GEOCHEMISTRY OF THE TERTIARY VOLCANISM OF NORTHERN-IRELAND

The Antrim Plateau (Northern Ireland) forms the southwestern part of t he British Tertiary Igneous Province and is dominated by basaltic lava s. Previous work divided the province into three stratigraphic formati ons, viz. the Lower, Middle and Upper Formations, with the Middle Form ation having a basaltic member known as the Causeway Member. New chemi cal and Sr-Nd isotopic data covering the spectrum of lava types are pr esented. 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 ma ximum around Nd; (2) some of the Upper Formation basalts and the major ity of the Causeway Member basalts are LREE-enriched with no convex sh ape in their REE pattern; and (3) some of the Causeway Member basalts are LREE-depleted and similar to N-MORB. Isotopic compositions of Sr a nd Nd show considerable variation throughout the three formations with epsilon(Sr)(t)= -22 to + 107 and epsilon(Nd)(t)= - 11 to + 8.5. The e xceptionally 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 de rived from a LREE-depleted mantle and with the exception of the Causew ay Member, most have convex-up REE patterns. These are interpreted as being due to residual garnet in the mantle source, which indicates mel ting at depths in excess of 80 km. This suggests that at the start of Tertiary volcanism, the melt regime was controlled by a thick lithosph ere which thinned with time such that the Causeway Tholeiites were pro duced at shallower levels. The return to the convex-up patterns of the Upper Formation means that simple models of lithospheric stretching a nd rifting are not able to explain the Antrim situation.