Petrogenesis and geodynamics of the Early Miocene Kashan granitoids in the central Urumieh-Dokhtar Magmatic Arc, Iran

Abstract

This study investigates the early Miocene granitoids (rhyolites, dacites, granodiorites, and diorites) from the Kashan area in the central Urumieh-Dokhtar Magmatic Arc of Iran, to constrain their sources and geodynamic processes. Zircon U–Pb ages, zircon Hf isotopic data, and whole-rock geochemistry are presented. The diorites yielded a zircon U−Pb age of 22 Ma. They are geochemically classified as non-adakitic rocks, characterized by low Sr (258.0–289.5 ppm) and high Y (28.21–29.33 ppm) with low Sr/Y (9.04–9.92). These rocks exhibit high-K calc-alkaline affinities, enrichment in light rare earth elements (LREEs), pronounced negative Nb−Ta anomalies, high initial ⁸⁷Sr/⁸⁶Sr ratios (0.706181–0.706269), coupled with negative εNd(t) (− 3.6 to − 3.0) and εHf(t) (− 3.5 to − 0.9) values. These features indicate that the diorites were derived from partial melting of an enriched subcontinental lithospheric mantle (the amphibole-bearing spinel stability field) metasomatized by subducted sediment-derived fluids. In contrast, the rhyolites, dacites, and granodiorites exhibit adakitic affinities, showing high Sr (377.2–595.4 ppm), low Y (4.36–13.48 ppm) with high Sr/Y (29.93–94.28). They display high SiO₂ (63.01–77.33 wt%) and low MgO (0.74 − 2.45 wt%), Mg^# (29–47), and Ni (4.56 − 22.13) values. Such signatures are consistent with partial melting of the thickened lower crust. Based on the regional geology and previous work, we propose a two-stage geodynamic model. The diorites formed during the initial Arabia-Eurasia collision related to the final breakoff of the Neo-Tethys slab, whereas the adakitic rocks were generated during the main collisional stage.