[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-13] Fine-grained hydrous peridotites discovered from the Hayachine ultramafic complex, Iwate Prefecture: Evidence for rock-hydrous melt reactions in the back-arc spreading

*Ayu Harano¹, Katsuyoshi Michibayashi^1,2, Yui Kouketsu¹, Kenta Yoshida², Takenori Kato³ (1.Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3.Institute for Space-Earth Environmental Research, Nagoya University)

Keywords:Olivine CPO, Hayachine-Miyamori ophiolite, Subduction zone, Amphibole

This study presents the structural and petrological characteristics of mantle peridotite in relation to the rock-hydrous melt reaction found in Hayachine ultramafic complex (Ozawza, 1988, 1990). We conducted sampling, microstructural observations, crystal orientation analyses, and major element composition analyses of the major constituent minerals in peridotite. We used 17 serpentinized peridotites preserving relatively better mantle textures. The peridotites consist of olivine, orthopyroxene, clinopyroxene, amphibole, and spinel. Based on the mineral mode, the peridotites were composed of lherzolite to harzburgite. The peridotites were classified into two types according to the olivine grain size: coarse-grained (ca. 2–3 mm) and fine-grained (ca. 0.3–0.5 mm) peridotites. The coarse-grained peridotites appear to have primary textures in the asthenospheric mantle, whereas the fine-grained peridotites were characterized by aggregates of orthopyroxene and amphibole with a large number of spinel inclusions. Based on olivine crystal-preferred orientations, the coarse-grained peridotites were classified into Group 1 and Group 2. The fine-grained peridotites were classified into Group 3, accordingly. In addition, the major element compositions of the minerals show different characteristics for each group. It suggests that these variations could result from rock-hydrous melt reactions under the back-arc spreading and the subsequent processes.