Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O’ne metaperidotites (Japan) |
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| Authors: | Mohamed Zaki Khedr Shoji Arai |
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| Institution: | (1) Department of Earth Sciences, Graduate school of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan |
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| Abstract: | The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan,
characterized by the high-P/T Renge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated,
have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate
dunites. We found a characteristic mineral assemblage, olivine + orthopyroxene + tremolite + chlorite + chromian spinel, being
stable at low-T, from 650 to 750°C, and high-P, from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo88–Fo91, and orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al2O3 (up to 0.25 wt%), Cr2O3 (up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO2 (up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the
orthopyroxenes, have high TiO2, 3.1 wt% (up to 5.7 wt%) on average, and high Cr# =Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe3+ =Fe3+/(Cr + Al + Fe3+) atomic ratio, <0.3]. The bulk-rock chemistry shows that the Happo-O’ne metaperidotites with this peculiar spinel are low
in TiO2 (0.01–0.02 wt%), indicating no addition of TiO2 from the outside source during the metamorphism; the high TiO2 of the peculiar spinel has been accomplished by Ti release from Ti-bearing high-T pyroxenes during the formation of low-T, low-Ti silicates (<0.1 wt% TiO2) during cooling. Some dunites are intact from hydration: their olivine is Fo92 and spinel shows high Cr#, 0.72. The Happo-O’ne metaperidotites (tremolite–chlorite peridotites), being in the corner of
the mantle wedge, are representative of a hydrous low-T, high-P mantle peridotite facies transitional from a higher T anhydrous peridotite facies (spinel peridotites) formed by in situ retrograde metamorphism influenced by fluids from the
subducting slab. They have suffered from low-T (<600°C) retrogressive metamorphism to form antigorite and diopside during exhumation of the Renge metamorphic belt. |
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