Composition and structure of the lithospheric mantle beneath NE Iran: Constraints from mantle xenoliths

• Primitive mantle-like composition of the upper lithospheric mantle beneath NE Iran;
• Subduction-related mantle metasomatism contributing to the genesis of the pyroxenites;
• NE Iranian lithospheric mantle reconstructed using data from unique xenolith suite.

A detailed study on petrology and mineral chemistry of 32 mantle xenoliths has been conducted to decipher the physical and chemical characteristics of the lithosphere beneath NE Iran. Spinel lherzolite, the most abundant xenolith type, is made up of olivine, orthopyroxene, clinopyroxene, and spinel. Clinopyroxenes in the spinel lherzolites display a primitive mantle-like composition, typical of non-cratonic peridotites. Pyroxenite, another major xenolith type, shows equilibrated textures and highly variable compositions including olivine websterite, websterite and clinopyroxenite. These pyroxenites, together with an equigranular dunite, delineate a clear metasomatic trend, characterized by systematic Mg#, Cr#, Al2O3, and TiO2 variations in the constituent minerals, coupled with light rare earth element enrichment and high field strength element depletion in clinopyroxene. The pyroxenites are therefore suggested to have formed by the interaction between garnet-bearing peridotites within the lithospheric mantle and melts from a stagnant slab within the asthenosphere. The lithospheric mantle may have undergone multiple stages of partial melting. The earliest stage, evidenced by the equigranular dunite, resulted in significant NiO depletion in olivine, low Al2O3 and TiO2 coupled with high Mg# and Cr# in clinopyroxene, and high Cr# in spinel. The second stage occurred more widely and gave rise to the large ion lithophile element depletion in clinopyroxenes of all rock types. The extent of melting is lower in the spinel lherzolites than that in the pyroxenites, implying that the partial melting was not caused by decompression and thus most likely related to Tethyan subduction. A third and more recent melting stage, responsible for the spongy texture in some clinopyroxenes, is attributed to the extensional tectonic regime that started in the middle Miocene in the region. Temperature estimates show that both the spinel lherzolites and pyroxenites equilibrated at ~ 900–1000 °C. Based on our new data and published data we propose the lithospheric structure beneath NE Iran. The xenolith-defined geotherm appears to be hotter than that typical in cratonic and oceanic settings, but colder than that suggested by seismic tomographic modeling for the entire Iranian Plateau. This difference is probably due to the widespread occurrence of pyroxenites within the lithospheric mantle, which we argue could have played a significant role in causing not only the seismic and gravitational anomalies but also late Cenozoic volcanism on the Iranian Plateau.