Eclogite-Facies Mineral Inclusions in Clinozoisite from Paleozoic Blueschist, Central Chugoku Mountains, Southwest Japan: Evidence of Regional Eclogite-Facies Metamorphism

Abstract

An eclogite-facies assemblage garnet + omphacite + rutile + glaucophane + quartz occurs as inclusions in clinozoisite porphyroblasts in a high-grade blueschist block in the Paleozoic Osayama serpentinite mélange of the Chugoku Mountains, southwest Japan. Textual relations, and parageneses and compositions of minerals, indicate that the high-grade block experienced peak eclogite-facies metamorphism (M 1) and subsequent blueschist-facies overprinting (M 2). Moreover, low-P amphibolite-facies minerals (M 0) that may represent remnants of pre-subduction conditions are locally preserved in the protolith. Omphacite + adjacent garnet inclusion pairs in clinozoisite give a minimum P = 1.1-1.3 GPa at T = ~480-550°C. Many petrologic similarities between the Osayama eclogite and the Hida Mountains eclogite provide evidence for a regional eclogite-facies metamorphism in Paleozoic terranes of southwest Japan. Considering available petrotectonic information regarding eastern Asia, we suggest two alternative models: (1) Japanese Paleozoic eclogites may represent "Pacific-type" subduction prior to collision between the Sino-Korean and Yangtze blocks, which occurred on both western and eastern sides of the Qinling-Dabie-Sulu belt; or (2) Japanese Paleozoic eclogites may represent part of a Paleozoic "Pacific-type" subduction chain, which is continuous from the China-Russia border through Japan southward to North Korea.