High Potash Volcanic Rocks and Pyroclastic Deposits of Damavand Volcano, Iran, an Example of Intraplate Volcanism

Damavand is a fascinating dormant stratovolcano, 60 km to the ENE of Tehran located in the Alborz Mountains. Damavand volcanic products consist of lava flows and pyroclastic fall, flow and surge deposits from different eruption cycles. The volcanic rocks ranges from trachyandesite to trachyadacite and minor basalt. The mineral assemblage consists of potash feldspar (Or43/7), Plagioclase (An25 to An59), amphibole, clinopyroxene (augite and salite), orthopyroxene (hypersthene and ferro-hypersthene), biotite (phologophite) and Fe-Ti oxides. Some of the lavas and pyroclastic deposits show calc-alkaline affinities. Lavas from different cycled are classified as shoshonitic types and most pyroclastic deposits are classified as High-K. In comparison to n-type MORB, three recent pyroclastic deposits in Damavand show an enrichment in LREE, LILE, Th, and P and are slightly depleted in MREE and HREE. Incompatible LILE (Rb, Ba and Sr) together with Th and U have not shown broad enrichment as a function of increasing SiO2 content. Variations in the Major and trace element compositions of Damavand rocks and pyroclastic deposits are difficult to explain by fractional crystallization mechanism. Scatter of several trace elements in plots against SiO2 and incompatible trace elements, also suggests that the petrogenesis is more complex than a simple fractionation process from a single composition parent. High K, Ba and Rb content in volcanic products could be due to enrichment of these elements in the source. Field observation such as limitation of magmatism in region suggest that decompression melting could be generate the Damavand Lavas and pyroclastic deposits.