Magmatic and metasomatic imprints in a long-lasting subduction zone: Evidence from zircon in rodingite and serpentinite of Kochi, SW Japan

- Characterisation of zircon grains in serpentinites, pyroxenites and rodingites
- Magmatic/metasomatic events in mantle wedge inferred from zircon U-Pb and REE data
- Dominantly positive εHf(t) values indicate zircon growth in an isotopically primitive source.

Southwest Japan, as an active convergent margin, is one of the world's natural laboratories to investigate magmatic and metasomatic processes associated with a long-lived subduction zone. Rocks that carry the imprints of mantle metasomatism in this region include serpentinite, jadeitite, omphacitite, albitite, and rodingite. Here we investigate rodingites, serpentinites and pyroxenites from the accretionary complex of Kochi in the Shikoku island from within the Kurosegawa serpentinite mélange. We present petrology, zircon U-Pb geochronology, rare earth element (REE) geochemistry and Lu-Hf isotope data which indicate multiple pulses of magmatism and metasomatism. The zircon grains in these rocks yield major age peaks at 485 Ma and 469 Ma, marking the timing of formation of the protoliths. The wide range of minor age peaks including those at 152 Ma and 104 Ma suggest continuous fluid- and melt-induced mantle metasomatism in an active subduction zone from Cambrian to Paleogene associated with subduction of the Pacific plate. Our data suggest that the serpentinite-rodingite association might belong to various stages of subduction with the 450 Ma marking the initial phase. This is also close to the serpentinite zircon age of ca. 490 Ma. The REE patterns of zircons in all the rocks analyzed in this study show a distinct negative Eu-anomaly, particularly in the case of hydrothermal zircons. However, few zircons show only a slight negative Eu-anomaly suggesting that they might have crystallised from melts with high H2O content generated by partial melting of mantle wedge during the interaction of fluids released by the downgoing slab and the overlying sediments. Although the εHf(t) values of zircons in our samples show wide variation from − 9 to 18, most zircon grains that are younger than 485 Ma are typically characterised by positive εHf(t) values (ranging from 2 to18). This feature indicates that the zircon grains crystallised during serpentinisation and rodingitisation from an isotopically primitive source. Our study provides insights into melt and fluid infiltration processes in a mantle wedge associated with long-lasting subduction along an active convergent margin.

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