Geochemistry of mantle peridotites from the Sistan zone (west of Zahedan and north of Sahl Abad), eastern Iran

Ophiolitic outcrops of Sistan zone are remnants of Neo-Tethyan oceanic lithosphere, once existed between Lut and Afghan continental blocks. Mantle peridotites including harzburgites and cpx-harzburgites are dominant ophiolitic outcrops. Petrographical evidence together with geochemical characteristics such as CaO (0.88 – 2.94 wt.%), Al2O3 (0.76- 3.51 wt.%) and MgO (38.79- 45.25 wt.%) variations suggest different degrees of mantle depletion. The study of chemical composition of Cr-spinel shows it’s highly sensitive nature to alteration. During alteration, Cr-spinel commonly displays increasing Fe, Mn, Cr# and in some cases Si and decreasing Al, Mg and Mg# values. Since the chemistry of Cr-spinel is commonly used as a petrogenetic tool so it should be regarded with caution. Based on the chemistry of fresh Cr-spinels of a cpx-harzburgite sample, the melting percent of about 11 to 11.5% has been obtained. Moreover, the Cr# (26.5 to 27.5), Mg# (64.9 to 65.8) and Al2O3 (42.3 to 43.4 wt.%) of Cr-spinels suggest abyssal peridotite or MORB-like geodynamic setting. Although, based on the whole rock chemistry, the mantle peridotites display various depletion degrees which may imply various mid-ocean ridge to supra-subduction zone tectonic settings of ophiolite generation.