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Geology; May 2005; v. 33; no. 5; p. 393-396; DOI: 10.1130/G21205.1
© 2005 Geological Society of America
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Jurassic oceanic lithosphere beneath the southern Ontong Java Plateau: Evidence from xenoliths in alnöite, Malaita, Solomon Islands

Akira Ishikawa1, Eizo Nakamura1 and John J. Mahoney2

1 Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan
2 School of Ocean and Earth Science and Technology, University of Hawai'i, Honolulu, Hawaii 96822, USA

The least metasomatized xenoliths of spinel lherzolite and gabbro recovered from alnöite intruding crust of the southern Ontong Java Plateau on the island of Malaita yield an Sm-Nd age of ca. 160 Ma and an initial {varepsilon}Nd value of ~+8. In contrast, the plateau basement is ca. 120 Ma with initial {varepsilon}Nd between +3.7 and +6.5. The xenoliths appear to represent normal Pacific oceanic lithosphere (uppermost mantle and lower crust) formed ~40 m.y. before the plateau, indicating that the southern part of the plateau was emplaced off axis on mature seafloor. The closest 160 Ma seafloor to Malaita is >1800 km to the north, off the northeastern margin of the plateau, implying the presence of either an intervening large-offset fracture zone or a triple junction. The presence of significantly older oceanic lithosphere beneath the plateau places restrictions on proposed origins for the plateau.

Key Words: Ontong Java Plateau • xenoliths • Sm-Nd age • oceanic lithosphere • mantle plume






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