OPHIOLITES AND GREENSTONES IN THE JAPANESE OROGENIC BELTS:MULTIPLICITY AND DIVERSITY OF THE ACCRETED OCEANIC LITHOSPHERE

Ophiolites in the circum-Pacific orogenic belts are called "cordilleran-type" with negative adjectives such as "incomplete", "dismembered" and "metamorphosed", contrasting to the typical "Tethyan-type" (Coleman, 1986). However, the circum-Pacific ophiolites have some positive significance in the global tectonics in view of their unique features: (1) Multiple tectonic superposition of ophiolites of widely varying ages. (2) Wider petrologic diversity of ophiolites with highly depleted mantle peridotite. (3) Occurrence of ophiolites with thick crustal sections. These features suggest their repeated generations and emplacements in the supra-subduction zone environments. The Japanese ophiolites are commonly soled with younger blueschist rocks and are tectonically underlain by much younger accretionary complexes. For example, in southwestern Japan, the Ordovician Oeyama ophiolite (>450 Ma) is underlain by the 320 Ma Renge blueschist and the late Permian (250 Ma) Akiyoshi accretionary complex. This relation suggests tectonic erosion or non-accretion during the intervening Siluro-Devonian time. Similar gap exists between the early Permian (280 Ma) Yakuno ophiolite and the underlying Jurassic Tamba accretionary complex (150 Ma). An accretionary complex is characterized by the "oceanic plate stratigraphy" composed of greenstone, chert, limestone, mudstone and sandstone in younging order. The basal greenstone commonly includes OIB with high Ti and Nb concentrations, but the ophiolite is almost devoid of OIB. In the present western Pacific, Izu-Mariana and Tonga areas are characterized by the existence of ophiolite outcrops in trench-slope, the absence of accretionary complexes, and the occurrence of submarine blueschist rocks, whereas the areas off northeastern Honshu and Hokkaido are characterized by the vast development of present-day accretionary complexes. These contrasting two stages may have been repeated in any segment of the Japanese orogenic belt throughout the Phanerozoic. The periods of oceanic island arc and marginal basin development (ophiolite formation) and tectonic erosion (blueschist metamorphism) might have alternated with the accretion periods, during which accretionary complexes were developed through off-scraping of oceanic sediments and seamounts.