Abstract Young volcanic centres of the Bransfield Strait and James Ross Island occur along back-arc extensional structures parallel to the South Shetland island arc. Back-arc extension was caused by slab rollback at the South Shetland Trench during the past 4 myr. The variability of lava compositions along the Bransfield Strait results from varying degrees of mantle depletion and input of a slab component. The mantle underneath the Bransfield Strait is heterogeneous on a scale of approximately tens of kilometres with portions in the mantle wedge not affected by slab fluids. Lavas from James Ross Island east of the Antarctic Peninsula differ in composition from those of the Bransfield Strait in that they are alkaline without evidence for a component from a subducted slab. Alkaline lavas from the volcanic centres east of the Antarctic Peninsula imply variably low degrees of partial melting in the presence of residual garnet, suggesting variable thinning of the lithosphere by extension. Magmas in the Bransfield Strait form by relatively high degrees of melting in the shallow mantle, whereas the magmas some 150 km further east form by low degrees of melting deeper in the mantle, reflecting the diversity of mantle geodynamic processes related to subduction along the South Shetland Trench.

We report new 40 Ar/ 39 Ar ages on submarine lavas from the Azores Plateau that yield ages of 6–4.9 Ma and 1.5 Ma to Holocene. An additional sample from the eastern plateau gives an age of 39 Ma. Thus, at least two, possibly even three eruptive phases occurred in the past 39 m.y. The lava compositions range from slightly incompatible trace element–enriched tholeiites to highly enriched alkali basalts similar to those erupted on the Azores islands. The less enriched tholeiitic lavas occur in the westernmost plateau and formed by relatively high degrees of partial melting. The highly enriched alkali basalts appear to be restricted to local volcanic structures, implying different phases of intraplate volcanism that are possibly related to the jump of the ultraslow-spreading Terceira Rift within the Azores Plateau. The abundance and widespread spatial distribution of volcanism with ages of 6–4.9 Ma in subaerial and submarine lavas implies the presence of a thermal or chemical melting anomaly and a period of enhanced volcanism that led to tholeiitic volcanism following, and being followed by, alkali-basaltic volcanism. The small-scale (Sr-)Nd-Pb isotopic heterogeneity of the Azores plume source implies limited mixing in the mantle, in contrast to observations made on other plume-related settings such as the Galapagos. We suggest that the Azores Plateau initially formed from interaction between geochemically and thermally anomalous mantle, possibly a small mantle plume, arriving underneath the lithosphere, and the local plate tectonics, i.e., the Terceira Rift axis, provided ascent paths for the generated magmas.