Evaluating physicochemical conditions of Miocene–Pliocene volcanic rocks in the middle part of the Urumieh-Dokhtar Magmatic Arc

Urumieh-Dokhtar Magmatic Arc (UDMA) resulted from subduction of the Arabian plate beneath the Eurasian plate. Miocene–Pliocene volcanic rocks in the middle part of UDMA located in the north of Isfahan (Central Iran) are mainly dacite. They are chiefly composed of phenocrysts of plagioclase, amphibole, and biotite lying in a matrix made of feldspar, quartz, opaque, glass, and microlite. Having a relatively invariable composition, plagioclases are all andesine. Amphiboles are calcic and tschermakite type. The chemical composition of biotites hardly varies. All of them are Mg-biotite. On average, they are composed of 55.3 % phlogopite, 15.7 % talc, 12.8 % Ti-phlogopite, 11.6 % eastonite, 3.7 % Ferri-eastonite, and 0.9 % muscovite. The chemical composition of biotite indicates the nature of calcalkaline for magmatic series of magma generating biotites. An estimation of the temperature and pressure of crystallization resulted from integrating amphibole and amphibole-plagioclase thermobarometry equations indicates that amphibole-plagioclase phenocrysts were crystalised in the temperature range of 763–823 °C with average of 797 °C and in the pressure range of 2.2–4.5 kbar with the average of 3.4 kbar. An estimation of the oxygen fugacity of the magma, based on chemical composition and Fe3+ content of biotite, shows the oxygen fugacity was equal to that of the FMQ buffer. Quantifying oxygen fugacity shows that logfO2 was about −15.