Prograde eclogites from the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt, central Shikoku, southwest Japan

Abstract

Abstract Prograde eclogites occur in the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt of central Shikoku. The Tonaru mass is considered to be a metamorphosed layered gabbro, and occurs as a large tectonic block (approximately 6.5 km × 1 km) in a high‐grade portion of the Sambagawa schists. The Tonaru mass experienced high‐P/low‐T prograde metamorphism from the epidote‐blueschist facies to the eclogite facies prior to its emplacement into the Sambagawa schists. The estimated P–T conditions are T = 300–450°C and P = 0.7–1.1 GPa for the epidote‐blueschist facies, and the peak P–T conditions for the eclogite facies are T = 700–730°C and P ≥ 1.5 GPa. Following the eclogite facies metamorphism, the Tonaru mass was retrograded to the epidote amphibolite facies. It subsequently underwent additional prograde Sambagawa metamorphism, together with the surrounding Sambagawa schists, until the conditions of the oligoclase–biotite zone were reached. The high‐P/low‐T prograde metamorphism of the eclogite facies in the Tonaru mass and other tectonic blocks show similar steep dP/dT geothermal gradients despite their diverse peak P–T conditions, suggesting that these tectonic blocks reached different depths in the subduction zone. The individual rocks in each metamorphic zone of the Sambagawa schists also recorded steep dP/dT geothermal gradients during the early stages of the Sambagawa prograde metamorphism, and these gradients are similar to those of the eclogite‐bearing tectonic blocks. Therefore, the eclogite‐bearing tectonic blocks reached greater depths in the subduction zone than the Sambagawa schists. All the tectonic blocks were ultimately emplaced into the hanging wall side of the later‐subducted Sambagawa high‐grade schists during their exhumation.