Ocean island basalts and sedimentary units in the accretionary complex of Kochi, SW Japan: Implications for convergent margin tectonics and arc subduction

Abstract

Ocean island basalts (OIBs) and associated sedimentary units form part of the ocean plate stratigraphy accreted onto island arcs or continental arcs and provide important insights into convergent margin processes. Here, we investigate some of the typical localities of Sanpozan, Shirakidani, and Kamiananai within the Chichibu composite belt in Kochi, SW Japan, where the accretionary complexes incorporate OIBs and sedimentary rocks that preserve the travelogue of an oceanic plate from a mid‐oceanic ridge (MOR) to subduction zone. We present petrology and geochemistry, as well as zircon U–Pb geochronology and REE data of the accreted OIB and sedimentary formations from Sanpozan, Shirakidani, and Kamiananai. The OIBs are enriched in LILE and LREE and depleted in HREE. The chondrite‐normalised REE patterns of zircon grains from OIBs and sedimentary rocks display LREE depletion and HREE enrichment, sharply positive Ce and Sm anomalies and negative Pr and Eu. LA‐ICP‐MS U–Pb geochronology of zircon grains from the OIBs shows three major populations with 206Pb/238U mean ages of 164–456, 724–955, and 1,638–2,202 Ma. The diverse ages suggest heterogeneous recycled source components in the plume‐related upwellings that generated the OIBs, as well as different times of formation and accretion of the Palaeozoic and Mesozoic oceanic islands. The dominant population of zircon grains from the sedimentary units shows 206Pb/238U mean ages of 147 and 288 Ma and indicate a major change in the provenance regime with subduction and the disappearance of older arcs attesting to the destruction of continental crust in convergent margins.