Mineral Chemistry and Thermobarometry of the Upper Eocene Volcanic Rocks in NE

Petrography and chemistry of minerals show that rocks of Upper Eocene in northeast of Tafresh are composed mostly of andesitic basalt, basaltic andesite and andesite volcanic rocks. Mineralog-ically these rocks are composed of phenocrystals of olivine, clinopyroxene and plagioclase and main texture of them is porphyry with cryptocrystalline or microcrystalline matrix. In addition, aphyric and pitted textures (amygdala) are also observed. According to the results of EPMA, phe-nocrystals of plagioclase in mentioned rocks include a range of anorthite to albite minerals. Alkali feldspars also contain a range of sodic to potassic minerals. Pyroxene crystals include hedenber-gite, augite and hypersthene. Olivine minerals are often of the ferrohornblendite type. Based on thermobarometry it is estimated that to form clinopyroxene crystals of basaltic andesite rocks, temperature between 750˚C to 1000˚C is needed. Andesitic basalt rocks at higher temperature (1100˚C) and andesite rocks at lower temperature (below 750˚C) are formed. According to the distribution of aluminum in clinopyroxenes, these minerals at pressures less than 5 kbar and wa-ter content between 5 % to 10 % are crystallized. The mineral composition indicates that thes