Investigation of petrology, tectonic setting and ore mineralization potential of basic rocks in north of the Alvand complex using amphibole and plagioclase mineral chemistry

The Alvand complex (Hamedan province) comprises several bodies ranging from acidic to intermediate and basic compositions intruded in the regional metamorphic sequence of the area (the Hamedan phyllites) during various magmatic episodes in Jurassic. In northern part of the complex (the Cheshmeh-Ghasaban area) there are some outcrops of hornblende gabbro and olivine gabbronorite. The olivine-gabbronorites are composed of olivine, clino- and ortho-pyroxene, plagioclase and hornblende. Apatite, magnetite, titano-magnetite, pyrite and chalcopyrite are the accessory minerals. The alterations include substitution of the olivine and pyroxene by bolingite, serpentine and chlorite. Electron microprobe analyses show that the amphiboles are of pargasite, and magnesio-hornblende. Variations of MgO, Al2O3 and Na2O and thermobarometry calculations based on amphibole and plagioclase compositions indicate that pargasites formed in average pressure and temperature of 11 kbar and 835°C, from metasomatized mantle derived melt as a result of asthenospheric mantle elevation and in extensional setting. Mafic magma contaminated with crustal material as it ascends. This situation can be provided in a rift setting (probably a back arc basin). Investigation of Fe and Ti concentration variations in the plagioclases indicates that differentiation trend of the parental magma was not consistent with concentration of these elements in the melt. High fO2 and slow cooling rate of the magma caused Fe-depletion during plagioclase crystallization. In addition, formation from a Fe-depleted source could be another factor that limited iron mineralization of the Cheshmeh-Ghasaban olivine-gabbronorite.