A major part of the Palaeogene North Atlantic Igneous Province (NAIP), the British Palaeogene Igneous Province (BPIP) has been extensively studied in the past, with work on the Hebridean islands of Skye, Mull and Rum making major contributions to research that underpinned much of modern igneous petrology in the early and mid 20th century.
The Antrim Lava Group in NE Ireland is the least studied part of the BPIP, yet it covers and area in excess of 4000 km2, with a thickness of > 700 m reported in some deep drillholes. The aim of this research project is to carry out a modern, yet stratigraphically constrained, geochemical, and geochronological investigation of this lava suite. The sequence of basalts are divided by a horizon of deep weathering, the Interbasaltic Formation, that marks a time break in basalt effusion that was partly punctuated by eruption of rhyolite and other differentiated compositions. U-Pb zircon dating of these rhyolites has yielded an important time marker (58.4 ± 0.7 Ma) for the basalt sequence, that we propose to expand with a detailed Ar/Ar dating study.
The Antrim lavas are mostly olivine tholeiites with only a few rocks showing quartz or nepheline in their normative composition. The Upper and Lower basalts are magnesian (> 9% MgO), however the Causeway Member of the Interbasaltic Formation is more evolved and forms a low-MgO group. Trace element data 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, with light REE showing low ratios (e.g. (La/Sm)CN ~ 0.83), and fractionated heavy REE showing high ratios (e.g. (Ga/Yb)CN ~ 1.6). In contrast, the Interbasaltic formation basalts are consistently depleted in REE ((La/Sm)CN ~ 1.71, (Ga/Yb)CN ~ 1.21) (Barrat & Nesbitt, 1995), invoking a varying source for the Antrim Lava Group.
It is clear that simple models of lithospheric stretching and rifting cannot explain the Antrim situation. The recent completion of the GSNI’s TELLUS project provides an excellent opportunity to readdress the Antrim lavas. Utilising the TELLUS data sets, our sampling will concentrate on identified key successions in the Upper and Lower Basalt Formations, along with the dolerite sills and dykes that crosscut them. The relationship between the latter intrusive features and the main lava flows is of particular interest, as are the geochemical trends in the previously un-investigated sills. Analyses will include major and trace element analysis as well as Sr, Nd and Pb isotopes. In addition, the uppermost and lowermost flows in the succession will be dated, using the 40Ar/39Ar method in order to obtain a better understanding of the time span between the commencement and cessation of eruptive activity. This will enable magma flux and eruptive rates to be determined, which, along with the geochemical analyses, should provide detail into the eruptive patterns and emplacement history.