TRANSITION BETWEEN THOLEIITIC AND ALKALI BASALTS - PETROGRAPHIC AND GEOCHEMICAL EVIDENCE FROM FANGATAUFA, PACIFIC-OCEAN, AND KERGUELEN, INDIAN-OCEAN

The transition from tholeiitic to alkali basalts, which is well establ ished in the Hawaiian series, is exposed in numerous other oceanic isl ands. Two case studies are presented here, which illustrate the more g eneral geochemical evolution with time from ''subalkaline'' to ''alkal ine'' compositions: Fangataufa atoll (French Polynesia, Pacific Ocean) and Kerguelen archipelago (T.A.A.F., South Indian Ocean). At Fangatau fa atoll, drillings by the French C.E.A. have reached the basaltic bed rock under coral cap rock formations. Two basalt types have been cored : tholeiites form the submarine volcanic sequence, while alkali basalt s constitute the top of the submarine zone and the whole subaerial seq uence. 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 w eak differentiation effects. In Kerguelen archipelago, two Miocene (26 -6 Ma) magmatic episodes are recorded in the southeastern province (Ro narc'h and Jeanne d'Arc peninsulae). Lower Miocene mildly alkaline bas alts (0.4-2.1 wt% K2O and Na2O/K2O ranging between 2 and 4) are overla in 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 trachyt e compositions, whereas trachy-phonolite and phonolite compositions co nstitute the Upper Miocene residual melts. The thin transition zone wi th interstratified tholeiitic and alkali basalts, which is well expose d at Hawaii, was not observed in both Fangataufa and Kerguelen. These two case studies confirm that alkali contents increase with time withi n 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 syste m.