Involvement of continental crust in the formation of the Cretaceous Kerguelen Plateau: New perspectives from ODP Leg 120 sites

The basaltic basement of the large igneous province formed by the Kerguelen Plateau and Broken Ridge in the southeastern Indian Ocean has been sampled by three Ocean Drilling Program cruises (Legs 119, 120 and 183). Although the Cretaceous parts of this plateau formed in the embryonic Indian Ocean basin, presumably by melting associated with the Kerguelen plume, trace element abundances and isotopic ratios of Sr, Nd and Pb of Cretaceous basalt from several drill sites indicate that continental lithosphere was involved in their petrogenesis. On the basis of relative depletions in Nb, Ta and Th, and isotopic characteristics similar to those of EMI ocean island basalt, lavas from Leg 120 Site 747 in the Central Kerguelen Plateau contain a component derived from lower continental crust. On the basis of relative abundances of Sr and Eu and EMI-like Pb isotopic ratios, the source of basalt from Leg 120 Site 750 in the northeastern part of the Southern Kerguelen Plateau also contained a component derived from lower continental crust; in this case, the crustal component formed as a plagioclase-rich, clinopyroxene-bearing cumulate. Basalts from Leg 120 Site 749 define two distinct isotopic (Sr, Nd and Pb) groups which differ from the isotopic fields for Site 747 and 750 basalts. Among Site 749 lavas, there is subtle evidence for a continental component, broadly similar (i.e. moderate Pb-206/Pb-204 similar to18.0) to that expressed more obviously in basalt from Leg 119, Site 738 on the southern edge of the Southern Kerguelen Plateau and Leg 183 Site 1137 on Elan Bank. The continental components in the Kerguelen Plateau basalts may have resided in a heterogeneous mantle plume that was formed, in part, by deep recycling of crust. It is more likely, however, that slivers of Gondwana lithosphere reside within the lithosphere and asthenosphere of the Indian Ocean mantle where they contaminate both plume-derived and mid-ocean ridge basaltic magmas.