Early Oligocene Continental Alkalibasalts of the Central Toveireh Area (Southwest of Jandaq, Isfahan Province, Iran)

Abstract

Abstract: The Early Oligocene alkalibasalts exposed in the Central Toveireh area located in the southwest of Jandaq city in Isfahan Province (Iran) and northwest of the Central-East Iranian Microcontinent (CEIM). Field studies reveal that these alkalibasalts crosscut the Eocene calc-alkaline volcanic rocks and granitoids and covered by the Miocene sedimentary rocks. The basaltic magma rose to the surface along the local faults. Based on the petrography, these alkalibasalts are composed of major minerals of olivine and plagioclase, and minor minerals of clinopyroxene, sanidine, Cr-spinel, and magnetite. The microscopic textures are porphyritic, microlithic porphyritic, trachytic, anti-rapakivi, corona, sieved texture, and poikilitic. Olivines are forsterite and chrysolite (Fo 0.90–0.75), plagioclases are labradorite to oligoclase (An63.1–20.7), alkali-feldspars are sanidine (Or50.3–61.5), clinopyroxenes are diopside and augite (Mg# 0.79 to 0.87), and Cr-spinels are hercynite (Cr# 0.24 to 0.25) in chemical composition, spinels (Mg# 0.72 to 0.76) are present as xenocrysts in some samples. The average contents of SiO2 and TiO2 of these rocks are 48.96 and 1.54 (wt %), respectively. The chondrite and primitive mantle-normalized diagrams characterized by enriched LREE relative to HREE, LILE enrichment, and absence of evident Eu anomaly. The normative content of nepheline reaches up to 14.6%. Modal and normative mineralogy, as well as geochemical data of minerals and whole rocks revealed that these rocks are sodic to highly sodic alkalibasalts formed in a within-plate continental tectonomagmatic setting after the cessation of subduction. The whole-rock chemical data indicate that the Central Toveireh alkalibasalts probably formed by relatively medium degrees of partial melting of an amphibole-bearing garnet lherzolite from the asthenospheric mantle of about 105 km in depth, which was previously enriched by subduction of the Neo-Thetyan slab. © 2022, Pleiades Publishing, Inc.