[S-CG08_29AM2] Collision, Subduction, and Metamorphic processes-II

★ [SCG08-10] Subduction origin for UHP chromitite from the Nishisongi metamorphic rocks, Western Kyushu, Japan

*Tadao NISHIYAMA¹, Yousuke MORIBE¹, Satoko ISHIMARU¹, Shoji ARAI², Yasushi MORI³, Miki SHIGENO³ (1.Graduate School of Science and Technology, Kumamoto University, 2.Department of Earth Sciences, Kanazawa University, 3.Kitakyushu Museum of Natural History and Human History)

Keywords:microdiamond, UHP chromitite, subduction zone, Nishisonogi metamorphic rocks, ultrahigh-pressure metamorphic rocks

Ultrahigh-pressure (UHP) chromitite from the Luobsa Ophiolite in non UHP terrane has been an enigima because of its peculiar occurrence. We newly found a UHP chromitite from serpentinite in the Nishisonogi metamorphic rocks (NMR), a member of the Nagasaki Metamorphic Rocks, in Western Kyushu, following our finding¹ of it from the Higo Metamorphic Rocks (HMR), Central Kyushu. The UHP chromitite from NMR documents well a fluid -chromite interaction, showing partial graphitization of microdiamond. Such a fluid - chromite interaction is not observed in HMR chromitite. The NMR are high P/ T ( epidote-glaucophane schist subfacies ) metamorphic rocks of Cretaceous in age, mainly consisting of pelitic and psammitic schists intercalating with minor basic schists². The peak metamorphic condition of the crystalline schists is estimated as 1.4 GPa and 520 ^oC by an assemblage of garnet with inclusions of chloritoid and omphacite, glaucophane, paragonite, and phengite in a garnet galucophanite³. Serpentinite and serpentinite melanges occur as elongated bodies or lenses concordant with schistosity trending N-S of the country schists¹. Jadeitite and omphacitite occur as tectonic blocks in the serpentinite melange, showing the peak condition of 1.5 GPa and 500 ^oC by coexistence of jadeite and quartz⁴. Microdiamond - bearing chromitite was found from serpentinite in a melange at Ooseto Town, Saikai City. Chromitite occurs as a thin layer several cm thick and meter-size long in a serpentinite with numerous magnesite (or ankerite) veins. The layer is strongly deformed to show a schlieren - like structure. The serpentinite consists of fine-grained antigorite with no relics of olivine and pyroxenes. The chromitite consists of an aggregate of rounded and fractured chromite crystals with small amounts of talc and magnesite as a matrix and veins. Microdiamond occurs as aligned crystals in narrow zones ranging from a few μm to several tens of μm in chromite. Chromite is zoned, consisting of Mg-rich core (Mg_0.33Fe²⁺_0.65Mn_0.03)(Cr_0.84Al_0.12Fe³⁺_0.04)₂O₄ and Fe-rich rim (Mg_0.06Fe²⁺_0.89Zn_0.02Mn_0.03)(Cr_0.85Al_0.12Fe³⁺_0.04)₂O₄. The microdiamond - bearing zones are conspicuously richer in Fe₂O₃ [ (Mg_0.03Fe²⁺_0.94Mn_0.04Zn_0.01)(Cr_0.67Ti_0.01Fe³⁺_0.310)₂O₄], observed as a brighter zone in a BSE image, than other part of chromite in the same grain. The network -like distribution of the zones clearly indicates fluid infiltration associated with the following exchange reaction of trivalent cations between chromite and the fluid: Cr³⁺ + Al³⁺( in chromite ) = Fe³⁺( in fluid) Microdiamond occurs either as polyhedral or as platy crystals, 1 to several μm across. Identification of diamond was carried out with an energy dispersive X-ray spectroscopy (EDS) analysis (carbon peak) and Raman spectroscopy with a He-Ne laser. We observed a broad Raman peak at 1331 cm^-1, which is comparable to the peak (1332 cm^-1) characteristic of diamond. Graphite peak at about 1600 cm^-1 is also observed, showing partial graphitization of microdiamond. These lines of evidence shows that the fluid infiltration may have occurred after inclusion of microdiamond. It is quite astonishing that microdiamond is preserved in such a completely serpentinized ultramafic rock. Chromite can be a good container of microdiamond to prevent graphitization during geologically long duration of exhumation and serpentinization. Our finding suggests the subduction origin of UHP chromitite from NMR rather than mantle migration origin⁵ in the case of the Luobusa Ophiolite.References 1. Nishiyama. T., Shiosaki, D., Eguchi, H., and Yoshiasa, A. JpGU Meeting, S-MP46 (2014);2. Nishiyama, T. Mem. Geol. Soc. Japan 33, 237-257 (1989) ;3. Moribe, Y. unpublished Mc thesis, Kumamoto University (2014);4. Shigeno, M., Mori, Y.,