Mineral, bulk rock, and isotope geochemistry of the Late Cretaceous Sabzevar ophiolite in NE Iran and the magmatic and tectonic evolution of a continental back-arc basin oceanic crust in the Mesozoic Tethyan orogenic belt

Abstract In current geological studies, intensified investigations have been directed toward oceanic back-arc basins, with deep examination of their mechanisms of genesis, magmatic evolution from inception to maturation, and the temporal dynamics governing the transition from basin opening to back-arc spreading. Surprisingly, scarce attention has been devoted to their continental counterparts. The Late Cretaceous Sabzevar ophiolite in northeastern Iran presents a great opportunity to explore the geological, geochemical, and geodynamic evolutionary patterns of a fossil back-arc basin crust. The Sabzevar back-arc seems to have opened from the middle Cretaceous and evolved until the latest Cretaceous, with different magmatic expressions. By examining the variable mineral compositions of olivine, spinel, clinopyroxene, orthopyroxene, and amphibole in the mafic–ultramafic rocks of the Sabzevar back-arc basin, we found that the observed compositional differences in these minerals resulted from fractionation trends of magmas and interactions between different melt pulses within crustal magma chambers. Fractionation of basaltic magmas in an open chamber system produced compositionally variable cumulus minerals. High-clinopyroxene TiO2 contents and intra-crystalline variations in clinopyroxene compositions in some rock samples indicate multiple injections of different pulses of magmas during the evolution of the Sabzevar oceanic crust. Wet gabbros exhibit significant geochemical variabilities in their minerals, which suggests the influence of both water-poor and water-rich magmas during their formation. Melt compositions in equilibrium with clinopyroxenes in ultramafic cumulates reveal two distinct types of melts: island-arc tholeiitic melts, and melts originating from a highly depleted mantle affected by slab-derived fluids. Our study reveals diverse compositional variations within the crustal sections of the Sabzevar ophiolites. Notably, we observe abundant occurrences of back-arc basin basalt–type igneous rocks, both Nb-depleted and Nb-enriched varieties, alongside island-arc tholeiite–type magmatic rocks. Enriched mid-oceanic-ridge basalt (E-MORB)-, oceanic-island basalt (OIB)-, and depleted MORB–like magmatic rocks are also present. Elemental ratios such as the Nb/Yb and Nd isotopic composition of magmatic rocks show variations, which indicates that both depleted and enriched mantle have been responsible for the formation of these rocks. Various subduction components, including subducting slab fluids and melts, have also generated rocks with different enrichment and depletion in some elements such as Th, U, K, and Sr, and the light rare earth elements. The enriched MORB- and OIB-like magmatic rocks could have resulted from the melting of a trapped, enriched sub-arc mantle, or the melting of an enriched asthenosphere during plume-ridge interaction, or the melting of enriched components dispersed in the depleted upper mantle. The tectonic significance of the Sabzevar ophiolite is closely tied to the timing of the continental back-arc basin opening and the relationship of this basin’s development to the initiation of the Zagros subduction zone within the Neotethyan oceanic realm.