Adakite magmatism within the Sabzevar ophiolite zone, NE Iran: U-Pb geochronology and Sr-Nd isotopic evidences

Abstract

Numerous post-ophiolite intrusions exposed into the late Cretaceous–Paleocene Sabzevar ophiolite belt, northeast Iran. The intrusions consist of calc-alkaline intermediate to felsic rocks with metaluminous to peraluminous in nature and adakitic affinities. Based on U-Th-Pb dating on separated zircons by SHRIMP Ⅱ, the emplacement age of these rocks into the ophiolite zone is constrained to ~45 Ma. All intrusions show similar MORB –like initial Nd and Sr values of 0.512911 to 0512846 and 0.704758 to 0.703790, respectively, with positive εNd (45 Ma) values of +5.26 to +6.45 implying their cogenetic nature. These isotopic signatures combined with geochemical evidences are corresponding to parental adakitic magmas derived from wet partial melting of a garnet-amphibolite source from Sabzevar subducted oceanic lithosphere. After emplacement in the deep magma storage region (equal to 700-900 MPa), this magma evolved to intensify adakitic signatures via amphibole–dominated magma fractionation.