Abstract

Aqkand area is located 48 km north of Zanjan and it is a part of the Tarom subzone within the Western Alborz-Azerbaijan structural zone. Acidic rocks exposed in this area consist of Oligocene rhyolite-rhyodacite, perlite, pitchstone and ignimbrite. Rhyolites and rhyodacites are exposed as domes and lava flows. Perlites are located at the lower parts of the rhyolitic-rhyodacitic domes. Geochemical investigations indicate calc-alkaline nature along with magmatic arc affinity for the study rocks. These rocks are characterized by LREE-rich patterns with high LREE/HREE ratio and distinctive negative Eu anomaly. Positive anomalies of Rb, Pb and Th in the spider diagrams suggest continental crust involvement in the genesis and evolution of the parental magmas. Based on tectonic setting discrimination diagrams, Aqkand acidic rocks are plotted within the Post-COLG setting. Overall petrographic, petrologic and geochemical evidences demonstrate that during the final stages of the continental collision, the thickened sub-continental lithosphere in the Alborz was delaminated and replaced by hot asthenospheric mantle. Rapid upwelling of hot asthenosphere gave rise to the partial melting of sub-continental lithosphere which produced the Oligocene basaltic rocks of the study area. Emplacement of basaltic magma in the upper part of the continental crust led to the partial melting of the crust and generation of the acidic magmas. Rapid cooling of acidic lavas led to the generation of glassy rocks such as obsidian which have been changed to perlite in contact with meteoric and warm waters.