Petrography, isocon analysis and singular value decomposition analysis of jadeitites and metasomatic rocks surrounding them from the Nishisonogi metamorphic rocks revealed that the metasomatic rocks developed outside the stability field of jadeite + quartz after jadeitites had been included by a serpentinite mélange in a subduction channel possibly during exhumation. The jadeitites occur at two localities, Tone and Mie in Nagasaki City, from a serpentinite mélange in the Nishisonogi metamorphic rocks. The jadeitite mineral assemblage is jadeite/omphacite + paragonite + phlogopite + albite + clinozoisite/epidote ± muscovite ± analcime. Jadeite core with fine-grained quartz inclusions and the inclusion-free rim is partially replaced by albite. These jadeitites are surrounded by metasomatic zones of albitites and/or a muscovite rock. The mineral assemblage of the albitites is albite + clinozoisite/epidote ± muscovite ± omphacite ± phlogopite ± amphibole ± chlorite, and that of the muscovite rock is muscovite + clinozoisite + chlorite. Because these zones have no high-pressure minerals, they represent products in a P–T regime outside the stability field of jadeite + quartz. Isocon analyses between the jadeitites and the metasomatic zones reveal that K2O, H2O, Sr and Ba were added to jadeitite and SiO2, Na2O and Fe2O3 were removed. REE-rich veinlets emanate from clinozoisite grains, suggesting REE mobility during the fluid–jadeitite interactions. The metasomatic zones developed by interaction between jadeitites and serpentinite via K-, Sr- and Ba-rich fluid, during exhumation from the stability field of jadeite + quartz through that of albite to that of analcime.