Petrogenesis of A-Type Khorasanlu Granitoid, Southeast of Zanjan: evidence for Eocene Extensional Magmatism, in Western Alborz, Iran

Khorasanlu intrusive body with monzonite to granite composition is located 75 km south-east of Zanjan city in the Western Alborz. This intrusive body has intruded into the Eocene rocks of the region. Based on petrographic and geochemical studies, the Khorasanlo intrusive complex have alkali-calcic to alkaline nature and are peraluminum and ferroan. Enrichment of light rare earth elements, relatively high amounts of HFSE compared to other granitoids, high Ga/Al ratio and placement of samples in the A-type granitoids field on the discrimination tectonic setting diagrams, indicate that the Khorasanlu intrusion mass is A2-type granitoid and was generated in the post-collisional setting. It seems that the tension resulting from the rollback or increase of the subducted plate dip and asthenosphere uplifting, after the compressional tectonics regim, caused the melting of the crust and generation of the Khorasanlo young alkaline granitoids.