Jadeitites and associated metasomatic rocks from serpentinite mélanges in the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan: a review

摘要

The Nishisonogi jadeitite is a peculiar example among Japanese jadeitites in terms of the age of formation and its genesis as metasomatic replacement (R–type). The Nishisonogi jadeitite is characterized by jadeite with a quartz–inclusion–rich core and quartz–free rim. The pressure–temperature (P–T ) conditions of the Nishisonogi jadeitite is constrained by the assemblage of jadeite + quartz and of clinozoisite + paragonite + quartz. The jadeitite occurs as tectonic blocks in a sepentinite mélange together with metasomatic rocks such as omphacitite, albitite and zoisite rock. The protolith of the jadeitite is likely plagiogranite or adakitic granite as suggested by the rare earth element (REE) pattern of zircon. The zircon has a euhedral core with a U–Pb age of 126 ± 6 Ma with exhibiting sharp and flat boundary with metamorphic rim, which has a U–Pb age of 84 ± 6 Ma. The rim age is almost consistent with the 95–90 Ma age of the peak metamorphism of the coherent schists, as a host of the serpentinite mélange, as determined by ~(40)Ar/~(39)Ar method applied to glaucophane and phengite. Some jadeitite blocks show lithological zonations with albitite and other rocks in scale of several tens of centimeters. These zonations likely formed in the stability field of albite during exhumation of the serpentinite mélange. The isocon analysis of whole rock compositions and the singular value decomposition analysis of mineral reactions show that SiO_(2), Na_(2)O, and Fe_(2)O_(3) evolved from the jadeitite towards serpentinite according to the retrograde reactions.