Spinifex‐like textured metaperidotites from the Higo Metamorphic Rocks, Japan, a possible high‐pressure dehydration product of antigorite serpentinite

Abstract

AbstractSpinifex‐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 high‐grade metamorphic rocks of the garnet‐cordierite zone, and Type II composed of Al‐spinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnet‐cordierite 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.