Introduction
Sabalan Volcano is located 40 km west-southwest of Ardabil city and 25 km southeast of Meshginshahr (Iran). This mountain is mainly located in Ardabil province, with some areas in the southwest of East Azerbaijan Province. According to recent structural studies of fractures and fault trends in Sabalan Mountain, strike-slip faults have led to the creation of extensional spaces and pull-apart basins in the region (Abdollahadi et al., 2024). During the Quaternary Period, the basic magmas of the region reached the surface along these extensional spaces. These magmas have been slightly contaminated with the lithospheric crust and have largely retained the characteristics of their asthenosphere origin. Afterward, the lithospheric crust has been delaminated, partially separated from the crust, and immersed in the asthenosphere. In the meantime, partial melting of this crust has caused the formation of volcanic rocks in Sabalan.
Study Method
About 100 samples were gathered from the study area to identify the texture of the rocks and their mineralogical composition. Next, 80 thin-section samples of volcanic rocks were prepared from them and studied with a polarizing microscope. The thermobarometric studies were conducted on the basalts and andesites of the studied area by preparing 5 thin-polished sections. Point analysis of minerals was performed using an electron microprobe analysis device, employing the XPMA method, in the Binaloud deposits. To this end, a HORIBA microscope (model: XGT-7200) with an accelerating voltage of 50KV and a current intensity of 1 mA was used. The point analysis device had a diameter of 10 μm and a duration of 80 s for each point.
Results
The Quaternary basic volcanic rocks of the region include trachyandesite, andesite, and trachydacite, which account for a significant volume of magma in the region. These rocks show a variety of porphyritic, flow, glomeroporphyritic, and sieve textures. The principal minerals of these rocks include plagioclase and one or more mafic minerals such as hornblende and pyroxene. Sieve texture is among the most important unequigranular textures in plagioclase crystals of Quaternary volcanic rocks. Zoning and degraded surfaces are observed at the margins of some plagioclase crystals of the rocks of the region. The plagioclase minerals of the igneous rocks of the region have a chemical composition range of labradorite to bitonite. Based on thermo-barometric calculations, plagioclase crystals of andesitic rocks (PL1) crystallized in the pressure range of 6 to 8.5 kbar and temperature of 1035 to 1,045℃; plagioclase of trachyandesitic rocks (PL2) crystallized in the pressure range of 5.5 to 7 kbar and temperature of 1035 to 1055℃; plagioclase of dacite rocks (PL3) crystallized in the pressure range of 5.5 to 8 kbar and temperature of 1035 to 1045℃; and plagioclase of rhyodacite rocks (PL4) crystallized in the pressure range of 6 to 8 kbar and temperature of 1035 to 1055℃.
From textural observations, a simple magma chamber model for the study area can be imagined based on the model proposed by Ranjit (2014). Based on this model, in the initial stage, the high-temperature water-saturated magma underwent extensive crystallization in the stable magmatic environment of the deep chamber, producing An-rich plagioclase. As this crystal-rich magma ascended to the shallow chamber, these crystals were subjected to different dissolution rates, leading to the development of coarse sieve (CS) texture with different sizes, shapes, and densities (number of CS per unit area). The variation in dissolution intensity can be attributed to differences in the decompression rate or the content of dissolved H₂O in the magma (Viccaro et al., 2014). Immediately after dissolution, many crystals may coalesce into glomerocrysts and later re-grow as single grains, while the remaining crystals grow as sheaths on CS nuclei. The crystals produced after decompression are devoid of CS morphology and form small to medium-sized phenocrysts (low An content) in the lava unit. The shallow chamber has been dynamically active by convection or the influx of some new magma, or a combination of both. These activities have limited the growth of pre-existing crystals and new crystals by non-uniform temperature increase and convection processes. They have developed fine sieve (FS) texture, fine oscillatory zones (FOZ), resorption (RS) surfaces, and flotation. The frequent occurrence of FS texture suggests the crystallization of individual grains because of multiple superheats. Therefore, evidence such as FS texture, FOZ, and flotation suggests that the crystals in the shallow chamber have undergone repeated dissolution-regrowth processes in an evolved magmatic environment. During the self-mixing process, the magma chamber may have undergone extreme cooling by degassing or water release and then intense air eruption that produced microlites, broken crystals, and swallowtails.
Conclusion
The Quaternary basic volcanic rocks of the Sabalan Volcano include trachyandesitic, andesite, and trachydacite. The main minerals of these rocks are plagioclase and one or more mafic minerals such as hornblende and pyroxene. Sieve, glomeroporphyritic, and regular textures are among the textures found in the plagioclase rocks of the studied region. The study and interpretation of these textures provide information about the effect of magmatic processes on the crystallization from the magma chamber to the magma eruption. The textures in the plagioclase rocks of the region are of the CS type, characterized by irregular, discrete, and sometimes zoned features, which are related to crystal growth. In this respect, glomerocrysts and microlites are also formed during dynamic magma processes such as convection currents, degassing, or explosive eruption, and in the final stage during or before the eruption of magma. The CS texture is formed more in the andesitic rocks of the region as a result of reduced pressure, temperature, and variations in the composition of the magma. Meanwhile, in the intermediate rocks of the region, the FS texture is common, which is due to the mixing of magma (the entry of calcium-rich basic magma into the intermediate magma chamber).