Mineral Chemistry of Sahneh-Harsin Peridotites; Evidences for Partial Melting on Mantle Peridotite over the Subduction Zone

Some Peridotite dismembered parts have outcropped in Southwest of Sahneh, as a part of Zagros ophiolite complex. These bodies contain olivine, pyroxene and spinel with some altered minerals such as serpentine and amphibole. Based on electron microprobe analyses, olivine composition is forsterite with Fo88-94. The composition of pyroxenes is diopside and clino­enstatite with low contents of TiO2 and Al2O3. The spinels are poor in Al2O3 with Cr# number between 0.41-0.51 which indicate the ophiolitic and depleted mantle origin. The serpentines have Al2O3 content in the range of 0.2 to 2.2 wt.% with SiO2 content between 34.15 to 44.07 wt.% and are chrysotile and lizardite. The low temperature of serpentine minerals (lizardite and chrysotile) and primary spinels indicates that the alteration was lower than 500 ºC. In the tectonic setting diagram, based on mineral chemistry, the higher content of Al2O3 and MgO in pyroxenes than to mid oceanic ridge ones and have the same properties with back arc basins. These evidences have been confirmed by spinel mineral chemistry. Accordingly based on the geodynamic situation of Kermanshah area and tendency of some samples to back arcs, it seems these peridotites have derived by high degree partial melting of depleted and sub-oceanic mantle in a supra-subduction zone-back arc and have same properties with residual mantle peridotites