Discovery of the Early Jurassic high-temperature pre-Sanbagawa metamorphism recorded in titanite

Abstract

The Eastern Iratsu body of the Sanbagawa metamorphic terrane in central Shikoku recorded an Early Jurassic metamorphic event older than the Sanbagawa metamorphic event. This study carried out the petrochronology of eclogitic high-pressure marble in the Eastern Iratsu body to determine the corresponding conditions of metamorphism and their variation in time. A combination of trace element (Zr) thermometry, host-inclusion mineral elastic barometry, and in-situ U–Pb dating was performed on the titanite in an eclogitic marble to reveal the metamorphic pressure/temperature conditions of 2.3–2.7 GPa and 1000–1200 °C high-temperature (HT) eclogite facies metamorphic conditions, which occurred at approximately 200 Ma. Moreover, certain titanite grains exhibited a distinctly younger age of 164 Ma with a similar Zr concentration. The 164 Ma domain of titanite is observed near the Na-enriched diopside, indicating the fluid-infiltration and corresponding titanite recrystallization. Some diopside in the matrix contains high-salinity aqueous fluid inclusions (freezing point depression = 3.6–3.7 °C) in the Na-enriched domain, which may infer the source of Na. Furthermore, titanite rims containing trace amounts of Zr yielded 126 Ma, which is comparable with that of the early Sanbagawa metamorphism; however, no P-T estimation was available.This newly identified Early Jurassic metamorphic event is much older than the previously reported metamorphic age of the Sanbagawa metamorphic belt, i.e., c. 120 Ma for early Sanbagawa metamorphism. Although the origin of the HP marble is unclear, the pre-Sanbagawa HT eclogite facies metamorphism reported here could provide a new insight for the subduction of the Izanagi plate and the corresponding geotectonic evolution of eastern Asia.