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- Title
Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks, Japan, a possible high-pressure dehydration product of antigorite serpentinite.
- Authors
Tadao Nishiyama; Hibiki Eguchi; Dai Shiosaki; Akira Yoshiasa; Nobutatsu Mochizuki; Yasushi Mori; Kazumasa Sugiyama; Hiroshi Arima; Kunio Yubuta
- Abstract
Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks (HMR), west-central Kyushu, Japan, may be formed by high-pressure dehydration of antigorite, and may indicate deep subduction of serpentinite reaching a pressure- temperature condition of 1.6 GPa and 740-750 °C. Three rock types have been identified based on mineral assemblage and rock texture: Type I (L) consisting of medium-grained (1-5 cm long) olivine + enstatite + chromite ±tremolite with secondary talc and anthophyllite that occurs in low-grade metamorphic rocks of the biotite zone, Type I (H) of coarse-grained (up to 10 cm long) olivine + enstatite (with clinoenstatite lamella) + chromite ±tremolite with secondary talc that occurs in highgrade metamorphic rocks of the garnet-cordierite zone, and Type II composed of Alspinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnetcordierite zone together with Type I (H). Olivines in all rock types are mostly serpentinized during exhumation. The chromite-olivine thermometer gives 560-690 °C for Type I (L) rocks, and the spinel-olivine thermometer gives 610-740 °C for Type II rocks. The peak metamorphic pressure will be higher than 1.6 GPa based on the location of the experimentally determined invariant point (P = 1.6 GPa and T = 670 °C) of antigorite + forsterite + enstatite + talc + H2O. This estimate is consistent with the occurrence of chlorite in Type II rocks, which is stable up to 890 °C at 2.0 GPa. The spinifex-like textured metaperidotites occur as small bodies in the low P/T type gneisses, implying tectonic juxtaposition of them probably during exhumation of the HMR. Recent findings of medium pressure (0.9-1.2 GPa) granulites and gneisses from the HMR may indicate that the HMR has a deep root into the wedge mantle from which the spinifex-like textured metaperidotites have derived.
- Subjects
KYUSHU Region (Japan); JAPAN; METAMORPHIC rocks; CHROMITE; SERPENTINITE; ANTIGORITE; DEHYDRATION; ENSTATITE
- Publication
Island Arc, 2021, Vol 30, Issue 1, p1
- ISSN
1038-4871
- Publication type
Article
- DOI
10.1111/iar.12382