Tectonic setting and source characteristics of the Abarsej Formation igneous rocks (Upper Ordovician), eastern Alborz, north of Shahrood

Outcrops of the Upper Ordovician igneous rocks, as basaltic lava and microgabbroic dyke and sill, present within the Abarsej and Ghelli formations in north and southwest of Shahrood. These rocks have basaltic, trachybasaltic and basaltic trachyandesitic composition with alkaline nature. Their parallel patterns on the primitive mantle and chondrite normalized spider diagrams indicate the common source of these rocks and the role of fractional crystallization in their generation. These rocks plot on the field of alkali basalts related to intracontinental rift on various discrimination tectonic setting diagrams. This tectonic setting corresponds to paleogeography of the Alborz during early Paleozoic (occurrence of Turan intracontinental Paleotethys rift) in Ordovician- Silurian times. Geochemical investigations indicate the magma forming of these basaltic rocks generated from 14-16% partial melting of an enriched garnet peridotitic astenospheric mantle source in 100-110 km depths. This magma has not contaminated by continental crust during ascending to the earth surface.