[1ch401-07] T4.［トピック］二次改変された過去の弧-海溝系の復元：日本およびその他の例

[T4-O-3] Boron isotope mapping of serpentinites in the Osayama Serpentinite Mélange, SW Japan

*辻森 樹¹、 山田 千夏¹、青木 翔吾²、青木 一勝³、常 青⁴、木村 純一⁴ (1. 東北大学、2. 秋田大学、3. 岡山理科大学、4. 海洋研究開発機構)

キーワード：蛇紋岩メランジュ、蛇紋石、ホウ素同位体比マッピング、微量元素濃度、大佐山蛇紋岩メランジュ

Based on the B-isotope characteristics, Yamada et al. (2019a) Lithos discriminated serpentinites in the Franciscan Complex into two groups. HP serpentinites showed lower δ¹¹B values (–12 to +8.8‰) than LP serpentinites (+7.2 to +24.4‰; mostly > +10‰). Yamada et al. (2019b) JMPS also showed a B-isotope difference of antigorite serpentinites among two different metamorphic units of the Itoigawa–Omi area; they found that low δ¹¹B (< +10‰) serpentinite in the blueschist- and eclogite-bearing unit can be distinguished from high δ¹¹B (> +10‰) serpentinite in the amphibolite-facies unit. These observations indicate that HP serpentinites were affected by forearc slab fluids with lighter B signature at deeper depths. In order to evaluate the origin of serpentinite in a blueschist-bearing serpentinite-mélange, we applied the isotope-mapping approach on a 1:5000 scale map of the Osayama Serpentinite Mélange (OSM), SW Japan. The OSM serpentinites show small chemical variations of relict Cr-spinel and no mappable systematics of those, excepting for sporadic occurrences of Na-bearing tremolite and Zn-rich metasomatic chromite rims. However, based on a 219-spots isotope analysis of 39 samples, we were able to map the distribution of two groups of serpentinites. Overall, OSM serpentinites show a wide range of B (9–913 µg/g; mostly > 100 µg/g). The B-isotope mapping revealed the distribution of lenticular antigorite serpentinite blocks with high-δ¹¹B (> +10‰) within low-δ¹¹B (< +10‰) serpentinites. The trace-elements geochemistry found a positive correlation between W and B; the correlation among W/Th and Ba/W ratios suggests an influence of sediments-derived fluids. Moreover, enrichments of As and Sb in the low-δ¹¹B serpentinite suggest an infiltration of As- and Sb-rich fluids from subducting sediments at blueschist-facies depths where sulfide breakdown occurs. We have also evaluated effects of contact metamorphism by a Cretaceous granite. The thermal effect is not extreme in the isotope changes, but in the contact aureole, our data suggests that deserpentinization reactions release boron and cause a small decrease of δ¹¹B. Our isotope-mapping approach offers an alternative and effective new method to constrain and map serpentinite mélange complexes.