四国西部三波川変成帯における中期中新世の熱変成作用  [in Japanese] Middle Miocene thermal metamorphism in the Sanbagawa metamorphic belt, western Shikoku, Southwest Japan  [in Japanese]

Geology, metamorphic petrology and K-Ar age of the Tsuzuragawa hornfels in the Sanbagawa metamorphic belt, western Shikoku, were investigated. The basic hornfels, originally Sanbagawa green schist, are divided into three metamorphic zones; plagioclase zone, hornblende zone and orthopyroxene zone, based on their mineral assemblages. The plagioclase zone is defined by the appearance of calcic plagioclase. The hornblende zone is characterized by the appearance of hornblende + plagioclase. The assemblage in pyroxene hornfels are clinopyroxene + orthopyroxene + plagioclase + ilmenite at the orthopyroxene zone. The P-T path of the Tsuzuragawa hornfels corresponds to a metamorphic facies series of typical contact metamorphism. One K-Ar hornblende age of hornblende-biotite hornfels is 15.1 Ma. The Tsuzuragawa and genetically related hornfels should be thermally formed by the infiltration of high-temperature fluid along a large fracture in the Sanbagawa metamorphic rocks in or slightly prior to the Middle Miocene volcanism and plutonism.