Petrological Evolution of the Upper Mantle Beneath the Southern Sanandaj-Sirjan Zone: Evidence from Kuhshah Peridotite Massif, Southeast Iran

The Kuhshah ultramafic complex is located in the south-east of SanandajSirjan metamorphic zone, near the probable remnants of Neotethys plate in Iran. It consists of highly depleted harzburgites, dunites, chromitite bands and altered gabbros. The ultramafic parts have been intruded by numerous clinopyroxenite dykes and veins. In the harzburgites, there are different generations of olivine, orthopyroxene and spinel. The chemical composition of the first generation minerals indicates that the harzburgites are depleted in incompatible elements, but the other generations show melt/rock interaction features which can be found in a suprasubduction zone setting. During subduction of the Neotethys plate beneath the Sanandaj-Sirjan zone, a back arc basin developed between the Sanandaj-Sirjan arc and the central Iranian microcontinent. Whole rock and mineral chemistry, specially, cromespinels, show that the harzburgites formed underneath this basin. After depletion, ascending melts with boninitic compositions reacted with these peridotites to form dunites and chromitite bands. The updoming and decompression of the mantle in the next stage, led to remelting at deeper levels and development of clinopyroxenite parental melts which intruded into the uppermost mantle. Then, the new basin closed and the Kuhshah peridotites were emplaced in present position.