Early Paleozoic jadeitites in Japan: An overview

抄録

Paleozoic jadeitite–bearing serpentinite–matrix mélange represents the oldest mantle wedge record of a Pacific–type subduction zone of proto–Japan. Most jadeitites are fluid precipitates (P–type), but some jadeitites are metasomatic replacement (R–type) which preserve relict minerals and protolith textures. The beauty and preciousness of some gem–quality, semi–translucent varieties of jadeitites in the Itoigawa–Omi area led to the designation of jadeitite as the national stone of Japan by the Japan Association of Mineralogical Sciences. Zircon geochronology indicates jadeitite formed prior to Late Paleozoic Renge metamorphism that formed blueschist and rare eclogite. For example, in the Itoigawa–Omi and Osayama localities, older jadeitites and younger high–pressure/ low–temperature metamorphic rocks in a single mélange complex imply different histories for the subduction channel and jadeite–bearing serpentinite–matrix mélange. This suggests that the jadeitite–hosted mélange (or serpentinized peridotite) can stay within the mantle wedge for a considerable time; thus recrystallization, resorption, and re–precipitation of jadeitite can continue in the mantle wedge environment. Therefore, studies of Paleozoic jadeitites in Japan have great potential to elucidate the earliest stages of orogenic growth (oceanward–accretion and landward–erosion) associated with the subduction of the paleo–Pacific oceanic plates, and to test geophysical observations of modern analogues from a mixture of fossilized mantle wedges and subduction channels.