ABSTRACT

The transition from tholeiitic to alkali basalts, which is well established in the Hawaiian series, is exposed in numerous other oceanic islands. Two case studies are presented here, which illustrate the more general geochemical evolution with time from “subalkaline” to “alkaline” compositions: Fangataufa atoll (French Polynesia, Pacific Ocean) and Kerguelen archipelago (T.A.A.F., South Indian Ocean). At Fangataufa atoll, drillings by the French C.E.A. have reached the basaltic bedrock under coral cap rock formations. Two basalt types have been cored: tholeiites form the submarine volcanic sequence, while alkali basalts constitute the top of the submarine zone and the whole subaerial sequence. Tholeiites contain less than 0.9 wt% K₂O with Na₂O/K₂O ranging mostly between 4 and 8, while alkali basalts contain up to 1.8 wt% K₂O with Na₂O/K₂O ranging mostly between 2 and 4. Both suites underwent weak differentiation effects. In Kerguelen archipelago, two Miocene (26–6 Ma) magmatic episodes are recorded in the southeastern province (Ronarc'h and Jeanne d'Arc peninsulae). Lower Miocene mildly alkaline basalts (0.4–2.1 wt% K₂O and Na₂O/K₂O ranging between 2 and 4) are overlain by Upper Miocene highly alkaline basalts (1.4–3.7 wt% K₂O and Na₂O/K₂O between 1 and 2). Lower Miocene magma type evolved towards trachyte compositions, whereas trachy-phonolite and phonolite compositions constitute the Upper Miocene residual melts. The thin transition zone with interstratified tholeiitic and alkali basalts, which is well exposed at Hawaii, was not observed in both Fangataufa and Kerguelen. These two case studies confirm that alkali contents increase with time within oceanic island basalts. Present data indicate that one single source is evidenced in the short-lived Fangataufa oceanic island, while more than one source was tapped in the long-lived Kerguelen magmatic system.