Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U–Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts

Abstract

In this paper we present new U–Pb zircon ages determined with Sensitive High-Resolution Ion MicroProbe (SHRIMP) for nine plutonic rocks or orthogneisses and one paragneiss from the Higo and Maana belts (here referred to as the paleo-Ryoke belt) in southwest Japan, and the South Kitakami belt in northeast Japan. Both belts are Paleozoic–Mesozoic continental terranes discontinuously distributed along the Median Tectonic Line (MTL), and structurally lying on the Sambagawa belt in the Japanese Islands. Three groups of U–Pb zircon ages showing the timing of different magmatic events were determined for the plutonic rock or orthogneiss samples: ca. 500 Ma (Early Ordovician, two samples), a mean age of 292.0±12.4 Ma (Late Carboniferous–Early Permian, one sample) and ca. 110 Ma (Early Cretaceous, six samples). Furthermore, zircons with core–rim microstructures from a garnet–biotite–cordierite paragneiss in the Higo belt were also dated with SHRIMP. The cores yielded 2155–184 Ma inherited U–Pb ages (mostly varying between 330 and 184 Ma), while the rims yielded a mean age of 116.5±18.7 Ma (Early Cretaceous) U–Pb recrystallization ages comparable with the igneous ages, limiting the sedimentary age of the protolith to between Early–Middle Jurassic (ca. 180 Ma) and Early Cretaceous time. These SHRIMP U–Pb ages of plutonism, metamorphism and sedimentation, together with previously reported radiometric ages, have revealed a strong similarity in the tectonic histories of the paleo-Ryoke and South Kitakami and Abukuma belts, suggesting that these belts can be correlated. At present the paleo-Ryoke belt and the South Kitakami and Abukuma belts are separated by more than 1000 km. However, these belts may have originally comprised a continuous continental terrane that was later displaced and juxtaposed, together with the underlying Sambagawa belt, against the Ryoke belt by large-scale sinistral faulting along the MTL and associated major strike–slip faults during the Latest Cretaceous.