Petrology, tectonic setting and ore mineralization potential of basic rocks in north of Alvand complex, using amphibole and plagioclase composition

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, titanomagnetite, 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. Thermobarometry calculations based on amphibole and plagioclase compositions obtained P-T averages of 835°C and 11 kbar for formation of the pargasites. Compistions of the amphiboles show that the studied mafic rocks formed ude to exnetion phases during subduction and upwelling of of asthenospheric mantle magmas. Fe and Ti concentration variations in the plagioclases indicate that differentiation trend of the parental magma was not consistent with concentration of these elements in the melt. High fO2 and also 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’s olivine-gabbronorite.