[SCG63-P04] Asthenospheric contribution to magmatism at the active rift zone in the northern Izu-Bonin arc
Keywords:back-arc basin, the northern Izu-Bonin arc, the active rift zone, slab-derived component
ODP Leg126 site 788, 790, and 791 reached the basaltic basement of the Sumisu rift (Gill et al., 1992). The geochemical data and stratigraphic relations of the basement indicate that the HZB is younger than the MZB. Estimated primary magma compositions suggest that segregation depth of primary magma for the basalts at the Sumisu rift exhibits 30 km (~ 1.0 GPa), whereas that at the Aogashima and Myojin rifts more than 45 km (~ 1.5 GPa). The correlation between Zr/Yb, Nb/Yb and Ba/Nb indicate that the MZB and LZB were produced by different degree of partial melting of a common source mantle. The MZB and LZB volcanism at the early stage of the back-arc rifting is best explained by a partial melting of subducted slab saturated with trace quantities of zircon under low-temperature condition in the mantle wedge. On the other hand, the HZB requires a partial melt of subducted slab accompanied by full dissolution of zircon under high-temperature condition in the mantle wedge. Spatial geochemical variation of the active rift zone basalts indicates that contribution of a slab melt component (high Th/Nb relative to Ba/Th) dominates in the Sumisu rift, whereas that of an aqueous fluid component dominates in the Aogashima rift. We propose that the back-arc rifting could have been caused by asthenospheric injection with high-temperature in the south during the syn stage.
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