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Eocene magmatism related to post-collisional extension in the Eastern Pontides (NE Turkey): 40Ar-39Ar geochronology, geochemistry and whole-rock Sr-Nd-Pb-Hf isotopes

2024    Emre AYDINÇAKIR

The mineral chemistry, whole rock geochemistry, ⁴⁰Ar / ³⁹Ar dating and Sr-Nd-Pb-Hf isotopes of the Eocene Narman (Erzurum) Volcanic rocks in the southeast of the Eastern Pontides Orogenic Belt (EPOB, NE Türkiye) were investigated. The Narman Volcanites consist of basaltic dyke, basaltic lava and basaltic volcanic breccia facies. Volcanites contain plagioclase (An_34-80), clinopyroxene (Wo_38-47En_41-50Fs_5-18), and olivine (Fo_68-90) as phenocrystals with magnetite/titanomagnetite micro phenocrysts. New ⁴⁰Ar-³⁹Ar ages suggest that these volcanic rocks erupted between 44.5 ± 0.1 Ma and 43.4 ± 0.1 Ma, within the Middle Eocene (Lutetian). Narman Volcanites have calc-alkaline character, with medium-high K content. Volcanites are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), while they are depleted in terms of high field strength elements (HFSE). Chondrite-normalized rare earth element distributions have concave shape with moderate enrichment (La_N/Lu_N=2.78-7.99), leading to consideration that the magmas forming the volcanics derived from similar sources. Isotopically, the rocks in the Narman Volcanites have low-medium initial ⁸⁷Sr/⁸⁶Sr values (0.70405-0.70485), initial ¹⁴³Nd/¹⁴⁴Nd values (0.512606-0.512848) and positive _ƐNd_i (+0.5 - +5.2). Depleted mantle Nd model ages were T_DM1 = 0.29–0.62 Ga and T_DM2 =0.43–0.83 Ga. (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i and (²⁰⁸Pb/²⁰⁴Pb)_i values vary between 18.246-18.709, 15.578-15.616 and 38.225-38.791, respectively. The initial (¹⁷⁶Hf/¹⁷⁷Hf)_i ratios for the volcanites are between 0.282770 and 0.283013, while the εHf values are +7.6 to +9.

All evidence supports the conclusion that the parental magma for the rocks probably derived from an enriched lithospheric mantle, previously metasomatized by fluids derived from subducted slab during asthenospheric upwelling due to fragmented asymmetric delamination in a post-collisional extensional tectonic environment.