Origin of spinel-hosted mineral inclusions in mantle peridotite from Setogawa in the Circum-Izu Massif Serpentine Belt, central Japan: Implications for the chromitite genesis

Abstract

Plagioclase-impregnated mantle harzburgites from Setogawa area in the Circum-Izu Massif Serpentine Belt (of Miocene in age), central Japan, contain chromian spinel (chromite) grains that have minute globular inclusions composed of micas, amphiboles and pyroxenes. They are similar in texture and chemistry to those in chromitites as well as in some plutonic rocks, such as dunites and troctolites, form the Moho transition zone (MTZ). The Setogawa harzburgite host is similar in petrography and geochemistry to the MTZ harzburgite from the Oman ophiolite, and is possibly a low-pressure restite after melt-assisted partial melting or reaction with a percolating magma. The orthopyroxene-melt reaction, which is essentially responsible for producing those rocks from the peridotite (harzburgite), can precipitate chromian spinel as well as generate chemically heterogeneous melts along decomposing orthopyroxenes. The spinel-hosted melt inclusions in podiform chromitites are most frequently trapped at the reaction front between the spinel-oversaturated, spinel-loaden magma and mantle orthopyroxene (=harzburgite). The spinel-loaden magma had been produced via the same reaction at lower levels and moved upward. The spinel quickly overgrows to include chemically heterogenous melt drops, some of which are high in Cr/Al ratio and precipitate high-Cr spinel via reaction with the wall spinel. The chemical character (Cr/Al ratio) of podiform chromitites would be determined by the accumulative contribution from the melt derived from orthopyroxene decomposition during the passage of the melt through the upper mantle. We can speculate that the chromian spinel grains containing similar melt inclusions from the stratiform chromitite are transported as xenocrysts of mantle origin. The parental magma for layered intrusions possibly reacted with the peridotite when it passed through the mantle before arriving at a crustal magma chamber, showing a possible genetic linkage between the podiform and stratiform chromitites.