Abstract

A new geological mapping of Raivavae island, Austral-Cook linear chain, French Polynesia, combined with 10 unspiked K/Ar ages measured on its lavas, shows that it was built during two successive volcanic phases : 10.6-7.4 Ma (dykes crosscutting Rairua submarine breccias and younger subaerial Rairua flows) and 6.4-5.4 Ma (Anatonu shield volcano and associated trachytic and phonolitic domes and plugs). Geochemical data from the present study and a previous one [Lassiter et al., 2003] demonstrate that the Rairua alkali basalts, picrobasalts and basanites are more enriched in incompatible elements (especially Th and Nb) than the predominantly tholeiitic Anatonu basalts. The isotopic signature of Rairua lavas displays a strong HIMU flavour, while that of Anatonu lavas is more subdued and intermediate between DMM, HIMU and EM end-members. Rairua mafic lavas show obvious petrologic and geochemical similarities with those of the neighbouring island of Tubuai. Both could result from the partial melting of a predominantly HIMU secondary plume, which formed sucessively Mangaia (19.4-18.4 Ma), the old lavas of Rurutu (12.7-12.1 Ma), Tubuai (10.0-8.8 Ma) and Rairua volcano. The geochemical signature of the younger Anatonu lavas is ascribed to the partial melting, within the same plume, of a distinct filament of more subdued composition. Alternatively, the proportion of pyroxenites with a HIMU character was lower as partial melting degrees increased, generating the Anatonu tholeiites.

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