Origin and tectonomagmatic setting of Quaternary volcanism in Alamut Valley

Quaternary volcanic activities in Alamut Valley occurred in three stages and covered Neogene red sedimentary rocks. The volcanic rocks are intermediate in composition and are classified as hornblende trachyandesite. The SiO2 content of the rocks varies in a range from 55.48-61.88 wt%. Chondrite normalized REE patterns exhibit a steep arrangement of LREEs and MREEs (La/Ho)N =85 and a relatively flat HREE (Ho/Lu)N =1.3., Large ion lithophile elements (LILE) do not show strong enrichment compared to LREEs in mantle normalized multi-element spider diagrams. Nb andTa are slightly depleted compared to neighboring light rare earth elements. These rocks have high Sr/Y (140-205), La/Yb (113-142) ratios, accompanied by high K2O/Na2O (0.7-0.9) and resemble C-Type adakites. Geochemical evidence suggest that these rocks are resulted from partial melting of thickened eclogitic lower crust in presence of abundant residual garnet. Partial melting of lower continental crust probably triggered by thickening of crust as a result of late Cenozoic orogenic phases especially the Passadenian Orogenic phase.