PRE-MIDDLE SILURIAN GRANITIC MAGMATISM AND ASSOCIATED METAMORPHISM INNORTHERN JAPAN - SHRIMP U-PB ZIRCON CHRONOLOGY

The results of SHRIMP U-Pb zircon dating are presented for igneous and metamorphic rocks of enigmatic age in northern Japan. The SHRIMP anal yses for the Tsubonosawa metamorphic rocks included in the Hikami Gran ite reveals (1) Ordovician sedimentation of the original rocks, includ ing Archaean-Proterozoic detrital zircons, and (2) thermal metamorphis m with partial melting or assimilation followed by intrusion of the Hi kami Granite at ca. 440 Ma (end of Ordovician). This study allows the reconstruction of Siluro-Ordovician palaeogeography and mapmatism-asso ciated metamorphism in East Asia. The SHRIMP results are compared with those of CHIME (chemical U-Pb total Th isochron method) chronology, w hich is evaluated as providing significant reconnaissance ages in some instances.