マグマ/かんらん岩相互反応の結果としてのポディフォーム・クロミタイト形成における壁岩組成コントロール

抄録

Podiform chromitites with dunitic envelopes, especially undeformed ones, usually cut mantle peridotite as dikes or pipes. Large-scale chromitites are usually set within the moderately refractory peridotite; i.e., the harzburgite with chromian spinel of intermediate Cr# (=Cr/[Cr+Al] atomic ratio), 0.4 to 0.6. Podiform chromitites are usually absent or very small in volume, in both fertile lherzolite and highly refractory harzburgite. This relationship indicates that the wall-rock chemistry controls to some extent the formation of chromitites, and is concordant with a petrogenetic model whereby podiform chromitite is formed by reaction between upwelling melt and mantle peridotite protolith and related melt mixing. Both the Cr# and the total amount of (Al+Cr) in the involved system, especially of orthopyroxene of the protolith, control the size and chemistry of the podiform chromitite, a reaction product. For fertile lherzolite wall, the Cr# of the system is too low to concentrate spinel because the mixed melt is relatively low in the degree of spinel-oversaturation. For highly refractory harzburgite wall, the amount of (Al+Cr) of the system is too low to precipitate large amounts of spinel. The moderately refractory harzburgite is satisfactory for the two criteria, and can be a host for large-scale podiform chromitites. Upper mantle beneath arc, especially arc proper and back-arc basin, and fast-spreading oceanic ridges is suitable for the podiform chromitite formation because the harzburgite with chromian spinel of intermediate Cr# is available there. Both melt/peridotite interaction and magma mixing can occur to precipitate large amount of chromian spinel at the Moho transition zone where upwelling melt is pooled within the harzburgite mantle.