Mantle, lithospheric and crustal controls on large igneous province magmatism: petrogenesis of the Antrim Lava Group, North Atlantic Igneous Province

Elliot J Carter , Michael Stock, Adam Beresford-Browne, Hannah Talbot, Mark Cooper

Continental large igneous province (LIP) volcanism is the result of long-lived interactions between mantle plumes, lithospheric topography, ascending melts, and crustal lithologies. Disentangling the competing effects of such factors is required to understand the petrogenesis of LIP magmas and ultimately the controls on their geochemical and isotopic diversity and on ore-forming processes. The British and Irish Paleogene Igneous Province (BIPIP), part of the North Atlantic Igneous Province, is among the best studied LIPs. Despite this, the largest onshore extrusive succession, the Antrim Lava Group (ALG), Northern Ireland, has been comparatively understudied in recent years. The ALG succession shows several notable features – including a near-hiatus in volcanism followed by coexisting eruption of depletion and enriched melts – which challenge existing models of magmatism developed in the Scottish Hebrides. New high quality major and trace element data have been collected from mafic and silicic units from across the Antrim Plateau. Combined with a large compilation of geochemical and isotopic data, these allow a self-consistent petrogenetic model for the ALG and the wider BIPIP. Geochemical indices of fractionation (MgO, CaO) correlate positively with stratigraphic height through both main lava successions suggesting two cycles of thermal maturation of the crust. In contrast La/Nd and Ba/Zr decrease with stratigraphic height indicating decreasing crustal assimilation during each maturation cycle of the system. Across the Antrim and Scottish centres, olivine-plagioclase-augite-melt (OPAM) barometry shows systematically lower storage pressures for more tholeiitic units relative to transitional/alkali basalts, which may relate to differences in melt flux or preservation biases. There is a systematic difference in composition between the two main ALG units in the east of the Antrim Plateau, with the Lower Basalt Formation (LBF) consistently enriched in incompatible elements including Ti, Zr and V relative to the Upper Basalt Formation (UBF). Forward melting models indicate that these compositional variations cannot be reconciled by variable melt fractions of a single mantle source and instead require that the eastern UBF and Causeway Tholeiite were derived from a source which was depleted relative to that which formed the LBF. These differences do not apply in the western plateau however where UBF compositions are indistinguishable from the LBF. Simultaneous, eruption of depleted and enriched UBF in the east and west, respectively, can be reconciled by a model of localised lithospheric thinning and source depletion focused over areas of highest magmatic flux, which matches geophysical observations; this can explain similar alkali to tholeiite transitions in other LIP successions. Finally, the near-hiatus which punctuates volcanism in Antrim requires an interval of low mantle Tp and is suggested to reflect a pulsing plume and regional shutdown of most magmatism except in localised areas of thinned lithosphere.