Petrogenesis of the Garmab area serpentinites from Ophiolite-Radiolithic zone of Kermanshah (west Iran) using chemistry of minerals and phase diagrams

The Garmab area is located in NW of Kamyaran and belongs to the Kermanshah Ophiolite Zone. This complex is located between the Sanandaj-Sirjan and the Zagros thrust zones. Peridotite rocks in the complex were replaced by serpentinite completely or partly due to hydrothermal alteration. Petrographic studies show that the serpentines formed by hydration of both olivine and orthopyroxene minerals. Electron microprobe and X-ray diffraction (XRD) analyses of the samples indicate that the Garmab serpentinites contain all the three serpentine polymorphs (chrysotile, lizardite, antigorite). Those serpentines replacing olivines have higher SiO2 and lower Al2O3, Cr2O3 and TiO2 than those replacing orthopyroxene. This clearly demonstrates their different sources. The calculated phase diagram for the studied samples shows that serpentinites probably formed in two metamorphism events; the first one includes alteration and hydration of harzburgite in T< 200 oC and P < 4 kbar, while the second one occurred due to increasing T and P to more than 400 oC and 4 kbar.