ZIRCON U-PB AGE AND GEOCHEMICAL CONSTRAINTS FROM THE NORTHERN SISTAN SUTURE ZONE on THE NEOTETHYAN MAGMATIC AND TECTONIC EVOLUTION IN EASTERN IRAN

The Sistan suture zone, eastern Iran is generally regarded as a deformed accretionary prism that was emplaced during the destruction in the late Cretaceous-Paleocene of a narrow arm of the Neotethys Ocean separating the Lut and Afghan continental blocks. Based on a detailed field investigation, here we report new results of zircon U-Pb age and whole rock geochemistry of representative magmatic rocks from the northern Sistan suture zone, which allow us to explore the Neotethyan magmatic and tectonic evolution in eastern Iran. The results include: (1) two gabbros in the Sistan ophiolite sequence that yield zircon U-Pb ages of 113±1 and 107±2 Ma, respectively; (2) four granodiorite intrusions that show adakitic geochemical characteristics and yield ages of ca. 86-71 Ma; (3) two A-type granites dated at ca. 56 Ma; and (4) the subsequent, widespread calc-alkaline volcanism that was emplaced from the middle Eocene to late Oligocene (ca. 45-25 Ma) not only within the suture zone but also to the west in the Lut block. Combining the above with literature data, we conclude that between the Lut and Afghan continental blocks the Neotethys Ocean should have closed before ca. 86 Ma, when the adakitic granodiorites started emplaced in the suture zone as a result of the Lut-Afghan continental suturing/collision. The compressional tectonic regime, later, switched to extensional from ca. 56 Ma so that eventually led to the voluminous volcanism in eastern Iran, which we ascribe to an orogenic collapse associated with delamination of thickened lithospheric root. This extensional regime succeeded to cause furthermore lithospheric thinning and asthenospheric upwelling that gave rise to the generation of intraplate basalts from the middle Miocene to Quaternary in eastern Iran.