TERTIARY MAGMATISM IN ALBORZ MOUNTAINS AND ITS RELATIONSHIP TO THE CONVERGENCE OF EURASIA WITH SOUTHERN PLATES

The Alborz Mountain Range which is a part of Alpine-Himalaya orogeny and is located in northern Iran at the south of the Caspian depression contains Precambrian to Quaternary sedimentary, igneous and metamorphic rocks. Tertiary magmatism was widespread in Alborz (and also many other parts of Iran), the products of which are numerous plutons, dykes, sills, volcanics and volcanosedimentary rocks. A majority of these magmatic rocks have been attributed to Eocene and Oligocene. Subduction of the Neotethyan Ocean slab beneath Central Iranian plate has played a major role in evolution of the Alborz Tertiary magmatism. Despite their great significance in unraveling the geodynamic setting of this important part of the Alpine-Himalaya range, detailed geochemical investigation has not been carried out on magmatic rocks from Alborz Mountains.

A comprehensive geochemical dataset obtained in the course of the present study show relationship of the Alborz magmatic rocks to subduction and orogenic settings. These rocks show spiked incompatible elements patterns commonly interpreted as evidence for the involvement of a fluid or fluid-rich melt derived from the subducted slab. High ratios of large ion lithophile elements (LILE, Pb) to high field strength elements (HFSE) can be seen in many of these rocks. Trace element patterns of these rocks are more akin to the active continental margin. Although signs of extensional and post-collisional (Miocene ?) magmatism are also found in Alborz, the subsequent magmatic rocks have been affected by previous subduction events.

In this study we try to demonstrate some geodynamic aspects of the Tertiary subduction-related magmatism in Alborz Mountains by combining our geochemical data from Central Alborz with the data from other parts of this belt.