The columnar jointed basalts of the Giant's Causeway in Northern Ireland are a localized group of flows, the Causeway Tholeiite Member within the Tertiary Antrim Lava Group. Major and trace element analyses, including rare-earth element (REE) data, are presented which show a range of compositions from hy-normative tholeiites with mid-ocean ridge basalt (MORB) characteristics, to qz-normative examples with SiO2 > 54% and K2O > 1.0%. Within the Causeway tholeiites are three well defined chemical sub-groups which can be correlated stratigraphically across the area. Zr/Nb data suggest a single mantle source for the lavas with the range of composition the result of variable degrees of partial melting of a progressively depleted mantle, followed by fractional crystallization of the magmas at higher crustal levels. Mass balance calculations are used to model the fractionation processes involved. The lavas of the Causeway result from tectonically controlled decompression melting with magma equilibration in ephemeral chambers at high crustal levels followed by eruption as sheet flows. Disruption of the contemporaneous drainage system and the subsequent flooding of hot flow surfaces modified the cooling regime and formed the characteristic multi-tiered flows with colonnade and entablature development.