Zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic constraints on petrogenesis of the Tarom-Olya pluton, Alborz magmatic belt, NW Iran

A petrological, geochemical and Sr–Nd–Pb isotopic study was carried out on the Tarom-Olya pluton, Iran, in the central part of the Alpine–Himalayan orogenic belt. The pluton is composed of diorite, monzonite, quartz-monzonite and monzogranite, which form part of the Western Alborz magmatic belt. LA–ICP–MS analyses of zircons yield ages from 35.7±0.8Ma to 37.7±0.5Ma, interpreted as the ages of crystallization of magmas. Rocks from the pluton have SiO2 contents ranging from 57.0 to 69.9wt.%, high K2O+Na2O (5.5 to 10.3wt.%) and K2O/Na2O ratio of 0.9 to 2.0. Geochemical discrimination criteria show I-type and shoshonitic features for the studied rocks. All investigated rocks are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), depleted in high-field strength elements (HFSEs), and show weak or insignificant Eu anomalies (Eu/Eu*=0.57–1.02) in chondrite-normalized trace element patterns. The Tarom-Olya pluton samples also show depletions in Nb, Ta and Ti typical of subduction-related arc magmatic signatures. The samples have relatively low ISr (0.7047–0.7051) and positive εNd(36Ma) (+0.39 to +2.10) values. The Pb isotopic ratios show a (²⁰⁶Pb/²⁰⁴Pb)i ratio of 18.49–18.67, (²⁰⁷Pb/²⁰⁴Pb)i ratio of 15.58–15.61 and (²⁰⁸Pb/²⁰⁴Pb)i ratio of 38.33–38.77. The εHf(t) values of the Tarom-Olya pluton zircons vary from −5.9 to +8.4, with a peak at +2 to +4. The depleted mantle Hf model ages for the Tarom-Olya samples are close to 600Ma. These isotope evidences indicate contribution of juvenile sources in petrogenesis of the Tarom-Olya pluton. Geochemical and isotopic data suggest that the parental magma of the Tarom-Olya pluton was mainly derived from a sub-continental lithospheric mantle source, which was metasomatized by fluids and melts from the subducted Neotethyan slab with a minor crustal contribution. Subsequent hot asthenospheric upwelling and lithospheric extension caused decompression melting in the final stage of Neotethyan subduction through slab rollback or shortly after the cessation of arc magmatism.