Late Carboniferous-Early Permian geodynamic evolution of NW Iran: Zircon U-Pb ages, Hf isotopes, and whole rock geochemistry of Salmas amphibolites

Abstract

We present new geochronological, petrological, and geochemical data from the amphibolites of the Salmas metamorphic complex in NW Iran. This region is where the Sanandaj-Sirjan magmatic-metamorphic zone, the Urmia-Dokhtar magmatic arc, and the Eastern Anatolian Plateau converge, creating a complex geodynamic context. The amphibolites alternate with gneisses and metamorphosed limestone layers and appear as enclaves of varying sizes within the gneisses. Fine- to medium-grained amphibole and plagioclase, exhibiting a granoblastic texture, are the dominant minerals, indicating basaltic and diabasic protoliths. The amphiboles show simple foliation along lineation, which is occasionally folded. These amphibolites are overlain by Permian to Jurassic sedimentary rocks and, in some places, by Miocene sediments with angular unconformity. Based on whole-rock geochemistry, the amphibolites have relatively high TiO₂ (1.23–2.62 wt%) and low MnO (0.18–0.21 wt%) contents, classifying them as ortho-amphibolites. The parental magma was sub-alkaline basaltic with tholeiitic affinities, formed in a within-plate tectonic setting. This is characterized by enrichment in LREE relative to HREE, a lack of Nb, Ta, and Ti anomalies, and the presence of negative Eu and positive Ba anomalies. The ɛHf(t) and ¹⁷⁶Hf/¹⁷⁷Hf ratios of dated zircons suggest a depleted mantle to lower crust origin for the parental magma of the amphibolites. U-Pb dating of zircon grains yields a mean age of 304.8 Ma, corresponding to Late Carboniferous-Early Permian magmatism related to the opening of the Neotethys Ocean. The thermal effects of Late Cretaceous to Early Cenozoic subduction-related magmatic events are recorded by overgrown metamorphic zircon around original magmatic grains.