09:00 〜 09:15
[SCG54-01] The birth of an oceanic island arc by subduction initiation
★Invited Papers
キーワード：沈み込み開始、伊豆小笠原マリアナ、マグマティズム、島弧
Geological and geophysical surveys in the Izu-Bonin-Mariana forearc have revealed oceanic crust generated in the initial stages of subduction and hence represent the earliest stage of island arc formation. Magmatism after subduction initiation generated forearc basalts, and subsequently, boninitic and tholeiitic to calc-alkaline lavas.
Details of the evolution and nature of volcanism following the initiation of subduction is crucial to our understanding of this specific magmatic environment. Volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge span the first 10 Myr of arc evolution. Dating results reveal that magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at 7–8 Ma. Following initial mid-ocean ridge basalt (MORB)-like spreading-related magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45Ma, volcanism switched to low-Si boninite and slab component was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. After 45 Ma, this signature was replaced by a Pacific MORB-dominated flux in the tholeiite and calcalkaline volcanic rocks which developed as subaerial volcanoes. During the same period, the mantle source switched from highly depleted harzburgite to a depleted MORB-type mantle-type source. These processes could be associated with the increasing depth of the leading edge of the slab through this 5Myr period.
This time-progressive development of arc volcanoes could be analogous to those documented from other forearc sections in the Western Pacific and some supra-subduction zone ophiolites. New results from these settings will be reported and discussed in terms of their similarity and differences with Izu-Bonin-Mariana forearc.
Details of the evolution and nature of volcanism following the initiation of subduction is crucial to our understanding of this specific magmatic environment. Volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge span the first 10 Myr of arc evolution. Dating results reveal that magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at 7–8 Ma. Following initial mid-ocean ridge basalt (MORB)-like spreading-related magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45Ma, volcanism switched to low-Si boninite and slab component was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. After 45 Ma, this signature was replaced by a Pacific MORB-dominated flux in the tholeiite and calcalkaline volcanic rocks which developed as subaerial volcanoes. During the same period, the mantle source switched from highly depleted harzburgite to a depleted MORB-type mantle-type source. These processes could be associated with the increasing depth of the leading edge of the slab through this 5Myr period.
This time-progressive development of arc volcanoes could be analogous to those documented from other forearc sections in the Western Pacific and some supra-subduction zone ophiolites. New results from these settings will be reported and discussed in terms of their similarity and differences with Izu-Bonin-Mariana forearc.