THREE DISTINCT METAMORPHIC UNITS IN THE CRETACEOUS SANBAGAWA HP BELT INFERRED FROM LA-ICP-MS ZIRCON U-PB DATING

To resolve issues associated with the deposition and metamorphic ages of the Cretaceous Sanbagawa HP metamorphic rocks in Japan, we performed laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis on psammitic schists of the Sanbagawa metamorphic rocks from the Besshi–Asemi-gawa region, central Shikoku, Japan. All analysed samples have young U–Pb ages of ca. 100–90 Ma regardless of their metamorphic grade, suggesting that the Sanbagawa metamorphic rocks from upper chlorite (low-grade) to oligoclase-biotite zone (high-grade) are metamorphic equivalents of the Cretaceous Northern Shimanto accretionary complex, which also accreted in trench and underwent prograde metamorphism after ca. 100–90 Ma. A combination of our results with previous geochronological data such as microfossil, K-Ar, Ar-Ar and U-Pb analyses for the Sanbagawa metamorphic rocks indicates that the Sanbagawa metamorphic belt in Shikoku is composed of three metamorphic units with different deposition/accretion and metamorphic ages. The ages of each unit are ca. 140-130 and 120-110 Ma, ca. 100-90 and 90-88 Ma, and ca. 80 and 80-60 Ma, respectively. In this study, each unit is called the Besshi, Asemi-gawa and Oboke units in ascending order by time. Moreover, our results suggest that two types of eclogites with different deposition/accretion and prograde metamorphic ages exist in central Shikoku.