TETHYAN EVOLUTION AND TECTONIC HISTORY OF SW ASIA: EVIDENCE FROM OPHIOLITES OF IRAN

Iran is constructed chiefly of Cadomian (~500-600 Ma) continental fragments (Cimmeria) that rifted from Gondwana as Paleotethys and Neotethys opened and re-amalgamated as these oceans closed. Iranian ophiolites can thus be divided into Paleozoic and Mesozoic ophiolites. Paleozoic ophiolites - remnants of Paleotethys – are aligned in two main zones in N Iran: Aghdarband, Mashad, and Rasht in the north and Jandagh-Anarak and Takab ophiolites to the south. Paleozoic ophiolites were emplaced when Cimmeria sutured to Eurasia near the end of Paleozoic time. Mesozoic ophiolites are related to subduction initiation and associated backarc basins on the SW flank of Eurasia. These are more abundant than Paleozoic ophiolites and can be divided into rare Jurassic and Early Creatceous ophiolites in SE Iran and abundant Late Cretaceous ophiolites, found in 6 regions: 1) outer Zagros ophiolites along the Main Zagros Thrust, and 2) inner Zagros ophiolites;; 3) Jurassic-Cretaceous Makran-Kahnuj ophiolites of SE Iran, 4) Cretaceous Birjand-Nehbandan-Tchehel-Kureh ophiolites in eastern Iran; 5) Late Cretaceous-Early Paleogene Sabzevar-Torbat-e-Heydarieh ophiolites of NE Iran; and 6) Late Cretaceous-Early Paleocene Maku-Khoy-Salmas ophiolites in NW Iran. Paleozoic ophiolites occasionally show mineral and chemical signatures reflecting formation above a subduction zone (SSZ) whereas Mesozoic ophiolites commonly show these signatures. Our geochronological data show that Paleozoic ophiolites formed from Devonian to Middle Permian time. Intra-oceanic subduction initiation in Devonian (U-Pb zircon age) could be responsible for the SSZ-related Darrehanjir gabbros in Aghdarband, a phase that was accompanied by formation of Nakhlak boninitic rocks in Jandagh-Anarak ophiolites. Following Carboniferous to Permian arc magmatism and then back-arc extension, BABB-like (SSZ-type) volcanic rocks erupted. Understanding the significance of Makran ophiolites requires collaborative studies on either side of the Iran-Pakistan border. Understanding the belt of Late Cretaceous ophiolites that straddles the Iran-Iraq border requires similar collaborative studies.