper Formations, with the Middle Formation having a basaltic member known as the Causeway Member. New chemical and Sr-Nd isotopic data covering the spectrum of lava types are presented. Three distinct geochemical types are recognised: (1) basalts of the Lower Formation and most of those from the Upper Formation are LREE-enriched but have characteristic convex-up REE patterns with a maximum around Nd; (2) some of the Upper Formation basalts and the majority of the Causeway Member basalts are LREE-enriched with no convex shape in their REE pattern; and (3) some of the Causeway Member basalts are LREE-depleted and similar to N-MORB. Isotopic compositions of Sr and Nd show considerable variation throughout the three formations with epsilon(Sr)(t)= -22 to + 107 and epsilon(Nd)(t)= - 11 to + 8.5. The exceptionally wide ranges of isotopic compositions are believed to have been produced by assimilation of a crustal (mainly Dalradian derived) component. The REE patterns indicate that most of the basalts were derived from a LREE-depleted mantle and with the exception of the Causeway Member, most have convex-up REE patterns. These are interpreted as being due to residual garnet in the mantle source, which indicates melting at depths in excess of 80 km. This suggests that at the start of Tertiary volcanism, the melt regime was controlled by a thick lithosphere which thinned with time such that the Causeway Tholeiites were produced at shallower levels. The return to the convex-up patterns of the Upper Formation means that simple models of lithospheric stretching and rifting are not able to explain the Antrim situation.