HR: 17:15h
AN: T44A-06    [Abstracts]
TI: Composition and Evolution of the IBM Arc Crust
AU: * Tatsumi, Y
EM: tatsumi@jamstec.go.jp
AF: IFREE/JAMSTEC, Natsushima, Yokosuka, 237-0061 Japan
AU: Takahashi, N
EM: narumi@jamstec.go.jp
AF: IFREE/JAMSTEC, Natsushima, Yokosuka, 237-0061 Japan
AU: Kodaira, S
EM: kodaira@jamstec.go.jp
AF: IFREE/JAMSTEC, Natsushima, Yokosuka, 237-0061 Japan
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: IFREE/JAMSTEC, Natsushima, Yokosuka, 237-0061 Japan
AB: The IBM oceanic arc is characterized by the occurrence of the middle crust with P-wave velocity of 6.0- 6.5 km/s and probably having andesitic compositions. It may be thus suggested that this arc could represent the initial stage of formation of the andesitic continental crust. Knowledge of arc crust composition and its temporal variation is crucial to understanding the evolution from the arc to the continental crust. We herein estimate the composition of the IBM arc crust based on the seismic structure of the arc and compositions of IBM arc magmas including the Tanzawa tonalite that would represent the obducted portion of the IBM middle crust. Two different processes of andesitic crust formation are assumed: (1) 30% melting of the pre-existing basaltic crust that was created by 20% fractionation of olivine from an inferred IBM primary basalt magma; (2) mixing of the differentiated basalt magma and the felsic magma produced by 15% melting of the basaltic crust, Both processes can reasonably explain compositions of the Tanzawa tonalite. Seismologically determined volume of the IBM middle crust and the inferred andesite formation processes provide estimates for volume of dunitic cumulate and gabbroic restites. The calculated volume of the restite layer is identical to that determined by seismic observation, suggesting that the dunitic cumulate would form a part of the upper mantle, not of the arc crust. It may be thus inferred that the bulk IBM arc crust possesses basaltic, not andesitic, compositions. In order for the IBM arc crust to evolve into an andesitic continental crust, further separation of mafic component from the arc crust such as delamination of the lower crust is needed.
DE: 1020 Composition of the continental crust
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 7205 Continental crust (1219)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Tectonophysics [T]
MN: Fall Meeting 2005