ALBERT

Description: Initial subduction-related boninitic magmatism occurred between 48 and 44 Ma in the Izu–Bonin–Mariana (IBM) arc. High-Mg adakites and low-Ca boninites have been dredged from the Bonin Ridge fore-arc seamount. Whole-rock 40 Ar/ 39 Ar ages suggest that the boninite (44·0 ± 1·4 Ma) and adakite (43·1 ± 1·0 and 40·8 ± 0·8 Ma) magmatism overlapped, or that the adakite magmatism occurred slightly later than the boninite magmatism. The low-Ca boninites are high-Mg andesites and exhibit U-shaped rare earth element (REE) patterns with an elevated average Mg# of 0·78 [Mg# = Mg/(Mg + Fe) molar ratio] and Ni content of 667 ppm. The high-Mg adakites are andesitic to dacitic in composition; they exhibit markedly high Sr contents and low Y contents and are highly enriched in light REE but depleted in heavy REE, with an average Mg# of 0·79 and Ni content of 433 ppm. A geochemical mass-balance model (Arc Basalt Simulator Version 3) indicates that both magma types could be generated by partial melting of a depleted mantle source fluxed by water-rich slab-derived melts in a hot subduction environment, comparable with the present-day South Chile (ridge subduction) or Southwest Japan (young slab subduction) arcs. An extremely high slab melt flux of 22% is required for the formation of the high-Mg adakite, whereas a low flux of 3% is sufficient for the low-Ca boninite. The low-Ca boninite requires a high-temperature shallow slab (854°C, 2·7 GPa on average), consisting of altered oceanic crust of the Pacific plate and volcaniclastic sediments from HIMU seamounts, and high-temperature shallow mantle melting (1216°C, 0·8 GPa) of depleted Indian mid-ocean ridge basalt (MORB)-type mantle. These modelled conditions are consistent with the occurrence of hot shallow mantle wedge melting in the initial subduction zone at the boundary between Pacific- and Indian-type mantle domains, as suggested by previous studies. In contrast, high-Mg adakite requires a higher temperature and deeper slab (929°C, 4·1 GPa), with the same slab components and slightly deeper but less hot melting (1130°C, 1·1 GPa) of HIMU-type depleted mantle, to satisfy the low Hf isotope ratios. This may occur because of the subsequent cooling of the mantle wedge by the establishment of the subduction system after the boninite magmatism and involvement of a small volume of an isotopically enriched mantle source embedded in the Indian-type mantle. The petrogenetic conditions provide constraints for reconstructing the tectonic settings of the early IBM arc. The hot subduction model would be consistent with the tectonic models with regard to the initiation of subduction associated with fore-arc spreading; this allowed the upwelling of the asthenospheric mantle to generate slab melts from the old Pacific plate slab and hot shallow mantle melting by slab melt fluxing for both boninite and adakite activities.