Petrography, minerals chemistry and genesis of peridotite in the Heydarabad ophiolitic melange (East of Iran)

The Heydarabad ophiolitic melange is hosted by the Nehbandan Ophiolite Complex (NOC), which crops out in the Sistan Suture Zone (SSZ) that marks the boundary between the Lut and Afghan continental blocks. The major rocks of this area include mantle peridotites, listwaenite, gabbronorite, cumulate and layered gabbro, pillow basalt and deep-sea pelagic sediments that were formed in the mid-ocean ridge (MOR). Olivine, orthopyroxene, clinopyroxene and spinel are rock forming minerals of Heydarabad ophiolite mantle peridotite.

After field studies, thin and polished sections were prepared for petrological and mineralogical studies. Whole-Rock major and some trace elements were obtained by X-ray fluorescence (XRF) spectrometer for SiO2, TiO2, K2O, Al2O3, FeO, CaO, Na2O, Cr2O3, MnO, MgO and NiO. Trace element and rare earth element (REE) concentrations were determined by using a Thermo Series X-I inductively Coupled Plasma Mass Spectrometer (ICP-MS). Also, 8 thin- polished samples (least fractured and altered) of wall rock were analyzed by electron probe microanalysis (EPMA), using a Cameca SX100 at the Iranian Mines and Mineral Industries Development and Renovation Organization (IMIDRO) with a 1-3 μm spot size, 20 nA beam current and a 15 kV acceleration voltage.

Most of the olivines from mantle peridotites are serpentinized. The detailed electron microprobe study revealed very high Mg# (62.4 to 67 wt.%), Cr# (18.1 to 34.5 wt.%) and very low TiO2 content (averaging 0.05 wt.%) for chromian spinels in mantle peridotites. The Fe+3 is very low (averaging 0.02 wt.%) in the chromian spinel of mantle peridotites which reflects crystallization under the low oxygen fugacity. The composition of olivine shows that forsterite type (Fo90.07-90.90), orthopyroxene is enstatite, clinopyroxene is diopside and spinels are Al-rich Cr-spinel. Tectonic environment discrimination diagrams for the harzburgite and lherzolite shows abyssal environment.

Hyderabad ophiolite melange consists of peridotite rock units (serpentine and listonite), lherzolite, gabbro, gabbronorite, plate dykes, and pillow lava, which are faulted. The mantle peridotite of this area is composed mostly of harzburgite and lherzolite. Based on mineral chemistry studies, harzburgite and lherzolite have olivine which is forsterite, orthopyroxene which is enstatite and clinopyroxene that is diopside. The peridotites of the study area contain Mg-rich olivines, Cr-spinels and Al-orthopyroxenes and are in the range of deep peridotites based on spinel and olivine chemistry. Also, high Mg value in mentioned minerals and high percentage of forsterite in olivines indicate the tectonic origin of these rocks. Orthopyroxenes and clinopyroxenes are Cr-rich, indicating limited partial melting of peridotites. Mineralogy studies show the relationship of these rocks with the oceanic environment. The composition of spinels in the peridotites shows that the type of high Al, with Mg and Cr is 62.4 to 67 wt% and 18.1 to 34.5 wt%, respectively. They are considered as spinels formed in Abyssal peridotites. Geochemical studies show that Hyderabad peridotites are mantle peridotites that are formed by melting 11 to 16% of a prepared mantle of lherzolitic spinel. The composition of chromium spinels in these peridotites is in the Alpine type range and their tectonic environment is more compatible with MOR peridotites.