Early Paleozoic tectonic reconstruction of Iran: Tales from detrital zircon geochronology

Abstract

In this study we use detrital zircons to probe the Early Paleozoic history of NE Iran and evaluate the link between sediment sources and Gondwanan pre-Cadomian, Cadomian and younger events. U–Pb zircon ages and Hf isotopic compositions are reported for detrital zircons from Ordovician and Early Devonian sedimentary rocks from NE Iran. These clastic rocks are dominated by zircons with major age populations at ~2.5Ga, ~0.8–0.6Ga, 0.5Ga and ~0.5–0.4Ga as well as a minor broad peak at ~1.0Ga.The source of 2.5Ga detrital zircons is enigmatic; they may have been supplied from the Saharan Metacraton (or West African Craton) to the southwest or Afghanistan–Tarim to the east. The detrital zircons with age populations at 0.8–0.6Ga probably originated from Cryogenian–Ediacaran juvenile igneous rocks of the Arabian–Nubian Shield; this inference is supported by their juvenile Hf isotopes, although some negative εHf (t) values suggest that other sources (such as the West African Craton) were also involved. The age peak at ca 0.5Ga correlates with Cadomian magmatism reported from Iranian basement and elsewhere in north Gondwana. The variable εHf (t) values of Cadomian detrital zircons, resembling the εHf (t) values of zircons in magmatic Cadomian rocks from Iran and Taurides (Turkey), suggest an Andean-type margin and the involvement of reworked older crust in the generation of the magmatic rocks. The youngest age population at 0.5–0.4Ga is interpreted to represent Gondwana rifting and the opening of Paleotethys, which probably started in Late Cambrian–Ordovician time. A combination of U–Pb dating and Hf-isotope data from Iran, Turkey and North Gondwana confirms that Iran and Turkey were parts of Gondwana at least until late Paleozoic time.