Zircon U-Pb geochronology and petrogenesis of Middle Eocene monzonitic plutons in NE Turkey: constraints for generation of post-collisional I-type shoshonitic magmas

We present U – Pb zircon geochronology and whole-rock Sr-Nd-Pb isotopic compositions of monzonitic plutons from NE Turkey to elucidate their genesis and provide significant insights into the generation of post-collisional I-type shoshonitic magmas. Zircon U – Pb dating of the monzonitic plutons yielded an emplacement of the Middle Eocene (44–39 Ma). Geochemically, the monzonitic plutons are I-type, metaluminous, and shoshonitic in composition, and have high contents of total alkalis and LILE, high ratios of LREE/HREE and LREE/HFSE, and show negative Nb-Ta-Ti and Eu (Eu_N/Eu* = 0.69–0.84) anomalies. The monzonitic plutons have homogeneous and moderately low initial ⁸⁷Sr/⁸⁶Sr (0.70464–0.70495), positive εNd_(t) values (0.33 to 1.28), and narrow variation of the initial Pb isotope contents with ²⁰⁶Pb/²⁰⁴Pb (18.74–18.82), ²⁰⁷Pb/²⁰⁴Pb (15.64–15.65), and ²⁰⁷Pb/²⁰⁴Pb (38.81–38.90). The combined use of whole-rock trace elements and isotopic compositions with regional geology suggests that parental magmas of the monzonitic plutons were generated from mainly melt-metasomatized sub-continental lithospheric mantle source and lesser crustal components in an extensional orogenic setting, and then monzonitic magmas have evolved by dominantly fractional crystallization in crustal levels.