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New Pb, Sr and Nd isotope data from volcaniclastic sediments recovered from the Tonga forearc and Lau Basin permit the isotopic evolution of a section of this arc system, next to the modern island of Ata, to be traced through the backarc basin rifting process from 7·0 Ma to the present. The new data suggest that the isotopic character of the mantle wedge remains constant, and of Pacific mid-ocean ridge basalt (MORB) character, during the early rifting phase. The isotopic evidence supports trace element data in showing an increase in the sediment contribution to arc petrogenesis about 2-3 m.y. after the start of Lau Basin rifting. Since 0·45 Ma the sediment contribution decreased to pre-rift values with the initiation of spreading in the adjacent backarc basin, where the high sediment influence is not seen in the isotopes. The isotopes show a relative increase in the volcaniclastic compared with pelagic sediment involvement during rifting. The inferred peak in sediment subduction is probably the result of a decoupling of the two plates owing to roll-back of the Pacific lithosphere at the time of arc rifting.

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Journal of Petrology

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