日本のオフィオライト

Abstract

The Japanese ophiolites are divided into nappe type and melange type by their occurrences. The nappe type formed in a short period (a few 10Ma), and emplaced shortly after its igneous formation keeping its original igneous stratigraphy. They generally escaped from post-emplacement, high-pressure metamorphism. The melange type is a mixture of ophiolitic fragments of various ages (a few 100Ma) and lithologies, and most of them are affected by high-pressure metamorphism. The age of igneous formation of the Japanese ophiolites ranges from early Paleozoic to Cenzoic. Large nappe-type ophiolites formed in Ordovician, Permian, Jurassic, and Cretaceous periods, corresponding to the world-wide ophiolite pulses. The Paleozoic ophiolites are mainly distributed in Honshu, while the Mesozoic ophiolites are in Hokkaido. The Cenozoic melange-type ophiolites occur in front of the Izu arc, which collided against Japan in Miocene.The residual peridotite of Japanese ophiolites ranges from fertile lherzolite to highly depleted harzburgite. Mafic-ultramafic cumulates show diverse crystallization sequences including plagioclase, clinopyroxene, and orthopyroxene types. MORBs are dominant among the ophiolitic volcanics, while picrites and alkali basalts are also common among melangetype ophiolites. A few ophiolites bear calc-alkali rocks or hydrous mantle peridotite suggestive of island-arc origin. Pervasive pre-emplacement metamorphism of the cumulate rocks in the amphibolite and granulite facies as well as the absence of sheeted dike complexes are common features of the Japanese ophiolites.