Triassic ~(40)Ar/~(39)Ar ages from the Sakaigawa unit, Kii Peninsula,Japan: implications for possible merger of the Central Asian Orogenic Belt with large-scale tectonic systems of the East Asian margin

摘要

The 218.4 ± 0.4, 228.8 ± 0.9 and 231.9 ± 0.7 Ma ~(40)Ar/~(39)Ar laser probe pseudo-plateau ages (2σ; 49-63% ~(39)Ar-release) of very low-grade meta-pelitic whole-rocks from the Sakaigawa unit date high-P/T metamorphism. We argue that this event occurred in a subduction-accretion complex, not along the East Asian continental margin, but on the Pacific side of the proto-Japan superterrane. Proto-Japan was a Permian magmatic arc, presently dispersed in the Japanese islands, which also contained older subduction-accretion complexes. The arc system was fringing but not yet part of the Eurasian continent. The Middle to Late Triassic high-P/T tectono-metamorphic event was partly coeval with proto-Japan's collision with proto-Eurasia along the southward extension of the Central Asian Orogenic Belt, causing the main metamorphism in the Hida-Oki terrane. It is possible that this system continued via the Cathaysia block (China) to Indochina. The Late Permian to Middle Triassic Indosinian event might stem from docking of Pacific-derived terranes with Southeast Asia's continental margin. The concept of the proto-Japan superterrane implies that the Qinling-rnDabie-Sulu suture zone joined the Central Asian Orogenic Belt to the east of the North China craton and did not continue to Japan, as commonly assumed.