Abstract
The metamorphic and plutonic rocks within a serpentinite mélange in the Nagato Tectonic Zone, Yamaguchi prefecture, southwest Japan have been known as a part of the early Paleozoic rock record in Japan. Thus, they play an important role in deciphering the tectonic evolution of proto-Japan. This study determined their U-Pb zircon ages to deduce the origin and geological significance of the Nagato Tectonic Zone. The results of the petrographic observation and LA-ICP-MS U-Pb zircon dating revealed the occurrences of the massive amphibolite originated from the 460 Ma mafic igneous rock, two-mica tonalites formed at 470-460 Ma in association with some xenocrystic zircon grains, and low-grade metasedimentary rocks including detrital zircon grains of 2460-400 Ma. The lithology, petrographic characteristics, and U-Pb zircon ages of these constituent rocks in the Nagato Tectonic Zone are well comparable with the blocks within serpentinite mélange in the Kurosegawa Tectonic Belt, rather than the Hida Gaien Belt which has been considered to form a single geological unit with the Nagato Tectonic Zone as the Sangun-Renge metamorphic belt or Nagato-Renge belt. It implies the Nagato Tectonic Zone could be regarded as an isolated fragment of the Kurosegawa Tectonic Belt. The features of detrital age spectra and Ordovician magmatism in the Nagato Tectonic Zone and Kurosegawa Tectonic Belt provide the possibility of the development of their granitoids and protoliths of metamorphic rocks at the active continental margin of the South China craton.
