The age, petrology, major and trace element geochemistry, and Sr–Nd–Hf–Pb isotopic geochemistry of basic and felsic rocks from the Val gabbro plutonic suite on the Kerguelen Archipelago, Southern Indian Ocean, are used to constrain the temporal and compositional relationships between sub-volcanic intrusions and flood basalt volcanism during the formation of a major oceanic island. The 4 km2 Val gabbro plutonic suite was emplaced at 24·25 ± 0·15 Ma (U–Pb zircon) into 25 Ma volcanic rocks of the Southeast Province, locally producing a large zone of overlying basaltic breccia. Cumulate basic–ultrabasic rocks are the dominant lithology in the intrusion, with horizontally layered peridotites at the base of the exposed part of the intrusion, overlain by vertically layered, coarse-grained plagioclase-bearing peridotites, melagabbros and equigranular gabbros. The intrusion was formed by repeated injections of relatively crystal-rich and crystal-poor magmas into an open-system magma reservoir. Strong geochemical and isotopic similarities between the fine-grained marginal microgabbros and cross-cutting felsic rocks and the hosting mildly alkalic basalts and trachytes of the Southeast Province indicate that they were derived from similar alkalic basaltic parental magmas, which were dominated by the enriched component of the Kerguelen mantle plume source. At 25 Ma, the change from tholeiitic–transitional to mildly alkalic basalts marks the terminal stage of flood basalt volcanism on the Kerguelen Archipelago. This compositional change was associated with deeper melting within the Kerguelen plume source, lower extents of melting, a decrease in magma supply, and the emplacement of high-level intrusions such as the Val gabbro plutonic suite.