Petrogenesis of olivine-phyric basalts from the Aphanasey Nikitin Rise: Evidence for contamination by cratonic lower continental crust

In this work we investigate the olivine-phyric basalt suite of the Aphanasey Nikitin Rise, an intraplate volcanic structure formed during the Late Cretaceous in the Indian Ocean. nle parental melt of the basalt suite has a hypersthene-normative tholeiitic composition with low H2O content (0.3-0.5 Wt %) and high SiO2/Al2O3 (3.5). nle basalt suite is characterized by Nb, Ta, Th and U depletion, and uniquely low Pb-206/Pb-204 and Nd-143/Nd-144 among the Cretaceous tholeiitic basalts of the Indian Ocean. Our modelling demonstrates that fractional crystallization of depleted mantle-derived melt and lower continental crust assimilation is a suitable model for the genesis of the parental magma of this suite. The continental crustal material involved is characterized by long-term Rb, U and Th depletion and probably remained isolated for >10(9) years in cratonic Gondwanan lithosphere. On a broader scale, two geochemical groups can be distinguished among tholeiites formed in the Indian Ocean basin during the period 115-75 Ma, from the Aphanasey Nikitin Rise, the southern Kerguelen and Naturaliste plateaux and the Broken Ridge. Both groups have a compositional range from hypersthene-normative basalt to basaltic andesite and are characterized by Nb-Ta depletion, extremely low epsilon (Nd t) (-2 to - 13) and high Pb-207/Pb-204 (15.525-15.750). The first group with high La/Th (15-19) is characterized by low Pb-206/Pb-204 (16.9-17.2) and Sr-87/Sr-86 (up to 0.706), Whereas the second group with low La/Th (5-9) has higher Pb-206/Pb-204 (17.7-18.1) and Sr-87/Sr-86 (up to 0.713). The tholeiite composition is likely to be controlled by contamination of the parental tholeiitic melts by continental crust derived from cratonic Gondwanan lithosphere. This conclusion, combined with other evidence for ancient crustal material in the Indian and Atlantic Oceans, indicates the impact of continental crust in the oceanic lithosphere.