Zircon U–Pb Age and Lu–Hf Isotopic Constraints on Granodioritic Rocks From the Qaradagh Batholith (NW Iran): Implications for Paleogene Magmatism in Northwest Iran and Southern Armenia

Abstract

ABSTRACT The Qaradagh Batholith in northwestern Iran, adjacent to the Meghri–Ordubad pluton (MOP) within the South Armenian Block, is dominated by granodiorite, which represents its earliest intrusive phase. Both the Qaradagh Batholith and the MOP formed in a subduction‐to‐post collisional setting linked to the closure of Neo‐Tethys Ocean between middle Eocene and early Miocene. U–Pb zircon dating of the granodiorite was conducted to constrain its emplacement age, yielding a 206 Pb/ 238 U weighted mean age of 43.81 ± 0.18 Ma and a U–Pb concordia age of 44.04 ± 1.00 Ma, placing intrusion in the middle Eocene, earlier than previously suggested Oligocene ages. In situ zircon Lu–Hf isotope analyses, conducted to determine magma source characteristics, yielded ε Hf ( t ) values between +8.7 and +11.1, indicating derivation from a depleted, juvenile mantle source with minimal crustal contamination. This is supported by low initial 87 Sr/ 86 Sr (0.7037–0.7041), positive ε Nd (i) (+2.69) and low zircon U and Th contents relative to Miocene intrusions in NW Iran, which have higher crustal contribution. Model T DM ages of zircons (256–354 Ma) indicate a Carboniferous–Permian source material, consistent with late Paleozoic subduction. These results place the Qaradagh granodiorite among the earliest manifestations of Paleogene magmatism in the region.