Petrography and Mineral Chemistry of Metalherzolites in the Posht-e-Badam Ophiolite (Yazd province)

The Posht-e-Badam ophiolite is situated in the 15 KM of the southwest of the Posht-e-Badam Robate village and in the northeast of Yazd province. The Posht-e-Badam ophiolite belongs to the Paleozoic and is a remnant of the Paleo-Tethys ocean. This ophiolite has passed three phases of metamorphism and consists of metaperidotite, metagabbro, amphibolite (ortho-amphibolite), rodingite and listwaenite. Metalherzolite is the most intact unites of the metaperidotite. Petrography study shows that metalherzolites consist of metamorphic minerals of olivine, tremolite, serpentine , chlorite, chromian magnetite and magnetite. The main textures of these rocks are nematoblastic, poikiloblastic, jack-straw and mesh texture. Mineral chemistry studies of metalherzolites indicate that olivines have Mg# (0.79-0.80), and are chrysolite in composition. Amphiboles are tremolite which present high values of Mg# (0.93-0.96). Chlorites are peninite in composition with Mg# (0.92-0.93). Spinels have Cr# 0.97, and are chromian magnetite in composition. Magnetite inclusions in metamorphic olivines reveals that the studied rocks were partly altered in greenschist facies. Tremolite with Jack-straw texture, as well as presence of Mg-chlorite and antigorite indicate progressive metamorphism in the amphibolite facies P-T conditions. Late-stage retrograde metamorphism led to formation of chlorite around of some of tremolites and partial serpentinization of metamorphic olivines. The phase relationships of metalherzolites of Posht-e-Badam ophiolite, in the CMASH (C = CaO, M = MgO, A= Al2O3, S = SiO2, H = H2O) model system, indicated that studied metalherzolites metamorphosed under lower amphibolite facies P-T conditions, followed by a retrograde metamorphism under greenschist facies conditions.