Petrology and geochemistry of Early Tertiary volcanism of the Mendejin area, Iran, and implications for magma genesis and tectonomagmatic setting

The Mendejin area, NW Iran, is part of the western Alborz-Azarbaijan zone which is one of the most structurally—and magmatically-active zones of Iran. Volcanic rocks with calc-alkaline and, locally, alkaline features cover an extensive part of this zone. The Mendejin volcanic rocks, Eocene-Oligocene in age, include tuffs and volcanoclastic rocks of dacite, andesite, basaltic andesite, and basalt composition. Felsic (andesite, dacite, and rhyodacite) and basic rocks (basalt, basaltic andesite and andesite) commonly occur in successive layers. This alternation along with multiple occurrences of various types of tuffs suggests prolonged and successive magmatic activity during Eocene-Oligocene in NW Iran. Fractional crystallization has been the most important factor controlling geochemical characteristics of the magma. However, absence of linear correlations on variation diagrams of some immobile elements (such as Al2O3, TiO2, P2O5 and Ga) and poorly-developed trends on variation diagrams of Na2O, MgO, MnO, CaO, Fe2O3, Nb, Nd, Y, La, Ce, Th, Hf, Sc, Zn, V, Ni and Co versus SiO2 indicate that, other than crystal (olivine, pyroxene, plagioclase, biotite, hornblende, zircon, monazite and apatite) fractionation, crustal processes (such as assimilation) have also affected the chemistry of the Mendejin magma. It appears that the basic magma has originated from the mantle whereas the felsic magma resulted from modification in the mantle-derived magma by assimilation in an active continental margin.