Evolution of N-Izu Rear Arc Magmas After the Backarc Shikoku Basin Formed: Site U1437, IODP Exp. 350
Abstract
IODP Site U1437 is located in the Izu rear-arc region, at 31.790 N, 139.026 E. The recovered core consists of seven lithologic units. We focused on the oldest four (Units IV through Unit VII) that include volcaniclastics and hyaloclastites with coarse lava clasts as well as tuff and lapilli-tuff, and have ages of about 6-17 Ma [1, 2]. Unit VII reflects the mantle source soon after Shikoku backarc basin opened between 24-15 Ma, and the younger units record the evolution of that mantle.
Major and trace elements of the Unit VII lava clasts differ from those of the adjacent Neogene rear-arc seamount chains (RASC) and the Quaternary volcanic front volcanoes (VF). The trace element characteristics indicate weak influences from the slab. Sr-Nd-Pb-Hf isotope ratios show a very narrow variation range, indicating the existence of a uniform and depleted Indian-type mantle source. On the contrary, younger units (especially Unit IV and Unit V) show a wider range of isotopic variation, reflecting an increased influence of slab material. The most striking feature is the co-existence of two types of magma in the upper units: one that remains similar to Unit VII, and another, starting >9 Ma, that resembles the RASC. We modeled both types using the Arc Basalt Simulator v. 4 (ABS4), coded by [3]. The results reveal that the depth of origin of the slab component for Unit VII was deeper than for the modern volcanic front, and therefore more melt-like (less H2O in the slab liquid), and the mass fraction of the slab component and percent of mantle melting both were smaller than for the modern volcanic front. In contrast, they are similar to results for the active rift and nearby back arc knolls 10-30 km behind the Izu volcanic front, although they differ from them in having less depleted mantle and shallower mantle melting. It is clear that the earliest rear-arc volcanism after back-arc basin opening had weak and deep slab contributions, and that the slab contribution became greater in the RASC. Both types coexisted from at least 9 to 6 Ma. The tectonic implications of this evolution will be discussed. [1] Tamura et al. (2015) Proc. IODP, Exp. 350. [2] Schmitt et al. (2018) Int. Geol. Rev. 60, 956-976. [3] Kimura et al. (2014) Geochem. Geophys. Geosyst. 15, 691-739.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51G0204M
- Keywords:
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- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8185 Volcanic arcs;
- TECTONOPHYSICS;
- 8413 Subduction zone processes;
- VOLCANOLOGY