OPHIOLITES AND ACCRETION HISTORY OF JAPAN (AND ORIGIN OF THE ALPINE TYPE LHERZOLITES IN THE PACIFIC RIM)

Phanerozoic ophiolites in Japan show extreme variations in their age and petrological-geochemical diversity. The early Paleozoic forearc-type ophiolites (Oeyama and Miyamori-Hayachine) represent the oldest oceanic lithosphere. The Yakuno ophiolite behind this forearc system in western Japan makes up the late Paleozoic marginal basin lithosphere. This tectonic situation resembles the modern Izu-Bonin-Mariana area, where the active volcanic arc and marginal basin occur behind the Eocene ophiolitic rocks in the forearc setting. The coeval, late Paleozoic accretionary complexes (Akiyoshi and Ultra-Tamba) of this forearc-arc-backarc system are tectonically underlain to the southeast by the Jurassic accretionary complex (Mino-Tamba), which includes greenstones (“plume (P) type ophiolites”) with ferropicritic rocks of oceanic LIP origin. The Jurassic Sorachi and Mikabu greenstones are also P-type ophiolites, and are tectonically underlain by the Cretaceous Kamuikotan and Sambagawa high-P schists, respectively. The Setogawa-Mineoka mélange is the Cenozoic accretionary type ophiolite (~16 Ma) set in the young part of the Cretaceous-Cenozoic Shimanto accretionary complex.

The Horoman peridotite body in Hokkaido is emplaced between the Poroshiri ophiolite (MOR-type? Cretaceous?) and the Cretaceous Hidaka accretionary complex (affected by low-P metamorphism in the Miocene). This massif consists mainly of fertile lherzolite with garnet pseudomorphs (symplectites), and has been identified as an “alpine-type, high-T solid intrusion”. However, why such fertile, deep-seated mantle lherzolite occurs between an ophiolite and an accretionary complex? A possible answer is non-magmatic spreading in the back-arc oceanic lithosphere as in oceanic core complexes, like the Godzilla Mullion in the Philippine Sea. Continental margin (CM) type ophiolites are characterized by sea-floor exposure of the fertile “subcontinental” lherzolite with eclogitic rocks and scarcity of melt products. Analogous ophiolites may also be formed by non-magmatic spreading in the intra-oceanic circumstances. The fertile lherzolite bodies in Japan, the Philippines, NE Russia and the western USA may be the oceanic version of CM-type ophiolites formed by “non-magmatic spreading” (NM type).