Abstract

Petrological and geochemical data for chromian spinel from the Tari-Misaka ultramafic complex, southwest Japan, are discussed. In this complex, the spinels form a chromitite pod coated by a dunite envelope and are collectively enclosed by clinopyroxene-bearing harzburgite. Chromitite and its dunite envelope are distinctly different from the surrounding harzburgite in terms of the chromian spinel chemistry. The Ti content is five to ten times higher in spinels of the enveloped chromitite than in those of the harzburgite, although the Cr ratio (Cr/(Cr + Al, the atomic ratio) for both is almost constant or varies only slightly (0.4-0.5). This eliminates a comagmatic origin for the harzburgite-dunite-chromitite association. Spinel chemistry for both major and trace elements is almost the same in the chromitite and its dunite envelope, except for the Mg ratio and the Co and Mn contents which have a changed subsolidus as a function of the spinel mode. Thus, the dunite and chromitite could be comagmatic, but fractional crystallization of the dunite after or before chromitite deposition is not possible. The chemistry of the chromian spinel and the mode of occurrence of the dunite-chromitite association are consistent with an origin by interaction of an exotic melt with wall-rock harzburgite. The dunite may be of replacive origin: the exotic melt would react with the harzburgite at the new contact to form dunite and a secondary Si-rich melt. The melt thus formed could mix with a successive inflow of a more primitive melt; the mixed melt would be expected to have chromite (chromian spinel) as the only liquidus phase, precipitating chromitite next to the replacive dunite (the dunite envelope).Podiform chromitite is most commonly formed in the mantle beneath island arcs. The spread in Cr ratios of spinel in podiform chromitites is almost comparable with that of spinel in primitive arc-related magmas. The presence of chromitite in the upper mantle beneath the Japan arcs is consistent with this interpretation.

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