Metamorphic petrology of the Omogo hornfels in central Shikoku, southwest Japan

The Omogo hornfels is one of a number of the Middle Miiocene hornfelses sporadically scattered throughout the Cretaceous high-pressure Sanbagawa metamorphic belt, central and western Shikoku, southwest Japan. It comprises very fine-grained basic and pelitic hornfels originating from the Sanbagawa metamorphic schists. The temperature gradient of the Omogo hornfels (>720°C/km) is steeper than those of typical contact aureoles (ca. 100-350°C/km), implying that the heat source of the Omogo hornfels was over in a shorter duration than those of common plutonic rocks. The high temperature gradient and lack of large basic intrusive rocks in the highest grade part support the idea that the Omogo hornfels was formed by episodic infiltration of high-temperature fluid along the schistosity of the Sanbagawa metamorphic rocks.