U-Pb zircon dating of the Sanbagawa metamorphic rocks in the Besshi—Asemi-gawa region, central Shikoku,Japan, and tectono- stratigraphic consequences

摘要

The aim of this study is to addresses unresolved issues associated with the depositional and metamorphic ages of the Sanbagawa metamorphic rocks (central Shikoku, Japan). We performed laser-abla- tion inductively-coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb analysis on psammitic schists from the Sanbagawa metamorphic rocks (sensu stricto) in the Besshi-Asemi-gawa region. Young U-Pb ages of ca. 100-90 Ma were found in all analysed samples, regardless of the metamorphic grade. These results suggest that the Sanbagawa metamorphic rocks from the upper chlorite to oligo- clase-biotite zones are metamorphic equivalents of the Cretaceous Northern Shimanto accretionary complex, which also underwent prograde metamorphism after ca. 100-90 Ma. Based on our results and previous geochronological data from the Sanbagawa metamorphic rocks, we suggest that the Sanbagawa metamorphic belt consists of three metamorphic units, which are characterized by different depositional and metamorphic ages. These three units, from old to young, are here referred to as the Besshi,Asemi-gawa, and Oboke units.