Majerad gabrodorites in the southeast of Shahrood: An evidence to the starting of opening of the supra-subduction basin of the Sabzevar Neotethyian branch in the middle Jurassic

Late Neoproterozoic Majerad basementic igneous - metamorphic complex with NE-SW trend located in the 150 Km southeast Shahrood and north edge of the central Iran structural zone. Same as the other basementic complex of southeast of Shahroud, in the north of Majerad pass, this complex crosscut by several small scale gabrrodioritic intrusions and numerous diabasic dikes. In these gabrodiorite intrusions observed field evidences of fractionation from gabbro to tonalite. These gabrrodiorits show granular, ophitic, subophitic and poikilitc textures and composed of augitic clinopyroxene, green hornblende, plagioclase and biotite (as an essential minerals). Magma forming of these rocks have calc-alkaline nature, and enriched in large ion lithophile elements (LILEs) and also light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs) and heavy rare earth elements (HREEs). For the first time carried out U-Pb age dating on zircons extracted from the mentioned gabbrodiorites, represented an average age around 166 million years  ago (equivalent to middle Jurassic) for their formation. These magmas originated from the partial melting of the metasomatized subcontinental lithosoheric mantle source with spinel- lherzolithic primary nature in an extensional intracontinental back arc basin tectonic setting located on the subduction zone of the Zagros neothetyan basin on the northern edge of central Iran structural zone during the middle Jurassic period.