Petrogenesis and Geodynamic Evolution of Eocene Volcanic Rocks in Eastern Meyami, Central Iran: Insights from Geochemistry and Sr-Nd-Pb Isotopic Constraints

The Eocene volcanic rocks of eastern Meyami in Semnan Province, with mafic to intermediate compositions, are emplaced within a shallow marine sedimentary sequence. Based on petrographic, geochemical, and Sr-Nd-Pb isotopic data, this study investigates the origin, magmatic evolution, and tectonic setting of these rocks. Results show that the rocks belong to the high-K calc-alkaline to shoshonitic series and are characterized by enrichment in LILE and LREE and depletion in HFSE. Sr-Nd isotopic data (Sr/ ۸۶ Sr = ۰.۷۰۳-۰.۷۰۵ and Nd = +۲ to +۵) indicate a depleted mantle (DM) source, while Pb isotopic patterns (with elevated ۲۰۸ Pb/۲۰۴ Pb) confirm the influence of subduction-related components. This evidence suggests that the magma was generated by partial melting of a metasomatized mantle wedge affected by fluids and melts derived from the subducted Neotethyan oceanic slab. Magmatic differentiation via fractional crystallization of clinopyroxene, olivine, and plagioclase played a key role in the formation of intermediate compositions. Tectonically, this assemblage formed in a back-arc environment during the transition from compressional to extensional regimes associated with Neotethyan subduction.

Keywords:

Eocene volcanic rocks , Isotope geology , Neotethyan subduction , Mantle metasomatism , Back-arc , Central Iran