Petrochemistry and geochronology of Middle Eocene high-K calc-alkaline I-type Kazıkbeli pluton, NE Turkey: implications from zircon U-Pb ages and Sr-Nd-Pb-Lu-Hf isotope data

Abstract

The Kazıkbeli pluton, located in the Kürtün (Gümüşhane) region of the Eastern Pontides, NE Turkey, was studied by employing zircon U-Pb geochronology, whole-rock geochemistry, Sr-Nd-Pb and zircon Lu-Hf isotope geochemistry. LA-ICP-MS U-Pb zircon dating revealed that the gabbroic diorite and granodiorite components of the pluton solidified during the Lutetian epoch of the Middle Eocene, at approximately 46.10 ± 0.26 Ma and 45.37 ± 0.29 Ma, respectively. The pluton is composed of calc-alkaline rocks, ranging from gabbroic diorite to granite (SiO₂ = 55.3 to 72.4 wt%). These I-type rocks are typically metaluminous but exhibit peraluminous tendencies in some cases. They are enriched in large ion lithophile elements and display concave chondrite-normalized rare earth element patterns (La_N/Yb_N = 4.39 to 6.67) with negative Eu anomalies (Eu_N/Eu* = 0.45 to 0.80). The initial εNd values ranged from 1.85 to 2.24, and the initial ⁸⁷Sr/⁸⁶Sr_(i) ratios ranged from 0.70458 to 0.70484, consistent with a mantle source in the isotope ratio diagram. The initial ²⁰⁶Pb/²⁰⁴Pb_(i), ²⁰⁷Pb/²⁰⁴Pb_(i), and ²⁰⁸Pb/²⁰⁴Pb_(i) ratios ranged from 18.505 to 18.571, 15.595 to 15.601, and 38.560 to 38.590, respectively. They have positive εHf_(i) values (5.75 to 8.03), located between depleted mantle and chondrite evolution values. Fractional crystallization of plagioclase, hornblende, and Fe-Ti oxide minerals played a significant role in the magma evolution, while assimilation and magma mixing may have had a minor influence. Collectively, the evidence suggests that the primary magma source for these plutonic rocks was an enriched lithospheric mantle. Subsequent differentiation within crustal magma chambers, with limited crustal contamination, shaped the final magmatic evolution.