Evidence for multi-stage mantle metasomatism at the Dehsheikh peridotite massif and chromite deposits of the Orzuieh coloured mélange belt, southeastern Iran

Abstract

The Dehsheikh peridotite massif is located in the Esfandagheh region in southeastern Iran at the NE edge of the main Zagros thrust and is part of the Orzuieh coloured mélange belt. The Dehsheikh massif consists of relatively unaltered harzburgite, lherzolite, dunite and several chromite deposits that are cut by pyroxenitic dykes. These peridotites display numerous evidences of mantle metasomatism at various scales. Dunite dykes, bands and patches as well as pyroxenites and chromite deposits were formed during several metasomatic stages. Textural features such as the incongruent melting of orthopyroxenes and interstitial spinels, olivines and pyroxenes suggest that mantle metasomatism has affected the Dehsheikh massif. The effects of the metasomatism can be traced in mineral chemistry and chemical zoning of the spinels and orthopyroxenes, as well as the REE patterns of the Dehsheikh harzburgites. These features indicate that the Dehsheikh peridotites were depleted in incompatible elements, light REE, as a consequence of partial melting prior to being metasomatized by migrating melts at mantle conditions. Our observations also suggest that ascending melts in the deeper levels of the upper mantle began to react with depleted peridotites and produced incongruently melted orthopyroxenes, dunites and impregnated harzburgites. The composition of the melts gradually changed toward boninite due to melt–peridotite reaction. These processes produced the chromite deposits and their dunitic envelopes in the upper levels of the mantle. Following the early pervasive metasomatism, pyroxenitic melts invaded the peridotite at shallower depths. These events suggest that the Dehsheikh peridotites were formed in a supra-subduction zone setting.