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Abstract

The laser-heating 40Ar/39Ar dating method was applied to volcanic rocks systematically collected from the back-arc region of the central part of the Izu-Bonin arc. Dating results combined with whole-rock chemistry and other geological information reveal the volcanic history of the back-arc region of the Izu-Bonin arc. In the back-arc seamount chains area, andesitic-basaltic volcanism initiated at c. 17 Ma, slightly before the Shikoku Basin ceased spreading, and continued until c. 3 Ma. Relatively old volcanism (>8 Ma) has been found only from the western part of the seamount chains, and younger volcanism mainly occurs in the eastern part of the chains, indicating the western margin of the active volcanic zone of the Izu-Bonin arc has migrated eastward with time. At around 2.8 Ma, volcanism initiated in the western part of the back-arc knolls zone. This volcanism is characterized by eruption of clinopyroxene-olivine basalt. In the first stage of rifting, this type of basalt erupted from N-S-trending fissures and/or vents aligned in this direction and formed N-S-trending ridges. Between 2.5 and 1 Ma, many small knolls were formed by eruption of basalt and minor felsic rocks. Volcanism younger than 1 Ma occurred only in the currently active rift zone and its adjacent area.

The active volcanic zone in the back-arc seamount chains area converged to the volcanic front with time from 17 to 3 Ma. Active rifting and rifting-related volcanism also migrated or converged eastward after 1 Ma. The observed temporal variation of locus of volcanism may be explained by rapid retreat of the Philippine Sea Plate relative to the Pacific Plate and resulting steepening of the subducting slab.

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