ND AND SR ISOTOPE CONSTRAINTS ON THE ORIGIN OF THE CORNUBIAN BATHOLITH, SW ENGLAND

Combined Nd and Sr isotope data for the Hercynian granites of SW Engla nd provide important new evidence for the origin of the Cornubian bath olith. The Nd isotope systematics are shown to be robust even in areas of extensive hydrothermal alteration. epsilon(Nd) values for the main plutons range from -4.7 to -7.1 whereas values for the Palaeozoic cou ntry rocks are significantly lowe;, typically -8 to -11. Depleted mant le Nd model ages (1.3-1.8 Ga) indicate a heterogeneous source region f or the granite magmas and imply that the age of the basement beneath s outhern England is significantly older than previously estimated. Calc ulated epsilon(Nd) 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. epsilon(Nd)-epsilon(Sr) data for the granites do not define a simple binary mixing array between a maf ic, depleted mantle melt and Proterozoic crust. The best fit model is consistent with partial melting of a composite lower crustal source co mprising immature metasedimentary and mafic meta-volcanic rocks, and m ixing with a relatively minor contribution of basaltic magma extracted from a slightly enriched mantle source. Such hybrid magmas are though t to have been generated during an Ivrea-type underplating of the lowe r crust during continental collision; their isotope signatures being d etermined by the relative proportions of crust and mantle material.