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 Control of Wall-rock Composition on the Formation of Podiform Chromitites as a Result of Magma/Peridotite Interaction
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.
資源地質 47(4), 177-187, 1997-08-30
The Society of Resource Geology