[S-CG46] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG46-P06] 中央パラワンオフィオライトの基底部かんらん岩に見られる変形・熱水変質作用：沈み込み開始過程への影響

*平内 健一¹、阿部 日誉里²、Pasco Julius³、Payot Betchaida³ (1.静岡大学理学部地球科学科、2.静岡大学大学院総合科学技術研究科理学専攻、3.フィリピン大学地質学研究所)

キーワード：熱水変質、マイロナイト、かんらん石、オフィオライト、沈み込み開始

Numerical simulations of intra-oceanic subduction initiation have shown that a pre-existing fault between oceanic plates is required to be very weak mechanically to allow subduction initiation in intra-oceanic settings (e.g., Gerya, 2022, Geosphere; Izumi et al., 2023, Tectonophysics). However, it remains unclear which rheological weakening mechanisms have operated at the incipient subduction-plate interface. To understand how deformation and hydrothermal alteration at the base of the immature mantle wedge influence intra-oceanic subduction initiation processes, structural and petrological analyses was conducted on basal peridotites from the Eocene central Palawan Ophiolite, Philippines. The central Palawan Ophiolite and underlying metamorphic sole have recorded horizontally-forced subduction initiation that occurred along extinct spreading ridges (Keenan et al., 2016, Proc. Natl. Acad. Sci. U.S.A; Valera et al., 2022, J. Metamorph. Geol.). Located adjacent to metamorphic sole amphibolites, the basal peridotites are mainly composed of dunite and harzburgite, which represent residual mantle peridotites with various degrees of partial melting (Pasco et al., 2023, Int. Geol. Rev.). Grain size reduction of olivine grains occurred by dynamic recrystallization during dislocation creep, and led to the formation of porphyroclastic tectonite, protomylonite, and mylonite. With decreasing distance from the amphibolites, the degree of mylonitization becomes significant, and the crystal-preferred orientation (CPO) pattern of olivine grains transitions from [100]{0kl} (D-type), [100](010) (A-type), or [100](001) (E-type) to [001](100) (C-type). Based on the effect of water and stress on the olivine CPO (Jung et al., 2006, Tectonophysics), this CPO transition was possibly driven by infiltration of a larger amount of water. Since the fabric strength for olivine grains decreases with grain size, we suggest that grain size reduction promoted a transition to a grain-size-sensitive mechanism (i.e., dislocation-accommodated grain-boundary sliding or diffusion creep), and thereby led to a significant decrease in flow strength. At temperatures below ca. 800 °C and places where slab-derived water was added, the mylonite underwent a dissolution-precipitation process, resulting in the formation of anastomosing networks of talc and tremolite above the stability field of serpentine (antigorite). As talc is one of the weakest hydrous phyllosilicates (Hirauchi et al., 2013, Geology), mylonitization and subsequent hydrothermal alteration occurring at the base of the mantle wedge are thought to facilitate underthrusting of the oceanic plate during subduction initiation.