[S-CG55] Oceanic plate as inputs to subduction zone: from oceanic spreading ridge to subduction trench

[SCG55-P04] Hydration processes of a forearc mantle wedge inferred from the Takadomari serpentinite body in the Kamuikotan Belt, central Hokkaido, Japan

*Shun Takamizawa¹, Yuji Ichiyama¹, Shusaku Yamazaki², Akihiro Tamura³, Tomoaki Morishita³ (1.Chiba University , 2.Civil Engineering Research Institute for Cold Region, 3.School of Geoscience and Civil Engineering, College of Science and Technology, Kanazawa University)

Keywords:Subduction zone, Mantle wedge, Slab-derived Fluid, Metasomatism, Kamuikotan Belt

Subduction zones are the place where the recycling of Earth's surface materials has been taking place. In particular, the water cycle, which is caused by the subduction of the hydrated oceanic crust, is thought to be related to hydration of mantle wedges and consequent igneous and hydrothermal activities in subduction zones, but the details of hydration processes have been unclear. The Kamuikotan Belt in central Hokkaido consists mainly of Cretaceous high-pressure metamorphic rocks and ultramafic rocks. Because the high-pressure metamorphic rocks are interpreted to be derived from ancient subducted slabs, the ultramafic rocks associated with them can be regarded as mantle wedge materials beneath a subduction zone. The Takadomari serpentinite body in the Kamuikotan Belt. Consist of highly depleted serpentinized peridotite, which is interpreted as the residual mantle after the release of high-Mg andesitic magma, such as boninite, beneath an oceanic island arc (Tamura et al., 1999). The Takadomari serpentinite body contact with the high-pressure metamorphic rocks by faults, and the degrees of antigoritization increase towards the boundary with the metamorphic rocks (e.g., Igarashi, 1985). The presence of amphibolite tectonic blocks with blueschist-facies overprints (e.g., Ishizuka and Imaizumi, 1980) suggests that high-pressure metamorphism also affected the Takadomari serpentinite body. In this study, field survey, petrographic observations, and major and trace element analyses of minerals for the Takadomari serpentinite body in the Kamuikotan Belt, were performed to understand the spatial and temporal hydration processes of a mantle wedge. In the Takadomari serpentinite body, highly depleted harzburgite and dunite (Spinel Cr# = 0.6-0.9) are highly serpentinized. In particular, antigoritization tends to be strong in the southern part of the body near the high-pressure metamorphic rocks. This trend is consistent with the observation of Igarashi (1985), but highly antigoriteized serpentinites are also broadly distributed the Takadomari serpentinite body. Antigoritization is more pronounced in dunite than harzburgite. The Takadomari serpentinites usually have talc and Ca amphibole with tremolite to magnesio-hornblende compositions, as replacements of pyroxene. The Ca amphibole in the Takadomari serpentinites can be divided into two types, based on the Na₂O content. Estimated temperatures and pressures using thermobarometries for the Na-rich Ca amphibole (ca. 830 ℃ and 1.2 GPa) are higher than those of the Na-poor Ca amphibole (ca. 780 ℃ and 0.5 GPa). The occurrence of the Na-rich Ca amphibole is restricted to the northern part of the Takadomari serpentinite body, and rare phlogopite-bearing serpentinite is also present. After the formation of Ca amphibole, subsequent cooling resulted in talc. Further cooling formed richterite, Fe-rich metamorphic olivine, and clinopyroxene in association with antigorite in serpentinites near the boundary with high-pressure schist. In these serpentinites, olivine with stripe structure or parting is also present. These mineral paragenesis indicate that the serpentinites experienced prograde and retrograde events at around 500-600 ℃. In contrast, Mg-rich metamorphic olivine associated with antigorite and magnetite, was formed by prograde events at lower temperatures (around 400 ℃). The petrological characteristics of the Takadomari serpentinites are closely similar to those of the Mariana forearc serpentinites, indicating that the Takadomari serpentinite body was part of a forearc mantle wedge. The trace element patterns of Ca amphibole and richterite are significantly enriched in fluid mobile elements, such as Cs, Rb, and Pb. Similar trends are also observed in the Mariana forearc serpentinite (Ichiyama et al., 2021), implying that the metasomatic agent for the Takadomari serpentinites body would have been fluids derived from subducted slabs. The slab-derived fluids at high temperatures were probably K-rich, as implied by the presence of phlogopite, but those at low temperatures were Na-rich due to the formation of richterite. The Takadomari serpentinite body, was finally emplaced on land together with the high-pressure Kamuikotan metamorphic rocks as an ancient forearc mantle wedge and an underlying slab.