seyed zahed mousavi

The Plio-quaternary sub-volcanic domes are the products of magmatism in the Turkish-Iranian plateau in the collision zone between Eurasia and Arabia. Intermediate-felsic volcanic rocks are found 50 km west of Ardabil. These volcanic domes make a significant part of the Sabalan volcanic, a Plio-quaternary stratovolcano in northwest Iran. The igneous rocks (adakitic) include dacite, trachyte, andesite, trachy-andesite, and trachydacite, associated with ignimbrite and pyroclastic equivalents. They mainly comprise phenocrysts and a microcrystalline groundmass of pyroxene, amphibole, and plagioclase, with biotite and titanomagnetite. These rocks are enriched in Light Rare Earth Elements (LRREs) and Large Ion Lithophile Elements (LILEs) and depleted from Heavy Rare Earth Elements (HRREs) and High-Field Strength Elements (HFSEs). In these rocks, the SiO2 content is 56-66 wt%, Na2O is > 3.5 wt%, Al2O3  > 15 wt%, Yb < 0.2 ppm, and Y < 7 ppm, which are typical of high silica adakitic rocks. The initial ratios of the 143Nd/144Nd range from 0.5127 to 0.5129 and the initial ratios of 87Sr/86Sr for the adakites range from 0.7035 to 0.7060, reflecting the heterogeneity of the mantle and different degrees of crystallization. These geological, geochemical, and Sr, and Nd isotopic data indicate that these rocks belong to the post-collisional adakite type, and are derived from low-degree partial melting of a subduction-metasomatized continental lithospheric mantle (eclogite or amphibolite garnet). In the studied area, mineralization related to Plio-quaternary adakitic rocks has not been observed.

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.