Origin of serpentinites in the Omi serpentinite melange (Hida Mountains, Japan) deduced from zoned Cr-spinel

Abstract

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+Fe^<2+>) 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.% Na_2O) 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-sub-duction origin. Serpentinites of the Omi serpentinite melange may have derived from the mantle wedge above a subduction zone and sub-sequently metamorphosed at low- to mid-temperature condition of probably eclogite or amphibolite facies.