Journal of Petrology
Peter D. Clift and Pieter Z. Vroon
Abstract
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
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 to pelagic sediment involvement during rifting. The inferred peak in
sediment subduction is probably the result of a decoupling of the two plates
due to rollback of the Pacific lithosphere at the time of arc rifting.
Key Words:
Isotopes/Pacific/Rifting/Subduction/Volcanism
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