From Rheic to Paleotethys: subduction history of the Shanderman Eclogites (NW Iran)

Abstract

Pre-Jurassic plate reconstructions rely mainly on ophiolites and metabasites found along suture zones and plate boundaries. Tethyan oceanic rocks have undergone at least four Wilson cycles, characterized by Gondwana-derived continental terranes detaching and later accreting onto Laurussia/Eurasia, offering an excellent case example to understand long-term tectonic processes. The oceanic rocks from present-day north Turkey to northeast Iran contain an excellent record of those cycles. In this paper, we focus on the origins and tectonometamorphic evolution of the Shanderman eclogites in NW Iran. We have performed thorough petrological, Sm-Nd isochron, and U-Pb/Lu-Hf on zircon analyses. Our results show that the Shanderman eclogites' protoliths, basalts with mid-ocean ridge signature, underwent a maximum pressure of 2.25 GPa (~80 km) at ca. 350 Ma based on a Sm-Nd isochron in garnet. In one of the studied samples, we found a series of inherited zircon grains with Precambrian age that suggests the basaltic rocks formed or evolved in proximity to a Gondwana-derived terrane. In addition, we found a population of Paleozoic zircon grains, including a single zircon with a U-Pb age of ca. 350 Ma, possibly indicating that the oceanic crust formed immediately before subduction and metamorphism. The population of zircon with ages of ca. 280 Ma is compatible with a hydrothermal origin. We interpret that the Shanderman eclogites represent a piece of the Rheic (aka, Ran or Prototethys) that formed in the Carboniferous while the Pontide-Kopeh Dagh terrane migrated northward during the opening of the Paleotethys. The final consumption of the Rheic ocean produced the collision between the Pontide-Kopeh Dagh terrane during the late Carboniferous. Finally in the Early Permian the Paleotethys began subducting beneath Eurasia.