OICC Press Iranian Journal of Earth Sciences 2228-785X 18 3 2026 09 30 Tectonomagmatic evolution implications of the Rashid Abad Eocene volcanic-pyroclastic rocks: evidence of an active continental margin in the subduction setting (THMP, Alborz-Azerbaijan Block, NW Iran) 10.57647/j.ijes.2025.16973 EN Mahsa Seyyedi Department of Geology, North Tehran Branch, Islamic Azad University, Tehran, Iran Mohammad Lotfi Department of Geology, North Tehran Branch, Islamic Azad University, Tehran, Iran Arash Gourabjeripour Department of Geology, North Tehran Branch, Islamic Azad University, Tehran, Iran Afshin Ashja Ardalan Department of Geology, North Tehran Branch, Islamic Azad University, Tehran, Iran Journal Article 2026 09 30 In the Western Alborz-Azerbaijan Block, Eocene volcanic activity generated a suite of volcanic-pyroclastic rocks and marine lavas of Middle Eocene age. The study area is located 45 km north of Zanjan, in the central part of the Tarom-Hashtjin subzone within the Western Alborz-Azerbaijan structural zone, northwestern Iran. This research aims to investigate the regional tectonomagmatic evolution of Eocene volcanic-pyroclastic rocks and their significance for mineralization in the study area. Petrographic and geochemical studies of these rocks in the Rashid Abad district illustrate a compositional range from acidic to intermediate-basic, ranging from rhyolite to basalt. Based on the geochemical data, the rocks exhibit a high-K calc-alkaline to shoshonitic series. According to the geochemical data, these rocks are mainly enriched in LREEs (e.g., Nd, Ce, La, and Sm) and LILEs (e.g., Rb, Cs, Pb, and K) but are depleted in HFSEs (e.g., Ti and Nb). Furthermore, the REE patterns indicate a negative Eu anomaly. These geochemical characteristics show the obvious features of the magmatic arc and magmatic activity related to subduction, which were derived from the involvement of released fluids from a subducted oceanic slab in the enrichment of magma elements. The ratios of Nb/U, Ce/Pb, Nb/U vs. Ce/Pb, and Na/Ta confirm the influence of crustal contamination in the area. The Nb/Y vs. Rb/Y ratios also demonstrated that the rocks were enriched in the subduction zone and experienced crustal contamination. The fractional crystallization (FC) trend was proved by the decrease in the concentrations of MgO, FeOt, Fe2O3, CaO, TiO2, Al2O3, Na2O, and P2O5 with increasing SiO2. The FC of pyroxene, plagioclase, and Fe-Ti oxides is the primary process in the magmatic evolution of the Rashid Abad volcanic and pyroclastic rocks. Moreover, the Ba/Rb vs. Rb ratio highlights the influence of crustal contamination associated with FC process within the upper crust. According to the tectonic discrimination diagrams, the rocks of the region exhibit characteristics of a magmatic arc setting related to an active continental margin arc in a subduction zone. Furthermore, the Nb/Y ratio, which varies below 1.72, confirms the presence of an active continental margin setting in the Rashid Abad district. The Eocene andesites and andesite basalts of the study area originated from the partial melting of a spinel lherzolite source, whereas dacitic and rhyolitic magmas formed by partial melting of the lower amphibolite crust. Based on these studies and results, it can be concluded that the primary magma of these rocks, of mantle origin, probably originated from partial melting of subducted oceanic crust and was metasomatized by fluids derived from a subducted slab. Finally, as the magma ascended through the crust or interacted with a shallow-level crustal magma chamber, it became contaminated with crustal material before undergoing fractional crystallization and eventually was suffered from FC.