Intra-oceanic island arc origin for Iratsu eclogites of the Sanbagawa belt, central Shikoku, southwest Japan

Atsushi Utsunomiya, Bor Ming Jahn, Kazuaki Okamoto, Tsutomu Ota, Hironao Shinjoe

Research output: Contribution to journalArticlepeer-review

38 Citations (Scopus)


New geochemical and Sr-Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O2 and H2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu-Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative εNd(t) value (~-5)), consistent with their origin as continent-derived trench-fill turbidites.

Original languageEnglish
Pages (from-to)97-114
Number of pages18
JournalChemical Geology
Issue number1-2
Publication statusPublished - Jan 7 2011


  • Eclogites
  • Isotopic geochemistry
  • Oceanic island arc
  • Sanbagawa belt

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology


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