Isotopic Evolution of the Tonga Arc during Lau Basin Rifting; Evidence from the Volcaniclastic Record

New Pb, Sr and Nd isotope data from volcaniclastic sedimentsrecovered from the Tonga forearc and Lau Basin permit the isotopicevolution of a section of this arc system, next to the modernisland of Ata, to be traced through the backarc basin riftingprocess from 7.0 Ma to the present. The new data suggest thatthe isotopic character of the mantle wedge remains constant,and of Pacific mid-ocean ridge basalt (MORB) character, duringthe early rifting phase. The isotopic evidence supports traceelement data in showing an increase in the sediment contributionto arc petrogenesis about 2–3 m.y. after the start ofLau Basin rifting. Since 0.45 Ma the sediment contribution decreasedto pre-rift values with the initiation of spreading in the adjacentbackarc basin, where the high sediment influence is not seenin the isotopes. The isotopes show a relative increase in thevolcaniclastic compared with pelagic sediment involvement duringrifting. The inferred peak in sediment subduction is probablythe result of a decoupling of the two plates owing to roll-backof the Pacific lithosphere at the time of arc rifting. KEY WORDS: isotopes; Pacific; rifting; subduction; volcanism ^*Corresponding author. Telephone: (508) 289 3437. Fax: (508) 457 2187. e-mail: pclift{at}whoi.edu