Abstract
Damavand Volcano, as the highest volcano in Iran and the Middle East, is located approximately 50 km northeast of Tehran. The volcanic rocks of the Haraz Road region mainly consist of olivine basalt, trachyandesite, and basaltic trachyandesite, with the primary mineral phases including plagioclase, pyroxene, and olivine. Petrographic studies indicate the presence of textures such as hyalomicrophaneritic, glomeroporphyritic, and vesicular in these rocks. The occurrence of disequilibrium features, including sieve textures in plagioclase, oscillatory and reverse zoning, amphibole opacization, and xenolith fragments, suggests a lack of equilibrium between crystals and melt, indicative of magma evolution in an open thermodynamic system. Examination of these features reveals that magma mixing, together with fractional crystallization, played a key role in the evolution and formation of the region’s volcanic rocks. Lava evolution primarily occurred through crystallization at varying pressures, injection of hot magma with similar composition into magma chambers, and mixing with resident magma. Thermometric calculations of plagioclase indicate that crystallization occurred at pressures of approximately 2.5–18 kbar and temperatures ranging from 750 to 1200 °C.
Keywords: Petrography, Plagioclase, Magma Mixing, Central Alborz, Haraz Road, Iran
1. Introduction
Damavand Volcano, as the highest Quaternary volcanic edifice in Iran and the Middle East, represents a key location for understanding tectono-magmatic processes in the Central Alborz region. The structural framework of the Central Alborz exhibits a V-shaped configuration, where northwest–southeast trending faults and folds in the western part are separated from northeast–southwest structures in the eastern section. The region remains tectonically active, as indicated by seismicity, GPS-based crustal motion, and geodetic measurements, reflecting an incomplete isostatic equilibrium. This dynamic environment has facilitated magma ascent, influencing the emplacement and evolution of young volcanic products, particularly in Damavand. Volcanic products are predominantly trachyandesitic to trachytic lava flows accompanied by limited but significant pyroclastic and epiclastic deposits, mainly concentrated in the southern and southeastern sectors. Major faults in the area, including Mashaa, Ask, Bayjan, Nova, Sefidab, Shahandasht, and Vararud, play a crucial role in controlling magma migration and vent distribution. Previous geophysical studies estimate the crustal thickness beneath the Central Alborz to range from 35 km (gravity modeling) to 58 ± 2 km (seismic data), with some studies suggesting anomalously thick crust (~65–67 km) beneath Damavand, indicative of a deep magmatic root and complex crustal-mantle interactions. Understanding the petrography and geochemistry of key minerals such as plagioclase and amphibole provides insights into magmatic evolution, crystal fractionation, magma mixing, and thermodynamic conditions during crystallization.
2. Materials and Methods
A total of 70 representative volcanic rock samples were systematically collected along the Haraz Road region. Thin sections were prepared from all samples and examined using a polarizing microscope to assess mineralogical composition, textures, and microstructures. The primary focus was on plagioclase and amphibole crystals, evaluating their zoning patterns, reaction rims, dissolution features, and textural relationships. Petrographic criteria for magma mixing, including oscillatory zoning, sieve textures, and xenocrystic inclusions, were identified. Amphiboles were studied for reaction rims, opacization, and evidence of chemical disequilibrium. Geochemical analyses were conducted to quantify major and trace elements, while thermobarometric calculations estimated crystallization temperatures and pressures of plagioclase and amphibole phases. Crystal size distribution (CSD) techniques were applied to quantify crystal populations and to interpret magmatic processes such as fractional crystallization, magma mixing, and multi-stage crystallization events.
3. Results and Discussion
Petrographic observations indicate three dominant volcanic rock types: trachyandesitic-basaltic lavas, trachyandesites, and olivine basalts. Trachyandesitic-basaltic samples exhibit porphyritic to microlitic textures with plagioclase ranging from labradorite to anorthite, commonly displaying oscillatory and normal/reverse zoning. Alkali feldspars are generally sanidine with variable crystal shapes. Pyroxenes are mainly augite, occasionally showing twinning, and amphiboles display partial to severe alteration, including opacized margins. Olivine basalts show porphyritic and microlitic textures with olivine, augite, and minor plagioclase phenocrysts; mafic phases often partially replaced by opaque minerals. Textural evidence such as sieve textures, zoned plagioclase cores and rims, and reaction rims on amphiboles support the occurrence of open-system magma processes, including repeated injections of hotter, more mafic magma into evolving magma chambers. Oscillatory zoning in plagioclase is attributed to variations in pressure, temperature, water content, and diffusion kinetics within the magma chamber. Sieve textures and dissolution features in plagioclase and olivine indicate magma mixing and transient disequilibrium during crystallization. Amphibole breakdown, formation of reaction rims, and opacization reflect sensitivity to temperature, pressure, and volatile content, consistent with observations in analogous volcanic systems. Thermobarometric results suggest plagioclase crystallization occurred at pressures of ~2.5–18 kbar and temperatures of ~750–1200 °C. CSD analyses reveal crystal population heterogeneity, confirming fractional crystallization, magma mixing, and polyphase growth events. The combined petrographic, textural, and geochemical evidence indicates that magma mixing and partial crystallization played critical roles in the evolution of Haraz Road volcanic rocks.
4. Conclusion
The volcanic rocks of Haraz Road, Central Alborz, record complex magmatic processes characterized by magma mixing, oscillatory zoning in plagioclase, dissolution features, and amphibole reaction rims. The interplay between repeated injections of hotter mafic magma, fractional crystallization, and variations in pressure, temperature, and water content controlled mineral textures and chemical compositions. Thermobarometric results confirm crystallization of plagioclase at 2.5–18 kbar and 750–1200 °C, consistent with open-system magmatic evolution. Crystal size distribution analyses support the occurrence of polyphase growth, magma mixing, and fractional crystallization. Collectively, petrographic and geochemical evidence highlights the dynamic magmatic environment of the Central Alborz and provides insights into crustal processes controlling volcanic activity in Damavand.