PETROLOGY AND GEOCHEMISTRY OF THE KERGUELEN ARCHIPELAGO BASALTS (SOUTH INDIAN-OCEAN) - EVOLUTION OF THE MANTLE SOURCES FROM RIDGE TO INTRAPLATE POSITION

Kerguelen basic lavas belong to three magmatic series: transitional, mildly alkaline and highly alkaline, showing from the first to the latter, a general increase in alkali and incompatible element contents as well as some of the incompatible trace element ratios [( Ce Yb)n, Nb Zr, Nb Y, Zr Y, Ti Zr]. Ba Nb, La Nb and all ratios involving Th are in the range of those observed for Dupal type OIBs although subtle differences exist mainly reflecting a slight Th enrichment in the Kerguelen basalts relative to other OIBs. New Sr, Nd and Pb isotope data strengthen the previously identified Dupal signature of Kerguelen rocks. While the transitional series, older than 26 Ma, shows slightly depleted Sr and Nd isotopic signatures ( 87Sr 86Sr < 0.705 and ε{lunate}Nd > 0), the alkaline series, between 26 and about 8 Ma, show a slightly enriched signature which is strengthened in the highly alkaline series ( 87Sr 86Sr up to 0.70056), younger than 12 Ma. Pb isotope ratios do not show any significant difference between these three series. These isotopic data coupled with trace element geochemistry indicate a mixing process between a depleted MORB component and an enriched OIB-type component, the Dupal plume. The Pb isotope characteristics indicate the presence of an old recycled component (continentally derived sediment or crust itself) incorporated into the deep plume source-region which may be responsible for this enrichment. The Kerguelen Islands show a geodynamic evolution from an early ridge centered stage, above or close to the South East Indian Ridge (SEIR) 45 Ma ago, to the present day intraplate setting. The geochemical and isotopic variations reflect this geodynamic evolution and correspond to variable degrees of mixing between the two extreme components (SEIR, MORB and the Kerguelen plume) whose relative proportions evolve with the archipelago position, i.e. with the distance from the ridge at the time of eruption. © 1990.