U-Hf-Pb Isotope Zircon Data on A Basaltic Dyke in the Pushtashan Ophiolite, NE Iraq: Implication for Tectonic Switching of the Neo-Tethyan

Abstract

Magmatic activity of varying ages along the Zagros Orogenic Belt reflects the transition from the Neo-Tethyan oceanic subduction to continental collision. Within the Pushtashan ophiolite of northeastern Iraq, two compositionally and temporally distinct basaltic units have been identified: older and younger basalts. The younger basalt occurs as dyke, that intrude the older basalts and are notably different from the older units in both age and tectonic setting. We present here new zircon U-Pb radiometric data with Hf isotopic data for the younger basalt. The U–Pb results yield a crystallization age of 74 ± 1 Ma. Initial εHf values show a range from-1 to +10, with an average of +5, indicating a magma source of juvenile mantle material with notable assimilation of older continental crust. When integrated with published U–Pb ages from other magmatic rocks across the Zagros Orogenic Belt, our results constrain a series of younger magmatic pulses formed during alternating extensional and compressional phases of the accretionary orogen. We propose that the 74 Ma Pushtashan basaltic dyke, along with coeval magmatic events in the Zagros Orogenic Belt, record tectonic switching between advancing and retreating subduction processes, which governed the alternating geodynamic regimes along the Neo-Tethyan margin.