Geothermobarometery–mineralogical and mineral chemistry studies of Dacite and trachydacite of Masahim volcano south and east of Shahr-e-Babak north

Massive of dacite and trachydacite are present around the Masahim volcanic of Shahr-e-Babak are, in Central Iran, Urumieh-Dokhtar volcano plutonic belt. Based on lithogical studies and the results of chemical analyses, they are composed of dacite and trachydacite and the main texture in these rocks are porphyry. Both contain plagioclase (albite to andesine), pyroxene (augite), mica (biotite), amphibol (tschermakite), quartz and opaque minerals, but k-feldespar is also present in the trachydacite rocks (sanidin, anorthoclase). Phenocrysts of plagioclase show disequilibrium textures such as oscillatory zoning and sieve. These minerals show variations in composition from the core to rim that may be resulted from changes in magma composition with pressure changes during the ascent of magma and variation in water pressure. The temperature obtained is about 650 to 750 °C. Mineral chemistry studies on clinopyroxenes show that they are Augite and the magma, where clinopyroxens were crystallized, contained about 10% H2O. Fe3+ values of the clinopyroxenes indicate high oxygen fugacity. Geothermal-barometric studies of clinopyroxenes show that the crystallization pressure of clinopyroxenes is about 6-10 kbar and the composition of clinopyroxenes shows temperature range of 1200-1150 ° C. These clinopyroxenes belong to the calc-alkaline orogenic series.