HR: 1340h
AN: T53A-1396    [Abstracts]
TI: The magmatic plumbing of the submarine Hachijo NW volcanic chain, Hachijojima, Japan: long distance lateral magma transport?
AU: * Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: Geological Survey of Japan, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Geshi, N
EM:
AF: Geological Survey of Japan, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Itoh, J
EM:
AF: Geological Survey of Japan, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Kawanabe, Y
EM:
AF: Geological Survey of Japan, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Tsujino, T
EM:
AF: Geological Survey of Japan, Central 7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: Recent geophysical observations on basaltic composite volcanoes in Izu-Bonin arc reveal the process of long distance lateral magma transport within the shallow <ETH> middle crust. Such intrusion events sometimes caused flank fissure eruption and also triggered a formation of collapsed caldera (Miyakejima 2000). To clarify a long-distance magma transport system of the basaltic composite volcano in volcanic arc from geological and petrological aspects, we investigated a submarine volcanic chain (Hachijo NW chain) nested Hachijo Nishiyama volcano, a frontal composite volcano in the northern Izu arc. The volcanic chain extends 15 km from Hachijo-Nishiyama volcano and is composed of ridges and many small cones with basal diameters generally less than 2km. Dredge sampling recovered basaltic lavas and spatters. A diving survey using a ROV (Hyper Dolphin) revealed pillow lava flows on steep slopes and accumulation of spatters and agglutinates near the eruption center. Basalts from the Hachijo NW chain generally have more primitive composition (up to nearly 7% of MgO) compared to the basaltic rocks from the Nishiyama. The bulk magma composition of Hachijo NW chain is controlled by fractionation of clinopyroxene, olivine and plagioclase while plagioclase accumulation was indicated by aluminum-rich character of the Nishiyama volcano and its subaerial satellite cones. Trace element ratios unaffected by melting or crystal fractionation (e.g., Nb/Zr) are not significantly different between the Nishiyama and the Hachijo NW chain. This implies that the sources of magma for these volcanic systems are basically identical. However, ratios affected by melting process are significantly different between the two. Hachijo NW chain shows lower LREE/HREE and Zr/Y, implying difference in degree of partial melting of the source. Other possible processes for producing these differences in trace element characteristics include crustal assimilation. These results obtained so far appear to indicate that the feeding system of the Hachijo NW chain is independent from that of the Nishiyama volcano, even though there are some transitional lavas implying interaction between the two systems. Magma erupted from the Nishiyama volcano seems to have plagioclase accumulation after crystal fractionation of plagioclase and mafic minerals possibly in a shallow crustal magma chamber. Magmas erupted in the Hachijo NW chain, however, experienced much less crystal fractionation in the crust during ascent. They erupted without plagioclase accumulation. The observed difference in their magmas precludes the possibility that the Hachijo NW chain magma was separated and transported from the magma system of Nishiyama volcano before the Nishiyama magma was affected by crystal fractionation and plagioclase accumulation.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8427 Subaqueous volcanism
DE: 8434 Magma migration and fragmentation
SC: Tectonophysics [T]
MN: Fall Meeting 2005