TECTONIC HISTORY OF IRAN: INITIAL DETRITAL ZIRCON RESULTS

An ongoing geochronological study of detrital zircon populations for the Phanerozoic succession of Iran yields insight into the plate tectonic evolution of the Middle East. In contrast to the better studied Alpine and Himalayan belts, the distribution and timing of multiple interactions among continental blocks in the Middle East remain poorly understood. Preliminary detrital zircon results place constraints on the identity and location of various blocks during opening and closure of Tethyan oceans and growth of the Alborz mountains, Zagros mountains, and central Iranian plateau. Ion-microprobe U-Pb analyses of 242 detrital zircon grains from seven sandstones (Cambrian, Permian, Triassic-Jurassic, and Neogene) yield concordant to moderately discordant dates between 3150 and 50 Ma. The Lower Cambrian Lalun and Barut formations of the Alborz exhibit age distribution peaks at 590, 765, 910, and 2510 Ma; a Permian sandstone from the Zagros yields peaks at 510, 770, 1020, and 2130 Ma. These three Paleozoic samples suggest a Gondwanan source presently to the south and west in Iran and in the Arabian-Nubian shield of Saudi Arabia and northeastern Africa. U-Pb zircon dates for two sandstones of the uppermost Triassic-Lower Jurassic Shemshak Formation of the Alborz have a broad range with primary age peaks at 220-250, 610, and 1860 Ma, and subordinate peaks at 310-750 and 2530 Ma. These dates indicate a Eurasian source (Turan plate) to the north and east, requiring collision of Iran and Eurasia by Late Triassic time. Two Neogene sandstones along the border between the Alborz and central Iranian plateau display the youngest and nearly the oldest dates (50 and 2850 Ma) of any samples, with an age distribution peak at 50-55 Ma and many subordinate peaks. This cosmopolitan assemblage is the result of sedimentary recycling of older Phanerozoic cover rocks. The youngest zircon dates for several sandstones are indistinguishable from the corresponding depositional ages, suggesting (1) rapid exhumation of a Late Proterozoic-Early Cambrian granitic belt shortly after Pan-African orogenesis and prior to Paleozoic passive-margin sedimentation in Iran, (2) rapid exhumation of a youthful Carboniferous-Triassic magmatic arc of southernmost Eurasia during Late Triassic continental collision with Iran, and (3) Eocene magmatism preceding the Iran-Arabia collision.