Ultramafic rocks of the Omi serpentinite melange are extensively serpentinized. However, newly discovered Cr-spinels in chromitite and massive serpentinite provide a key to decipher the origin and metamorphism of serpentinites. Cr-spinels show distinct chemical zoning: the cores have high Cr# [=Cr/(Cr+Al) atomic ratio] (0.70-0.77); Mg# [=Mg/(Mg+Fe2+) atomic ratio] decreases remarkably towards the rim through a transition zone between core and ferritchromite rim. Moreover, the cores of zoned Cr-spinel in chromitite contain abundant tiny inclusions of mainly pargasitic amphibole (up to 3.8 wt.% Na2O) and rare diopside. These inclusions were completely replaced by secondary tremolitic amphibole and chlorite at the rim, and by dolomite at the Ti-enriched metasomatized rim. Although the Mg# of the core (0.20-0.43) is significantly lower than those of Cr-spinel in both Alpine and abyssal peridotites, the observed mineral inclusions and the analysed Cr# at the core are interpreted to be relics of igneous stage. Compositional characteristics and the occurrence of hydrous mineral inclusions of the zoned Cr-spinel exhibit critical features that suggest a supra-subduction origin. Serpentinites of the Omi serpentinite melange may have derived from the mantle wedge above a subduction zone and subsequently metamorphosed at low- to mid-temperature condition of probably eclogite or amphibolite facies. [飛騨山地, 青海蛇紋岩メランジュを構成する超苦鉄質岩は著しく蛇紋岩化し, その起源や変成作用を明らかにするための情報に乏しかった. しかし, 塊状のアンチゴライト蛇紋岩と, それに伴うクロミタイト脈から, 初生的なCr/(Cr+Al) 原子比 (0.70-0.77) をコアに保持した組成累帯クロムスピネルと, 包有物としての初生的なパーガス閃石 (～3.8wt.% Na2O) を初めて見出した. 蛇紋岩の源岩は変成作用を被っており, クロムスピネルのコアは低Mg# (0.20-0.43) で特徴づけられる. さらに, リムにおいて初生鉱物包有物のドロマイト化と Ti の付加が部分的に認められる. これらの特徴から, 青海蛇紋岩メランジュを構成する蛇紋岩は, 沈み込み帯のマントルかんらん岩を起源とした低～中程度の温度の変成かんらん岩が, より低温で蛇紋岩化したものと推測される.]