GEOCHEMICAL CHARACTERISTICS OF LAVAS FROM BROKEN RIDGE, THE NATURALISTE PLATEAU AND SOUTHERNMOST KERGUELEN PLATEAU - CRETACEOUS PLATEAU VOLCANISM IN THE SOUTHEAST INDIAN-OCEAN

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, (207)pb/Pb-204 and (208)pb/Pb-204 isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic Lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with epsilon(Nd)(T) = - 13 to - 7, (Sr-87/Sr-86)(T) = 0.7090 to 0.7130 and high Pb-207/Pb-204 relative to Pb-206/Pb-204. T, addition, the low-epsilon(Nd)(T) Naturaliste Plateau samples are elevated in SiO2 (> 54 wt%), In contrast to ''DUPAL'' oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La/Nb > primitive mantle values); the lowest epsilon(Nd)(T) and highest Th/Ta and La/Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction, Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-Pb-206/Pb-204 Indian mid-ocean ridge basalts, Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive epsilon(Nd), relatively high-Pb-206/Pb-204 plume composition, or a plume source dominated by mantle with epsilon(Nd) of -3 to similar to 0.