Evidence for an early-MORB to fore-arc evolution within the Zagros suture zone: Constraints from zircon U Pb geochronology and geochemistry of the Neyriz ophiolite (South Iran)

Abstract

Late Cretaceous Neyriz ophiolite, as a part of the Outer Zagros Ophiolitic Belt, represents a remnant of the southern Neo-Tethyan oceanic lithosphere exposed along the Zagros suture zone in the south of Iran. Neyriz ophiolite is composed of dismembered lithological units including variably depleted mantle peridotites as well as intrusive and extrusive rocks preserving a geochemical evolution from mid-ocean ridge (MORB) to subduction-related (boninitic) signatures. Initially, the southern Neo-Tethyan oceanic lithosphere formed in the mid-ocean ridge spreading centre between the Arabian plate to the south west and the Eurasian plate to the north east. The inception of intra-oceanic subduction in the northern margin of the Arabian plate caused a slightly metasomatised supra-subduction-zone (SSZ) mantle lithosphere, producing MORB-like melts with a high degree of partial melting (~25–30%), and complementary depleted residual clinopyroxene (Cpx)-rich harzburgite. The zircon U-Pb geochronology for plagiogranite and gabbro intrusions reveals formation ages of 100.1 ± 2.3 Ma to 93.4 ± 1.3 Ma, respectively (Cenomanian). At a later stage, the rapid slab rollback and associated fore-arc spreading led to asthenospheric diapirism and arc-wedge mantle corner flow, which produced boninitic-like melts with advanced degrees of shallow partial melting (~35%), and ultra-depleted residual Cpx-poor harzburgites. The pervasive interaction between boninitic-like melts and refractory Cpx-poor harzburgites produced dunites as replacive lenses. Thus, this study postulates that Neyriz ophiolite formed from the Neo-Tethyan intra-oceanic subduction system, and preserves evidence of the evolution from an early-MORB oceanic crust to an extending fore-arc basin during subduction rollback processes. This tectonic environment is common in the Eastern Mediterranean region, where SSZ ophiolites have resulted in extended protoarc-forearc settings above subducted slabs.