Eocene volcanism in the Tashvir region: Indication of extension in the Northern subzone of the Tarom range in northwestern Iran

The present study aims to characterize the igneous rocks of the Tashvir range. The Tarom mountains are located at the southwest end of the Alborz mountain range with an approximate northwest-southeast orientation. Tashvir region is part of the northern edge of the Tarom mountains and 90 km northeast of Zanjan city. Igneous rocks of this range mainly include basalt, basaltic andesite, and interlayers of tuff. Petrographically, these rocks contain olivine and clinopyroxene phenocrysts with a zoning structure. Since they have olivine and nepheline (less than 5%) in their norms, they can be considered alkali olivine basalts. A remarkable feature of olivines in the Tarom basalts is the presence of carbonate fragments that have reacted with magma and completely turned into clinopyroxene needles. The reduction in SiO2 concentration and the reduction in Mg# concentration suggest the assimilation of carbonate rocks by basaltic magma. Also, the pattern of enrichment in large-ion lithophile elements (LILEs), the positive anomalies of Ce, P, and Sm, the depletion of Nb, Ta, Zr, Hf, and Th elements, and Low K2O/Na2O and Ba/Rb ratios in the rock samples of the Tashvir range indicate magma contamination with the lower crust. The evidence of crustal contamination and Nb/Y and Zr/Y values in the samples indicate that the primary origin of the basalts in the Tashevir region is due to plume sources. The alkaline nature, shoshonitic, peraluminous nature, and low Ti content of the rocks of the Tashvir area, along with enrichment in the rare earth elements (REE) and other elements such as Ba, Rb, Sr as well as the magma origin and depth of magma generation suggest that these magmas belong to an original rift environment. The tectonic setting of the Tarom range is divided into two parallel sub-ranges with arc-type eruptions and rift zones, where the expansion of this rift zone has stopped before developing into a back-arc basin.