HR: 10:50h
AN: V52A-03    [Abstracts]
TI: Two contrasting magmatic types coexist after the cessation of back-arc spreading
AU: * Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: Geological Survey of Japan/AIST, Central 7 1-1-1 Higashi, Tsukuba, 305-8567, Japan
AU: Yuasa, M
EM: yuasa-m@aist.go.jp
AF: Geological Survey of Japan/AIST, Central 7 1-1-1 Higashi, Tsukuba, 305-8567, Japan
AU: Taylor, R N
EM: rex@noc.soton.ac.uk
AF: National Oceanography Centre, Southampton, European Way, Southampton, SO14 3ZH, United Kingdom
AB: A characteristic of the Izu-Bonin arc is its voluminous back-arc volcanism. In the central part of the arc this volcanism is represented by a series of seamount chains extending across the arc. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active back-arc rifting just behind the Quaternary volcanic front. Enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that these seamount chains have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism. A number of unexplored seamounts of various sizes (East Shikoku Basin seamounts) are also found in the region between the western end of the back-arc seamount chains and the Kinan Seamount Chain (KSC), which traces along the extinct spreading center of the Shikoku Basin. We have recovered relatively fresh volcanic rocks from these unsampled seamounts using a Benthic multi-coring system. Preliminary data indicate that these volcanoes, like the seamount chains, show variable enrichment in LILE and depletion in HFSE, and are distinct from the MORB-like crust of the Shikoku Basin. Ar-Ar ages range from 15 to 12 Ma, indicating eruption after Shikoku Basin spreading. These observations imply that arc magmatism occurred as far as 270km west of the current volcanic front immediately after the Shikoku Basin ceased its spreading. Although contemporaneous and in a similar area, the KSC have strikingly distinct chemical characteristics to the East Shikoku Basin seamounts. The KSC lavas are alkaline basalt and are enriched not only in LILE, but also in HFSE and other highly incompatible elements. In terms of their isotope composition, they have slightly enriched Nd but comparable Sr and Pb to Philippine Sea MORB. A remarkable feature of the KSC lavas is a clear temporal variation of incompatible element concentrations such as K₂O. Their K₂O content increases with time and reaches a maximum at ca. 7 Ma when the chain ceased its activity. These observations imply that KSC magmas were produced by a small degree of partial melting of Philippine Sea MORB source. The degree of melting became smaller with time, probably reflecting decreasing temperature and/or upwelling rate of mantle beneath the dying spreading center. Unlike the other seamounts, these lavas show no influence from the on- going subduction to the East. The Izu-Bonin arc to back-arc system gives us the chance to examine magmatism in the period following back- arc spreading but with on-going subduction. This stage in arc evolution is characterised by widespread across arc volcanism and the occurrence of two contrasting and contemporaneous magmas.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting