HR: 1340h
AN: T53A-1397    [Abstracts]
TI: Compositionally Diverse Magmatism in Torishima Volcano, Izu-Bonin Arc: Implications for the Genesis of Silicic Magma in the Intra-Oceanic Arc
AU: * Tani, K
EM: kentani@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Tamura, Y
EM: tamuray@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Suzuki, T
EM: belltree@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Chang, Q
EM: qchang@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Fiske, R S
EM: rfiske@volcano.si.edu
AF: Smithsonian Institution, MRC-119, Washington, DC 20560 United States
AB: Torishima, located 600km south of Tokyo, is an active Quaternary volcano of the Izu-Bonin arc. The total volume of this volcano is 466 cubic km, making it the second largest volcano within the arc. Due to the inaccessibility of this isolated island, the precise geological and petrological studies of this volcano have only begun recently. The results revealed that this volcano had compositionally diverse magmatism in its volcanic history that ranges from basalt to rhyolite. Volcanic history of this island can be mainly separated into three stages; stratovolcano stage, syn-caldera stage, and post-caldera stage. The stratovolcano stage basaltic lava and dike constructed main body of the island. Then simultaneous to the caldera formation, voluminous andesite to dacite pyroclastic eruptions occurred. In addition, abundant rhyolitic pumice was collected from the submarine slope of the volcano. The post-caldera eruptions are characterized by the large subaerial effusive lava flows of basalt to basaltic andesite magmas. The syn-caldera stage andesite and dacite occur as pumice and volcanic glass. These andesite and dacite are characterized by their high MgO (<5.2 wt.%) and Ni (<48 ppm) contents and their disequilibrium phenocrysts assemblages. From petrological and geochemical evidences, these andesite and dacite were produced by the magma mixing of two different end-member magmas; an unevolved basaltic magma of high-Fo ol, high-Mg cpx, and high-An pl phenocrysts, and a rhyolitic magma that have anhydrous phenocryst assemblage of low-Mg cpx, low-An pl, opx, and titanomagnetite. The basaltic end-member magma has similar geochemical and petrological characteristics as to the stratovolcano stage basalt, but has more unevolved composition. The rhyolitic end-member magma has comparable composition and petrography as to the submarine rhyolitic pumice. Tamura and Tatsumi (2002) proposed that a dehydration melting of the hydrous tonalitic middle crust as a possible origin for the rhyolitic magma in the intra-oceanic arc. If so, the rhyolitic end-member magma of Torishima may represent the partial melt of the tonalitic middle crust. To test this hypothesis, dehydration melting experiments of tonalite (SiO2=61wt.%) was done at 3 kb, 900-1000C. The partial melts produced in these experiments have equivalent compositions and residual phase assemblage as to the rhyolitic end-member magma. Other previously proposed processes to produce rhyolitic magma in the intra-oceanic arc, such as partial melting of the mafic lower crust or fractional crystallization from the basaltic magma, are insufficient to explain the major and trace element compositions and anhydrous phenocryst assemblage of the silicic end-member magma. The silicic magmatism of the Torishima volcano is most likely to have generated from the partial melting of the tonalitic middle crust. The basaltic magma played an important role not only as a heat source for the partial melting but also to produce compositionally diverse silicic magmas through magma mixing processes.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
SC: Tectonophysics [T]
MN: Fall Meeting 2005