Combined Nd and Sr isotope data for the Hercynian granites of SW England provide important new evidence for the origin of the Cornubian batholith. The Nd isotope systematics are shown to be robust even in areas of extensive hydrothermal alteration. ENd values for the main plutons range from −4.7 to −7.1 whereas values for the Palaeozoic country rocks are significantly lower, typically –8 to –11. Depleted mantle Nd model ages (1.3–1.8 Ga) indicate a heterogeneous source region for the granite magmas and imply that the age of the basement beneath southern England is significantly older than previously estimated. Calculated ENd values for the Palaeozoic country rocks at the time of granite intrusion suggest that they were unlikely to have been the main source material for the magmas. END-ESr data for the granites do not define a simple binary mixing array between a mafic, depleted mantle melt and Proterozoic crust. The best fit model is consistent with partial melting of a composite lower crustal source comprising immature metasedimentary and mafic meta-volcanic rocks, and mixing with a relatively minor contribution of basaltic magma extracted from a slightly enriched mantle source. Such hybrid magmas are thought to have been generated during an Ivrea-type underplating of the lower crust during continental collision; their isotope signatures being determined by the relative proportions of crust and mantle material.