Potassium (K)-rich volcanic rocks occur within Permian sedimentary rocks in SW England and are approximately contemporaneous with the emplacement of the Cornubian granite batholith. The volcanic rocks have chemical characteristics of subduction-related magmas and may have been derived by small amounts of partial melting of heterogeneous large-ion lithophile (LIL) enriched mantle with the assemblage olivine–pyroxene–garnet–phlogopite–titanate. The LIL enrichment may have occurred during shallow or oblique subduction of oceanic lithosphere below SW England during the Devonian and Carboniferous. Such LIL-enriched mantle may have contributed some components to the Cornubian granite batholith.