Magma signification and geothermobarometry of Vanako gabbro, southeast Sahneh, using the pyroxene and plagioclase composition

The Vanako gabbroic body, southwest of Sahneh, extended along the Zagros ophiolite belt, in the west of Iran. The body contains plagioclase and pyroxene with minor amount of olivine and compositionally lie in the gabbro and olivine-bearing gabbro fields. Based on microprobe analyses, plagioclase composition is labradorite with An60.31-67.53 which has crystallized in temperature of about 950 oC and low pressure. The composition of pyroxene is diopside and it has developed in medium to low pressure and temperature ranging from 800-1120 oC. The low values of Fe3 in pyroxenes and moderate values of An in the plagioclases point to the low ƒO2 in the composition of the parent magm. Also, the high values of Al2O3 in the pyroxenes and rare earth elements pattern normalized with chondrite infer that these rocks have been generated from fractionation of MORB type magma. The geodynamics situation of the Kermanshah area and the tendency of some samples to calc-alkaline affinity are further evidence to support a supra-subduction zone for the origin of these rocks rather than Mid-Oceanic Ridge basin.