Cadomian basement evolution and Cenozoic crustal growth in Iran: Constraints from zircon Hf isotopes

Iran is located in the Arabia-Eurasia collision zone and comprises several Gondwana-derived terranes with a drifting history close connected with the evolution of Tethys oceans. New combined zircon U-Pb and Hf isotopic results from the Late Neoproterozoic-Eocene igneous rocks, compiled with our published data, provide more evidence for further understanding the magmatic and crustal evolution in Iran and the adjacent Caucasus region. The widespread Late Neoproterozoic to Cambrian (ca. 600-500 Ma) granitoids and associated volcanics, which represent the crystalline basement of Iran, are regarded as part of the Cadomian magmatic arc resulting from the subduction of the Prototethys along the northern Gondwanan margin. The Cadomian basement rocks yield variable zircon Hf isotopic results of ɛ_Hf(T) values from +11 to -6 at 595-511 Ma and Hf crustal model (T_DM^C) ages of ca. 0.9-1.9 Ga, implying that the magmas were formed by the melts of mixed depleted-mantle and Archean crustal sources. The Carboniferous (349-311 Ma) A-type granitoids in northwestern Iran have well constrained the timing of initial phase of the Neotethyan opening and yield zircon ɛ_Hf(T) values from +5 to -1 and T_DM^C ages of ca. 1.0-1.4 Ga, suggesting the formation of these rocks involving the reworked Cadomian basement. In the Caucasus region, the Carboniferous granitoids with a subduction-related feature are exposed with obvious crustal contamination during their formation, showing relatively negative zircon ɛ_Hf(T) values from +2 to -12 at 331-309 Ma and older T_DM^C ages of ca. 1.2-2.1 Ga. The long-lasting Neotethyan subduction beneath Iran led to the occurrence of the Urumieh-Dokhtar magmatic arc with major activities during the Eocene to Oligocene (ca. 55-25 Ma), and this stage of magmatic flare-up event also widely took place in the Sanandaj-Sirjan zone, Alborz-Sabzevar and Lut-Sistan regions of Iran, and Armenia. The Eocene-Oligocene igneous rocks, along with the subsequent Miocene-Quaternary samples, have dominantly positive zircon ɛ_Hf(T) values from +15 to -2 and much younger T_DM^C ages of ca. 0.2-1.2 Ga. Therefore, the results have an implication that the mantle input has significantly contributed to the magma genesis and the growth of newly formed crust during the Cenozoic in the central part of the Neotethyan orogenic belt.