Thermobarometry and petrogenesis of Sabalan volcanic rocks: based on mineral chemistry

The Sabalan is one of the Pliocene-Quaternary volcanoes in northwestern Iran. Based on the activity history, the Sabalan volcanic rocks can be classified into two rock groups: Paleo- and Neo- Sabalan volcanic rocks. Paleo-Sabalan was constructed during the Pliocene-Pleistocene inmultiple eruptions between 4.5 and 1.3Ma, whereas Neo-Sabalan activity initiated after a series of violent explosive eruptions during the middle to late Pleistocene (545 to 149 ka; Ghalamghash et al, 2016). The Paleo-Sabalan volcanic rocks are trachy-andesitic to andesitic and rarely trachy-dacitic and Neo-Sabalan volcanic rocks are generally dacitic to andesitic.

Field investigation and sampling, petrography and mineral geochemistry are different part of our studies during this project. Mineral chemistry have been performed on two thin-polished sections of Paleo-Sabalan olivine basalt and Neo-Sabalan trachyandesite in Iran Mineral Processing Research Center. In this center, a Micro-probe model EPMA Sx100 model made by Cameca company with a voltage of 15 kV (Kv: 15 Kev), current intensity of 20 nm (nA), a diameter of 5 um and a detection limit of 500 ppm for microscopy and point chemical analysis Used. Sample preparation for microprobe studies has also been done in Iran Mineral Processing Research Center using carbon coating device.

Plagioclase composition in the Paleo-Sabalan volcanic rocks in the oligoclase to andesine range, and in the Neo-Sabalan volcanic rocks in the range of oligoclase, to andesine. K-feldspars are sanidine and orthoclase. The Olivine and pyroxenes of the Paleo-Sabalan volcanic rocks have chrysotile and diopside-augite compositions, respectively. The most of the mica in Neo-Sabalan lava are Biotite. The most amphibole of Neo-Sabalan lavas are Magnesiohornblende, and rarely are edenite. Based on the mineral chemistry diagrams of Sabalan volacic rocks have a sub-alkaline and calc-alkaline nature that have formed in the orogenic environment. Paleo-Sabalan clinopyroxenes have crystallized at 5 to 10 kbar pressure (equal 17.5 to 35 Km depth). The amphibole phenocrysts have crystallized at a pressure of 1 to 3 kbar (equal 3.5 to 10.5 Km depth) and a temperature of about 725 to 750 °C.

Considering the crystallization temperature of amphibole (725 to 750° C) and biotite modeling with FMQ buffer oxygen fugacity at the time of biotite crystallization was estimated to be -16 to -16.50. Probably, high oxygen fugacity and the presence of aqueous minerals indicate the effect of water-rich old subduction environment on the source of primary magma of Sabalan volcano.