Mineralogy, thermobarometery and magmatic series of volcanic rocks in Kuh-e Dom, Ardestan

The studied area is located in the north-east of Ardestan in Isfahan province. In Iran’s geological classification it lies in the central Iran zone. The volcanic rocks of the Kuh-e Dom area are predominantly rhyolite, rhyodacite, dacite, tuff, ignimbrite, andesite, banakite and basaltic-andesite. The major rock–forming minerals are quartz, plagioclase, alkali feldspar (orthoclase, albite), amphibole, biotite, clinopyroxene and olivine. Apatite, zircon and opaque minerals are common accessory minerals. Biotite, clinopyroxene, plagioclase, alkali feldspar and chlorite in various rock samples were chosen for electron microprobe analysis. The results suggest that feldspar phenocrysts show a large compositional range from andesine-labradorite to labradorite-bytonite in banakite and basaltic-andesite and oligoclase-andesine and sanidine in rhyolite and dacite. Clinopyroxene phenocrysts show a compositional range diopside to augite in banakite and basaltic andesite. Phenocryst of biotite in the studied rocks is essentially of similar composition, and they are phlogopite (Mg-rich). This observation suggests that clinopyroxene and phlogopite formed from a Mg-rich magma. Also mineral chemistry of volcanic rocks of Kuh-e Dom indicates that these rocks were generated from calc-alkaline to shoshonitic magma and are related to continental arc orogenic magmatism. Barometery of pyroxene indicates that this mineral formed from a water rich magma in pressure of 5 kbar. On the basis of geothermometery studies, crystallization temperature has estimated to be 720-800 °C for acidic rocks and 1170 °C for basic rocks. The geochemical investigations show two stages in Kuh-e Dom magma evolution: Stage I is represented by clinopyroxene and phlogopite formation in a magma chamber. Stage II of magma evolution is represented by crystallization of matrix during and after magma eruption. Probably sanidine with small amounts of apatite and magnetite crystallized from the quenched magma at this stage.