U-Pb geochronology and geochemistry of the Torud igneous rocks: Implications for post-collision Eocene magmatism in northeast Iran

Abstract

New geochemical and U-Pb isotopic data from the Torud volcanic and subvolcanic rocks and their associated dikes exposed along the southern margin of the Sabzevar-Torud zone provide new evidence for Early-Middle Eocene, post-collision magmatism in northeast Iran. The 52–46 Ma (late Ypresian-Lutetian) U-Pb age interval of zircons separated from these rocks confirm the paleontological age of nummulite-bearing limestone interlayers. Inherited zircons separated from these igneous rocks have a much wider range of U-Pb ages that include Archean, Paleoproterozoic, Mesoproterozoic, Carboniferous, Permian, Triassic, Middle Jurassic, and Late Cretaceous. Most volcanics and subvolcanic rocks display high-K calcalkaline and shoshonitic trends. Distinct crystal fractionation patterns of the volcanic and subvolcanic rocks suggest magmatic differentiation in separate magma chambers. The studied rocks, depleted in the HFSE and enriched in the LILE, display nearly homogeneous isotopic Sr (0.7039–0.7055) and Nd (0.5126–0.5129) ratios and positive ɛNd values (0.08–0.56) indicating partial melting from an enriched lithospheric mantle source that was slightly contaminated with crustal material. Fluids, especially those released from the subducted slab, affected the composition of the source for the studied rocks. The magmatism occurred in the Torud area after the Late Cretaceous-Paleocene collision of the Central Iran microcontinent and Binalud-Kopeh-Dagh blocks and the Early-Middle Eocene break-off of the subducted slab.