Tectonomagmatic development of the Eocene Pasevh pluton (NW Iran): Implications for the Arabia-Eurasia collision

The Pasveh pluton is one of a number of intrusive complexes in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), NW Iran. The major part of this complex is composed of high-K, calc-alkaline granitic rocks. The geochemical data indicate that the granitic rocks, although compositionally similar, are not cogenetic and can be divided into three groups. The first group (mesocratic granitoids) is represented by the largest outcrop in the Pasveh pluton. It is mainly composed of metaluminous magnesian monzodiorite to granodiorite with abundant MME and gabbroic enclaves. Zircon LA–ICP–MS U–Pb dating yielded Eocene crystallization ages (ca. 40 Ma) for this group. The monzodiorite (εNd_t = +2.0 to +2.5), granodiorite (εNd_t = +1.5 to +1.6), MME and gabbroic enclave (εNd_t = +3.3 to +4.7) have distinct Nd isotopic compositions. The second group is composed of Eocene (39.7 ± 0.6 Ma), weakly peraluminous monzogranites that have Carboniferous and Cretaceous inherited zircons. Leucocratic granitic dikes of adakitic composition comprise the third group of Pasveh silicic rocks. The dikes are slightly younger (37.5 ± 0.3 Ma) than host rocks and cross-cut the mesocratic granitoids. Our results indicate that the underplating of basaltic magmas related to extensional stress in the overlying Iranian plate during the Eocene induced partial melting of the lower crust. Differences in the amount of partial melting of the source rocks are likely responsible for the volume and heterogeneity of their melt compositions. Subsequent to melt generation, it is possible that the silicic magmas mixed with mafic magmas in variable proportions to produce the hybridized rocks and enclaves. The intrusion of leucogranite dikes after the emplacement of the Pasveh pluton suggests that during or slightly after Neo-Tethys subduction there was another period of magmatism that was likely related to the melting of the subducted slab.