Document Type : Original Research Paper


1 Department of Geology, Science and Research Branch, Islamic Azad University, Tehran, Iran

2 Department of Geology, Islamshahr Branch, Islamic Azad University, Tehran, Iran

3 School of Geology, College of Science, University of Tehran, Tehran, Iran


This study investigates petrology and major, minor, and rare earth elements ‎geochemistry of ‎East Iranian Eocene–Oligocene volcanic rocks in Sistan suture ‎zone, to examine their ‎petrogenesis and magma evolution. The volcanic rocks include andesite, trachy-andesite, dacite ‎and rhyolite. ‎These calc-alkaline rocks of high-K series are enriched in Large ‎Ion Lithophile ‎Elements of Rb and Ba and depleted in High Field Strength Elements  ‎(e.g. Ti and Nb). The REE ‎pattern shows Eu negative anomaly. These ‎features are comparable with rocks in subduction ‎zone of continental ‎margin setting. ‎The petrographic features and the geochemical variation ‎of major oxides and trace ‎elements against SiO2 can be related to fractional ‎crystallization in parent magma. The trace ‎element ratios, e.g. Nb/La, Nb/U, Ba/Rb, Nb/Y and Rb/Y, verify crustal ‎contamination with a remarkable upper ‎crustal contamination as the main ‎process in the ‎formation of volcanic series. ‎The ratios of Nb/Ba and Nb/Zr indicate that magmatic activity in Momen Abad had ‎resulted ‎from sub-continental lithospheric mantle. In addition, Nb/Y and Zr/Yb versus ‎Ta/Yb reveals an ‎E-MORB like mantle source. A spinel lherzolite composition ‎with significant role of the ‎AFC processes are deduced from the concentrations of Yb and ‎Y, and the ratios of Th/Yb and ‎La/Yb. ‎The Ba/Nb, Ba/Th and Th/Nb ratios presume a metasomatized mantle source ‎resulted from ‎the melting of upper crust sediments inherited from the subduction of ‎Neotethys ocean beneath ‎the Lut Block. ‎