The Călimani Mountains represent the largest and most complex volcanic structure at the northern part of the Călimani–Gurghiu–Harghita range in Romania. Sixty-eight K-Ar ages (thirty-three new) provide constraints on the eruptive history of the Călimani volcanic structure between 11.3 and 6.7 Ma. The oldest rocks are from shallow exhumed intrusions, which pierced the basement between 11.3–9.4 Ma. The oldest stratovolcano was centered on the presently recognizable main volcanoes, Rusca-Tihu and the Călimani Caldera and grew very large (ca. 300 km3), generating a large-volume (26 km3) debris avalanche. Debris avalanche blocks dated between 10.2–7.8 Ma, suggest an edifice failure event at 8.0±0.5 Ma. The Drăgoiasa Formation (9.3–8.4 Ma), Budacu Formation (9.0–8.5 Ma), Lomas Formation (8.6 Ma), a number of Peripheral Domes (8.7–7.1 Ma) and Sărmas basalts (8.5–8.3 Ma) were also active before the debris avalanche event. Volcanic activity continued from the Rusca-Tihu Volcano between 8.0–6.9 Ma, generating the “Rusca-Tihu Volcaniclastic Formation”. The Călimani Caldera structure including pre-caldera and post-caldera stages was generated between 7.5–6.7 Ma, with an inferred collapse event at 7.1±0.5 Ma. Monzodioritic-dioritic bodies in the central part of the caldera show ages between 8.8–7.3 Ma, implying several episodes of intrusions. Fractional crystallization was important in the generation of different magma series at lower crustal to shallow crustal depths, where plagioclase was the main crystallizing phase. Crustal assimilation affected most of the analysed samples to some degree through assimilation-fractional-crystallization (AFC) processes. Isotopic enrichment of the most basic rocks suggests that contamination processes affected the source of most parental magmas, except those of the Lomas Formation. The initial stages of volcanism were most complex from the petrological point of view. The Drăgoiasa Formation (represented only by felsic rocks), for instance, suggests either fractionation from a basic parental magma and mixing with partial melts of (lower) crustal origin, or represents direct melting of the garnet bearing lower crust. The Lomas Formation represents the most primitive magma, which reached the surface recording minimal interaction with crustal material and most closely characterizes the isotopic composition of the mantle source beneath the Călimani Volcano. The youngest volcanic rocks represented by the Călimani Caldera structure were derived from magmas that show a lower degree of partial melting and were largely affected by assimilation processes.