Petrology and geochemistry of chromian spinel-bearing serpentinites in the Hida Marginal Belt (Ise area, Japan): characteristics of their protoliths

The Ise area is located in the western part of the Hida Marginal Belt (central Japan), which includes several sporadic exposures of ultramafic rocks, sometimes forming a serpentinite melange of the Paleozoic age. Ultramafic rocks in the Ise area, enveloped by Paleozoic-Mesozoic sedimentary rocks, are completely serpentinized; however, the abundance of bastite after orthopyroxene suggests harzburgite protoliths. The bastite- and mesh-textured serpentines are distinguished from each other in Al2O3, Cr2O3 and NiO contents. The bastite-textured serpentine is high in Al2O3 (up to 4.0 wt%) and Cr2O3 (up to 1.2 wt%), but low in NiO (<0.3 wt%) relative to the mesh-textured one. Relic chromian spinel, vermicular in shape, shows an inter-grain chemical homogeneity and is sometimes altered to ferritchromite at the margin. It has a narrow range of Cr# [ = Cr/(Cr + Al) atomic ratio] from 0.38 to 0.51 and low Y-Fe {=[Fe3+/(Cr + Al + Fe3+) atomic ratio, <0.03]}, similar to chromian spinel in Kotaki and Oeyama ultramafic masses. It is also similar in chemistry to spinels in forearc and abyssal peridotites, suggesting two possibilities for the derivation of the Ise serpentinite's protoliths. The degree of melting using Cr# versus TiO2 of chromian spinel is similar to 20-25%, which is in accordance with the harzburgite protoliths obtained by whole-rock chemistry models. We found that Cr, Al, Ni and Ca, preserved in bastite and mesh-textured serpentine, are conservative during serpentinization, confirmed from the similarity in the whole-rock Al and Ca of the Ise serpentinites to Horoman harzburgites. The occurrence of dolomite can stabilize the Ca and limited its mobility to escape outside the serpentinites. The harzburgite protoliths were possibly serpentinized by slab-derived fluids at a shallow depth relative to the Happo-O'ne serpentinites during the exhumation process.