The geochemical composition of Ocean Island Basalts (OIBs) from the Azores reflects the spatial distribution, shape, and temporal evolution of small-scale geochemical heterogeneities within their mantle plume source. Here, we investigate the time-related evolution of volcanism at São Jorge Island, Central Azores. New field observations, a magnetic survey, 40Ar/39Ar and 14C ages and geochemical data indicate that the fissural volcanic activity at São Jorge produced at least four main mafic volcanic complexes (V. C.). The oldest V. C., São João, produced the thickest lava piles at ca. 1.3 Ma in the eastern part of the island. After a period of quiescence, the Serra do Topo V. C. was produced at ca. 0.8-0.5 Ma in the central part of the island. The Rosais V. C. was emplaced between ca. 0.4 and 0.1 Ma on the entire island. Finally, the Holocene Manadas V.C. volcanism became active in the western part of the island and includes three historic eruptions (1580, 1808 and 1964 CE).

Magmas were formed at low melting degrees from a peridotitic mantle with possible minor contributions of recycled components. Olivine compositions and whole-rock trace element ratios discard a significant contribution from pyroxenitic source rocks. Melting temperatures (ca. 1420-1480 °C) were slightly higher than those of the ambient upper mantle. The four V.C. are characterized by distinct geochemical compositions in terms of incompatible trace elements and Sr-Nd-Pb isotopic ratios. The oldest V.C., São João, is characterized by Pb isotopic compositions (e.g., markedly negative Δ7/4 and 8/4 values) plotting well below the Northern Hemisphere Reference Line (NHRL). The Upper Pleistocene (Rosais V.C.) lavas from the north-western cliffs have compositions similar to enriched mantle (EM) end-member basalts (e.g., high 207Pb/204Pb at moderate 206Pb/204Pb; high Ba/Nb), which are rare among northern hemisphere OIBs. Finally, high 206Pb/204Pb (up to 20), reflecting contribution from a HIMU-type component characterizes the Holocene Manadas lavas and is occasionally found in lavas from other volcanic complexes from 1.3 to 0.1 Ma. These findings indicate that magmas from São Jorge and the nearby Central Azores islands were sourced from a strongly heterogeneous mantle plume, which displayed localized filaments of heterogeneous material that were rapidly exhausted (in ca. 0.2 Ma). The dominant component at São Jorge and in the Central Azores in general appears to be the HIMU-type endmember, which instead is not significant in the Eastern Azores. Possibly, the Central and Eastern Azores were produced by distinct branches of the Azores mantle plume, as would also be consistent with previous tomographic studies.