Geochemistry and petrogenesis of alkaline rear-arc magmatism in NW Iran

Abstract

High Na- to K- alkaline magmatism is common in the rear-arc region of the Cenozoic Urumieh-Dokhtar Magmatic Belt of Iran, but their geochemical signatures, as well as their formation mechanisms, have been poorly studied thus far. In the Kleybar area of NW Iran, Middle Eocene magmatic activity comprises silica-undersaturated to -saturated gabbros, monzogabbros, monzosyenites to nepheline-monzosyenites, and nepheline syenite as well as tephritic to trachy-basaltic and trachy-andesitic dikes. New 40Ar39Ar results show a restricted age range (39.2–43.6 Ma) for the intrusive rocks (43.16 ± 0.43 and 43.34 ± 0.43 Ma for gabbros, 43.56 ± 0.44 for monzogabbros, 39.22 ± 0.48 and 42.09 ± 0.42 Ma for nepheline syenites, 42.17 ± 0.42 Ma for nepheline-monzosyenites), whereas a cross-cutting trachy-andesitic dike yielded an 40Ar39Ar age of 41.78 ± 0.42 Ma. The Kleybar rocks are enriched in alkalis (K2O and Na2O) with variable K2O/Na2O ratios (0.14 to 2.93), light rare earth elements and large ion lithophile elements such as Th, Rb, K, U and Pb. High field strength elements such as NbTa are depleted in these rocks.The isotopic compositions of the different rock types are quite variable: gabbros and monzogabbros have 87Sr/86Sr(t) = 0.70419–0.70436, εNd(t) = +3.1 to +3.8 and εHf(t) = + 7.8 to +10.1; nepheline monzosyenites and nepheline syenites have 87Sr/86Sr(t) = 0.70359–0.70566 (except a nepheline syenite with a ratio of 0.74833 and very high Rb/Sr), εNd(t) = +1.5 to +4.2 and εHf(t) = +4.6 to +8.8; and trachy-basaltic to trachy-andesitic to tephritic dikes have 87Sr/86Sr(t) = 0.70437–0.70469, εNd(t) = +2.8 to +4.2 and εHf(t) = +8.3 to +10. In the thorogenic-Pb isotope diagram, the Kleyber igneous rocks define an array above the Northern Hemisphere Reference Line (NHRL), with ∆8/4 (deviation from the NHRL) of ~40–60. These samples also plot above the NHRL in uranogenic Pb space (∆7/4–5-10). The positive ∆8/4 Pb and ∆7/4 Pb may reflect the involvement of subducted terrigenous sediments in their mantle source during the subduction of the Neotethyan oceanic lithosphere. Two nepheline syenite samples have significantly higher thorogenic and uranogenic Pb isotopic compositions, that may reflect assimilation of surrounding clay-rich sedimentary rocks.Modelling of trace elements compositions using less fractionated Kleybar trachybasalt and fine-grained monzogabbro samples indicate that a 96:4 mixture of the depleted mantle and subducting (trench)-sediment melts with 6% aggregated fractional melting closely matches the trace-element abundances of the Kleybar trachybasalt and monzogabbro. Together with previous studies on high-K volcanic rocks from NW Iran, our results indicate that Neotethyan slab retreat and related extension of the Iranian continental lithosphere in the rear-arc region of the Urumieh-Dokhtar Magmatic Belt generated alkali-rich magmatic rocks throughout the NW Iran rear-arc during Middle-Late Eocene.