New high-precision Pb-Sr-Nd isotope, major and trace element and mineral chemistry data are presented for the submarine stage of ocean island volcanism on Santiago, one of the southern islands of the Cape Verde archipelago. Pillow basalts and hyaloclastites in the Flamengos Valley are divided into three petrographic and compositional groups; the Flamengos Formation lavas (4·6 Ma) dominate the sequence, with the younger Low Si and Coastal groups (2·8 Ma) found near the shoreline. Olivine and clinopyroxene compositions and isotopic data for minerals and their host melts indicate disequilibrium between some crystals and the melt. Intra-sample disequilibrium suggests homogenisation of liquids but eruption before complete equilibration between crystals and melt preserves the heterogeneity. Pressures of crystallization for clinopyroxene (0·4-1·1 GPa) indicate stalling and crystallization of the magmas over a range of depths in the lithosphere. Major element compositions indicate melting of a carbonated eclogite source. Sr-Nd-Pb isotope data suggest the involvement of FOZO-like and EM1-like components in the mantle source, which are simultaneously available at all depths in the melting column. The Flamengos Valley lavas display large compositional variations, often between stratigraphically adjacent flows; these frequent abrupt changes of magma composition suggest stalling and crystallization of discrete magma batches on transport through the lithosphere.