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

Abstract

In this work we investigate the olivine-phyric hasalt suite of the Aphanasey Nikitin Rise, an intraplate volcanic structure formed during the Late Cretaceous in the Indian Ocean. The parental melt of the basalt suite has hyperthene-normative tholeiitic composition with low H2O content (0·3-0·5 wt %) and high SiO2/Al2O3 (3·5). The basalt suite is characterized by Nb, Ta, Th and U depletion, and uniquely low 206 Pb/204 Pb and 143 Nd/144 Nd 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 >109 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 εNdt (-2 to - 13) and high 207 Pb/204 Pb (15·525-15·750). The first group with high La/Th (15-19) is charaterized by low 206 Pb/204 Pb (16·9-17·2) and 87 Sr/86 Sr(up to 0·706), whereas the second group with low La/Th (5-9) has higher 206 Pb/204 Pb (17·7-18·1) and 87 Sr/86 Sr (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.