2004 Volume 110 Issue 10 Pages 591-597
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.
