The Eocene bimodal Piranshahr massif of the Sanandaj-Sirjan Zone, NW Iran: A marker of the end of the collision in the Zagros orogen

The bimodal Piranshahr massif is composed of coeval but geochemically unrelated mafic (40.7 ± 0.2 Ma zircon U-Pb sensitive high-resolution ion microprobe (SHRIMP) age) and A-type felsic rocks (41 ± 0.5 Ma Rb-Sr and 41.3 plusmn; 0.8 Ma zircon U-Pb SHRIMP age). The mafic rocks consist of two geochemical types of gabbros that derived from different magmas. The more abundant gabbros are moderately alkaline, have ratios of large ion lithophile elements to REE and high field strength elements to REE similar to those of intraplate mantle magmas, ⁸⁷Sr/⁸⁶Sr_{41 Ma} ≈ 0.7036 and ε(Nd)_{41 Ma} ≈ +7.2. The less abundant gabbros have calc-alkaline affinities, ⁸⁷Sr/⁸⁶Sr_{41 Ma} ≈ 0.7043 and ε(Nd)_{41 Ma} ≈ +4.78. Felsic rocks are metaluminous A2-type annite-fayalite-hedenbergite, hypersolvus, leucocratic, coarse-grained agpaitic syenites, pulaskites and granites, with ⁸⁷Sr/⁸⁶Sr_{41 Ma} ≈ 0.7048 and ε(Nd)_{41 Ma} ≈ +3.6-4.5. Syenites, pulaskites and granites are genetically related. Pulaskites probably represent alkali-enriched water-rich residual melts from which an F-rich vapour phase was later separated. Granites were probably generated during open-system processes, in which F-rich hydrous alkaline fluids released from the syenites acted upon pre-existing felsic rocks. The c. 41 Ma age of the post-collisional Piranshahr massif indicates that the related collision probably occurred at 50-60 Ma (i.e. Late Palaeocene or Early Eocene), thus resolving a much debated question.