Mafic lamprophyre dykes in northern England were emplaced soon after the final closure of the Iapetus Ocean at the end of the Caledonian orogeny. High Mg/(Mg+Fe) ratios and Cr and Ni contents are consistent with equilibrium with mantle peridotite. Incompatible trace element abundances suggest that the mantle sources were metasomatized prior to the melting events. Three components are recognized in the lamprophyre chemistry: (i) a depleted mantle source, taken to be that overlying the subducting lithosphere; (ii) a H₂O-rich subduction zone component, related to the dehydration of the subducting oceanic crust; and (iii) a CO₂-rich phase thought to result from the degassing of the mantle after ocean closure. This multi-component origin may be applicable to other lamprophyres of calc-alkaline affinity.