The Oligocene basaltic lavas of east and southeast of Shahroud: Implication for back-arc basin setting of Central Iran Oligo-Miocene basin

Some basaltic lava, with olivine basalt to basaltic composition, have been erupted within the Oligocene gypsiferous red marls (Lower Red Formation), in east and southeast of Shahroud. These rocks display microlitic porphyry, glomeroporphyry and hyaloporphyry textures. Olivine, plagioclase and pyroxene (in olivine basalts) and plagioclase and pyroxene (in basalts) are the main minerals. Enrichment in LREE and LILE and depletion in HREE, with positive anomalies in Pb, Sr and Cs, and the absence of negative anomalies in Eu and HFSE, indicate an alkaline nature and a sub-lithospheric enriched mantle source for these rocks. The rocks studied plot in back-arc basin realm in petrogenetic and tectonomagmatic diagrams and seem to be generated from about 10% partial melting of an enriched garnet-lherzolitic mantle source. This tectonic setting, implies an extensional proto-back-arc basin in Oligo-Miocene, behind the Uroumiyeh-Dokhtar magmatic arc, in some parts of Central Iran, that had been accompanied by the deposition of continental sediments (Lower Red Formation), shallow sea deposits (Qom Formation) followed by continental sediments (Upper Red Formation) and basic alkaline mantle magmatism.