AGES OF PLUTONIC ACTIVITY IN THE SANANDAJ-SIRJAN ZONE, IRAN: IMPLICATION FOR PLATE CONVERGENCE AND ZAGROS COLLISION

Despite modern advances in plate tectonics aided by geodynamic modeling, the relative timing of plate convergence, ocean closure and continental collision remain a fundamental problem in the study of orogenic belts. The Zagros orogen formed by the collision of Arabian and Eurasian plates and final demise of the Neotethys ocean that was closed via northward subduction of its lithosphere beneath the Iranian Plateau. The resulting Cenozoic magmatic arc stretches with a NW-SE trend along the entire length of the orogen from eastern Turkey through Iran to western Pakistan. The Zagros orogen provides one of the best records of fold-and-thrust tectonics and foreland basin development worldwide.

An important but poorly studied structural element of the Zagros orogen is the Sanandaj-Sirjan Zone (SSZ) that extends for nearly 1500 km sub-parallel to, and to the southwest of, the Cenozoic magmatic arc. It is characterized predominantly by Mesozoic, medium-grade, metasedimentary rocks intruded by widespread granitoid plutons. The two prominent Boroujerd and Alvand plutonic complexes dominate the northern sector of the SSZ.

We report new U-Pb zircon age data from the Boroujerd and Alvand plutonic complexes that indicate the bulk of magmatism in northern SSZ occurred within a relatively short time interval in the Middle Jurassic. This indicates that a convergent tectonic regime had already been established across the Neotethyan domain by this time. Volumetrically small but important felsic intrusions were emplaced throughout the northern SSZ during late Eocene. These rocks have distinct lithologic and geochemical characteristics than those of the Jurassic intrusions and probably reflect a thermal episode at the peak of Zagros continental collision.