MIDDLE MIOCENE THERMAL METAMORPHISM DUE TO THE INFILTRATION OF HIGH-TEMPERATURE FLUID IN THE SAMBAGAWA METAMORPHIC BELT, SOUTHWEST JAPAN

The Middle Miocene Tobe hornfels in the Sanbagawa metamorphic belt, we stern Shikoku, southwest Japan, is characterized by an abnormally stee p metamorphic gradient compared with other hornfelses associated with intrusive bodies. The basic hornfels, originally Sanbagawa greenschist rocks, is divided into the following three metamorphic zones: plagioc lase, hornblende, and orthopyroxene. The plagioclase zone is defined b y the appearance of calcic plagioclase, the hornblende zone by the ass emblage of hornblende+calcic plagioclase+quartz, and the orthopyroxene zone is characterized by the assemblage of orthopyroxene + clinopyrox ene + plagioclase + quartz. Calcic amphibole compositions change from actinolite to hornblende as a result of the continuous reactions durin g prograde metamorphism. Petrographical and thermometric studies indic ate a metamorphic temperature range of 300-475 degrees C for the plagi oclase zone, 475-680 degrees C for the hornblende zone, and 680-730 de grees C for the orthopyroxene zone. The temperature gradient based on petrological studies is approximately 5 degrees C/m, which is unusuall y high. Geological and petrological studies demonstrate that the hornf elses were formed by the focusing of high-temperature fluids through z ones of relatively high fracture permeability. The steep thermal gradi ent in the Tobe hornfels body is consistent with a large fluid flux, g reater than 8.3 X 10(-7) m(3) m(-2)S(-1), over the relatively short du ration of metamorphism, approximately 100 years.