Journal of Petrology Volume 42 Number 2 Pages 277-319 2001
© Oxford University Press 2001
Petrogenesis of Olivine-phyric Basalts from the Aphanasey Nikitin Rise: Evidence for Contamination by Cratonic Lower Continental Crust
1VERNADSKY INSTITUTE OF GEOCHEMISTRY AND ANALYTICAL CHEMISTRY, KOSYGIN ST. 19, 117975, MOSCOW, RUSSIA
2DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, CP 160/02, UNIVERSITÉ LIBRE DE BRUXELLES, AVENUE F.D.ROOSEVELT, 50, B-1050 BRUSSELS, BELGIUM
3UNIVERSITÄT GÖTTINGEN, GOLDSCHMIDTSTRASSE 1, 37077 GÖTTINGEN, GERMANY
4INSTITUTE OF PRECAMBRIAN GEOLOGY AND GEOCHRONOLOGY, MAKAROV EMB. 2, 119034, ST PETERSBURG, RUSSIA
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. The 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). The basalt suite is characterized by Nb, Ta, Th and U depletion, and uniquely low 206Pb/204Pb and 143Nd/144Nd 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 
Nd t (-2 to -13) and high 207Pb/204Pb (15·525–15·750). The first group with high La/Th (15–19) is characterized by low 206Pb/204Pb (16·9–17·2) and 87Sr/86Sr (up to 0·706), whereas the second group with low La/Th (5–9) has higher 206Pb/204Pb (17·7–18·1) and 87Sr/86Sr (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.
KEY WORDS: Aphanasey Nikitin Rise; tholeiites; granulite; Gondwana; Dupal isotope anomaly; Indian Ocean
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