Comparision of the Mafic Igneous Rocks the Ghelli Formation (Upper Ordovician) and the Gorgan Schists in the Eastern Alborz Zone

On the basis of stratigraphical studies, parental rocks of the Gorgan schists have same petrological features and fossil contents as the Ghelli formation in the eastern Alborz Zone and despite to the previous theories, these schists are not Precambrian in age. Also, the mafic igneous rocks of these two rock units have considerable petrological and geochemical similarities. These mafic igneous rocks include basalt, trachybasalt and basaltic trachyandesite in association with microgabroic sills, dykes and small intrusions. Investigations on, and comparison of their petrological and geochemical features, indicate similarities in age, petrology, magma sources, geochemistry and tectonic setting. It seems that the parental rock of the Gorgan schists and the rocks forming the Ghelli formation were sedimentary and igneous rocks, deposited during the initial stages of extensional movements and rifting of the Paleotethys ocean during Ordovician in the eastern Alborz. However the, parental rocks of the Gorgan schists were metamorphosed in the greenschist facies in middle-upper Triassic (early Cimmerian orogeny), due to vicinity to the oceanic trench of the Paleotethys subduction zone. Geochemical features of the igneous rocks in these two units indicate 12-16% partial melting of an enriched garnet lherzolite mantle source (such as the Oceanic Island Basalt source) for generation of the original magma in the garnet peridotite stability field at 100-110 Km depth..