AbstractThe Kerguelen Islands (6,500 km2) are the third largest oceanic archipelago after Iceland and Hawaii. One of their most remarkable geological features is the existence of several igneous complexes thought to represent the deep levels of volcano‐plutonic structures of which the upper part has been eroded. These plutonic bodies have ring shapes with diameters ranging from 1 to 15 km. The oldest (39–23 Ma) have transitional petrological features while the youngest (26 Ma–Recent) are typically alkaline. This sequence is also observed in the surrounding flood‐basalts. Both the basalts and the plutonic rocks have a mantle origin. The alkaline plutonic suites evolve towards silica‐over or undersaturated rock types depending on whether they are located to the west or to the east of a NNW–SSE boundary. The divergence in alkaline trends may be related to magmatic differentiation or linked to the evolution of two different initial magmas, one being mid‐alkaline and the other high‐alkaline. In both hypotheses fluids, particularly H2O, pl