Metamorphic pressure-temperature conditions of garnet granulite from the Eastern Iratsu body in the Sambagawa belt, SW Japan

抄録

Several coarse-grained mafic bodies with evidence for eclogite-facies metamorphism are present in the Besshi area of the Sambagawa subduction-type metamorphic belt, SW Japan. Among them the granulite-bearing Eastern Iratsu metagabbro body involves an unresolved problem of whether it originated in the hanging-wall or footwall side of the subduction zone. The key to settle this problem is its relationship with the adjacent Western Iratsu metabasaltic body, which includes thick marble layer and certainly has the footwall ocean-floor origin. Several previous studies consider that the Western and Eastern Iratsu bodies were originally coherent in the footwall side and formed the shallower and deeper parts of a thick oceanic crust, respectively. The validity of this hypothesis may be assessed by deriving pressure-temperature history of the Eastern Iratsu body, or especially the pressure (depth) condition of the granulite-facies metamorphism before the eclogite-facies overprinting because, if the pressure was relatively high, the oceanic crust assumed in the above hypothesis might be too thick to tectonically achieve the present-day adjacence of the two bodies on the geological map. This study petrologically analyzes a garnet-bearing granulite from the Eastern Iratsu body and newly reports stable coexistence of garnet and orthopyroxene in the sample. By utilizing a garnet-orthopyroxene geothermobarometer, the minimum P-T conditions of the granulite-facies stage was estimated to be 0.8 GPa (∼ 27 km in depth) and 780 °C. If the Western and Eastern Iratsu bodies were assumed to have formed a coherent oceanic crust before their subduction, the original thickness of it was >27 km and this demands unusually strong ductile shortening (<1/9) or unrealistically large vertical displacement on intraplate faulting, suggesting invalidity of the assumption. The Western and Eastern Iratsu bodies, therefore, are originally bounded by subduction-boundary fault and the obtained pressure of 0.8 GPa can be interpreted to represent that of the hanging-wall lower continental crust in the subduction zone, where the Eastern Iratsu body originated. After the granulite-facies metamorphism, the Western Iratsu body, which was located near the footwall surface, initiated subduction and was subsequently juxtaposed with the above-located Eastern Iratsu body at the corresponding depth (∼ 27 km or greater) along the subduction boundary.