Permian basic/ultrabasic lavas from south-west England may be divided into a ‘basaltic’ and a K-rich group. Both groups have enrichment of large-ion lithophile (LIL) elements relative to high field strength (HFS) elements, and the K-rich group show large degrees of LIL enrichment (c.50–500 times primordial mantle) in association with varied transition element concentrations. Samples from both groups ⁸⁷Sr/⁸⁶Sr_i = 0.704–0.705 and ¹⁴³Nd/¹⁴⁴Nd_i = 0.5123–0.5127 and plot close to the mantle array on an ɛ_Sr−ɛ_Nd diagram. These data are interpreted in terms of derivation of the lavas from magmas resulting from partial melting of mantle which had experienced less (for the basaltic group) or more (for the K-rich group) enrichment in LIL elements as a result of migration of mantle melts. Such enrichment accompanied or followed subduction of oceanic lithosphere below south-west England. The resultant magmas experienced fractional crystallization of olivine and pyroxene prior to eruption.