Back - arc magmatism during Eocene subduction and closure of eastern Tethys: An insight from Alborz Magmatic Belt (NW Iran)

Abstract

The Pargeh plutonic complex in Central Alborz, Iran, was emplaced during the Early Eocene and consists mainly of olivine gabbros and monzodiorites with granular to ophitic textures. The olivine gabbros contain olivine, clinopyroxene, plagioclase, biotite, and accessory magnetite, ilmenite, and apatite, whereas the monzodiorites are composed of clinopyroxene, plagioclase, biotite, and > 10% alkali feldspar, together with accessory oxide phases and apatite. Apatite UPb dating yields emplacement ages of 57.1 ± 3.5 Ma (olivine gabbro) and 53.2 ± 2.7 Ma (monzodiorite), indicating Early Eocene magmatism. The rocks display high-K signatures, enrichment in large-ion lithophile elements (LILE), and depletion in high-field-strength elements (HFSE), along with negative NbTa anomalies and enrichment in Sr, Ba, U, and Pb—features typical of subduction-related geochemical influence. Whole-rock isotopic compositions (εNd(t) = +1.5 to +1.9; ⁸⁷Sr/⁸⁶Sr(i) = 0.7049–0.7053) indicate derivation from a metasomatized subcontinental lithospheric mantle (SCLM) influenced by subduction-related fluids. The parental mafic magmas—generated by low-degree partial melting of this hydrated mantle—underwent limited crustal assimilation and fractional crystallization within deep crustal zones, producing monzodioritic derivatives. These features collectively suggest that the Pargeh intrusion formed in an extensional, supra-subduction back-arc environment.