Petrology and geochemistry of intrusions in the Karaj-Taleghan axis (Central Alborz)

The Intrusive bodies of the Karaj-Taleghan Axis, situated in the Central Alborz zone have lithological compositions including gabbro, monzogabbro, monzodiorite, monzonite andfelsic dikes of alkali feldspar syenite to syenite. Furthermore, they are accompanied by pyroclastic rocks of the Karaj formation, for which Oligocene age is considered. Geological surveys, mineralogical and geochemical studies indicated that the parental magma of this complex had shoshonitic and meta-aluminous nature. Petrological evidence show that fractional crystallization from gabbro to monzonite had main role in genesis of these rocks. Enrichment in LILE) Ba, Rb and Th(and depletion in HFSE (Nb and Ti) in Rare Earth Elements distribution patterns of spider diagrams, indicate an subduction-related environment for formation of these rocks. Also they are enriched in LREE rather than HREE, the characteristics which are usually observed in the rocks from subduction environments and active continental margins. Chemical and petrological studies indicate a common source for these igneous rocks and the main role of fractional crystallization in the evolution of magma, although, minor amounts of assimilation and contamination of magma by crustal rocks occurred. The alkaline magma formed by low degree partial melting of an enriched sub-continental lithospheric garnet-lherzolite mantle. Contribution of both the lithospheric and asthenospheric mantles in petrogenesis of the Karaj-Taleghan basic rocks might be attributed to a tensional geodynamic setting with change in subduction dip in the form of roll back, dominated at the final stage of the subduction of Neotethyan plate beneath Iran in upper Oligocene (Chattian).