Decoding the evolution of Paleo-Tethys: Geochemical and geochronological reinterpretation of the Paleozoic ultra-mafic to felsic rocks in Northeast Iran

Abstract

The tectono-magmatic evolution of NE Iran records the Paleozoic–early Mesozoic formation and consumption of the Paleo-Tethys Ocean. Debates focus on how the regionally-distributed basement exposures and their diverse evolution settings—such as rifting, oceanic-plateau formation, arc magmatism, and collision—are connected. The compositionally diverse meta-igneous rocks comprise the early Paleozoic Neyshabur complex, the Mississippian–Cisuralian Vakilabad complex (‘Mashhad metamorphics’), the Cisuralian Fariman complex, and the Frasnian–Permian Darreh Anjir complex; the age assignment is preliminary. Combining new field, geochemical, and geochronologic data with published ones, we specify the connection and evolution of these complexes. The Neyshabur complex has dominantly alkaline and subordinate tholeiitic rocks dated at 490–435 Ma (UPb zircon). In terms of high field strength element rations, such as Nb/Y vs. Zr/Y, they record long-lasting, mantle-plume triggered continental rifting along the northern Gondwana margin. A ∼ 364 Ma (UPb apatite) age dates hydrothermal alteration and deformation, possibly recording the integration of this complex into an oceanic accretionary wedge. Circa 195 Ma (Ar/Ar amphibole and plagioclase) metamorphism dates collisional-wedge formation, i.e., the collision of the Central Iranian (Gondwana) and Turan Blocks (Eurasia). In the Vakilabad complex, 281–268 Ma (Ar/Ar amphibole) ages of meta-komatiite/gabbro match those (∼276 Ma; cumulate) of ultra-mafic to mafic rocks of the southern Fariman complex; a ∼ 320 Ma (UPb titanite) age of tholeiitic basalt in the Vakilabad complex supports the presence of older rocks, suggested by Carboniferous radiolarian cherts. 188–183 Ma (Ar/Ar mica) metamorphism again dates collisional-wedge formation. The Darreh Anjir-complex rocks with a supra-subduction zone signature record subduction beneath an oceanic plateau within the Paleo-Tethys Ocean. The Vakilabad and the southern Fariman complexes comprise an oceanic plateau built by a mantle plume. This plateau accreted to the hanging-wall plate prior to the terminal collision, detached from the lower plate, blocked the subduction, and caused slab break-off and slab-window formation; the slab window allowed plume material to ascend. The northern Fariman complex documents this process: the rising plume material— depleted by the extraction of OIB-like magma—mixed with fluids from the subducting slab, creating boninite-like magmas. Calc-alkaline andesitic magmas, likely originating from the mantle wedge, coexisted with the OIB-like and boninite-like magmas. Our study provides an evolution model that integrates the various complexes of NE Iran into a common Late Cambrian to Late Triassic evolution of the Paleo-Tethys.