Geochemistry and magmatic zircon U–Pb dating of amphibolite blocks in the Omi serpentinite mélange, north central Japan Possible subduction of the Cambrian oceanic crust

摘要

Early Paleozoic serpentinite melanges in Japan preserve the oldest high–P metamorphic rocks in the circum–Pacific orogenic belt. To understand the tectonic regime at the subduction initiation of the proto–Japan convergent plate boundary, whole–rock geochemistry, and zircon U–Pb geochronology were investigated for amphibolite blocks in the Omi serpentinite mélange, central Japan. The studied amphibolites from two different localities have the mineral assemblage of albite + clinozoisite + amphibole ± rutile ± titanite, which characterize epidote–amphibolite facies metamorphism. Whole–rock trace element concentrations of the amphibolites suggest that gabbroic protoliths formed possibly in an oceanic setting. The zircon U–Pb weighted mean ages obtained from two amphibolite samples indicate that the protolith was formed in the Cambrian. The protolith ages of the studied amphibolites are comparable with those of reported Early Paleozoic ophiolite and high–pressure rocks in Paleozoic serpentinite mélanges in Japan. This fact implies that the young hot oceanic crust was subducting into the East Asian convergent plate margin during the Cambrian.