Quaternary Journal of Iran

Mineral Chemistry, Pressure, and Thermobarometry of Plio-Quaternary Intermediate and Acidic Lavas Around Beygz Mountain: A Comparative Study with Similar Occurrences in the Arasbaran Magmatic Belt, NW Iran

Document Type : Original Article

Authors

Rahim Dabiri 1
Keivan Shiaian 2
Abdollah Yazdi 3

1 Department of Geology, Islamic Azad University, Mashhad Branch, Mashhad, Iran

2 Department of Geology, Tabas Branch, Islamic Azad University, Tabas, Iran

3 Department of Geology, Kohnooj Branch, Islamic Azad University

10.22034/irqua.2024.2042013.1029
Abstract
Abstract

1-Introduction
Based on the geological map of northwestern Iran, as well as the characteristics of the rock structures and the trends of the major faults, there exists a block known as the “Arasbaran Volcanic Belt,” which is bordered by the Tabriz, Miyaneh, Talash, Allahyarlu-Hawai, and Arax faults. The area studied within this block has attracted the attention of many researchers due to its location in a metallogenic zone. A few researchers have focused on the magmatism of this region, identifying it as a result of subduction. The purpose of this research is to investigate the mineralogy and thermobarometry of Plio-Quaternary intermediate and acidic lavas around Beygz Mountain in this block, and to compare these findings with similar occurrences, such as the Sablan Volcano, the Andrian Neogene volcanic rocks (which are part of the Arasbaran magmatic zone), and the Cone Arvana Volcano, located in the eastern part of the Sahand volcanic complex southwest of Bostanabad.

2-Materials and methods
To investigate the various volcanic sequences and their lateral variations, a series of surveys were conducted using both transverse and longitudinal profiles. More than 300 rock samples were collected during these surveys. Based on microscopic examinations, 33 samples with the least alteration effects were selected for geochemical analysis. Additionally, 13 samples were chosen for electron microprobe analysis (EPMA).


3-Results and discussion
Ten distinct lithological types can be identified in the Quaternary volcanic rocks of this area. The intermediate rocks include basaltic trachy-andesite, trachy-andesite, pyroxene andesite, and hornblende andesite, with trachy-andesites being the most abundant. Most of these rocks exhibit a porphyritic texture with a cryptocrystalline or microcrystalline matrix, and they may also display trachytic and vesicular (amygdaloidal) features. The primary minerals in these rocks are feldspar, amphibole, and pyroxene, respectively. The acidic rocks consist of trachydacite, dacite, and rhyodacite, which constitute a smaller volume in the area. These rocks typically have a porphyritic texture with a microcrystalline and trachytic matrix. The main minerals present include feldspar, amphibole, biotite, and quartz. In the older Sablan volcanic rocks, plagioclase, pyroxene, amphibole, biotite, and a small amount of potassium feldspar and quartz can be observed. The younger Sablan volcanic and subvolcanic rocks exhibit porphyritic and hyaloporphyritic textures, with phenocrysts of plagioclase, biotite, hornblende, and quartz found in glassy, microlithic, and fluidal textures.
The volcanic rocks of Arvana contain phenocrysts of plagioclase, hornblende, biotite, and occasionally quartz and feldspar. The primary textures of these rocks are hyaloporphyric and microlithic porphyritic. The Andrian volcanic rocks are primarily composed of alkali feldspar, plagioclase, calcic amphibole, clinopyroxene, and biotite, with textures mainly being microlitic porphyritic to trachytic. In some of the intermediate rocks of the Beygz Mountain area, particularly the andesites, disequilibrium textures such as sieve textures, corroded or bayed margins, opacification of ferromagnesian minerals, and rounding of minerals are observed. These features are indicative of disequilibrium and complex physical-chemical variations in magmatic systems, resulting from the disequilibrium between phenocrysts and the matrix of andesite rocks. Thermobarometry in the Beygz Mountain area has been conducted by calculating the temperature and pressure of amphibole and pyroxene formation. The average pressure obtained for the studied amphibole is approximately 7.5 kbar, with formation temperatures ranging from 1043 °C to 1020 °C. Measurements based on clinopyroxene composition indicate that the formation pressure of this clinopyroxene is around 7 kbar, with a temperature of 1071 °C. Old Sablan pyroxenes crystallized at pressures ranging from 5 to 10 kilobars and at depths of approximately 17.5 to 35 kilometers. The amphibole phenocrysts of young Sablan crystallized at pressures of 1 to 3 kilobars, at depths of about 3.5 to 10.5 kilometers, and at temperatures ranging from 725 °C to 750 °C. Based on the amphibole-plagioclase thermometer and Al barometry data, hornblende in the Arvana volcanic cone has a crystallization temperature between 899 °C and 707 °C, with pressures ranging from 0.5 to 2.9 kbar. In the volcanic rocks of Andrian, thermobarometric measurements have been determined using several methods, yielding a temperature range of 700 °C to 800 °C and pressures between 3 to 6 kilobars.

4- Conclusion
Disequilibrium factors such as plagioclase sieve textures, opacification of ferromagnesian minerals, and rounding of amphibole minerals, along with the incompatibility of pyroxene compositions with the surrounding melt and the reverse zoning of pyroxenes, indicate a disequilibrium between the melt and crystals. These features suggest that magmatic variation processes have occurred within an open thermodynamic system. It is believed that the arrival of new and hot magma pulses caused the magma chamber to migrate from deeper levels (approximately 22.5 km depth and 7.5 kbar pressure) to shallower depths (around 21 km depth and 7 kbar pressure). This increase in temperature and decrease in pressure disrupted the stability conditions of amphibole and facilitated the growth of augite. The occurrence of augite, along with the growth of alkali feldspars on some plagioclase, may indicate magma migration and disequilibrium conditions (approximately 7.2 kbar pressure and 1050 °C temperature). Trachy-andesite volcanic rocks and some acidic samples from the region show evidence of crustal contamination. Based on the temperature and pressure of intermediate and acidic magmas in the Ahar block, it can be inferred that the magma chamber in the Beygz Mountain area is located at greater depths, temperatures, and pressures, and that repeated injections have occurred within the magma chamber. The presence of mafic lavas in this area, along with evidence of magmatic mixing and the injection of mafic magmas into felsic magma, further supports the notion of injection within the magma chamber.

Keywords

Mineralogy
Thermobarometry
Disequilibrium
Beygz mountain
Arasbaran magmatic belt

Subjects

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Quaternary Journal of Iran
Volume 9, 3, 4
October 2023
Pages 472-496

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