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Petrological, geochemical and zircon U–Pb and Hf isotope studies were carried out on the Hajseyran pluton, along the Tarom–Hashtjin metallogenic zone, as part of the Alborz–Azerbaijan magmatic belt, which is located in the central part of the Alpine–Himalayan orogenic belt. The Hajseyran pluton, which is made up of syenite, monzonite, quartz-monzonite and granite, yields a zircon U–Pb age of 39.20 ± 0.56 Ma (for a quartz-monzonite sample) with shoshonitic features. Enrichment in large ion lithophile elements, depletion in high-field-strength elements and negative anomalies of Nb, Ta and Ti in Chomalou samples suggest a subduction-related arc magmatic setting. The Chomalou epithermal Pb–Zn–Cu deposit occurs close to the Hajseyran pluton with the quartz–base metal sulfide veins predominantly hosted by Eocene volcanic rocks. The ore-forming processes include chalcopyrite and sphalerite–galena formation, with the latter the major mineralisation in this region. Microthermometric results from the liquid–vapour fluid-inclusion assemblages show that the homogenisation temperatures in the first-stage quartz range from 230 to 310 °C, in the sphalerite and second-stage quartz from 161 to 293 °C and in the late-stage quartz from 175 to 262 °C. The salinities of fluids decrease from early-stage quartz to late-stage quartz. The calculated stable isotope values of ore–fluid components (δ18Owater = +3.28‰ to −3.68‰; δ34SH2S = +5.5‰ to +7.8‰) show that magmatic fluids have played a significant role during mineralisation. The Chomalou deposit is interpreted as a volcanic-hosted intermediate-sulfidation type of epithermal mineralisation formed through shallow-level emplacement of the Hajseyran pluton, which formed via partial melting of phlogopite–amphibole-bearing sub-continental lithospheric mantle. The Neotethyan slab roll-back and concomitant asthenospheric upwelling led to the decompression melting of metasomatised sub-continental lithospheric mantle and the formation of several K-rich plutons and associated epithermal deposits in the Alborz–Azerbaijan magmatic belt. The late Eocene Hajseyran pluton shows a shoshonitic, arc-related signature. The Chomalou Pb–Zn–Cu deposit is an intermediate-sulfidation style of epithermal mineralisation. Fluid inclusion and stable isotope data suggest the evolution of ore-fluid system from magmatic to meteoric. Decompression melting of metasomatised sub-continental lithospheric mantle via slab roll-back led to the formation of Hajseyran pluton. [ABSTRACT FROM AUTHOR]