Volcanic rocks of Cambrian age from Zafra (Ossa-Morena Zone, Iberian Massif) are the result of rift processes that affected Cadomian arc units accreted to the NW edge of Gondwana during the Neoproterozoic–Early Cambrian transition. Tephrite to rhyolite volcanics define an alkaline transitional association (Coombs type). Basic–ultrabasic rocks exhibit typical alkaline REE-patterns, strongly enriched in LREE with respect to HREE. Two parental magmas are identified, one with a mantle signature, lack of Nb negative anomaly and εNd500Ma +3.8 to +4.2; another with crustal contribution, minor Nb negative anomaly and εNd500Ma +0.8 to +1.8. Intermediate-acid rocks show variable REE fractionation and share geochemical characteristics of both basic–ultrabasic groups with restricted εNd500Ma +2.2 to 3.1 and general absence of Nb negative anomaly. Basic–ultrabasic melts resulted from different amounts of partial melting of a homogeneous source and segregation at the garnet–spinel transition zone. We argue that the “Hales transition” recently recognized in reflection seismic experiments of SW Iberia might image such a source region. Mantle-derived magmas ponded at the base of the crust and weakly interacted with crustal rocks/melts, whilst intermediate-acid rocks were generated by plagioclase±clinopyroxene±amphibole fractionation. Melt ascent took place through fractures, with limited crustal interaction. Based upon the new geochemical results and complementary cartographic and geophysical data, a model is presented for the Cambrian break-up of North Gondwana due to magma ascent from the mantle.